POSTHARVEST QUALITY OF ‘NEW’ POTATOES: EFFECTS OF IMPROVED WATER AND FERTILIZER USE EFFICIENCY DURING PRODUCTION AND APPLICATION OF

‘RAPID CURING’ DURING STORAGE

By

MILDRED N. MAKANI

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2014

© 2014 Mildred N. Makani

To my son, Michael Takudzwa. With love.

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ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. Steven Sargent, for his continuous guidance,

support, and encouragement throughout my research project. I would also like to thank

my co-advisor, Dr. Lincoln Zotarelli, for all the help with my field trials and building

confidence in me. I also thank my committee members, Dr. Donald Huber and Dr.

Charles Sims for their guidance and suggestions.

My sincere gratitude goes to the Hastings team – Allison Beyer, Douglas Gergela,

Dana Burhans, Scott Taylor, Bart Harrington for all the help setting up my field trials and

ensuring access to all necessary resources. Much thanks to everyone in Dr. Zotarelli’s

lab for all the support, particularly Patrick Moran, Charles Barrett, and Christian

Christensen. I also owe my sincere thanks to Joel Reyes-Cabrera and Adrian Berry for

all the help in both the production and postharvest experiments. Special thanks to Kim

Cordasco, James Lee, and all fellow postharvest graduate students for their assistance

with laboratory methods and statistical analysis. I would also like to express my sincere

gratitude to Dr. Dufault and Kristin Beckham for giving me access to their lab for my

microscopy work.

Finally, I would like to thank my mom, my son Michael Takudzwa, and the rest of

my family for their continuous support and unconditional love. Special thanks to my

friends Yvonne Zisengwe, the Barwa, Rukuni, Muyengwa, and Mukandavire families for

all the support through the years.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF ABBREVIATIONS ........................................................................................... 14

ABSTRACT ................................................................................................................... 15

CHAPTER

1 INTRODUCTION .................................................................................................... 17

2 LITERATURE REVIEW .......................................................................................... 20

The Irish Potato ...................................................................................................... 20 U.S. Production and Consumption .......................................................................... 22

Florida’s ‘New’ Potatoes ......................................................................................... 23 Production and Consumption ........................................................................... 23 Irrigation in the Tri-County Agricultural Area ..................................................... 25

Nitrogen Fertilizer Application in the Tri-County Agricultural Area .................... 27 Potato Growth, Development and Tuber Harvest Maturity...................................... 28

Plant Growth and Development ........................................................................ 28

Tuber Maturity Indices ...................................................................................... 29

Plant Vine Killing and Tuber Harvesting ........................................................... 31 Tuber Yield and Harvest Quality Response to Crop Production Practices ............. 32

Response to Irrigation Method ......................................................................... 32

Response to Nitrogen Fertilizer Application Method and Rate ......................... 34 Postharvest Handling and Storage of New Potatoes .............................................. 36

Tuber Skin and Wound Injury ........................................................................... 36 ‘Rapid Curing’ As a Means of Prolonging Shelf-life In New Potatoes ............... 37 Storage of New Potatoes.................................................................................. 38

Effect of Preharvest Factors on Tuber Postharvest Quality .................................... 40 Research Objectives ............................................................................................... 41

3 EFFECT OF DRIP IRRIGATION AND HARVEST TIME ON YIELD AND TUBER STORAGE QUALITY OF TWO TABLESTOCK POTATO CULTIVARS. ................ 43

Introduction ............................................................................................................. 43 Materials and Methods............................................................................................ 45

Plant Material ................................................................................................... 45 Experimental Site and Field Trials .................................................................... 46 Plant Vine Kill and Tuber Harvest Times .......................................................... 47

Tuber Yield and At Harvest Quality .................................................................. 47 Postharvest Storage Analysis ........................................................................... 48 Statistical Analysis ............................................................................................ 49

Results – ‘Fabula’ ................................................................................................... 50

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Total Rainfall Received .................................................................................... 50 Effect of Irrigation Method and Harvest Time On ‘Fabula’ Tuber Yield and At

Harvest Quality .............................................................................................. 50

Effect of Irrigation Method And Harvest Time on ‘Fabula’ Tuber Storage Quality ........................................................................................................... 51

Initial Storage Quality ................................................................................. 51 Storage Quality .......................................................................................... 52

Discussion – ‘Fabula’ .............................................................................................. 54

Results – ‘Red LaSoda’ .......................................................................................... 59 Effect of Irrigation Method and Harvest Time On ‘Red LaSoda’ Tuber Yield

and At Harvest Quality .................................................................................. 59

Effect of Irrigation Method And Harvest Time on ‘Red LaSoda’ Tuber Storage Quality ............................................................................................. 59

Initial Storage Quality ................................................................................. 59 Storage Quality .......................................................................................... 60

Discussion – ‘Red LaSoda’ ..................................................................................... 61

Conclusions ............................................................................................................ 63

4 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. I. ‘FABULA’ .......................................................... 75

Introduction ............................................................................................................. 75

Materials and Methods............................................................................................ 76 Experimental Site and Field Layout .................................................................. 76

Tuber Yield and At Harvest Quality Analysis .................................................... 78 Postharvest Analysis ........................................................................................ 80 Statistical Analysis ............................................................................................ 81

Results .................................................................................................................... 82 Weather Conditions .......................................................................................... 82

Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality ........................................................................................................... 83

Nitrogen Assimilation and Biomass Accumulation ..................................... 83

Tuber Yield and At Harvest Quality ............................................................ 84

Effect of Nitrogen Application Method and Rate on Tuber Storage Quality ...... 86 Initial Storage Quality ................................................................................. 86 Storage Quality .......................................................................................... 87

Discussion .............................................................................................................. 89 Conclusions ............................................................................................................ 97

5 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. II. RED LASODA’. ............................................. 113

Introduction ........................................................................................................... 113 Materials and Methods.......................................................................................... 114

Plant Material ................................................................................................. 114

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Statistical Analysis .......................................................................................... 114 Results .................................................................................................................. 114

Weather Conditions ........................................................................................ 114

Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality ......................................................................................................... 114

Nitrogen Uptake and Biomass Accumulation ........................................... 114 Tuber Yield and At Harvest Quality .......................................................... 116

Effect of Nitrogen Application Method and Rate on Tuber Storage Quality .... 118

Initial Storage Quality ............................................................................... 118 Storage Quality ........................................................................................ 119

Discussion ............................................................................................................ 120

Conclusions .......................................................................................................... 126

6 EVALUATION OF SKIN ADHESION STRENGTH AND ‘RAPID CURING’ AS A MEANS OF MINIMIZING QUALITY LOSSES DURING STORAGE OF ‘NEW’ POTATOES. ......................................................................................................... 140

Introduction ........................................................................................................... 140

Materials and Methods.......................................................................................... 143 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In

Relation To Nitrogen Treatment and Harvest Time ..................................... 143

Plant Material ........................................................................................... 143 Vine Kill and Tuber Harvesting ................................................................. 143

Tuber Skin-set Tests ................................................................................ 144 Experiment 2: Evaluation Of Tuber Wound Periderm Suberization ................ 145

Determination of ‘Rapid Curing’ Conditions ............................................. 145 Effect of Storage Condition On Wound Periderm Suberization ................ 146 Tissue Preparation And Histochemical Analysis ...................................... 147

Statistical Analysis .......................................................................................... 148 Results .................................................................................................................. 148

Weather Conditions ........................................................................................ 148 Tuber Decay Incidence .................................................................................. 149 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In

Relation To Nitrogen Treatment and Harvest Time ..................................... 149

Experiment 2: Evaluation Of Tuber Wound Periderm Suberization ................ 151 Determination of ‘Rapid Curing’ Conditions ............................................. 151 Effect of Storage Condition On Wound Periderm Suberization ................ 151

Discussion ............................................................................................................ 152 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In

Relation To Nitrogen Treatment and Harvest Time ..................................... 152 Experiment 2: Evaluation Of Tuber Wound Periderm Suberization ................ 155

Conclusions .......................................................................................................... 156

7 SUMMARY AND FINAL CONCLUSIONS ............................................................. 167

APPENDIX: SUPPORTING DATA .............................................................................. 171

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LIST OF REFERENCES ............................................................................................. 172

BIOGRAPHICAL SKETCH .......................................................................................... 191

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LIST OF TABLES Table page 3-1 Effect of irrigation method on yield and incidence of physiological disorders in

‘Fabula’ tubers harvested 3 weeks after vine kill ................................................ 65

3-2 Analysis of variance for irrigation method and harvest time on selected quality parameters for ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH ....... 66

3-3 Effect of harvest time on peel and pulp dry matter contents of ‘Fabula’ tubers’ initial storage quality (season 2) ......................................................................... 67

3-4 Effect of harvest time after vine kill on tuber peel and pulp ascorbic acid contents of ‘Fabula’ tubers’ initial storage quality (season 2) ............................. 67

3-5 Effect of irrigation method and harvest time on firmness of ‘Fabula’ tubers, stored for 14 d at 10°C, 80-85% RH (season 2) ................................................. 67

3-6 Effect of irrigation method and harvest time on tuber peel and pulp ascorbic acid content of ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH (season 2) ........................................................................................................................ 68

3-7 Effect of irrigation method on yield and incidence of physiological disorders in ‘Red LaSoda’ tubers harvested 3 weeks after vine kill ....................................... 69

3-8 Analysis of variance for irrigation method and harvest time for selected quality parameters for ‘Red LaSoda’ tubers stored for 14 d at 10°C, 80-85% RH ...................................................................................................................... 70

3-9 Effect of harvest time on cumulative fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2) ................................................. 71

3-10 Effect of irrigation method and harvest time on tuber firmness of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2) ........................ 71

4-1 N fertilizer rates and application methods for ‘Fabula’ potato fertigation trial in 2013 (season 1) and 2014 (season 2) ................................................................ 99

4-2 Plant aboveground (AG) and tuber biomass, on a dry weight basis, at full flower of ‘Fabula’ potatoes grown under five nitrogen treatments in 2013 (season 1) and 2014 (season 2). ........................................................................ 99

4-3 Plant aboveground and tuber nitrogen uptake of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2). ........................................................................ 99

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4-4 Total Kjeldahl nitrogen of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2)......................................................................................................... 100

4-5 Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 1) ............................... 101

4-6 Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 2) ............................... 101

4-7 Tuber external disorders in ‘Fabula’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill (season 1 and 2) ................. 102

4-8 Analysis of variance of season, fertilizer treatment and harvest time effect on tuber initial storage quality in ‘Fabula’ ............................................................... 102

4-9 Effect of fertilizer treatment and harvest time on pulp dry matter content of ‘Fabula’ tubers’ initial storage quality (season 2) .............................................. 103

4-10 Effect of fertilizer treatment on soluble solids content (SSC), total titratable acidity (TTA), and pH of ‘Fabula’ initial storage quality (season 1 and 2) ......... 103

4-11 Analysis of variance of season, fertilizer treatment, and harvest time effect on tuber quality in ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ........................... 104

4-12 Effect of fertilizer treatment and harvest time on tuber peel dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ............................................... 105

5-1 Plant aboveground (AG) and tuber biomass, at full flower of ‘Red LaSoda’ potatoes grown under five nitrogen treatments (season 1 and 2). .................... 129

5-2 Plant aboveground and tuber nitrogen uptake of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2). ...................................................................... 129

5-3 Total Kjeldahl nitrogen of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2)......................................................................................................... 130

5-4 Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 1) ................. 131

5-5 Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 2) ................. 131

5-6 Tuber external disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill in 2013 (season 1) and 2014 (season 2) ................................................................................................ 132

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5-7 Analysis of variance of season, fertilizer treatment, and harvest time on tubers’ initial storage quality in ‘Red LaSoda’ ................................................... 132

5-8 Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 1). ............ 133

5-9 Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 2). ............ 133

5-10 Analysis of variance of fertilizer treatment, harvest time on tuber quality in ‘Red LaSoda’ tubers stored at 10°C, 80-85% RH for 14 d ................................ 134

5-11 Effect of harvest time on tuber peel dry matter content of ‘Red LaSoda’ stored for 14 d at 10°C, 80-85% RH ................................................................. 135

6-1 Skin-set readings of non-vine killed ‘Fabula’ tubers grown under five nitrogen treatments and harvested over two weeks (91 to 105 days after planting), during season 1. ............................................................................................... 160

6-2 Effect of vine kill on skin-set readings of ‘Fabula’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting). ................................................................................... 160

6-3 Effect of vine kill on skin-set readings of ‘Red LaSoda’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting). ......................................................................... 161

6-4 Average thickness of suberized wound periderm layer of ‘Red LaSoda’ stored for 7 days at 15, 20, and 25ºC, 90-95% RH. .......................................... 161

A-1 Tuber classification used to evaluate marketable and unmarketable yield after harvest, 3 weeks after vine kill. ................................................................. 171

A-2 Effect of interaction of fertilizer treatment and harvest time on tuber pulp dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ........................ 171

A-3 Effect of an interaction of fertilizer treatment, harvest time and storage time on soluble solids content (SSC), and total titratable acidity (TTA) and pH of ‘Fabula’ tubers stored at 10°C, 80-85% RH for 14 d. ........................................ 171

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LIST OF FIGURES Figure page 3-1 Total and cumulative rainfall during growing A) season 1, and B) season 2 in

Hastings, FL. ...................................................................................................... 72

3-2 Effect of harvest time on firmness of ‘Fabula’ tubers’ initial storage quality (season 2)........................................................................................................... 73

3-3 Fresh weight loss in ‘Fabula’ tubers stored for 14 d at 10ºC, 80-85% RH. ......... 73

3-4 Effect of harvest time on firmness of ‘Red LaSoda’ tubers’ initial storage quality (season 2).. ............................................................................................. 74

4-1 Rainfall and cumulative evapotranspiration in Hastings, FL, during growing A) season 1, and B) season 2 ............................................................................... 106

4-2 Mean air and soil temperatures experienced during growing A) season 1, and B) season 2 ...................................................................................................... 107

4-3 Tuber specific gravity of ‘Fabula’ potatoes grown under five fertilizer treatments......................................................................................................... 108

4-4 Effect of harvest time on the peel dry matter content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill. ................................................................... 108

4-5 Effect of harvest time on the peel and pulp ascorbic acid content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill. ........................................................ 109

4-6 Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 1). ......... 110

4-7 Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 2). ......... 111

4-8 Firmness of ‘Fabula’ tubers stored for 14 d at 10ºC and 80-85% RH ............... 112

5-1 Tuber specific gravity of ‘Red LaSoda’ potatoes grown under five fertilizer treatments during A) season 1, and B) season 2.............................................. 136

5-2 Peel dry matter content of ‘Red LaSoda’ tubers’ initial storage quality, harvested 1-3 weeks after vine kill (season 1). ................................................. 137

5-3 Pulp dry matter content of ‘Red LaSoda’ tubers’ initial storage quality grown under five fertilizer treatments (season 1). ....................................................... 137

5-4 Firmness of ‘Red LaSoda’ tubers’ initial storage quality harvested 1-3 weeks after vine kill (H1-3) (season 1) ......................................................................... 138

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5-5 Tissue pH of ‘Red LaSoda’ tubers’ initial storage quality, grown under five fertilizer treatments during two seasons ........................................................... 138

5-6 Fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH during A) season 1, and B) season 2. ........................................................ 139

6-1 Modified skin-set tester used to measure resistance to periderm injury. .......... 162

6-2 Layout of trays with tubers in racks for ‘rapid curing’ at 20ºC, 90-95% RH.. ..... 162

6-3 Total rainfall experienced during tuber harvest period in season 1 and 2. ........ 163

6-4 Tuber decay in A) ‘Fabula and B) ‘Red LaSoda’ tubers harvested 1 to 3 weeks after vine kill (H1-H3) during two seasons. ............................................ 164

6-5 Fresh weight loss in wounded ‘Red LaSoda’ tubers stored for 7 days at 15, 20, and 25ºC, 90-95% RH. ............................................................................... 165

6-6 Wound periderm suberization of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ............................................................................................................. 165

6-7 Wound periderm suberization of ‘Red LaSoda’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ................................................................................................... 166

6-8 Fresh weight loss of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ..... 166

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LIST OF ABBREVIATIONS

°C Degree Celsius

d Days

DAP Days after planting

kg.ha-1 Kilograms per hectare

mg 100-1 Milligrams per 100 grams

µm Micrometer

N·m Newton meter

FAO Food and Agricultural Organization

FAWN

USDA

Florida Automated Weather Station

United States Department of Agriculture

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

POSTHARVEST QUALITY OF ‘NEW’ POTATOES: EFFECTS OF IMPROVED WATER AND FERTILIZER USE EFFICIENCY DURING PRODUCTION AND APPLICATION OF

‘RAPID CURING’ DURING STORAGE

By

Mildred N Makani

December 2014

Chair: Steven A. Sargent Cochair: Lincoln Zotarelli Major: Horticultural Sciences

Two tablestock potato cultivars were evaluated for yield and tuber storage quality

response to irrigation method, N-fertilizer rate, application method, and harvest time. In

spring 2011 and 2012, ‘Fabula’ and ‘Red LaSoda’ were irrigated using seepage (SP),

surface drip (SD), or sub-surface drip (Sub-SD) irrigation. To increase tuber skin-set,

plant vines were killed before tuber harvest, 1 to 3 weeks later (H1, H2 and H3) and

stored for 14 days at 10ºC, 80-85% relative humidity. In ‘Fabula’, SD and SP produced

comparable yields, averaging 26,544 kg ha-1. However, soil moisture fluctuations under

SP resulted in higher tuber physiological disorders. ‘Red LaSoda’ was not well adapted

to the soil wetting patterns of drip irrigation, resulting in significantly lower yields and

quality compared to SP. In both cultivars, generally H2 and H3 tubers maintained better

storage quality.

In spring 2013 and 2014, effects of N-fertilizer rate and application method on

method on yield, storage quality and tuber skin-set (in non- and vine killed tubers) was

evaluated. N was applied in standard granular form (224 kg N ha-1) or through SD

irrigation (fertigation rates: 0, 112, 224, 336 kg N ha-1). In both cultivars, N fertigation

16

rate of 224 kg N ha-1 had comparable yields, and similar or better storage quality than

the other N treatments. In non-vine killed ‘Fabula’, N stress at 0 and 336 kg N ha-1 likely

delayed tuber development, resulting in lower skin-set. High rainfalls during the harvest

period delayed skin maturation in both cultivars, resulting in similar skin-set in non- and

vine killed tubers. A drier harvest period resulted in higher skin-set, as early one week

after vine kill, in vine killed tubers.

Therefore, irrigating with SD, using 224 kg N ha-1 has the potential to produce

comparable yields to higher N rates, while maintaining tuber postharvest quality in

‘Fabula’. In ‘Red LaSoda’, although drip irrigation produced tubers of comparable

storage quality, SP resulted in higher yields. In both cultivars, tubers harvested two to

three weeks after vine kill generally had higher skin-set and better storability. Storage at

10ºC, 80-85% RH also promoted wound-healing while minimizing weight loss.

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CHAPTER 1 INTRODUCTION

The internal and external quality of potato tubers is very important as it affects

marketability and processing quality. Quality encompasses a combination of

characteristics that give a commodity its degree of excellence and value as food

(Abbott, 1998; Kader 2002). The relative importance of quality parameters depends on

the intended end use (Feltran et al., 2004; Singh and Kumar, 2004).

A study conducted by Jemison et al. (2008) showed that consumers select

tablestock potatoes based on visual characteristics such as shape and color. Therefore,

tubers that are free from any type of blemishes tend to attain a higher market price

(Jemison et al., 2008; Singh et al. 2004). On the other hand, while tuber external

appearance is important, consumers are also concerned with the processing, sensory

and storage quality of tubers. The degree of maturity at harvest is generally recognized

as an important determinant of storability and processing quality (Kumar et al, 2004).

Potato tuber maturity can be viewed as three main components: compositional,

physiological, and physical (Bussan, 2003). Compositional maturity refers to the sugar

concentration of tubers, with mature tubers having minimum levels of sucrose sugars

(Iritani and Weller, 1980; Sowokinos, 1978). Physiologically mature tubers have

reached maximum dry matter content, which usually coincides with maximum starch

accumulation (Sabba et al., 2007; Sowokinos, 1978). The sensory and processing

quality of tubers is greatly influenced by the degree of compositional and physiological

maturity at harvest (Rosenthal, 1999; Sowokinos et al., 2000; Thompson et al., 2008).

Meanwhile, tuber external appearance is greatly influenced by physical maturity, which

18

is the resistance of the periderm (skin) to excoriation (Halderson and Henning, 1993;

Lulai and Orr, 1994; Wilcoxon et al., 1985).

Any form of stress during growth, such as non-uniform soil moisture or nutrient

imbalances, affects various physiological processes. This, in turn, affects leaf growth

and carbon partitioning, thereby negatively affecting tuber maturity and harvest quality

(Tyner, 1997; Wigginton 1974). Storability of tubers is largely determined by the maturity

and condition of the tubers going into storage (Iritani and Weller, 1980). A higher degree

of quantitative and qualitative losses are associated with storage of immature tubers

(Burton, 1989; Iritani and Weller, 1980; Makani, 2010; Wichrowska et al., 2009).

Greater awareness of how preharvest and postharvest factors affect tuber quality

has resulted in increased research on ways of maximizing yields while maintaining tuber

quality. However, most of the research has been devoted to medium and late season

cultivars, commonly known as the fall crop. The fall crop is grown in the fall season and

takes a minimum of four months to mature. Due to longer growing seasons, the crop is

harvested when the tuber skin is set and the vines have commenced natural

senescence. Curing the tubers briefly at high temperatures and relative humidity further

increases physical maturity (skin-set) and heals existing wounds, minimizing storage

losses.

The crop grown in Florida has a shorter growing season of between 90 to 110

days (Mossler and Hutchinson, 2008). These early to medium season cultivars reach

compositional and physiological maturity before the inclement weather conditions of

high rainfall and temperatures set in (Park, et al., 2006). However, the tubers are

harvested before reaching physical maturity, and are therefore characterized by a thin

19

periderm which is easily damaged during harvesting and handling (Brecht, 2012;

Chiputula, 2009; Suslow and Voss, 1996). Due to the relatively rapid loss of quality,

these ‘new’ or immature potatoes are stored for very short periods, without curing,

before being distributed to their respective U.S. and Canadian markets (Emekandoko et

al., 2006).

Until recently, production and postharvest handling of new potatoes has been

based on knowledge gained from the behavior of the fall crop. However, applying

findings from studies of the fall crop may cause yield and quality losses in new potatoes.

Therefore, the overall objective of this study was to determine how preharvest factors

affect tuber yield, harvest quality and storability of two tablestock potato cultivars,

commonly grown in Florida.

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CHAPTER 2 LITERATURE REVIEW

The Irish Potato

A native of the Andean highlands of Peru, the commercial potato (Solanum

tuberosum L.) is an annual, herbaceous, dicotyledonous plant belonging to the family

Solanaceae, section Tuberarium. It was derived from a hybrid cross between S.

tuberosum and S. andigenum, and is the most frequently cultivated potato belonging to

the genus Solanum (Salaman et al., 1954).

The aerial parts of the potato plant range from 30 to 80 cm in length, with some

cultivars reaching two meters. White, yellow, purple, blue or variegated inflorescences

are borne on the stems; with many cultivars producing fruit (Linsinka and Leszczynki,

1989). The seed found in the fruit can be used for propagation in breeding; while

vegetative propagation using tubers is the commonly used method in commercial

production (Loria, 2001). Extensive breeding programs have produced many

commercial cultivars which vary in time of maturity (early to late season cultivars), yield

potential, appearance, tuber quality and disease resistance.

The tuber, which is the edible portion, is an enlarged underground stem that

forms at the end of stolons. It maintains the characteristics of the above ground stem,

such as nodes, internodes, scale leaves, and lenticel pores. There are two ends to a

tuber – the bud end and stem end, the latter of which is attached to the stolon. The

tuber consists of four primary zones of tissue (Dean, 1994).The first zone, the periderm

or skin, is the outermost layer and cultivars range in color from brown russet, white, red,

pink, purple or yellow. The skin texture ranges from smooth to netted. The cortex, which

lies between the periderm and the vascular tissue, is the second tissue zone. It contains

21

the highest starch concentration. Beneath the cortex lies the perimedullary zone, which

comprises the largest amount of storage tissue. The pith is the central part of the tuber,

and has the lowest starch concentration. The tuber flesh, comprising of the cortex,

perimedullary zone and pith can be white, cream, yellow, purple or striated in color

(Burton, 1989). Tuber shapes are round, oval, oblate, or a combination.

The internal and external quality of potato tubers is very important as it influences

the marketability, processing and sensory quality. Generally, blemish-free, well-shaped

tubers help growers obtain the best market price, while tubers with a good cooking

quality are a favorite among consumers and processors. Nutritionally, the tuber is a low-

fat (0.1%) vegetable; with the digestible dry matter consisting mainly of carbohydrates

(Brown, 2005; Mackay et al., 1987). Raw potatoes have an average energy content of

about 80 kcal per 100 g fresh weight basis (Li, 1985).

Small amounts of antioxidant carotenoids are present in the flesh of all tubers,

ranging from 50 to100 µg per 100 g fresh weight, in white-fleshed tubers. Yellow to

orange-fleshed tubers have a considerably higher amount, averaging 2000 µg per 100 g

fresh weight (Brown, 2005). Significant amounts of iron, thiamine, nicotinic acid and

riboflavin are also found in the vegetable crop. Ascorbic acid (vitamin C) is one of the

most important nutrients in potato. Despite the modest contents (10-30 mg per 100 g

FW), potato tubers are a major source of ascorbic acid because of the large quantities

consumed annually (Tudela et al., 2002). The nutrient, as with other antioxidants, is

found in greater quantities in the potato skin. The potato is a major food source in many

parts of the world largely because of its ability to produce large yields per unit land area

(Dean, 1994).

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U.S. Production and Consumption

Potatoes are grown in over 130 countries across the globe, with the USA

currently being the fourth largest producer – growing the crop commercially in 36 states

(FAO, 2010). In 2013, the U.S. potato industry generated $4.3 billion on over 400,000

ha of harvested land, making potato the top vegetable crop grown (USDA, 2014).

Location of potato production in the U.S. is based on consumer need, new markets and

technological developments (Dean, 1994). The crop is grown year round, with the fall

crop, which is harvested from August through November, contributing about 90% of the

total national production. The primary state producers of the fall crop (also known as fall

crop) include Idaho, Washington, and Wisconsin, producing potatoes worth over $1.9

billion, 56% of the national production in 2012 (USDA, 2012).

However, the spring crop, harvested between April and June, realizes a higher

market price compared to the fall crop. This is because, unlike the fall crop which is

normally stored for months, the spring crop is freshly harvested and marketed, and

therefore meets specific market requirements. Florida, together with Arizona, California,

North Carolina and Texas, are the main producers of the high-value spring crop.

The U.S. potato industry has two main markets: the tablestock and processing

market. Attributes of the cultivar which help determine final use of the tubers include

skin type, set, color, shape and specific gravity. Other desirable characteristics include

yield, adaptability and disease resistance (Pack et al., 2006). Since the late 1980s, the

consumption of potato as food has shifted from tablestock potatoes, to primarily

processed products. The national average per capita utilization of potatoes in 2012 was

36 kg per person in processed products, and 16 kg per person for tablestock potatoes

(USDA, 2013). Processors used 12.8 million metric tons (MT) of potatoes to produce a

23

variety of products. Frozen french fries and other frozen products utilized close to 60%

of the processing potatoes, with the rest of the share going to chips, shoestrings

dehydrating, canning, starch and flour production (USDA, 2013). Though a 15%

increase from 2011, only 5.4 million MT were used as tablestock potatoes, with the

common cooking methods being boiling, baking, roasting, frying, microwaving and

steaming.

Irrespective of the lower demand for tablestock potatoes, the industry still

accounts for a big portion, with 30% of the national produce of 2012 being utilized as

tablestock potatoes (National Potato Council, 2013). Less than 6 % of the national

produce was used in the seed industry and for livestock feed (USDA, 2013).

Florida’s ‘New’ Potatoes

Production and Consumption

When a potato cultivar is released, it is classified either as ‘early’, ‘medium’, or

‘late’ season. This is based on vine growth and tuber bulking observations over many

years. Florida potatoes are early to medium season cultivars with an average growing

season of between 90 – 110 days (Mossler and Hutchinson, 2008). They are commonly

known as ‘new’ or ‘early’ potatoes because of their characteristic immature, thin skin

which easily detaches from the underlying cortical tissue during harvest and handling.

‘New’ potatoes can be defined as potatoes harvested before they are completely

mature, marketed immediately after harvesting and whose skin can be easily removed

without peeling (UNECE of Geneva, Standard FFV-30/2001).

With a $135 million market value, the Florida potato industry is relatively small

when compared to other state producers such as Idaho and Washington. However,

Florida contributes a third of the high-value crop produced in the winter or spring season

24

(Mossler and Hutchinson, 2008). In the 2013 growing season, Florida’s new potatoes

attained the highest market price of $0.38/kg, while the national average was $0.20/kg

(USDA, 2014).

Florida’s new potatoes are grown all over the state, with an average of 14,933 ha

across the southern and northern counties being used for production. Eighty percent of

potatoes grown in Florida are harvested between May and June (spring growing

season), with a smaller proportion being harvested between January and April (winter

growing season) in the southern parts of state. The lead state producer is the Tri-

County Agricultural Area (TCAA) of Putnam, St. Johns, and Flagler counties in the

northeast, accounting for approximately 85% of the nation’s spring potato production,

and an average yield of 32,630 kg ha-1 (Hochmuth and Hanlon, 2011; USDA, 2012)

Florida is one of the top shipping states in the nation, with most of the growers

establishing pre-season contracts mainly for the processing industry (USDA, 2010).

Cultivars with a high specific gravity, such as ‘Atlantic’ and ‘Harley Blackwell’ are

produced commercially for use in the chipping industry. Tablestock potatoes are also

high value crops, with grower production marketing costs at an average of $4,140 per

acre (Van Sickle et al., 2009). The leading tablestock cultivars grown in Florida include

‘Red LaSoda’, ‘LaRouge’ and ‘LaChipper’, which are all early to medium season

cultivars (Hutchinson et al., 1999).

With potatoes being grown in sandy soils with low water holding capacities and

nutrient content, growers in the TCAA typically irrigate the plants and apply inorganic

fertilizers to maximize yields.

25

Irrigation in the Tri-County Agricultural Area

Water is essential for crop production, accounting for approximately 34% of the

nation’s surface and groundwater usage in the 21st century (USDA, 2012). Florida uses

about 45% of the total freshwater allocated to Agriculture in the country; ranking 9th

nationally and 2nd in the eastern US total irrigated acreage (Marella, 1999; Smajstrla et

al., 1997). Potatoes are the major irrigated crop in the TCAA, with a daily average water

withdrawal of approximately 782 million m3 (Tripensee, 1995; SJRWMD, 1995). An

increase in resident population and erratic rainfall distribution, with 60% of the rain

falling between June and September, has led to increased water demand in the area

(Fereres et al., 2003). Extended periods of dry weather, with some areas experiencing

rainfall deficits of 30 cm, have additionally resulted in a significant decrease in ground

and surface water levels (SJRWMD, 2012).

To supplement the rainfall, majority of the potato crop in the TCAA is irrigated,

with seepage irrigation being the most common method used (Smajstrla et al., 1991;

SJRWMD, 1995). The irrigation season is concentrated between February and May due

to rising daily temperatures and erratic rainfalls. There are two main types of seepage

systems: semi-closed pipeline and open ditch, with the former system being the

common type used by growers in the TCAA. Seepage irrigation is a sub-surface

irrigation system where potatoes are planted on high row beds and shallow lateral

ditches supply irrigation water and facilitate drainage. Water is pumped from deep wells,

raising the natural water table to just below the plant root zone with water moving up by

capillary action to the root zone. The height of the perched water table is maintained by

a loamy fine sand restricting layer, which lies 1 to 1.6 m below the soil surface in the

area (Munoz-Arboleda et al., 2008).

26

Seepage irrigation is a popular method in Florida because it is less expensive to

install and operate (Locascio, 2005). However, the system demands high natural water

tables, or else large quantities of water are required to raise the water table. In addition,

to ensure uniform soil wetting, fields must be relatively level and soils uniformly porous

(Smajstrla et al., 2002). Semi-closed seepage irrigated systems have been reported to

be only 30-70% efficient (Smajstrla et al., 1991). These shortfalls, coupled with

increased competition for water resources have resulted in a growing need to evaluate

drip irrigation as an alternative method for potato production in the TCAA.

Micro irrigation systems such as drip have already proved to be more water use

efficient, using 30-50% less water compared to seepage systems (Niebling and Brooks,

1995; Reyes-Cabrera et al., 2014). With drip irrigation, drippers are placed on the soil

surface (surface drip), or buried (sub-surface drip), with the water being discharged

close to the plant root zone, at a controlled rate. The soil surface is only wetted within

30-60 cm of the water source, thereby maximizing the amount of moisture in the crop

root zone, and minimizing evaporation (Haman et al., 1996). Drip irrigation also reduces

nutrient leaching, erosion, deep percolation, runoff and pumping costs, saving an

average of $110 ha-1 when compared to seepage irrigation (Dukes et al., 2004;

Smajstrla et al., 1991).

Originally, the drip irrigation systems were not popular due to their high

installation and maintenance costs when compared to other conventional methods like

seepage and sprinkler irrigation. However, improvements in the system, such as better

tubing material that can be used over many growing seasons, has renewed interest in

the irrigation method (Camp, 1998). Burying the drip tape below the soil surface, as

27

practiced in sub-surface drip irrigation, further protects the tape from damage and

further reduces evaporation or runoff (Shock, 2006).

Nitrogen Fertilizer Application in the Tri-County Agricultural Area

Carbon, hydrogen, and oxygen, which are assimilated from carbon dioxide and

water uptake, constitute about 95% of the potato biomass. The remaining comes from a

range of macro- and micro-elements which are absorbed from the soil solution (Bucher

and Kossman, 2007). Nitrogen (N), phosphorus (P), and potassium (K) are important as

they greatly affect tuber growth, development and quality. In nutrient deficient soils,

application of organic or inorganic fertilizers improves plant growth and increases tuber

yield (Allison et al., 2001; Trehan and Sharma, 2003; White et al., 2007).

N is a constituent of amino acids, proteins, and nucleic acids, making it the fourth

most abundant element found in tubers. It is absorbed by the plant primarily in the form

of ammonium (NH4+) or nitrate (NO3-) and is the most limiting nutrient in sandy soils (Li

et al., 1999). The nutrient influences biomass accumulation and partitioning, dry matter

content, specific gravity, sugar content, tuber defects and maturity (Marschner, 1995;

Shock et al., 1990). In the past years, excessive N fertilizer application was justified as a

means of ensuring high yields, and rates as high as 300 kg ha-1 were applied in the

1980s (Hochmuth at al., 1993; Weingartner et al., 1999). This liberal application of N

and irrigation water increased nitrate leaching and runoff, in addition to other losses

through gaseous exchange and fixation, thereby reducing the amount of N available to

plants. Further research concluded that 150-250 kg N ha-1 was adequate for most soils

(White et al., 2007). The current University of Florida-IFAS recommended N fertilizer

rate in the TCAA is 224 kg ha-1 (Hochmuth and Hanlon, 2000; Hutchinson et al., 2008).

28

Policymakers and producers are continuously developing new N management

strategies for potato that will further improve nutrient use efficiency and conserve the

environment. Nutrient use efficiency in soils is greatly influenced by fertilizer application

timing and placement (Baligar and Bennett, 1986; Joern and Vitosh, 1995; White et al.,

2007). The current grower practice in the TCAA is band-application of granular N

fertilizer at crop emergence, followed by a side dress at tuber initiation. There is a

period of about 45 days between planting and rapid N uptake by the plant, during which

the risk of leaching is high (Zebarth and Milburn, 2003). More precise application timing

and placement of fertilizer is therefore critical to achieve maximum nutrient use

efficiency.

Applying fertilizers through fertigation has been proposed as a means to meet

production and environmental goals of both growers and policymakers. Fertigation is

the delivery of dissolved fertilizer through drip tapes. This method allows growers to

better synchronize fertilizer application with plant demands, by splitting N applications

throughout the growing period (Shock et al., 2006; Westermann et al. 1988). Fertilizer

applications through drip tapes also allow application where roots are concentrated,

further improving efficiency (Schwankl and Prichard, 2001) and therefore possibly

reducing the amount of fertilizer required to achieve good yields (Shock, 2006).

Potato Growth, Development and Tuber Harvest Maturity

Plant Growth and Development

The life cycle of the potato plant can be divided into five main growth stages, with

early season cultivars reaching maturity in less than 4 months, while late cultivars can

take up to 7 months, depending on the prevailing weather conditions (Kay, 1973; Rowe,

1993; Mossler and Hutchinson, 2008).

29

The first plant growth stage, sprout development (I), begins after planting, when

eyes break dormancy and produce sprouts. This is followed by a 15 to 30 day long

vegetative stage (II), during which leaves, roots and stems develop, and photosynthesis

commences. Tuber initiation stage (III), with duration of 10 to 14 days, begins when

tubers develop at the stolon tips but are not appreciably enlarging.

The tuber bulking stage (IV) is the longest stage, with duration of 60 to over 120

days. During this stage, tuber cells expand with the accumulation of water, nutrients and

carbohydrates, increasing tuber size and weight. The final development phase, the

maturation stage (V), begins with canopy senescence. Leaves become chlorotic and

then necrotic, falling off as the stage progresses. Photosynthesis decreases, tuber

growth slows, with any further increases in tuber dry matter resulting from translocation

of photosynthates from the vegetative tops into the tubers. Tuber maturity is reached

when dry matter content reaches a maximum, reducing sugars content is at a minimum,

and the tuber skin has set.

Tuber Maturity Indices

Tuber maturity at harvest is generally recognized as an important determinant of

tuber storage ability and processing quality (Kumar et al, 2004). Maturity can be viewed

as composed of three components: compositional, physiological, and physical maturity

(Bussan, 2003). Compositional maturity refers to the sugar concentration of tubers, with

mature tubers having minimum levels of sucrose sugars (Iritani and Weller, 1980;

Sowokinos, 1978). Tuber starch content is a measure of physiological maturity.

Physiologically mature tubers have reached maximum dry matter content, which usually

coincides with maximum starch accumulation (Sabba et al., 2007; Sowokinos, 1973).

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The sensory and processing quality of tubers is greatly influenced by the degree of

compositional and physiological maturity of the tubers at harvest.

Physical maturity refers to resistance to excoriation of the periderm, which is a

measure of skin maturation (Halderson and Henning, 1993; Lulai and Orr, 1994;

Wilcoxon et al., 1985). The periderm is a protective tissue comprising of three parts: the

phellogen (cork cambium), the phellem (cork), and the phelloderm. The phellogen,

which is comprised of actively dividing meristematic cambium cells originates from the

epidermis, and produces phellem cells outwardly and phelloderm cells towards the

cortex (Fahn, 1990). As phellem cells develop, they become suberized and die, to form

the protective layer known as the ‘skin’ (Ginzberg et al., 2009).

Suberin, a complex biopolyester, is deposited on the cell walls when plants are

exposed to biotic or abiotic environmental stress stimuli (Schreiber et al., 2005). It is

comprised of two major domains, a phenolic domain and an aliphatic domain, in

addition to soluble waxes found within the matrix (Lulai and Corsini, 1998). Suberization

of the tuber skin provides a barrier to desiccation of internal tissues and impedes

microbial invasion.

An immature periderm contains a phellogen layer with thin radial cell walls that

fracture easily during harvesting and handling, resulting in the slipping of the phellem

layer (Lulai and Freeman, 2001; Suslow and Voss, 2000). As the periderm matures,

meristematic activity slows, phellogen cell walls thicken with the physiological bonding

process known as ‘skin-set’ occurring during the final stages of periderm maturation.

Tuber physical maturity is of primary importance in Florida’s new potatoes

because the tubers reach horticultural maturity before the periderm has fully matured.

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The tubers have a poorly developed skin that rubs off easily during harvest and

packaging (Sabba and Lulai, 2002). This can potentially lower the market value of fresh

market potatoes as appearance is one of the key factors influencing consumer

preference. In addition, exposure of the internal tissue increases the occurrence of

water loss and disease pathogen entry.

Plant Vine Killing and Tuber Harvesting

In order to promote skin-set and develop resistance to excoriation, potato plant

vines are killed 7 to 21 days before harvest, a process termed ‘vine kill’ (Bohl, 2003;

Plissey, 1993; Mossler and Hutchinson, 2008). Vine kill induces tuber maturity,

encourages tubers to loosen from stolons, and helps ease harvest by reducing plant

vine quantity (Mossler and Hutchinson, 2008). Vines can be killed mechanically by vine

pulling or cutting, chemically using herbicides, or a combination of both methods

(Haderlie, et al., 1989).

Chemical vine killing using non-selective herbicides is the preferred method in

North America (Haderlie et al., 1989). Slow-acting herbicides are applied as split-

applications at the first indication of tuber maturity, when the plant tops begin to

senesce. Faster acting herbicides, such as diquat dibromide (Syngenta, NC), are

applied once and require at least 7 days between application and tuber harvest

(Hochmuth, et al., 1999; Hutchinson, et al., 1999). However, due to a number of other

factors such as cultivar and plant growth stage, the actual number of days from

application of the desiccant to the harvesting of potatoes ranges from 7 to 21 days.

Sample tubers are tested by growers for adequate skin-set by applying thumb

pressure and lateral force to the periderm – with the tubers only been harvested when

there is reduced skin slipping (Bowen et al., 1996). However, delaying tuber harvesting

32

presents problems especially at the end of the growing season, with high rainfall

occurrences causing losses through tuber decay.

Tuber Yield and Harvest Quality Response to Crop Production Practices

Potato tuber yield and harvest quality can be influenced by preharvest

environmental and production practices, such as irrigation method, fertilizer rate and

application method.

Response to Irrigation Method

The potato plant is characterized by a shallow root system, with more than 90%

of the total root volume in the top 25 cm of soil, making it relatively sensitive to

inadequate or excessive amounts of soil moisture (Munoz-Arboleda, 2004). Water

availability is one of the critical factors influencing tuber yield and quality. Small

deviations from optimum water application results in water stress, which impairs the

plant’s photosynthetic, transpiration, respiratory, and cell enlargement activities.

(Fabeiro et al., 2001; Kumar et al., 2004; Ojala et al., 1990; Shock et al., 2007).

Differences in water application and soil wetting patterns among different

irrigation methods have varying effects on soil moisture levels throughout the growing

season. Seepage irrigation artificially raises the natural water table to just beneath the

plant root zone, with water moving up by capillary action to the root zone. In drip

irrigation, water is applied close to the root zone, moving down gravitationally (surface

drip) or up by capillarity (sub-surface drip). An inefficient soil wetting pattern could

trigger moisture stress, affecting leaf growth and carbon partitioning, thereby reducing

yield, tuber growth, skin development, compositional and nutritional qualities (Wigginton

1974; Tyner, 1997).

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Generally, total and marketable tuber yields decline when plants are grown under

inadequate soil moisture levels for an extended period of time. Water stress and soil

moisture fluctuations have been associated with secondary growth, growth crack, tuber

malformations, and other physiological disorders such as brown center and hollow heart

(Shock et al., 2006). The stage of growth determines the degree of sensitivity to water

stress, with most researchers agreeing that flowering and tuber initiation stages are the

most sensitive. (Affleck et al., 2008; Deblonde and Ledent 2001; Tekalign and Hammes,

2005).

Higher yields for sub-surface drip (Sub-SD) irrigated potato plants, compared to

surface drip (SD) and sprinkler methods have been reported (De Tar et al, 1996;

Sammis, 1980). On the other hand, other studies concluded no yield differences

between different irrigation methods. Niebling and Brooks (1995) reported similar yields

when Sub-SD was compared to wheel line irrigation. However, Sub-SD method still

proved to be more water use efficient in that study, using 30-50% less water. A higher

root concentration under drip irrigation enables plants to utilize soil moisture more

efficiently and thereby reduce soil matric potential to lower levels with no yield reduction

(Shalhevet et al., 1983). Limited water table management in seepage irrigation has also

been reported to cause soil moisture fluctuations, resulting in 58% decrease in US

Number 1 grade tubers and a marked increase in physiological disorders (Hochmuth

and Hanlon, 2011; Robins et al., 1956).

Soil conditions also affect the degree of skin-set, with moderate soil moisture (65-

80% field capacity) during tuber bulking favoring periderm maturation (Braue et al.,

1983; Stark and Love, 2003; Yagamuchi et al., 1964;). Previous research has also

34

shown that excessive vine growth at time of vine kill, due to excessive water application

during growing season, affects the desiccation rate and subsequent skin-set. Plants

approaching maturity that have begun to senesce require less time to desiccate

compared to immature vines (Haderlie et al., 1989; Sanderson et al., 1984; Kempenaar

and Struik, 2008; Lulai and Orr, 1993).

Conflicting reports on the effects of soil moisture levels on tuber dry matter

content have been published. Vos (1999) noted that dry soil conditions, particularly at

the end of the growing season, increased the dry matter content. On the other hand,

inadequate soil moisture levels during midseason tuber growth have been reported to

reduce tuber dry matter and moisture content (Jefferies and Mackerron, 1989; Lynch et

al, 1995). Other quality attributes affected by preharvest cultural practices include the

tuber nutritional composition. An increase in ascorbic acid content has been associated

with less irrigation frequency (Lee and Kader, 2000). With the limited work currently

done on how irrigation method affects tuber maturity and harvest quality, there is need

for more research focus in that area.

Response to Nitrogen Fertilizer Application Method and Rate

The demand for N varies according to the crop growth stage, with a low N

requirement during the early stages of vegetative growth, and uptake increasing slightly

during tuber initiation. Potatoes have a high N requirement during the tuber bulking

stage, with over 50% of the total required N being used in this stage (Shock, 2006;

Westermann et al., 1994). N influences biomass partitioning between vines and tubers,

tuber bulking rates and subsequent yields, tuber maturity and quality.

The availability of N to the plant is greatly determined by the application method

and timing of application. By applying fertilizer through drip irrigation, scheduling can be

35

managed more efficiently to meet crop N demands. Fertigation also allows precise

deposition of N in the plant root zone, thereby increasing nutrient use efficiency and

minimizing nutrient losses (Schwankl and Prichard, 2001). According to Shock (2006),

an added advantage to this application method is less N will be required for plant growth

and development.

Insufficient N during critical growth periods reduces tuber yield and size due to

lower bulking rates (Errebhi et al., 1998; Westermann and Kleinkopf, 1985; Westermann

et al., 1988). Previous research has already shown higher marketable yields when

fertigation is used, compared to granular application (Janat, 2007; Obreza and Sartain,

2010). Total yields (Dahlenburg et al., 1990; Hochmuth and Hanlon, 2011; Williams and

Maier, 1990) and tuber size (Dubetz and Bole, 1975; Murphy et al., 1967) also

increased with increasing N rates due to higher crop growth at higher N rates, leading to

more synthesis and photosynthates translocation to the tubers. According to

Dahlenburg et al. (1990), the minimum fertilizer amount required to ensure that N is not

limiting total tuber yield is 80 to 120 kg ha-1, while Hochmuth and Hanlon (2011)

reported yield declines at rates greater than 224 kg ha-1.

N is also indirectly related to tuber quality through its effect on relative tuber

maturity and compositional quality parameters. Excess N favors vine growth over tuber

growth and maturation, thereby decreasing tuber quality, delaying senescence and

tuber maturity (Love et al., 2005; Zvomuya et al., 2003). Higher N rates have also

reported to decrease both reducing sugars and ascorbic acid content (Kumar et al.,

2004; Teich and Menzies, 1964). However, conflicting reports have been given on the

effect of N on tuber specific gravity, dry matter content and starch. Some studies

36

observed an increase with increasing N rate (Dahlenburg et al., 1990; Wszelaczynska

and Poberezny, 2011), while others saw a decrease (Bombik et al., 2007; Laurence et

al., 1985; MacLean, 1984; Teich and Menzies, 1964) or no significant change

(Winterton, 1975).

Currently, limited research has been carried out to determine how fertilizer

application methods and N fertigation rates affect nutrient use efficiency, tuber yield and

harvest quality of new potatoes.

Postharvest Handling and Storage of New Potatoes

Freshly harvested new potatoes are washed to remove sand from the field,

before being fan-dried and packed into various commercial and consumer packages.

Washers may be in the form of flumes, barrel-type or brush washers. Brush washers,

which consist of cylinder brushes or studded rubber rolls, are the most common among

potato growers (Talburt and Smith, 1987). While these types of washers are a favorite

among growers because a minimum amount of water is used, their biggest

disadvantage is the high degree of tuber skin and wound injury associated with them. In

addition, the thin skin of new potatoes is also associated with high water permeability,

which causes great tuber weight loss if kept under inadequate storage conditions (Vogt

et al., 1983).

Tuber Skin and Wound Injury

Skin and wound injury causes postharvest losses through increased weight loss

and rots, lowering the market value of tubers if excessive (Chiputula, 2009; Brecht,

2012). To promote wound-healing and improve skin-set after harvest, tubers are held at

relatively high temperatures and high humidity before storage. This process is termed

curing and is commonly used with the fall crop before the long-term storage.

37

Curing promotes wound healing and periderm suberization, which helps reduce

qualitative and quantitative tuber losses during storage. Tubers are held between 15-

18°C and 80-95% relative humidity (RH) for an average of 2 weeks (Hide et al., 1994;

Suslow and Voss, 2000). Under these conditions, periderm maturation is enhanced

through increased suberin deposition on the tuber skin. Soluble waxes that are

imbedded in the suberin matrix help to prevent desiccation of the tuber inner tissues

during postharvest storage. In the case of skinned or wounded tubers, a wound

periderm develops over the existing wound; a process termed wound-healing (Lulai,

2001).

Due to high early season prices in Florida, new potatoes are stored briefly,

without curing, before being distributed to their respective markets. Lack of curing

causes quality losses along the distribution chain because the tubers are stored under

varying conditions from producer to retail market to consumer. Lulai (1994) observed up

to 28 times more tuber water loss from tubers with immature periderms when compared

to cured tubers.

Research has shown that early cultivars can also be cured for 8 days at 15°C

and 95% relative humidity. This enables storage time of up to 5 months at 4°C and 95 to

98% relative humidity (Suslow and Voss, 1996).

‘Rapid Curing’ As a Means of Prolonging Shelf-life In New Potatoes

The first step in wound healing is the formation of a closing layer at the wound

site. This is formed when suberin is deposited on the exposed cortical cells. Following

that, new cells are formed beneath the closing layer and suberized shortly after (Burton,

1989). Factors affecting rate of suberin deposition on the tuber skin or wound site

38

include physiology of the tuber, type and severity of wound (Lulai, 2008; Wigginton,

1974).

The humidity should be ideal, preventing tissue desiccation on wound sites, and

at the same time not too high as to induce cell proliferation. Rate of wound healing

increases as temperature increases, reaching a maximum at 25°C (Lulai, 2008; Burton,

1989). Lulai (2008) noted that it took 1-8 days under controlled laboratory conditions

(high temperature and relative humidity) for suberin barriers to develop. On the other

hand, it took double the time when tubers were stored under commercial conditions of

10-16°C. Studies conducted by Kim and Lee (1993) showed that when tubers were

cured at 13°C and 90% RH, it took 12 days for a good periderm layer to form. This was

reduced to 6 days, when cured at 18°C under the same humidity conditions.

Rate of wound healing has been reported to also increase with an increase in

tuber physiological age, with more mature tubers healing at a faster rate (Kumar and

Knowles, 2003; Thomson et al. 1995). In addition, it has been noted that tubers with an

immature periderm or freshly harvested have a higher rate of wound healing compared

to tubers which have been previously stored (Wigginton, 1974).

‘Rapid curing’ could be a possible means of reducing quality loss of new potatoes

in transit or during storage. It would involve storing tubers for a shorter duration under

higher temperatures and RH compared to standard curing conditions. Further studies

are required to determine if rapidly curing tubers before storage under commercial

conditions improves the shelf life of new potatoes.

Storage of New Potatoes

The potato is a semi-perishable commodity, which requires good storage

conditions to maintain quality throughout the distribution chain (Kumar et al., 2007).

39

Storage conditions should help minimize water loss and respiration, keep sugars at a

minimum and maintain external appearance (Plissey, 1976). Regulation of temperature

and humidity is very important as the tuber is made up of about 80% water, and

therefore quality can easily be compromised through moisture loss (Rastovski et al.,

1981). Moisture loss in tubers during storage depends on the permeability of the

periderm, decreasing with increased skin maturity (Makani, 2010), and the extent to

which the tuber water is free to move outside (Afek et al., 1998).

The potential of tuber moisture loss to the storage environment is determined by

the difference in vapor pressure or water potential difference (Es and Hartmans, 1987;

Kleinkopf, 1995). Vapor pressure deficit (VPD) is a measure of the difference between

the actual water vapor present in the atmosphere, and the potential amount that could

exist at the same temperature, without condensation. A high VPD increases moisture

loss from tuber tissue, as it is transferred into the storage air. The two critical elements

that affect VPD in storage are temperature and relative humidity.

Inadequate storage temperatures hinder tuber wound healing, affect disease

spread and severity and tuber respiration rates. Respiration increases with increasing

temperature, resulting in tuber weight loss with increased storage time (Smith, 1975;

Suslow and Voss, 2000). At a constant temperature, increasing humidity lowers the

VPD, thereby minimizing water loss and shrinkage. High humidity maintains potato

firmness by maintaining cell turgor, and minimizes conversion of starch to sugars (Afek

et al., 2000). Quality losses, such as specific gravity, dry matter content (Burton, 1989;

Lammerink, 1989) and reducing sugars (Iritani and Weller, 1980), have also been

observed with increased storage time. Increased storage time has been reported to

40

also favor the biochemical transformation of organic acids; reducing malic, tartaric and

fumaric acids, while increasing citric acid concentration (Wichrowska et al., 2009).

Previous studies have shown that storage of new potatoes at 10°C to 13°C and

80-85% RH can maintain tuber quality for a few weeks (Makani, 2010)

Effect of Preharvest Factors on Tuber Postharvest Quality

Increased understanding of how best management practices (BMP) help

optimize crop production while conserving the environment has triggered growing

interest in both growers and researchers to fully explore the potential of more efficient

water and fertilizer application methods. Therefore, to date, most research has focused

on determining the effect of irrigation method on potato yield and harvest quality.

However, considering that storage is one of the most important and difficult stages of

maintaining good tuber quality (Wszelaczynska and Poberezny, 2011), it becomes

necessary for more research to focus on how these production factors also affect

storage ability.

Production practices during growth, particularly water availability and fertilization,

have been shown to affect tuber storage quality. High N rates and excessive water

application promote vegetative growth, delay tuber maturation and result in immature

tubers at harvest. In storage, higher weight loss (Dahlenburg et al., 1990; Leszczynski,

2000), disease incidence (Jablonski, 2006), and decreased starch content (Bombik et

al., 2007) was observed in tubers fertilized with high N rates. Different results were also

reported for starch content, where higer N rates were associated with a higher content

during storage (Wszelaczynska and Poberezny, 2011). Dahlenburg et al. (1990) also

concluded that N rate did not have a significant effect on weight loss during long term

storage.

41

However, to date, none of the studies have determined how an interaction of

irrigation method, fertigation rate, harvest time, and postharvest storage affect tuber

yield and quality of Florida’s new potatoes.

Research Objectives

The main goal of this research was to determine the potential of drip irrigation

and fertigation as viable options to promote sustainable agriculture, through improved

water and nutrient use efficiency, while maintaining postharvest tuber quality.

Objective 1. Determine the effect of irrigation method and harvest time on tuber

yield, quality, and storability of two widely grown tablestock cultivars.

Hypothesis 1. Minimizing non-uniform soil wetting and nutrient leaching,

promotes improved tuber growth, development, and quality in sub-surface drip irrigation,

when compared to surface drip or seepage methods.

Hypothesis 2. Sub-surface drip irrigated tubers harvested two weeks after vine

killing will have a prolonged shelf-life and better compositional and nutritional quality in

both cultivars.

Objective 2. Determine the effect of fertilizer N application method, rate, and

harvest time on tuber yield and storage quality of potatoes.

Hypothesis 1. Improving N fertilizer placement and timing in fertigation

application method produces higher yields and better tuber quality, compared to band-

application of granular fertilizer.

Hypothesis 2. More precise application on N fertilizer to root zone, matching crop

growth demands, in fertigation results in higher nutrient use efficiency, optimum yields

and better tuber quality.

42

Objective 3. Determine the effect of N rate, plant vine kill and harvest time on

tuber physical maturation (skin-set) in two tablestock cultivars.

Hypothesis. Lower N rates minimize vine growth and facilitate early senescence

of vegetative tops, resulting in physically mature tubers; increasing in skin-set with

increased harvest time.

Objective 4. Evaluate ‘rapid curing’ as a means of minimizing tuber storage

losses, through rapid wound-healing and suberization.

Hypothesis 1. Increasing the curing temperature and humidity would increase the

rate at which wound-periderm formation and suberization occur.

Hypothesis 2. Rapid curing accelerates wound-healing and suberization, which

minimizes quality losses during storage of new potatoes, compared to non-cured tubers.

43

CHAPTER 3 EFFECT OF DRIP IRRIGATION AND HARVEST TIME ON YIELD AND TUBER

STORAGE QUALITY OF TWO TABLESTOCK POTATO CULTIVARS.

Introduction

The potato (Solanum tuberosum L.) plant is characterized by a shallow root

system with more than 90% of the total root volume in the top 25 cm of soil (Munoz-

Arboleda et al., 2006). This makes it very sensitive to inadequate or excessive amounts

of soil moisture, with small deviations from optimum water application greatly influencing

tuber yield, grade and quality (Fabeiro et al., 2001, Shock et al., 2007). Excessive soil

moisture results in oxygen depletion and nutrient leaching, while inadequate moisture

impedes nutrient uptake and plant growth.

Water stress and soil moisture fluctuations have been associated with decreased

yields and increased incidence of physiological disorders (Lynch et al., 1995;

MacKerron and Jefferies, 1988; Ojala et al., 1990; Shock et al., 1990; Yuan et al.,

2003). The stage of growth determines the degree of sensitivity to water stress, with

most researchers agreeing that tuber initiation and bulking are the most critical. (Affleck

et al., 2008; Alva et al., 2008; Deblonde and Ledent 2001; Ojala et al., 1990).

To ensure adequate water supply, growers in the Tri-County Agricultural Area

(TCAA) of northeast Florida supplement rainfall with irrigation. Seepage irrigation is the

commonly used method because it is inexpensive and easy to manage. Water is

pumped from deep wells, raising the natural water table to just below the plant root

zone, with water moving up by capillarity. The irrigation method requires high natural

water tables, or large quantities of water to raise the water table (Locascio, 2005).

Fields must be relatively leveled and soils uniformly porous, to ensure uniform soil

wetting (Smajstrla et al., 2002). Increased drainage flow, nutrient leaching and run-off

44

losses are associated with seepage irrigation, resulting in only 30-70% water use

efficiency (SJRWMD, 1990). These shortfalls have resulted in a growing need to

evaluate drip irrigation as an alternative method for potato production in the TCAA.

Micro irrigation systems are more water use efficient, using 30-50% less water

compared to seepage systems (Niebling and Brooks, 1995). Drip irrigation is the slow,

even application of low-pressure water to soil and plants using plastic tubing, called drip

tape (Shock et al., 2013) .The tape is placed on the soil surface, above the seed piece

(surface drip) with water moving down gravitationally, or buried below the seed piece

(sub-surface drip) with water moving up by capillarity. The soil surface is wetted only

within 30-60 cm of the water source, thereby saving water by directing it more precisely

and maximizing the amount of moisture in the crop root zone (Dukes et al., 2004; Shock

et al., 2013; Smajstrla et al., 1991).

Due to differences in water application and soil wetting pattern, different irrigation

methods have varying effects on soil moisture levels throughout the growing season.

Some of the work conducted in potato indicated higher yields for sub-surface drip

irrigated plants, compared to surface drip (Sammis, 1980; De Tar et al., 1996). Other

studies have reported similar yields for seepage and sub-surface irrigated potatoes

(Smajstrla et al., 1995). Soil moisture stress also affects quality attributes of tubers. Dry

soil conditions have been reported to increase (Lynch et al, 1995) or decrease (Jefferies

and Mackerron, 1989) dry matter content, depending on the plant growth stage. An

increase in ascorbic acid content has been associated with less irrigation frequency

(Lee and Kader, 2000). By affecting tuber maturity and harvest quality; soil moisture can

influence storability and processing quality of tubers (Kumar et al., 2004; Lulai and

45

Freeman, 2001). However, there is currently limited research in that area, particularly

for early to medium season potatoes commonly grown in Florida.

The objective of this study was to evaluate the effect of irrigation method and

harvest time on tuber yield, harvest quality, and postharvest storability of two tablestock

cultivars. The underlying hypothesis was that by minimizing non-uniform soil wetting

and nutrient leaching, sub-surface drip irrigation promotes improved tuber growth,

development, and quality, when compared to surface drip or seepage irrigation. The

second hypothesis was that sub-surface drip irrigated tubers harvested two weeks after

vine kill have a prolonged shelf-life and better compositional and nutritional quality in

both cultivars.

Materials and Methods

Plant Material

Two tablestock potato cultivars, ‘Fabula’ and ‘Red LaSoda’, were grown at the

UF/IFAS Florida Partnership for Water Agriculture and Community Sustainability,

Cowpen Branch in Hastings, FL, during spring 2011(season 1) and 2012 (season 2).

The light yellow skinned and fleshed ‘Fabula’ is an early to medium season cultivar

characterized by a long dormancy period. It has high yield potential and disease

resistance. ‘Red LaSoda’ is a red skinned, white fleshed, early to medium season

cultivar. Although generally susceptible to physiological disorders, it also has very good

yield potential. Both cultivars have low to medium specific gravity and are therefore

used primarily used for boiling and baking (Zotarelli et al., 2012; www.europotato.org;

www.inspectio.gc.ca ).

46

Experimental Site and Field Trials

The experimental site is characterized by Ellzey fine sands (taxonomic

classification sandy, siliceous, hyperthermic Areni Endoaqualfs), which are poorly

drained, moderately permeable soils, with a high water table, normally within 25 cm of

the surface for half of the year (Ou et al., 1995, USDA, 1981 ). The field experiments

were set up as a randomized complete block design with treatments in a split plot

design, replicated four times. Irrigation method – seepage (SP), surface drip (SD) and

sub-surface drip (Sub-SD) – were the main plots, while potato cultivar was the sub-plot.

The field was prepared by disking and plowing the soil, after which rows (78-m long,

0.35-m high) were formed. The rows were hilled to improve drainage and ease during

harvesting. The land was divided into 8 beds, each comprising of 16 rows, with a large

buffer plot (37-m width) placed between the seepage and drip irrigation beds, to

minimize water flow from the seepage into the drip section.

For the SP treatments, shallow water furrows were placed at the border of each

bed to supply irrigation water and remove drainage. The semi-closed system was used,

with water being withdrawn from a confined aquifer, pumped through polyvinyl chloride

pipes into the water furrows, and seeping laterally to raise the water table. In drip

treatments, drip lines with 16-mm inner diameter, 8-mm thickness, 20-cm emitter

spacing and a flow rate of 500 L h-1 100 m-1 were used (RO-DRIP, John Deere Water,

Moline, IL, USA). For Sub-SD treatments, the drip tape was placed 5 cm below the

projected placement of the seed piece (15-cm depth), before planting. In SD, the tape

was placed 5 cm above the seed piece after planting.

Potato seed pieces were planted on February 17, 2011 and January 17, 2012,

using 20-cm in-row spacing. UF/IFAS recommended fertilizer rates (N-P-K) were

47

applied at pre-plant (56-112-168 kg ha-1), as an initial side dress at emergence (84-0-

140), and final side dress when plants were 15-20 cm tall (84-0-0). All pesticide and

herbicide applications followed the Florida potato production recommendations (Zotarelli

et al., 2012).

Rainfall data were obtained from a weather station located at the site. Daily

reference evapotranspiration (ETo) was obtained from the Florida Automated Weather

Network (FAWN; www.fawn.ifas.ufl.edu).

Plant Vine Kill and Tuber Harvest Times

To promote natural plant vine senescence, irrigation was turned off 2 wks before

the projected vine kill time of approximately 90 days after planting (DAP) (Mossler and

Hutchinson, 2008). Therefore, 89 and 92 DAP in season 1 and 2, respectively, plant

tops were killed off using a single application of the chemical desiccant glufosinate

ammonium (Rely® 280 Herbicide; Bayer Crop Science, Research Triangle Park, NC,

USA), using a rate of 1535 ml ha-1. Harvest times were selected based on number of

weeks after vine kill, ranging from one to three weeks (H1 to H3). Tubers were hand-

harvested to avoid any form of skin injury, with average-sized tubers (150-200 g)

selected for further postharvest storage tests.

Tuber Yield and At Harvest Quality

Total and marketable yields were assessed at the final harvest time (three weeks

after vine kill) from 36 m2 sections in each plot. Tubers were graded according to size,

with marketable tubers falling under A1, A2, or A3 categories, based on the USDA

Standards for Grading of Potatoes, 2011 (Table A-1). Tubers with external physiological

disorders (greening, growth cracks, misshapen and decay) were separated and

quantified. Tuber internal quality was evaluated from a sub-sample of 20 marketable

48

tubers from each plot. Each tuber was cut into quarters and rated for incidence of brown

center, hollow heart, internal heat necrosis, and corky ring spot.

Tuber specific gravity was measured at all three harvest times. For each

irrigation treatment replicate (n=4), approximately 2 kg sample of randomly selected

tubers was weighed, transferred to another tared basket and weighed under water. The

weight in air and in water was used to calculate the specific gravity (specific gravity =

weight in air / (weight in air – weight in water)).

Postharvest Storage Analysis

Freshly harvested tubers at each harvest were placed in mesh produce bags and

transported to the Postharvest Horticulture Laboratory at the University of Florida,

Gainesville. Upon arrival, average sized tubers (150-200g) were carefully hand washed,

placed on paper towels and fan dried for not more than 30 min. Tubers were stored in

simulated commercial storage conditions of 10°C, 80-85% RH, for a total of 14 d.

Irrigation treatment replicates (n=4) were maintained in order to track the effects of

preharvest treatments on tuber storage quality. Tuber dry matter content, firmness,

ascorbic acid content and fresh weight loss were determined every 7 d during storage.

To determine dry matter content, tubers (n=3) were sliced longitudinally (2-cm

thick) using a food slicer. Each slice was cut into small pieces to promote uniform

drying. Samples were weighed before and after drying for 48 h in an oven set at 65°C.

Dry matter content was expressed as a percentage of the initial fresh weight. To further

quantify changes in dry matter content during the second season, the slice were

separated into peel and pulp (inner tissue) using a hand-slicer.

To measure firmness, potatoes (n=3) were sliced longitudinally (2-cm thick) using

a food slicer. Firmness measurements were made using a computer-controlled TA. HD.

49

Plus Texture Analyzer Machine (Texture Technologies Corp, NY) equipped with a 50 N

load cell. Samples were punctured at the center (pith), using a 4 mm diameter probe.

Force-deformation curves were recorded at 4 mm deformation. The maximum force

required to penetrate the tuber samples was recorded in Newtons (N).

Ascorbic acid content was measured using 2 g of sample tissue according to

AOAC (1984). Tubers (n=3) were washed, dried and a 2-cm thick slice cut longitudinally

before being blended (Hamilton Beach Model 908, Proctor-Silex Inc., NC, USA). The

samples (2 g) were mixed with 20 ml HPO3 and frozen at -30ºC. At analysis, samples

were thawed and centrifuged at 15,000 rpm for 20 min (Beckman model J2-21;

Beckman Coulter Inc., CA, USA). The supernatant was filtered and used to measure

ascorbic acid content using the direct colorimetric method. This measures the extent to

which a 2,6 dichlorophenol-indophenol solution is decolorized by ascorbic acid (Nourian

et al., 2002). In the second season, the slice was separated into peel and pulp (as

explained under dry matter content), and each component was analyzed separately.

Tubers (n=5) were weighed on a digital scale every 7 d during storage, to

determine fresh weight loss, which was expressed as a percentage of the initial fresh

weight.

Statistical Analysis

The two cultivars, ‘Fabula’ and ‘Red LaSoda’, were analyzed separately using

Analysis of Variance (SAS Institute Inc. Version 9.3, Cary, NC, USA). A general linear

mixed model (PROC GLIMMIX) was performed to determine preharvest and

postharvest main and interaction effects on tuber quality. The analysis was

implemented as a completely randomized design with split plot, where the main plots

were irrigation method, harvest time and storage time. Treatment means were

50

separated using the Tukey’s test and confidence limits of 95% calculated for each

mean.

Results – ‘Fabula’

Total Rainfall Received

Differences in rainfall amount and distribution were observed between the two

seasons. Season 1 had cumulative rainfall amount of 116 mm, with 85% of this total

occurring before mid-season (55 DAP) (Figure 3-1 A). The rest of the season

experienced a total amount of only 18 mm, with 30% (5 mm) falling between harvest 1

(H1) and harvest 2 (H2). The following season, cumulative rainfall increased to 186 mm,

with 65% (121 mm) occurring after mid-season (57 DAP) (Figure 3-1 B). The harvest

period was wetter, with 41% of the total seasonal rainfall (76 mm) occurring a week

before vine kill and between H2 and H3.

Effect of Irrigation Method and Harvest Time On ‘Fabula’ Tuber Yield and At Harvest Quality

There was a significant interaction between irrigation method and season (p <

0.05) on total and marketable yields; therefore each season was analyzed separately

(Table 3-1). In season 1, seepage (SP) and surface drip (SD) produced similar total

yields of 26,104 and 26,983 kg ha-1, respectively, while sub-surface drip (Sub-SD)

yields were significantly lower at 18,198 kg ha-1. The following year, Sub-SD’s total yield

decreased by 32%, maintaining significantly lower yields than the other irrigation

methods. Similar patterns were observed in the marketable yields; SP and SD averaged

17,822 kg ha-1 (67% of total yield) and 20,322 kg ha-1 (78% of total yield), in season 1

and 2 respectively. On the other hand, although accounting for about 73% of the total

yield in both years, Sub-SD marketable yield was still significantly lower.

51

There were no significant differences in the incidences of internal physiological

disorders due to irrigation method, in both years (Table 3-1). However, significant

differences in the incidence of external disorders were observed. In season 1, SP and

SD had significantly higher totals which averaged 6,081 kg ha-1, while Sub-SD had a

lower incidence of 2,788 kg ha-1. In SP, 56% of the total external disorders were growth

cracks. In contrast, 81% and 71% of the total external disorders were tuber decay for

SD and Sub-SD, respectively. In season 1, the drip irrigation methods had maintained

high moisture levels in the soil layers where tubers grew, during tuber bulking (Reyes-

Cabrera et al., 2014) To reduce tuber decay, the volume of the water applied with drip

was reduced at the tuber bulking stage by 35% in season 2. This significantly reduced

decay in the drip methods; total physiological disorders averaged 1,393 kg ha-1 and

2,442 kg ha-1 for the drips and seepage, respectively.

Specific gravity was measured at all three harvest times. Irrigation method,

harvest time or their interaction had no significant effect on tuber specific gravity in both

seasons. The average specific gravity was 1.055 for all irrigation methods, at all harvest

times.

Effect of Irrigation Method And Harvest Time on ‘Fabula’ Tuber Storage Quality

Initial Storage Quality

No significant differences were observed in whole tuber dry matter content

(DMC) between irrigation method and/ or harvest time during season 1; average was

16.3%. The following year, with the tuber peel and pulp being analyzed separately,

there was a significant interaction between irrigation method and harvest time on initial

storage quality of peel DMC (Table 3-3). Generally, the peel DMC decreased with

increased harvest time for all irrigation methods. Sub-SD’s peel DMC significantly

52

declined from 9.5% at H1 to 7.6% at H3, while SP tubers lost only 0.4% in the same

time period. Lower contents were also observed in the drip methods at H2 and H3

ranging from 7.6 to 8.5%, while SP tubers averaged 9.1%. No significant differences

were observed in the pulp DMC of freshly harvested tubers, which averaged 14.6% for

all irrigation methods and harvest times.

Tuber firmness was significantly affected by irrigation method, harvest time and

season. No significant differences were observed in season 1; average firmness was

21.4 N. Significant differences were observed the following season; generally tubers lost

firmness with increased harvest time for all irrigation methods (Figure 3-2). At H1, Sub-

SD tubers (23.5 N) were 5.7 N firmer than the other irrigation methods. However, Sub-

SD tubers also had the most significant decrease of 9.4 N, with increased harvest time,

with no significant differences between irrigation methods by H2.

There were no significant differences in tuber ascorbic acid content (AAC)

between irrigation methods and/or harvest time in season 1; average content was 21.5

mg 100 g-1 in whole tubers. When the peel and pulp were analyzed separately in

season 2, an interaction of irrigation method and harvest time affected pulp AAC (Table

3-4). At H1, SP tubers had the lowest pulp AAC of 19.0 mg 100 g-1, while drip methods

averaged 24.4 mg 100 g-1. The pulp AAC decreased with an increase in harvest time

after vine kill, for all irrigation methods. Overall, the pulp AAC was significantly higher

than peel AAC, with the former ranging from 8.1 to 25.0 mg 100 g-1, and the latter from

6.2 to 9.7 mg 100 g-1.

Storage Quality

There was a significant interaction between harvest time and storage on tuber

fresh weight loss, with no significant differences between seasons (Figure 3-3). H1

53

tubers lost the most weight throughout storage, regardless of the irrigation method. The

average weight loss for H1 tubers at 7 d was 3.8%, while H2 tubers averaged 1.4% for

the same time period. By the end of storage (14 d), cumulative weight loss in H1 tubers

had significantly increased to 5.3%.

Significant differences in peel dry matter content between irrigation methods

were observed at 0 d storage time (results explained above, under initial storage

quality). The peel DMC did not change with increased storage time. Meanwhile, the

average pulp dry matter content was 14.8%, with no significant differences between

irrigation method, harvest time, or storage time.

In season 2, tuber firmness was affected by irrigation method, harvest time, and

storage duration. Generally, H1 tubers were firmer before storage (0 d), ranging from

17.1 to 23.1 N, while the other harvests ranged from 12.5 to 14.6 at the same time

period (Table 3-5). H1, Sub-SD tubers had the highest firmness of 23.1 N at 0 d, with no

significant difference between SP and SD, which averaged 17.4 N (results explained

above, under initial storage quality). However, the firmness decreased significantly at 7

d for all irrigation methods, with no significant differences between irrigation method or

harvest time by 14 d. No significant differences in firmness among irrigation methods

were observed at H2 and H3, throughout the storage duration.

No differences in AAC for whole tubers were observed in season 1, with an

average storage content of 20.1 mg 100 g-1. The following season, pulp AAC of H1

tubers was affected by irrigation method, harvest time, and storage time (Table 3-6).

Generally, H1 tubers had a higher pulp AAC at d 0, with an average of 22.6 mg 100 g-1,

while the other harvests ranged between 8.1 and 13.3 mg 100 g-1during the same time

54

period (Results explained above, under initial storage quality). While ascorbic acid

content declined at 7 d for all H1 tubers, both drip irrigated tubers maintained a higher

pulp AAC than SP tubers throughout storage. There were no significant changes in the

peel AAC throughout storage, with an average content of 7.2 mg100 g-1.

Discussion – ‘Fabula’

Application efficiencies of irrigation methods are highly dependent on system

design and ability to adequately supply moisture to the plant root zone. In this study, SP

and SD irrigation methods outperformed Sub-SD in total and marketable yields (Table

3-1). This contradicts some previous studies which reported equal or higher yields in

sub-surface irrigation, compared to surface drip (Dukes and Scholberg, 2005; Onder et

al., 2005; Starr et al., 2008). According to the authors, placement of the drip tape

beneath the soil surface allowed maintenance of optimum soil moisture in the root zone,

which improved water and fertilizer use efficiency. However, it has also been reported

that uniformity of water application of irrigation methods is also dependent on soil

hydraulic properties, rate of transmission of water to the soil root zones, and depth of

drip tape for Sub-SD irrigation (Patel and Rajput, 2007; Smajstrla et al, 2002; Starr et

al., 2008 ). Slower capillary movement from the emitter to the root zone (Starr et al.,

2008), and drier soils at 15 cm drip tape depth (Patel and Rajput, 2007) likely caused

moisture stress and reduced yields in Sub-SD, observed in this study.

The soil wetting patterns of the irrigation methods also caused differences in

incidence of physiological disorders (Table 3-1). High moisture levels in the root zone

for drip irrigation methods may have accounted for the higher decay incidence observed

in season 1. Prolonged exposure of tubers to moist soils delays tuber periderm

maturation and increases susceptibility to decay bacteria (Adams, 1975; Lulai, 2001).

55

On the other hand, limited water table management in SP irrigation likely caused soil

moisture fluctuations, which resulted in high incidence of growth cracks, supporting

previous reports (Robins et al., 1956; Hochmuth and Hanlon, 2011).

Specific gravity is an important parameter associated with the processing quality

of tubers (Yuan et al., 2003). It is greatly affected by moisture stress during plant growth

(Ojala et al., 1990). Previous irrigation studies have shown a decrease in specific gravity

with increased water application (Hang and Miller, 1986; Shock et al., 1998; Yuan et al.,

2003) or decreased soil water potential (Miller and Martin, 1987; Shock et al., 1998;

Stark and McCann, 1992). In other studies, reductions in specific gravity were due to

vine killing actively growing plants (Rowberry and Johnson, 1966; Wright and Hughes,

1964) or delaying tuber harvesting (Stark et al., 2009). This present study suggests that

all three irrigation methods supplied an adequate amount of moisture at the critical

growth stages, achieving compositional and physiological maturity by vine killing time. A

combination of adequate tuber maturity and soil conditions under all irrigation methods

possibly minimized starch respiration and potential losses in specific gravity with

increased harvest time.

Closely related to specific gravity, dry matter content largely determines the

processing weight and quality of tubers. Any form of stress during vegetative growth,

particularly tuber bulking, causes reduction in dry matter accumulation (Hughes, 1974;

Fabeiro at al., 2001; Mackerron and Davies, 1986). Results from this study suggest that

all three irrigation methods were able to provide adequate moisture for whole tuber dry

matter accumulation. When the peel and pulp were analyzed separately, the peel dry

matter content decreased with increased harvest time for all irrigation methods (Table 3-

56

3), as seen in previous studies (Jewell and Stanley, 1989; Rowberry and Johnson,

1966). The higher dry matter content in SP irrigated tubers at H2 and H3 was likely due

to a slower senescence rate, because of more lush growth at the time of vine kill,

resulting in continuous translocation of photosynthates to tubers during the harvest

period. No significant difference in the pulp dry matter content suggests that dry matter

translocation under unfavorable conditions occurs in the peel first.

There was a significant interaction between harvest time and irrigation method on

firmness (initial storage quality) of tubers during season 2 only (Figure 3-2). Higher

firmness is related primarily to tuber moisture retention (Rocha et al., 2003). According

to Adams (1975), drier soils encourage suberization and more mature tuber periderms.

Therefore, the wet harvest period in season 2 probably delayed tuber maturation and

increased moisture loss in tubers for all irrigation methods. According to Lulai and Orr

(1994), an influx of water can occur if immature tubers are kept in wet soils, explaining

the higher firmness at H1 for all irrigation methods. By inducing stress, vine killing

promotes suberin deposition on the periderm, resulting in a more mature periderm

(Adams, 1975; Tyner, 1997). Therefore, significantly higher firmness in Sub-SD tubers

at H1 indicates delayed physical maturation, which caused higher water influx and tuber

turgor.

On the other hand, there were indications that SP irrigation delayed tuber

compositional maturity, particularly ascorbic acid content. Dale et al. (2003) stated that

harvesting before full maturity and foliage senesce causes a decline in ascorbic acid

content. Excess water application in SP irrigation likely favored vegetative growth over

tuber development, resulting in significantly lower pulp ascorbic acid content at harvest

57

1 (Table 3-4). In the more controlled drip treatments, soil moisture levels gradually

decreased prior to harvest, encouraging senescence of the plant tops and tuber

maturity. Delaying the harvest time decreased the pulp ascorbic acid content for all

irrigation methods due to reduction in biosynthesis when plant tops undergo desiccation

(Kelly and Somers; 1949; Gonella et al., 2009).

During storage, significant changes with increased storage time were only

observed in tuber fresh weight loss, firmness, and pulp ascorbic acid content. H1 tubers

lost the most weight throughout storage, regardless of the irrigation method, indicating

that moisture loss was highly dependent on maturity of the periderm (Figure 3-3). This is

supported by previous studies which showed increased tuber skin maturity with

increased time after vine kill (Adams, 1975; Tyner, 1997; Makani, 2010, Nipa et al.,

2013).

This is further supported by significant losses in firmness with increased storage

time for all H1 tubers during season 2 (Table 3-5), which supported findings by Gamea

et al. (2009). Cargill (1976) stated that tubers were noticeably softer at 5% or more

weight loss. In this study, H1 tubers had the highest cumulative weight loss of 5.3% by

the end of storage, accounting for the decrease in firmness.

There were indications that tuber maturity also played a significant role in pulp

ascorbic acid content losses during storage (Table 3-6). Due to less mature tubers at

harvest 1, SP irrigated tubers consistently had the lowest ascorbic acid content in

storage. Storage of these immature tubers likely increased metabolism or oxidation of

ascorbic acid to dehydroascorbic acid (DHAA) in the pulp, causing a significant decline

as early as 7 d (Augustin 1975; Watada, 1987; Dale et al., 2003; Phillips at al. 2010).

58

The rapid decline by 7 d, with no significant change thereafter can be attributed to an

immediate reaction between ascorbic acid and dissolved oxygen in the pulp tissue

(Polydora et al., 2003). Initial concentration of ascorbic acid is one of the key factors

determining the rate of loss during storage (Cvetkovic and Jokanovic, 2009). Higher

ascorbic acid content in the pulp likely resulted in more losses occurring in the tissue

during storage, compared to the peel.

Tuber dry matter content was maintained throughout storage, in both seasons.

Starch is the major component of potato tubers, constituting about 70 - 75% of the dry

matter content (Lisinska and Leszczynski, 1989). As the primary respiratory substrate,

starch composition in tubers has a direct linear relationship to the dry matter content.

Previous research showed that low temperature storage (4°C or less) stimulates starch

breakdown, reducing dry matter content (Isherwood, 1973; Malone et al., 2006). On the

other hand, storage at higher temperatures resulted in an increase in dry matter content

with storage time (Kaaber et al., 2001; Nipa et al., 2013). This was attributed to higher

water loss, which resulted in increased starch concentration in cells. However, because

most of the published work was done on the main potato crop, there is currently limited

information on effect of short-term storage on dry matter content of new potatoes, which

are not cured and are stored for shorter periods. Storage at 10°C, 80-85% RH for 14 d

maintained dry matter content of ‘Fabula’ new potatoes, regardless of irrigation method

or harvest time.

59

Results – ‘Red LaSoda’

Effect of Irrigation Method and Harvest Time On ‘Red LaSoda’ Tuber Yield and At Harvest Quality

There was a significant interaction of irrigation method and season on total and

marketable yields; therefore each season was analyzed separately (Table 3-7). In

season 1, SP irrigated tubers produced the highest total yield of 40,788 kg ha-1 while

drip treatments averaged 25,414 kg ha-1. In season 2, Sub-SD produced the lowest total

yields of 15,813 kg ha-1, while SP had the highest of 31,501 kg ha-1. Tuber marketable

yields in each year followed a similar trend to the total yield pattern.

A higher incidence of brown center disorder in SP and Sub-SD resulted in

significantly higher total internal physiological disorders in both seasons, compared to

SD. No differences in total external disorders among irrigation methods were observed

in season 1; average was 6,638 kg ha-1. However, growth cracks accounted for 24% of

the total external disorders in SP, while they averaged 5% in the drip methods. Similar

to ‘Fabula’, a high incidence of decay was observed in season 1, primarily in the drip

irrigation methods. Although the irrigation scheduling was adjusted in season 2, decay

incidence still remained high in the drip irrigated treatments.

There were no significant differences in specific gravity for irrigation methods,

harvest time or the interaction; average value was 1.056.

Effect of Irrigation Method And Harvest Time on ‘Red LaSoda’ Tuber Storage Quality

Initial Storage Quality

No significant differences in tuber firmness were observed during season 1;

average firmness was 21.4N (Table 3-8). In season 2, harvest time had an effect on

tuber; generally, tubers harvested 1 week after vine kill (H1) had the highest firmness,

60

for all irrigation methods (averaged 22.3 N). Firmness decreased to an average of

14.1N by the H2 (Figure 3-4).

No significant differences were observed in the dry matter content, in both years.

In season 1, the average whole tuber DMC was 16%, while in season 2, the average

peel and pulp DMC was 11.5% and 14.7%, respectively.

There also were no significant differences in tuber ascorbic acid content between

irrigation method, harvest time, and the interaction. The average whole tuber AAC was

27.4 mg 100 g-1 in season 1, while the average peel and pulp contents in season 2 were

12.0 and 28.2 mg 100 g-1, respectively.

Storage Quality

There was a significant interaction between irrigation method, harvest time and

storage time on fresh weight loss in ‘Red LaSoda’ tubers, in both years (Table 3-9).

Generally, H1 tubers lost the most weight compared to the other harvests. All irrigation

methods lost significantly more weight with increased storage time, for H1 and H2

tubers. H1, Sub-SD and H2, SP tubers lost the most weight of 4.1% and 3.8%,

respectively, by the end of storage.

There was a seasonal effect on tuber firmness; significant differences were

observed in season 2 only (Table 3-10). Significant differences were only observed at 0

d (initial storage quality), where H1 tubers had the highest average firmness of 22.3 N

for all irrigation methods. Firmness decreased at 7 d for all irrigation methods for H1;

average by end of storage was 14.7 N. The average firmness for all irrigation methods

in season 1, at all harvest times was 21.5 N.

There were no significant differences in tuber dry matter and ascorbic acid

contents during storage. The average whole tuber DMC in season 1 was 16%; peel and

61

pulp contents in season 2 were 12% and 15%, respectively. Average whole tuber AAC

in season 1 was 27.4 mg 100 g-1, while peel and pulp contents were 12.5 and 27.6

mg100 g-1, respectively.

Discussion – ‘Red LaSoda’

Results from this study show cultivar differences in response to the water

delivery systems of irrigation methods. SP irrigation outperformed the drip methods

(Table 3-7), contradicting some studies which reported improved yields with micro-

irrigation (Yuan et al., 2003; Onder et al., 2005). High yields are achieved when soil

moistures levels conducive for plant growth are maintained in the crop root zone (Patel

and Rajput, 2007). Sub-SD irrigation likely did not irrigate the entire root zone, which

may have caused periods of moisture stress. This resulted in yield reductions in Sub-SD

drip irrigated plants.

Soil moisture fluctuations under SP irrigation possibly caused higher incidence of

growth cracks, while the water delivery method in the drip irrigation resulted in tuber

decay, as observed in ‘Fabula’. However, when the irrigation adjustments were made in

season 2, decay incidence remained high in both drip methods. This indicates that ‘Red

LaSoda’ is not adapted to the soil wetting patterns of the drip irrigation methods (Reyes-

Cabrera, et al., 2014) and further adjustments are required in the irrigation scheduling to

maintain a comparable yield level to seepage irrigation.

Results in this study indicate that plant growth under all three irrigation methods

was sufficient for dry matter accumulation (Hughes, 1974; Fabeiro at al., 2000;

Mackerron and Davies, 1986), and ascorbic acid biosynthesis (Wang et al., 2011).

Unlike ‘Fabula’ tubers, ‘Red LaSoda’ has a shorter seed dormancy period, which likely

resulted in more compositionally and physiologically mature tubers at vine kill time. This

62

probably minimized tuber stress, with no significant losses in tuber total solids during

the harvest period in this particular cultivar. Previous work done with SP irrigated ‘Red

LaSoda’ showed no significant differences in peel or pulp dry matter contents of tubers

harvested between one and three weeks after vine kill (Makani, 2010).

On the other hand, physical maturity of tubers may have been affected by the

weather conditions, resulting in the year-to-year differences observed in tuber firmness

(Figure 3-4). The higher rainfalls in season 2 possibly resulted in less physically mature

tubers, characterized by immature periderms. Previous studies have shown that when

tubers are kept under adverse environments such as dry soils, moisture is easily lost

through the periderm of immature tubers (Adams, 1976; Lulai et al., 2006). Similar to

‘Fabula’, rains that occurred just prior to vine kill likely resulted in an influx of moisture,

which accounted for firmer tubers at H1. Firmness decreased due to moisture loss as

the soils dried out with increased delay in tuber harvesting.

During storage, a significant interaction between irrigation method, harvest time

and storage time affected fresh weight loss only (Table 3-9). As observed in previous

studies, the higher permeability of the immature periderms of H1 tubers resulted in

higher weight loss with increased storage time (Gamea et al., 2009; Lulai and Orr, 1994;

Makani, 2010; Nipa et al., 2013). This possibly accounted for the decrease in firmness

during storage, observed in H1 tubers (Table 3-10). The higher weight loss during

storage of SP and Sub-SD irrigated tubers was an indication of delayed periderm

maturation.

63

No significant changes in dry matter or ascorbic acid content were observed

during storage indicating minimum physiological and metabolic losses at 10°C, 80-85%

RH storage.

Conclusions

The purpose of this study was to determine the potential of drip irrigation as a

viable option which improved water use efficiency, while maintaining or improving

storage quality in ‘Fabula’ and ‘Red LaSoda’.

Data showed that adequate water supply in seepage and surface drip irrigation

resulted in significantly higher yields than sub-surface drip irrigation in ‘Fabula’. When

water amount applied through the drips was decreased in season 2, the amount of

decay in both drip irrigation methods declined significantly, while seepage irrigation

maintained a high incidence of growth cracks in both seasons.

In storage, harvest time had a great effect on ‘Fabula’ tuber quality, with

significant differences observed in all attributes. Generally, tubers harvested one week

after vine kill were physically (periderm), compositionally (dry matter, ascorbic acid

content) and physiologically (firmness) less mature than the later harvests. This resulted

in significant losses in tuber quality with increased storage time; while tuber storage

quality was maintained when tubers were harvested two to three weeks after vine kill for

all irrigation methods.

In ‘Red LaSoda’, the water delivery method of the drip irrigations resulted in

lower yields and high tuber decay in both growing seasons. This showed ‘Red LaSoda’

was not well adapted to the soil wetting patterns of drip irrigation methods. In storage,

generally the less mature tubers harvested one week after vine kill lost the most weight.

In addition, a delay in periderm maturation in surface drip and seepage tubers at

64

harvest 1 and 2, respectively, likely caused higher weight loss during storage. The loss

in moisture with increased storage time in tubers harvested one week after vine kill also

attributed to a decrease in firmness during storage. No significant changes during

storage were observed in the other quality attributes, indicating good storage ability in

‘Red LaSoda’, especially when harvested two to three weeks after vine kill.

This study therefore concludes that surface drip irrigation can be used as a viable

option for ‘Fabula’ to replace the less water use efficient seepage irrigation method.

Comparable yields and acceptable harvest and storage quality of tubers can be

obtained with surface drip irrigation. On the other hand, for ‘Red LaSoda’, further

adjustments are required in the drip irrigation scheduling to maintain comparable yields

to seepage. The ideal harvest time recommended for both cultivars is two to three

weeks after vine kill, because the tubers are more mature and therefore maintain high

quality during storage at 10°C, 80-85% RH, irrespective of the irrigation method used.

65

Table 3-1. Effect of irrigation method on yield and incidence of physiological disorders in ‘Fabula’ tubers harvested 3 weeks after vine kill

Physiological Disorders Year

Irrigation Method

Total Yield Marketable Yield Internal External (kg ha-1) (kg ha-1) (%)y (kg ha-1) (%) (kg ha-1) (% )

Season 1 SP 26,104 az 17,696 a 67.8 306 a 1.2 6,038 a 23.1 SD 26,983 a 17,947 a 66.5 214 a 0.8 6,123 a 22.7 Sub-SD

18,198 b 13,429 b 73.8 122 a 0.7 2,788 b 15.3

Season 2 SP 26,104 a 19,704 a 75.5 245 a 0.9 2,442 a 9.4 SD 25,728 a 20,959 a 81.5 92 a 0.4 1,303 b 5.1 Sub-SD 12,299 b 8,785 b 71.4 92 a 0.8 1,483 b 12.1 z Means within a column (irrigation method) followed by the same small letter, within a particular year, do not differ significantly according to

Tukey’s test at 5% level. yPercent of total yield

Adapted from “Drip as alternative irrigation method for potato in Florida sandy soils” by Reyes-Cabrera et.al., 2014, American Journal of Potato Research.

66

Table 3-2. Analysis of variance for irrigation method and harvest time on selected quality parameters for ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH

Dry matter content (%) Ascorbic acid content (mg 100 g-1) Weight loss (%) Main Effects Whole Peel Pulp Firmness (N) Whole Peel Pulp

Irrigation (I) ns * ns * ns ns * ns Harvest (H) ns * ns * ns ns * * Storage (S) ns ns ns * ns ns * *

Year (Y) - - - * - - - ns Interactions:

I x H ns * ns * ns ns * ns I x S ns ns ns * ns ns * ns H x S ns ns ns * ns ns * * I x Y - - - * - - - ns H x Y - - - * - - - ns S x Y - - - * - - - ns

I x H x S ns ns ns * ns ns * ns I x H x S x Y - - - * ns - - ns

* Significant at p<0.05; ns: not significant; - data collected in one season

67

Table 3-3. Effect of harvest time on peel and pulp dry matter contents of ‘Fabula’ tubers’ initial storage quality (season 2)

Peel Dry Matter Content (%) Pulp Dry Matter Content (%) Harvest timex SPy SD Sub-SD SP SD Sub-SD

H1 9.6 aAz 9.3 aA 9.5 aA 15.0 aA 16.6 aA 15.0 aA

H2 9.0 bA 8.1 bB 8.5 bAB 14.5 aA 15.0 aA 15.3 aA

H3 9.2 bA 8.0 bB 7.6 cB 14.2 aA 14.0 aA 14.0 aA

z Peel or Pulp means within a column (irrigation method) followed by the same small letter, or by the same

capital letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively

Table 3-4. Effect of harvest time after vine kill on tuber peel and pulp ascorbic acid contents of ‘Fabula’ tubers’ initial storage quality (season 2)

Peel Ascorbic Acid Content (mg 100 g-1)

Pulp Ascorbic Acid Content (mg 100 g-1)

Harvest Timex SPy SD Sub-SD SP SD Sub-SD

H1 6.8 aAz 7.7 aA 8.1 aA 19.0 aB 23.8 aA 25.0 aA

H2 7.0 aA 6.1 aA 6.2 aA 12.2 bA 12.5 bA 13.3 bA

H3 7.8 aA 8.6 aA 7.6 aA 8.1 cA 10.5 bA 10.4 bA

z Peel or Pulp means within a column (irrigation method) followed by the same small letter, or by the same

capital letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively

Table 3-5. Effect of irrigation method and harvest time on firmness of ‘Fabula’ tubers, stored for 14 d at 10°C, 80-85% RH (season 2)

Storage Harvest Timex

time (d)

Bioyield Force (N) SPy SD Sub-SD

H1 0 7 14

17.7 aBz 17.1 aB 23.1 aA 13.6 bA 14.4 bA 16.6 bA 14.7 bA 13.9 bA 14.5 bA

H2 0

7 14

12.5 aA 13.2 aA 13.2 aA 12.8 aA 13.0 aA 13.7 aA 13.2 aA 12.5 aA 14.4 aA

H3 0

7 14

14.3 aA 14.4 aA 14.1 aA 14.5 aA 13.5 aA 12.8 aA 14.6 aA 13.3 aA 13.5 aA

z Means within a column followed by the same small letter for each irrigation method at each harvest time,

or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively.

68

Table 3-6. Effect of irrigation method and harvest time on tuber peel and pulp ascorbic acid content of ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH (season 2)

Storage

Harvest Timex

Time (d)

Peel Ascorbic Acid Content (mg 100 g-1)

SPy SD Sub-SD

Pulp Ascorbic Acid Content (mg 100 g-1)

SP SD Sub-SD

H1 0 7 14

6.8 aAz 7.7 aA 8.1 aA 6.8 aA 7.5 aA 7.9 aA 7.1 aA 7.4 aA 7.8 aA

19.0 aBz 23.8 aA 25.0 aA 12.7 bB 18.8 bA 18.2 bA 11.8 bB 17.9 bA 16.3 bA

H2 0

7 14

7.0 aA 6.1 aA 6.2 aA 6.6 aA 6.7 aA 6.3 aA 6.5 aA 7.6 aA 6.0 aA

12.2 aA 12.5 aA 13.3 aA 13.8 aA 12.3 aA 11.9 aA 12.4 aA 13.4 aA 13.9 aA

H3 0

7 14

7.8 aA 8.2 aA 7.6 aA 7.0 aA 8.6 aA 7.7 aA 7.1 aA 7.4 aA 7.1 aA

8.1 aA 10.5 aA 10.4 aA 10.6 aA 10.9 aA 11.6 aA 10.4 aA 12.3 aA 12.4 aA

z Means within a column followed by the same small letter for each irrigation method, at each harvest time, or by the same capital letter within a

row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively

69

Table 3-7. Effect of irrigation method on yield and incidence of physiological disorders in ‘Red LaSoda’ tubers harvested 3 weeks after vine kill

Physiological Disorders Year

Irrigation Method

Total Yield Marketable Yield Internal External (kg ha-1) (kg ha-1) (%)y (kg ha-1) (%) (kg ha-1) (% )

Season 1 SP 40,788 az 29,242 a 71.7 826 a 2.0 7,484 a 18.3 SD 23,092 b 14,182 b 61.4 0 c 0.0 6,296 a 27.3 Sub-SD

27,736 b 18,323 b 66.1 214 b 0.8 6,133 a 22.1

Season 2 SP 31,501 a 23,343 a 74.1 1,254 a 3.9 5,350 a 17.0 SD 21,837 b 15,813 b 72.4 61 c 0.3 3,989 a 18.3 Sub-SD 15,813 c 10,291 c 65.1 245 b 1.6 3,718 a 23.5 z Means within a column (irrigation method) followed by the same small letter, within a particular year, do not differ significantly according to

Tukey’s test at 5% level. yPercent of total yield

Adapted from “Drip as alternative irrigation method for potato in Florida sandy soils” by Reyes-Cabrera et.al., 2014, American Journal of Potato Research.

70

Table 3-8. Analysis of variance for irrigation method and harvest time for selected quality parameters for ‘Red LaSoda’ tubers stored for 14 d at 10°C, 80-85% RH

Dry matter content (%) Ascorbic acid content (mg 100 g-1) Weight loss (%) Main Effects Whole Peel Pulp Firmness (N) Whole Peel Pulp

Irrigation (I) ns ns ns ns ns ns ns * Harvest (H) ns ns ns * ns ns ns * Storage (S) ns ns ns * ns ns ns *

Year (Y) - - - * - - - ns Interactions:

I x H ns ns ns ns ns ns ns * I x S ns ns ns ns ns ns ns * H x S ns ns ns * ns ns ns * I x Y - - - ns - - - ns H x Y - - - * - - - ns S x Y - - - * - - - ns

I x H x S ns ns ns ns ns ns ns * I x H x S x Y - - - * ns - - ns

* Significant at p<0.05; ns: not significant; - data collected in one season

71

Table 3-9. Effect of harvest time on cumulative fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2)

Storage Harvest Timex

Time (d)

Fresh Weight Loss (%) SPy SD Sub-SD

H1 7 14

2.4 bBz 1.6 bC 3.0 bA 3.3 aB 2.2 aC 4.1 aA

H2 7

14

1.5 bB 1.1 bB 1.1 bB 3.8 aA 1.9 aB 2.0 aB

H3 7

14 1.0 aA 0.4 aB 0.7 aAB

1.0 aA 0.8 aA 1.0 aA zMeans within a column followed by the same small letter for each irrigation method, or by the same

capital letter within a row at the same harvest level do not differ significantly according to Tukey’s Multiple Range (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively

Table 3-10. Effect of irrigation method and harvest time on tuber firmness of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2)

Storage Bioyield Force (N) Harvest Timex

Time (d)

SP SD Sub-SD

H1 0 7

14

21.4 aAz 22.9 aA 22.5 aA 15.5 bA 16.1 bA 16.8 bA 14.8 bA 13.7 bA 15.6 bA

H2 0

7 14

14.3 aA 14.1 aA 13.8 aA 15.1 aA 13.2 aA 13.9 aA 15.9 aA 14.9 aA 14.5 aA

H3 0

7 14

13.9 aA 15.4 aA 15.1 aA 13.9 aA 13.4 aA 14.6 aA 15.5 aA 14.9 aA 15.1 aA

z Means within a column followed by the same small letter for each irrigation method at each harvest time,

or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip

x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively

72

A)

B) Figure 3-1. Total and cumulative rainfall during growing A) season 1, and B) season 2 in

Hastings, FL.

0

20

40

60

80

100

120

140

0

20

40

60

80

Vin

e k

ill

Ha

rvest 1

Ha

rvest 2

Ha

rvest 3

Ra

infa

ll (m

m)

Days after planting

Rain Cumu. Rain

0

20

40

60

80

100

120

140

160

180

200

0

20

40

60

80

Vin

e-k

ill

Ha

rvest 1

Ha

rvest 2

Ha

rvest 3

Ra

infa

ll (m

m)

Days after planting

Rain Cumu. Rain

73

Figure 3-2. Effect of harvest time on firmness of ‘Fabula’ tubers’ initial storage quality

(season 2). Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively

Figure 3-3. Fresh weight loss in ‘Fabula’ tubers stored for 14 d at 10ºC, 80-85% RH. Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively

0

5

10

15

20

25

H1 H2 H3

Bio

yie

ld F

orc

e (

N)

Harvest time after vine kill (weeks)

Seepage

Surface Drip

Sub-SD

0

1

2

3

4

5

6

7

0d 7d 14d

Fers

h w

eig

ht lo

ss (

%)

Storage time (days)

H1

H2

H3

74

Figure 3-4. Effect of harvest time on firmness of ‘Red LaSoda’ tubers’ initial storage

quality (season 2). Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively.

0

5

10

15

20

25

30

H1 H2 H3

Bio

yie

ld F

orc

e (

N)

Harvest time after vine kill (weeks)

Seepage

Surface Drip

Sub-SD

75

CHAPTER 4 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND

HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. I. ‘FABULA’

Introduction

Optimizing water and nitrogen (N) management is critical to the potato yield and

tuber quality (Joern and Vitosh, 1995). Drip irrigation has already proved to be a water

use efficient method, producing high yields and good tuber harvest quality (Sammis,

1980; Smajstrla et al., 1995; De Tar et al, 1996). In addition to water, drip irrigation is

capable of supplying dissolved or liquid fertilizer close to the area where plant roots are

concentrated, a method commonly known as fertigation (Phene et al., 1994; Wadell et

al., 1999). Through fertigation, both the timing and rate of nutrient application can be

better controlled to match plant growth needs (Shock et al., 2006; Papadopoulos, 1988).

Nitrogen (N) is a very important macronutrient for potatoes because it influences

biomass partitioning between vines and tubers, tuber bulking rates and subsequent

yields and tuber quality. Excess N favors vine growth over tuber growth and maturation,

thereby decreasing tuber quality, delaying senescence and tuber maturity (Zvomuya et

al., 2002; Love et al., 2005). On the other hand, low N fertilizer limits tuber size, reduces

marketable yield and triggers early plant senescence (Belanger et al., 2002). The IFAS

guidelines for N fertilizer in potatoes is 224 kg ha-1, which most growers apply in

granular form at pre-plant (30 to 40 d before planting), at emergence, followed by two

to three side-dressings after plant emergence. The common application methods of

banding or broadcasting the fertilizer across the beds, increase leaching occurrences,

especially in years with high rainfall, thereby decreasing N availability to the plant

(Hochmuth and Hanlon, 2000; Hutchinson et al., 2008).

76

Fertigation has the potential to minimize N leaching, together with reducing other

losses through volatization, erosion and deep percolation (Shock et al., 2006; Wadell et

al., 1999). Another advantage of fertigation is the ability to split N applications over the

growing season to better match plant growth requirements. This improves N utilization,

minimizes ground water contamination and possibly reduces the amount of fertilizer

required to achieve good yields and tuber quality (Westermann et al., 1988;

Papadopoulos, 1990; Wadell et al., 1999; Shock et al., 2006). Applying N fertilizer

through fertigation could therefore possibly help meet production and environmental

goals of both growers and policymakers. It is therefore important to understand how

interactions of fertilizer application methods, rates and other critical preharvest factors

affect tuber yield, harvest quality, and storability.

The purpose of this experiment was to determine how N fertilizer application

method, rate and harvest time affect tuber yield harvest quality and storability of

surface-drip irrigated potatoes. The first hypothesis was that improving N fertilizer

placement and timing in fertigation application method produces higher yields and better

tuber quality compared to band-application of granular fertilizer. The second hypothesis

was that more precise application of N fertilizer to root zone, and matching crop growth

demands, in fertigation results in higher nutrient use efficiency and optimum yields and

tuber quality.

Materials and Methods

Experimental Site and Field Layout

The study was carried out at the UF/IFAS Florida Partnership for Water

Agriculture and Community Sustainability, Cowpen Branch in Hastings, FL, during the

spring seasons of 2013 (season 1) and 2014 (season 2). Tablestock potato cultivars

77

‘Fabula’ and ‘Red LaSoda’ were evaluated for response to five nitrogen fertilizer

treatments, three harvest times, and 14 d storage duration. The field experiment was a

completely randomized design with treatments in a split plot design and four

replications. The main plot was fertilizer treatment and the sub-plot was cultivar.

The field (79-m long, 18-m wide) was prepared by disking and plowing the soil,

after which rows (78-m long, 0.35-m high) were established, and three cross-cuts

formed to lay the polypipes for drip irrigation. The rows were hilled to improve drainage

and ease during harvesting. The land was divided into 40 plots, each comprising of 4

rows, with the cultivars and fertilizer treatments being randomly assigned. Potato seed

pieces were planted on January 28, 2013 and January 29, 2014, using 20-cm in-row

spacing and 15-cm depth. Phosphorus (P) fertilizer at 112 kg ha-1 rate and Potassium

(K) at 168 kg ha-1 rate was applied pre-plant; an additional side-dress of 140 kg ha-1 of

K was applied at plant emergence. Drip lines (16-mm inner diameter, 8-mm thickness,

30-cm emitter spacing) with a flow rate of 0.5 L h-1 100 m-1 (RO-DRIP, John Deere

Water, Moline, IL, USA) were buried 5 cm above the seed piece after planting. Four

irrigation events, each 25 mins long, were applied each day; with irrigation commencing

38 and 30 days after planting (DAP) in season 1 and 2, respectively.

There were a total of four fertigation treatments with N rates of 0, 112, 224, and

336 kg ha-1 (F-0, F-112, F-224, F-336) and one granular treatment of 224 kg N ha1 (G-

224) (Table 4-1). The fertigation rates were selected to determine if the fertilizer

application method would result in a lower (0 and 112 kg ha-1) or higher (336 kg ha-1)

rate than the current IFAS recommended rate of 224 kg N ha-1. The granular treatment,

78

at 224 kg N ha-1, was used to compare current grower fertilizer application methods with

the fertigation application method.

The fertilizer amount in each fertigation treatment was split over five weeks

matching the plant growth pattern and projected demands. During each application,

granular ammonium nitrate fertilizer (34-0-0) was completely dissolved in water before

being pumped into the main irrigation pipe. Five polypipes were connected to the main

irrigation line, each delivering a level of fertilizer treatment to the respective plots.

Valves were used to close the other polypipes while applying a particular treatment. To

minimize residual fertilizer build-up in the main irrigation line, water was flushed through

in between treatments. To determine effect of application method, the recommended

rate of 224 kg N ha-1 in the form of granular ammonium nitrate was also applied as a

fifth fertilizer treatment. The fertilizer was banded and incorporated into the soil at plant

emergence and when plants were 15-20 cm long.

All pesticide and herbicide applications followed the Florida potato production

recommendations (Hutchinson et al., 2004). Weather data (temperature, wind speed,

relative humidity, and solar radiation) was obtained from a weather station located at the

site. Daily reference evapotranspiration (ETo) was obtained from the Florida Automated

Weather Network (FAWN; www.fawn.ifas.ufl.edu).

Tuber Yield and At Harvest Quality Analysis

The two inner rows were divided into four sections, each 3 m long, where plant

establishment, biomass, and tuber sampling was carried out. A stand count was done

when the plants were well established, with average number of plants per plot used to

calculate plant density. Plant vines and tuber biomass accumulation was evaluated at

full flower (84 and 92 DAP in season 1 and 2, respectively), by randomly selecting two

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plants per plot. The plant vegetative material (vines) and tubers were bagged separately

in paper bags. The total tuber fresh weight for the sample plants in each plot was

measured, before a sub-sample was selected and weighed again for dry matter

calculations. All vine and tuber samples were dried in an oven at 65ºC, until they had

reached constant weight. The dried tissue samples were later ground using a tissue

grinder (Laboratory Mill Model 4, Arthur Thomas Company, Philadelphia, PA). Samples

were sent to the Analytical Research Laboratory (Gainesville, Florida) in season 1 and

Water Agricultural Laboratory (Camilla, Georgia) in season 2, for total N quantification

using the Kjeldahl method.

To promote natural plant vine senescence, irrigation was turned off 2 wks. before

projected vine kill time, which should be a minimum of 90 days after planting (DAP)

(Mossler and Hutchinson, 2008). Therefore, 91 and 98 DAP in season 1 and 2,

respectively, plant tops were killed off using a single application of the chemical

desiccant glufosinate ammonium (Rely® 280 Herbicide; Bayer Crop Science, Research

Triangle Park, NC, USA). The herbicide was applied at a rate of 1535 ml ha-1, with the

first harvest being carried out one week after spray. Harvest times were selected on the

basis of number of weeks after vine kill, ranging from one to three weeks (H1 to H3). At

each harvest, one of the flagged 3-m long sections was harvested using a mechanical

harvester to lift the tubers, followed by bagging in mesh produce bags and transported

to the Postharvest Laboratories at the University of Florida Horticultural Department in

Gainesville, Florida.

Potato yields and harvest quality were determined from a final mechanical

harvest (bag and tag harvester) from 6-m long rows in each plot, at three weeks after

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vine kill (H3). Total and marketable yields were determined from this sample plot.

Tubers were graded according to size, with marketable tubers falling under A1, A2, or

A3 categories (Appendix A-1), based on the USDA Standards for Grading of Potatoes

(2011). Culls (greening, growth cracks, misshapen and rots) were separated at the

grading line and reported as a percentage of total yield. Tuber internal quality was

evaluated from a sub-sample of 20 marketable tubers from each plot. Each tuber was

cut into quarters and rated for incidence of brown center, hollow heart, internal heat

necrosis, and corky ring spot.

Separate sub-samples from the marketable yield were used to measure tuber

specific gravity at all three harvest times. For each fertilizer treatment replicate (n=4),

approximately 2 kg sample of randomly selected tubers were weighed, transferred to

another tared basket and weighed under water. The weight in air and in water was used

to calculate the specific gravity (Specific Gravity = Weight in air / (Weight in air – Weight

in water)).

Postharvest Analysis

The tubers harvested from 3-m long sections at H1 to H3 were transported to the

Postharvest Laboratories at the University of Florida Horticultural Department in

Gainesville, Florida. Upon arrival, average sized tubers for each fertilizer treatment were

selected and carefully hand washed, placed on paper towels and fan dried for not more

than 30 min. Tubers were stored at 20°C, 80-85% RH overnight, then transferred to

simulated commercial storage conditions of 10°C, 80-85% RH the next morning, for a

total of 14 d storage duration. Fertilizer treatment replicates (n=4) were maintained in

order to track the effects of preharvest treatments on tuber storage quality. Tuber dry

matter content (peel and pulp), firmness, ascorbic acid content (peel and pulp), and

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fresh weight loss were determined before storage (initial storage quality) and every 7 d

during storage, using the methods outlined in chapter 3.

Soluble solids content, total titratable acidity and pH was determined from 2-cm

thick slices cut longitudinally from 3 tuber replicates in each plot, before being blended

(Hamilton Beach Model 908, Proctor-Silex Inc., NC, USA). The homogenate was

centrifuged at 15,000 rpm for 20 min using a Beckman model J2-21 centrifuge

(Beckman Coulter Inc., Fullerton, CA, USA), filtered through cheesecloth and the

supernatant used for further analyses. Two to three droplets of the supernatant were

placed on the prism of an electronic refractometer, the °Brix reading noted and later

used to determine the soluble solids content. Titratable acidity and pH readings were

determined from 3 ml of the supernatant, diluted with 50 ml deionized water. The

mixture was filtered and titrated with 0.1 mol L-1 sodium hydroxide solution, using

phenolphthalein as an end point indicator, in a digital pH-meter with a potentiometer

capable of measuring pH values of up to 8.1. Total titratable acidity (TTA) was

calculated as the number of milliliters of 0.1 N sodium hydroxide multiplied by a

conversion factor of 0.067, based on malic acid, which is the predominant acid in potato

(Ranganna 1986; Board, 1988). These variables were also measured every 7 d in

storage together with the other quality parameters.

Statistical Analysis

Data was analyzed using Analysis of Variance (SAS Institute Inc. Version 9.3,

Cary, NC, USA). A general linear mixed model (PROC GLIMMIX) was performed to

determine preharvest and postharvest main and interaction effects on tuber quality. The

analysis was implemented as a completely randomized design with split plot, where the

main plots were fertilizer treatment, harvest time and storage time. Treatment means

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were separated using the Tukey’s test and confidence limits of 95% calculated for each

mean.

Results

Weather Conditions

The cumulative rainfall amount decreased from 454 mm in season 1 to 394 mm

in season 2, while cumulative crop evapotranspiration (ETc) increased by 7% in season

2. Differences in the distribution of rainfall events were also observed between the two

years. In season 1, 83 % of the total rainfall occurred after mid-season (56 DAP), which

coincided with tuber bulking and maturation stage. The period between vine kill and final

harvest (H3) received 56% of the total seasonal rainfall, with majority (268 mm)

occurring the week between vine kill and the first harvest (H1). Rainfall amounting to 23

mm had also occurred just prior to vine kill (Figure 4-1A).

In season 2, more rainfall events occurred earlier in the season, with only 37%

occurring after mid-season (60 DAP). The period between vine kill and final harvest was

drier, when compared to the previous season. Only 9% of the total seasonal rainfall

occurred during this time, with 98% of this amount falling in the week between harvest 1

and 2 (Figure 4-1B).

The average air and soil temperatures increased as the season progressed, in

both years. In season 1, the average air and soil temperatures were 15ºC and 17ºC,

respectively by mid-season, increasing to an average of 21ºC for both by the end of the

season (Figure 4-2 A). Season 2 had slightly cooler temperatures at the beginning of

the season; the remainder of the season generally had similar temperatures to those in

season 1 (Figure 4-2 B).

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Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality

Nitrogen Assimilation and Biomass Accumulation

Early season rainfall and cooler temperatures during season 2 affected seed

germination and plant establishment, resulting in significantly lower plant population of

27,479 plant ha-1, compared to 48,959 plant ha-1 in season 1.

For aboveground biomass accumulation, the zero N fertigation rate, F-0, had a

significantly lower content, in both seasons, which averaged 14.4 g plant-1, while the

rest of the treatments averaged 36.5 g plant-1 (Table 4-2). The F-0 treatment was also

the only fertigation treatment significantly different from the granular G-224, in both

seasons. Similar patterns were observed in the tuber biomass, averaging 60.7 g plant-1

for higher fertilizer rates (F-112 to F-336), compared to 37.2 g plant-1 for F-0. Generally,

the harvest index (tuber biomass / plant total biomass) decreased with increasing N

rate. In season 1, F-0 and F-112 had a higher harvest index of 0.71, while the higher N

fertigation and granular rates averaged a ratio of 0.62. F-112 harvest index declined in

season 2, making it significantly lower than the other fertigation treatments.

There was a significant interaction between fertilizer treatment and season on N

uptake and assimilation in both aboveground and potato tubers. The lowest N uptake

was observed in F-0, in both seasons; average was 0.22 and 0.33 g N plant-1, in

aboveground and tubers, respectively (Table 4-3). For F-112, aboveground N uptake

(0.82 g N plant-1) was significantly lower than the higher fertigation rates (average 1.53

g N plant-1); which resulted in significantly lower total N uptake compared to higher N

rates. The granular treatment, G-224, had similar tuber N uptake with all fertigation

treatments.

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In season 1, significantly lower N content of 2.1% was also observed in F-0

aboveground tissue, while F- 224 and F-336 had twice the amount; F-112 had contents

similar to the higher treatment rates (Table 4-4). The F-0 treatment was also the only

fertigation treatment with significantly lower N content than the granular treatment.

Similar results were observed with the tuber N content in season 1. In season 2,

significant differences in aboveground N content were only observed for F-0 (2.0%) and

F-336 (4.4%). F-112, F-224, and F-336 had similar amounts, averaging 3.3%. However,

in the tubers, both F-224 and F-336 were significantly higher than F-0 – similar to

season 1’s pattern. There was no increase in N content at treatment rates higher than

112 kg ha-1 (F-112), for both tops and tubers, in both years.

Tuber Yield and At Harvest Quality

Fertilizer treatment had a significant effect on tuber total and marketable yields,

with seasonal differences; therefore each season was analyzed separately (Table 4-4

and Table 4-5).

In season 1, F-0 treatment had a significantly lower total yield of 12,069 kg ha-1,

than all other fertigation treatments, which averaged 26,006 kg ha-1 (Table 4-5).

Increasing the fertigation rate above112 kg ha-1 (F-112), did not result in any significant

change in total yields. In addition, yields similar to those of the granular application

method, G-224, were obtained by all fertigation treatments. For the marketable yield, F-

0 had 6,422 kg ha-1 (53% of total yield), which was similar to F-112 (62% of total yield),

but lower than F-224 and F-336, which averaged 20,597 kg ha-1 (75% of total yield). G-

224 produced marketable yields similar to all fertigation treatment.

Total yield declined for all fertigation treatments in season 2. However, similar to

season 1, F-0 total yields of 5,394 kg ha-1 were again lower than the other fertigation

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treatments, which averaged 15,245 kg ha-1 (Table 4-6) The higher fertilizer rates also

produced yields similar to those of the lower rate F-112. However, for the application

methods, G-224 total yields increased from 21,684 kg ha-1in season 1 to 26,562 kg ha-1

in season 2, resulting in significantly higher yields than F-0 and F-112. In both years, F-

224 produced total yields similar to the granular application method.

The marketable yield for F-0 declined to 2,033 kg.ha-1 (38% of total yield) in

season 2, making it significantly lower than all treatments. Similar to the previous year,

there was no increase in marketable yield with increased rates above F-112 treatment,

all averaging 12,142 kg ha-1 (69% of their total yields). In addition, although G-224

outperformed F-0 and F-112 in season 2, F-224 still maintained a similar marketable

yield, as seen in season 1.

There was a significant interaction between fertilizer treatment and season on

incidence of external physiological disorders. Generally, F-0 had the least incidence of

total external physiological disorders in both years (Table 4-5 and 4-6). Tuber decay

accounted for the greatest incidence of external physiological disorders in higher N

treatments (Table 4-7). During season 1, all fertilizer treatments had over twice the

amount of decay seen in F-0. The decay incidence was significantly lower in season 2,

with no difference between treatments and averaging 399 kg ha-1. No significant internal

physiological disorders were seen in any of the treatments.

Significant differences in specific gravity between fertilizer treatments were only

observed in season 1. F-0 had a ratio of 1.050, which was significantly higher than F-

336 (1.044). A lower specific gravity of 1.042 was also observed with the granular G-

224, while F-0, F-112 and F-224 produced higher averages of 1.048 (Figure 4-3). Due

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to a greater number of undersized tubers obtained with F-0 during season 2, specific

gravity was not measured for that treatment. However, no differences were observed

among the other treatments, with an average ratio of 1.042.

Effect of Nitrogen Application Method and Rate on Tuber Storage Quality

Initial Storage Quality

To determine the effect of fertilizer treatment and harvest time, tuber initial

storage quality was evaluated. All quality parameters, except firmness, were affected by

at least one of the preharvest treatments (Table 4-8).

There were no significant differences in pulp dry matter content in season 1 - all

treatments averaged 14.1%. The following year, at H1 the F-224 treatment had a lower

content of 13.7%, compared to F-0, which averaged 16.8% (Table 4-9). F-0 content

decreased by 3.6% with increased harvest time, resulting in no significant differences

between fertilizer treatments at H2 and H3. All fertigation treatments also produced pulp

dry matter contents similar to G-224, at all harvest times

Significant differences were observed in peel dry matter content during season 1

only. H1 tubers had a significantly lower content of 9.8% compared to the later harvests

which averaged 13.1% (Figure 4-4). No differences were observed the following year,

with an average content of 11.1%.

Harvest time affected both the peel and pulp ascorbic acid content, with no

significant differences between fertilizer treatments in both seasons. Generally, H1

tubers had the highest content of 17.4 mg 100 g-1 and 24.4 mg 100 g-1 for the peel and

pulp, respectively. The ascorbic content in both tissues decreased significantly at H2,

with no further changes to the end of the season (Figure 4-5).

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Fertilizer treatment affected soluble solids content (SSC), total titratable acidity

(TTA), and pH (Table 4-10). F-0 and F-112 had a lower SSC average of 3.04%,

compared to 4.13% for F-336. F-224 and F-336 produced similar SSC to G-224. TTA

also followed a similar pattern, with F-0 tubers measuring a lower percentage of 0.12%,

while the higher rates F-224 and F-336 had 0.15%. There were no significant

differences between fertigation treatments F-112 and F224, and granular G-224. Tuber

pH increased with increased fertilizer rate; F-0 having a more acidic reading of 2.7,

while F-224 and F-336 averaged 4.0. There were no pH differences between the

granular application method G-224 and fertigation treatments F-224 and F-336, while

the other treatments had significantly lower readings.

Storage Quality

Quality parameters affected by storage were fresh weight loss, dry matter

content, firmness, soluble solids content, and titratable acidity (Table 4-11). There was a

significant interaction of fertilizer treatment, harvest time, storage duration, and season

on fresh weight loss. In season 1, all H1 fertilizer treatments lost similar amounts of

weight with increasing time, with no significant differences throughout storage (Figure 4-

6 A). The average cumulative weight loss was 2.4% and 3.6% at 7 d and 14 d,

respectively. A significant interaction between fertilizer treatment and storage time

affected H2 (Figure 4-6 B) and H3 tubers (Figure 4-6 C), where F-0 lost significantly

more weight with increased storage time. By the end of storage, F-0 had lost an

average of 3.2%, while the other treatments had significantly lower weight loss average

of 1.3 %. In season 2, fertilizer treatment had no effect on weight loss, with all

treatments losing similar amounts of weight with increased storage time. However, H1

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and H2 tubers lost the most weight by the end of storage, with an average of 4.4% loss

by 14 d, while H3 treatments lost 2.5 % (Figure 4-7 A-C).

Peel dry matter content was affected by fertilizer treatment, harvest time, and

storage duration (Table 4-12). This was attributed to differences observed in H1 tubers,

where the content increased at 7d for all fertilizer treatments. F-224 and G-224 later

decreased at 14 d, resulting in a significant lower average content of 11.3% by the end

of storage, while others maintained an average of 14.4%. No significant changes were

seen in H2 and H3 tubers throughout storage, with an average of 10.6% and 11.4%,

respectively.

Meanwhile, the pulp dry matter content was affected more by a significant

interaction between harvest time and fertilizer treatment, with no changes occurring with

increased storage time. Generally, at H1 F-112, F-336 and G-224 maintained a lower

content of 14.7% throughout storage, compared to F-0, which averaged 16.1%

throughout storage (Table A-2). However, due to a decrease in F-0 initial content with

increased harvest time, as reported before, no significant differences between fertilizer

treatments at the last two harvest times were observed throughout storage.

Prior to storage, there were no differences in firmness for all fertilizer treatments,

at all harvest times. However, a significant interaction between harvest time and storage

time affected tuber firmness, with no difference between fertilizer treatments throughout

storage. Generally, firmness increased with increased time, at all harvest times (Figure

4-8). H1 tubers significantly increased from 11.8 N to 13.3 N at 7 d, later decreasing

again to 12.8 at 14 d. On the other hand, H 2 and H3 tuber firmness only increased

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significantly towards the end of storage, with no differences among all harvest times at

the end of 14 d storage.

Fertilizer treatment had a greater effect on tuber pH, soluble solids and titratable

acidity, with no changes with increased storage time being observed at all harvest

times. Similar to observations in the initials, higher readings were recorded at the high

fertilizer treatments, for all three quality parameters (Table A-3).

Discussion

Any factor influencing soil moisture, such as rainfall, irrigation and

evapotranspiration, will affect soil N availability and general plant growth. Higher rainfall

in season 1, with majority occurring during the harvest period, likely affected

effectiveness of vine kill and delayed tuber maturation. On the other hand, the low plant

population in season 2 was likely a result of poor seed germination and plant

establishment due to heavy early season rainfall and cool temperatures. The heavy

rainfall may have also caused N fertilizer movement beyond the crop root zone, thereby

depleting available soil N and reducing total yields. Similar to reports by Fan and

Mylavarapu (2010), delayed N application as a sidedress likely maintained adequate N

fertilizer in the root zone, thereby optimizing yields in granular treatment, in both

seasons.

In this study, higher N fertilizer rates had a positive effect on N uptake, N content

and plant biomass accumulation. Generally, N uptake increased with increased N rate;

the zero N treatment had the lowest uptake in both aboveground plant material and

tubers (Table 4-3). The zero N treatment also had significantly lower N content at full

flower, than the 336 kg N ha-1 treatment, for both aboveground and tubers. The high N

content at the highest N rate can be accounted for by the plant taking up more N than is

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needed to satisfy its immediate growth requirements (Millard and Marshall, 1986).

However, the other treatments matched the N contents in the highest treatment,

indicating sufficient N for plant growth and biomass accumulation at the lower rates of

112 and 224 kg N ha-1.

This was further supported by the findings showing a significantly lower biomass

accumulation in aboveground and tubers for the zero N treatment, compared to higher

rates of 224 and 336 kg N ha-1 (Table 4-2). Higher biomass accumulation with

increased N rate was also reported by Hutchinson et al. (2003) and Kelling et al. (1997).

Nitrogen stress at the low rates reduced leaf area and canopy development, resulting in

low photosynthates production and subsequent translocation from the tops into the

tubers (Ojala, et al., 1990). However, the harvest index (HI), that is the measure of

partitioning efficiency of dry matter to the tubers, decreased with increased N fertilizer

(Millard and Marshall, 1986; Vos, 1999; Zvomuya et al., 2003; Kumar et al., 2007). This

is likely due to increased biomass partitioning to tubers when plants are subjected to N

stress under the low rates (Belanger et al., 2001). Comparing the application methods,

the lower fertigation rates of 112 and 224 kg N ha-1 had similar biomass, N uptake, and

N content to the granular treatment (G-224), indicating efficiency of the fertigation

method.

Similar to other studies, the response pattern of yield to fertilizer rate was the

opposite of the harvest index, with higher yields being produced at higher N rates. As

expected, the zero N treatment produced significantly lower total yields than higher

fertigation rates (Tables 4-5 and 4-6). Inadequate supply of N during tuber initiation

reduced leaf area and canopy development, depressing yields due to poor tuber set

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(Roberts et al., 1982; Ojala et al., 1990). Increasing the fertilizer rate to 112 kg N ha-1

increased the total yield, which remained constant at higher N fertigation rates, in both

seasons. This is supported by previous studies done where optimum yields were

obtained at N lower rates, with little or no yield penalty at higher rates (Leep, 1988; Vos,

1997; Dahlenburg et al., 1990; Hutchinson et al., 2003; Shock et al., 2006; Hochmuth

and Hanlon, 2011). In addition, early maturing cultivars, such as ‘Fabula’ have been

reported to generally require less N due to lower yield potentials and earlier harvest

times, compared to late maturing cultivars (Rosen et al., 2008). Therefore, this indicates

that 112 kg N ha-1 fertigation was sufficient to ensure that N was not limiting tuber total

yield, while higher rates promoted excessive vine growth at the expense of tuber

bulking. However, in the event of heavy early season rains, as those experienced in

season 2, the fertigation rate of 112 kg N ha-1 also proved to be too risky, as it resulted

in significantly lower yields compared to season 1.

Tuber marketable yields followed a similar trend, with the zero N treatment

consistently producing equal or lower yields than the other fertigation treatments. In

season 1, 35% of the total yield for the zero treatment was under-sized, while

undersized tubers in the other treatments were only 10-13% of the total yield (Table 4-

5). The low average tuber size with low N rates is in agreement with earlier studies

(Dahlenburg et al., 1990; Hutchinson et al., 2003), and is a result of reduced tuber

bulking rate (Ojala et al., 1990). Meanwhile, tuber decay was significantly higher in the

other treatments, lowering the marketable yields. Generally, this can be attributed to

high rainfall amounts which occurred during harvest time in season 1. A higher

incidence of decay in high levels of N has been reported in other studies too

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(Shekhawat et al., 1982; Kumar et al., 1991). Hochmuth and Hanlon (2011) attributed

tuber decay to decreased phenolic content which resulted in higher bacterial infiltration

and activity in tubers grown under high N rates. Meanwhile, McGuire and Kelman

(1984) stated that decreased membrane permeability increased susceptibility to soft rot

bacteria. This can be supported by the fact that potatoes grown with higher N rates,

which likely have a less mature skin, consistently had a higher decay incidence than the

zero N treatment. However, the drier weather conditions in season 2, particularly during

the tuber bulking stage, resulted in a greater number of under-sized tubers in all

treatments, resulting in significantly lower yields again in the zero treatment (Table 4-6).

On the other hand, fertigation treatments of 112 kg N ha-1 and higher rates

produced equal or higher total yields than the granular application method in season 1,

showing the potential of fertigation as a strategy to reduce N application (Table 4-5).

High yields at lower N rates in fertigation were attributed to better N uptake due to split

applications matching crop growth demands, as reported in other studies (Shock et al,

2006; Rosen et al., 2008). A lower harvest index for 112 kg N ha-1 treatment in season 2

(Table 4-2), possibly affected yield, resulting in lower total and marketable yields

(together with 0 kg N ha-1) than the granular treatment (Table 4-6). This could have

been due to N leaching when the heavy early season rainfall occurred, resulting in

nutrient stress during the vegetative growth stage, particularly tuber bulking, leading to a

reduction in dry matter accumulation (Yungen et al., 1958; Hughes, 1974; Mackerron

and Davies, 1986; Fabeiro at al., 2000).

Tuber parameters influencing processing quality were also monitored before

storage (initial storage quality) and during postharvest storage. Preharvest factors had

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an effect on all initial storage quality parameters, except for firmness; meanwhile only

peel dry matter content, pulp ascorbic acid content, tuber fresh weight and firmness

were affected by storage. Many studies have reported a reduction in specific gravity at

high N rates, supporting observations in this study, where the zero treatment had a

significantly higher ratio than the highest fertigation treatment of 336 kg N ha-1 (Figure

4-3) (Dahlenburg et al., 1990; MacLean, 1984; Porter and Sisson, 1993; White and

Sanderson, 1983). This decline in specific gravity at high N is attributed to excess N

being available particularly during late tuber bulking, possibly delaying tuber maturity

and/or the higher N uptake resulting in higher protein content and lower total soluble

solids (Kunkel and Dow, 1961; Vakis, 1978; Papadopoulos, 1988). Meanwhile, some

authors have not detected significant effects of N rate on specific gravity, as seen during

season 2 of this study (Westermann and Kleinkopf, 1985; Millard and Marshall, 1986).

The results in this study showed that potatoes grown with the granular treatment had a

significantly lower specific gravity than all fertigation treatments, except 336 kg N ha-1.

This further indicates the inadequacies of applying high N rates through drip irrigation.

Evaluation of tuber dry matter content, which is closely related to specific gravity,

over three harvest times showed a greater effect of harvest time on the quality

parameter both before and during storage. The peel content in this study consisted of

primarily the periderm and a few directly underlying layers of cortical cells. Meanwhile

the pulp was comprised of all the inner tissue, that is, the cortex, xylem, perimedullary

and pith. The peel and the pulp are comprised of very different types of cells, and

therefore, it is expected for them to react differently to the same treatment.

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In season 1, only the peel dry matter content (initial storage quality) of H1 tubers

was affected, with no differences among fertilizer treatments (Figure 4-4). The opposite

trend happened in season 2, where pulp tissue had a higher content in the zero N

compared to fertigation 224 kg N ha-1 at H1 (Table 4-9). This temporal variation in tuber

tissue affected indicates that response of dry matter content to both N fertilizer and

harvest time largely depends on seasonal weather conditions, particularly rainfall. The

rainfall occurrences between vine kill and H1 likely caused increased water content in

the peel tissue, in order to maintain cell turgor pressure against increased negative

solute potential, resulting in a lower peel dry matter content (Westermann et al., 1994).

On the other hand, fertilizer treatment had a greater effect on the pulp dry matter

content during season 2. Previous studies, where the peel and pulp have not been

separated, have shown increased dry matter content with increased N rate, up to the

point of ‘over-fertilization’, with no significant increase thereafter (Westermann et al,

1994; Joern and Vitosh, 1995) or a marked decline (Wszelaczynska and Poberezy,

2011). Findings from this study suggest that 224 kg ha-1 might have been the point of

over-fertilization, resulting in a decreased rate of dry matter translocation to the tubers

during growth. However, weather conditions could have a played a greater role in this,

as this was observed only during the drier season 2. In addition, this was only observed

at the first harvest time, with no significant differences in pulp dry matter among the

fertilizer treatments at the later harvests.

In storage, the significant increase in peel dry matter content at 7 d of H1 tubers,

with no significant differences between fertilizer treatments, was likely due to immaturity

of the tubers (Table 4-12). Water loss may have resulted in increased starch

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concentration in cells, which constitutes about 70 - 75% of the dry matter content

(Lisinska and Leszczynski, 1989; Kaaber et al., 2001; Nipa et al., 2013). Increased cell

suberization and wound periderm formation has also been reported to account for

increased dry matter (Blekinsop et al., 2002). Meanwhile, storage time had less effect

on the pulp dry matter content, with no significant change throughout for all fertilizer

treatments (Table A-2).

Similar to tuber dry matter content, harvest time had an effect on ascorbic acid

content of tubers’ initial storage quality (Figure 4-5). The decrease in content with

increased delay in tuber harvesting was likely due to a reduction in biosynthesis as the

vines begin to die (Kelly and Somers; 1949; Namek and Moustafa, 1953; Gonella et al.,

2008). According to Mozafar (1993), the response of ascorbic acid to increased N rates

is to reach a maximum biosynthesis point, after which level the content starts to decline.

High N rates cause a delay in maturity and subsequent stage at which tubers exhibit

maximum ascorbic acid content (Shekhar et al., 1978). Our results therefore indicate

that all fertilizer treatments were within the ideal range for adequate nutrient

biosynthesis, similar to findings by Baker et al. (1950). The decline in ascorbic acid

content with increased N rate observed in many other studies, contradicting our studies,

may be due to a shorter growing season for ‘Fabula’, compared to late season cultivars

such ‘Russet Burbank’. In storage, losses of ascorbic acid through oxidation and

formation of dehydroascorbic acid have been reported (Augustin 1975; Watada, 1987;

Perkins 1993; Dale et al., 2003; Phillips at al. 2010). However, unlike in most of these

studied where tubers were stored for months, the short storage conditions and duration

carried out in this study may have prevented an accelerated breakdown.

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Fresh tuber weight loss during storage was affected by both harvest time and

fertilizer treatment during season 1. The weight loss with increased storage time for all

H1 tubers, irrespective of the fertilizer treatment (Figure 4-6 A), was likely due to an

immature skin not being an effective vapor barrier (Sabba and Lulai, 2002). Skin

maturity increased with a delay in harvest time, resulting in lower weight loss at later

harvests (Adams, 1975; Tyner, 1997; Blenkinsop et al. 2002, Nipa et al., 2013). The

higher weight loss observed in the zero N treatment, in H2 and H3, suggests a delay in

increased tuber maturation after vine kill for tubers produced at this fertilizer level

(Figures 4-6 B and C). This short term effect may have been over-looked in the

previously published studies where weight loss increased with increased N rate, during

six to nine months storage of cured potatoes (Jablonski, 2006; Wszelaczynska and

Poberezny, 2011). However, rate of N fertilizer did not significantly affect tuber weight

loss the following season, confirming work by Dahlenburg et al. (2000). This was likely

due to the drier conditions during vine kill and harvest time promoting uniform suberin

deposition in all fertilizer treatments.

The changes in weight loss seemed to affect tuber turgidity, and subsequently

firmness during storage. The increase in firmness at 7 d storage supports findings by

Gamea et al. (2009), who accounted it to high elasticity of the tubers due to higher

percentage weight loss in the first 25 d of storage (Figure 4-8). A slower rate of weight

loss in the more mature tubers from the later harvests likely accounted for changes in

firmness being observed only towards the end of storage.

Titratable acidity, pH and soluble solids content affect the biochemical quality of

potatoes, which ultimately affect the shelf-life and cooking quality. The pH determines

97

deterioration potential by fermentation and enzyme activity (Cecchi, 1999). The

maximum activity of phosphorylase (starch breakdown) is reached at pH 5.5, while

invertase (sucrose breakdown) is pH 4.7 (Pressey, 1966; Iritani and Weller, 1973). The

pH increase with increased fertilizer rate, before storage (Figure 4-10) and maintaining it

throughout storage, is indicative of a higher tendency of deterioration in the higher

fertilizer treatments. However, for ‘Fabula’, all fertilizer treatments maintained a more

acidic environment, minimizing enzyme activity throughout storage, indicating a lower

fermentation potential. Meanwhile, although the zero N treatment had significantly lower

organic acids and soluble solids at harvest (Figure 4-10), there were no changes in all

treatments throughout storage. This indicates a low respiration rate and/or minimum

build-up of sugars in all treatments, during storage (Chitarra and Chitarra, 1990).

Conclusions

This study revealed the potential of drip irrigation as a means of maximizing

yields, tuber quality and storability in Florida potatoes. Generally, tuber yields were

greatly influenced by the N fertilizer rate and/or application method, while harvest time

had a greater effect on tuber quality before, and during storage.

The fertigation rates of 112 and 224 kg N ha-1 had similar N uptake, biomass

accumulation and N content, to that of the granular treatment. In addition, 112 kg N ha-1

fertigation rate was enough to ensure that nitrogen was not the limiting factor, producing

total yields similar to the higher rates. However, N leaching due to heavy early season

rainfall in season 2 reduced both total and marketable yields for this lower N rate. This

indicates that 112 kg N ha-1 fertigation might be too risky, in the event of seasonal

weather differences, particularly heavy early season rainfall. On the other hand, the

fertigation rate of 224 kg N ha-1 was not affected by the varying weather conditions,

98

maintaining similar yields and a higher specific gravity than the granular method

throughout this study.

The initial and storage quality for most parameters of the tubers was highly

dependent on the harvest time. Any differences in quality among the fertilizer treatments

were generally observed in tubers harvested one week after vine kill. Tubers harvested

at this stage are physiologically and compositionally less mature than the later harvests,

resulting in greater storage losses. However, the storage conditions and duration used

in this study proved to be highly effective in minimizing quality loss, especially when

tubers are harvested two to three weeks after vine kill. Generally, the fertigation rates of

112 and 224 kg N ha-1 proved to maintain a similar or better storage quality than the

granular treatment for most of the quality parameters.

This study therefore concludes that fertigation using surface drip in ‘Fabula’

potato has the potential to produce high yields and tuber quality. There were indications

that nitrogen fertigation rates as low as 112 kg ha-1 could produce good yields and tuber

quality. However, given the varying seasonal weather conditions experienced in Florida,

particularly of low or high rainfall events, a fertigation rate of 224 kg N ha-1 is

recommended as it proved to produce more consistent yields and tuber quality. In

addition, the ideal harvest time recommended is two to three weeks after vine kill,

because the tubers are more mature and therefore maintain acceptable quality when

stored at 10°C, 80-85% RH, irrespective of the fertilizer rate applied.

99

Table 4-1. N fertilizer rates and application methods for ‘Fabula’ potato fertigation trial in 2013 (season 1) and 2014 (season 2)

Treatment Application Method N Rate (kg.ha-1)

F-0 Fertigation 0

F-112 Fertigation 112

F-224 Fertigation 224

F-336 Fertigation 336 G-224 Granular 224

Table 4-2. Plant aboveground (AG) and tuber biomass, on a dry weight basis, at full flower of ‘Fabula’ potatoes grown under five nitrogen treatments in 2013 (season 1) and 2014 (season 2).

Dry Biomass Weight (g plant-1) Season 1 Season 2 Treatmentz AG Tuber Total H.I.x AG Tuber Total H.I.

F-0 14.2 by 37.2 b 51.4 b 0.72 a 14.5 b 19.1 b 33.6 b 0.76 a F-112 28.3 ab 64.0 a 92.3 ab 0.69 a 38.1 a 61.3 a 99.4 a 0.62 c F-224 39.1 a 60.9 a 100.0 a 0.60 b 42.2 a 64.2 a 106.4 a 0.66 b F-336 35.5 a 64.8 a 100.3 a 0.63 b 39.7 a 57.1 a 96.8 a 0.69 b G-224 30.5 a 54.9 a 85.4 ab 0.64 b 38.5 a 58.8 a 97.3 a 0.65 bc zFertilizer treatments of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

.

y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level. xHarvest index: Tuber biomass/Total biomass

Table 4-3. Plant aboveground and tuber nitrogen uptake of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).

Nitrogen Uptake (gN plant-1) Season 1 Season 2 Treatmentz AG Tuber Total AG Tuber Total

F-0 0.24 cy 0.29 c 0.53 c 0.20 c 0.37 c 0.57 c F-112 0.79 b 0.93 b 1.72 b 0.85 b 1.06 b 1.92 b F-224 1.48 a 1.05 ab 2.53 a 1.59 a 1.81 ab 2.34 a F-336 1.33 a 1.38 a 2.71 a 1.71 a 1.16 b 2.40 a G-224 1.29 a 1.05 ab 2.34 a 0.95 b 2.09 a 2.40 a zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level

100

Table 4-4. Total Kjeldahl nitrogen of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).

Treatmentz

Total Kjeldahl Nitrogen (%)

Season 1 Season 2

AG Tuber AG Tuber

F-0 2.1 by 2.0 b 2.0 b 1.3 b F-112 3.1 ab 2.5 ab 2.5 ab 1.8 ab F-224 4.1 a 3.1 a 3.1 a 2.0 a F-336 4.1 a 4.7 a 4.4 a 2.1 a G-224 4.4 a 2.8 ab 2.8 ab 1.9 ab zFertilizer treatments of R-0, R-112, R-224, and r-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level

101

Table 4-5. Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 1)

Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu

Treatmenty (kg ha-1) (kg ha-1) (%x) (kg ha-1) (%) (kg ha-1) (%)

F-0 12,069 bz 6,422 b 53.2 4,301 aa 35.6 1,347 c 11.2 F-112 22,171 ab 13,698 ab 61.8 3,185 b 14.5 5,288 a 23.9 F-224 27,697 a 21,382 a 77.2 2,848 b 10.3 3,468 ab 12.5 F-336 27,624 a 19,812 a 71.7 2,928 b 10.6 4,884 ab 17.7 G-224 21,684 ab 15,715 ab 72.5 2,715 b 12.5 3,254 b 15.0 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

xPercent of total yield

wMarketable yield: tuber grades A1, A2, A3.

vUnmarketable grades A4, B, C.

uGreening, growth cracks, misshapen, decay

Table 4-6. Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 2)

Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu

Treatmenty (kg ha-1) (kg ha-1) (%x) (kg ha-1) (%) (kg ha-1) (%)

F-0 5,394 c 2,033 c 37.7 3,229 b 59.9 132 b 2.4 F-112 13,254 b 8,473 b 63.9 3,926 ab 29.6 855 a 6.5 F-224 17,144 ab 12,939 ab 75.5 3,596 b 21.0 609 a 3.6 F-336 19,337 ab 15,015 ab 77.6 3,578 b 18.5 745 a 3.9 G-224 26,562 a 21,029 a 79.2 4,466 a 16.8 1,068 a 4.0 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

xPercent of total yield

wMarketable yield: tuber grades A1, A2, A3.

vUnmarketable grades A4, B, C.

uGreening, growth cracks, misshapen, decay

102

Table 4-7. Tuber external disorders in ‘Fabula’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill (season 1 and 2)

Fertilizer treatmenty

External Physiological Disorders (kg ha-1)

Season 1 Season 2

GRx GC Misshapen Decay GR GC Misshapen Decay

F-0 198 bz 0 b 78 a 1,072 c 0 a 0 a 0 a 132 a F-112 734 a 62 b 125 a 4,367 a 0 a 151 a 0 a 705 a F-224 477 ab 0 b 81 a 2,909 b 0 a 0 a 151 a 459 a F-336 617 a 763 a 308 a 3,196 b 286 a 0 a 0 a 459 a G-224 242 b 0 b 0 a 2,737 b 738 a 88 a 0 a 242 a

z Means within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

xPhysiological disorders: GR – greening; GC – growth cracks; Misshapen; Decay

Table 4-8. Analysis of variance of season, fertilizer treatment and harvest time effect on tuber initial storage quality in ‘Fabula’

Dry matter Ascorbic acid Main Effects Peel Pulp Peel Pulp Firmness SSC TTA pH

Year (Y) * * ns ns ns ns ns ns Fertilizer (F) ns * ns ns ns * * * Harvest (H) * * * * ns ns ns ns Interactions: Y x F ns * ns ns ns ns ns ns Y x H * * ns ns ns ns ns ns H x F ns * ns ns ns ns ns ns Y x H x F ns * ns ns ns ns ns ns

* Significant at p<0.05; ns: not significant

103

Table 4-9. Effect of fertilizer treatment and harvest time on pulp dry matter content of ‘Fabula’ tubers’ initial storage quality (season 2)

Harvest Timex

Pulp Dry Matter Content (%)

Fertilizer Treatmenty

F-0 F-112 F-224 F-336 G-224

H1 16.8 aAz 14.7 aAB 13.8 aB 13.9 aAB 14.6 aAB H2 13.3 bA 14.3 aA 14.1 aA 14.1 aA 14.6 aA H3 13.1 bB 14.3 aA 14.7 aA 14.5 aA 15.0 aA z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital

letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.

Table 4-10. Effect of fertilizer treatment on soluble solids content (SSC), total titratable acidity (TTA), and pH of ‘Fabula’ initial storage quality (season 1 and 2)

Fertilizer treatmenty

SSC TTA pH

% %

F-0 2.73 b 0.12 b 2.70 cz

F-112 3.35 b 0.13 ab 3.47 b F-224 3.85 ab 0.15 a 3.88 ab F-336 4.13 a 0.14 a 4.02 a G-224 4.15 a 0.15 a 4.13 a z Means within each column followed by the same small letter do not differ significantly according to

Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

.

104

Table 4-11. Analysis of variance of season, fertilizer treatment, and harvest time effect on tuber quality in ‘Fabula’ stored at 10°C, 80-85% RH for 14 d

Dry matter Ascorbic acid Main Effects Peel Pulp Peel Pulp Weight

loss Firmness SSC TTA pH

Year (Y) ns * ns ns * ns ns ns ns Fertilizer (F) * * ns ns * ns * * * Harvest (H) * * * * * * ns ns ns Storage (S) * ns ns * * * ns ns ns

Interactions:

Y x F ns * ns ns * ns ns ns ns Y x H * * ns ns * ns ns ns ns Y x S ns ns ns ns * ns ns ns ns F x H * * ns ns * ns ns ns ns F x S * ns ns ns * ns ns ns ns H x S * ns ns * * * ns ns ns

Y x F x H ns ns ns ns * ns ns ns ns Y x F x S ns ns ns ns * ns ns ns ns Y x H x S ns ns ns ns * ns ns ns ns F x H x S * ns ns ns * * ns ns ns

Y x F x H x S ns ns ns ns * ns ns ns ns * Significant at p<0.05; ns: not significant

105

Table 4-12. Effect of fertilizer treatment and harvest time on tuber peel dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d

Harvest Timey

Storage time (d)

Peel Dry Matter Content (%)

Fertilizer Treatmentx

F-0 F-112 F-224 F-336 G-224

H1 0 10.9 bAz 11.1 bA 10.4 aA 10.8 bA 11.3 bA 7 13.6 aA 13.6 aA 13.9 aA 13.9 aA 14.7 aA 14 14.4 aA

14.3 aA 11.4 bB 14.1 aA 11.1 bB

H2 0 10.3 aA 10.8 aA 11.3 aA 10.1 aA 10.2 aA 7 10.0 aA 11.6 aA 11.9 aA 11.5 aA 10.6 aA 14 10.7 aA

11.4 aA 11.7 aA 10.6 aA 10.1 aA

H3 0 10.7 aA 11.4 aA 11.3 aA 11.7 aA 11.5 aA 7 11.1 aA 10.5 aA 11.3 aA 12.1 aA 11.8 aA 14 11.7 aA 11.0 aA 11.5 aA 12.4 aA 11.7 aA z Means within a column followed by the same small letter for each irrigation method, at each harvest

time, or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05).

y Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.

xFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

106

A)

B) Figure 4-1. Rainfall and cumulative evapotranspiration in Hastings, FL, during growing

A) season 1, and B) season 2

0

50

100

150

200

250

300

350

400

450

500

0

20

40

60

80

Vin

ekill H1

H2

H3

Ra

infa

ll (m

m)

Days after planting

Rain Cumu. Rain Cumu. Etc

0

50

100

150

200

250

300

350

400

450

500

0

20

40

60

80

Vin

ekill H1

H2

H3

Ra

infa

ll (m

m)

Days after planting

Rain Cumu. Rain Cumu. Etc

107

A)

B) Figure 4-2. Mean air and soil temperatures experienced during growing A) season 1,

and B) season 2

0

5

10

15

20

25

30

0

20

40

60

80

Vin

e k

ill

H1

H2

H3

Te

mp

era

ture

°C

Days after planting

Air-2m Soil-10cm

0

5

10

15

20

25

30

0

20

40

60

80

Vin

e k

ill

H1

H2

H3

Tem

pera

ture

°C

Days after planting

Air-2m Soil-10cm

108

Figure 4-3. Tuber specific gravity of ‘Fabula’ potatoes grown under five fertilizer treatments. Means separation: same letters do not differ significantly according to Tukey’s Test (p<0.05)

Figure 4-4. Effect of harvest time on the peel dry matter content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill (H1-3). Means separation: same letters do not differ significantly according to Tukey’s Test (p<0.05)

a

ab

ab

bc

c

1.038

1.04

1.042

1.044

1.046

1.048

1.05

1.052

F-0 F-112 F-224 F-336 G-224

Specific

gra

vity

Nitrogen Fertilizer Treatment

b

a

a

0

2

4

6

8

10

12

14

16

H1 H2 H3

Peel dry

matt

er

conte

nt (%

)

Harvest time after vine kill (weeks)

109

Figure 4-5. Effect of harvest time on the peel and pulp ascorbic acid content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill (H1-3). Means separation: same small letter (peel ascorbic acid content), or same capital letter (pulp ascorbic acid content) do not differ significantly according to Tukey’s Test (p<0.05)

a

b

b

A

B B

0

5

10

15

20

25

30

H1 H2 H3

Ascorb

ic a

cid

conte

nt

(mg/1

00g F

W)

Harvest time after vine kill (weeks)

Peel

Pulp

110

A)

B)

C) Figure 4-6. Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time

after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 1), where A) harvest 1, B) harvest 2; C) harvest 3

0

1

2

3

4

5

6

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 1: H1

F-0

F-112

F-224

F-336

G-224

0

1

2

3

4

5

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 1: H2

F-0

F-112

F-224

F-336

G-224

0

1

2

3

4

5

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 1: H3

F-0

F-112

F-224

F-336

G-224

111

A)

B)

C) Figure 4-7. Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time

after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 2), where A) harvest 1, B) harvest 2, C) harvest 3

0

1

2

3

4

5

6

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 2: H1

F-0

F-112

F-224

F-336

G-224

0

1

2

3

4

5

6

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 2: H2

F-0

F-112

F-224

F-336

G-224

0

1

2

3

4

5

6

0d 7d 14d

Fre

sh w

eig

ht

loss (

%)

Storage time (days)

Season 2: H3

F-0

F-112

F-224

F-336

G-224

112

Figure 4-8. Firmness of ‘Fabula’ tubers stored for 14 d at 10ºC and 80-85% RH

10.5

11

11.5

12

12.5

13

13.5

14

0d 7d 14d

Bio

yie

ld forc

e (

N)

Storage time (days)

H1

H2

H3

113

CHAPTER 5 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND

HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. II. RED LASODA’.

Introduction

Differences in response to water and nitrogen management among potato

cultivars has been reported by several authors (Westermann et al, 1994; Arsenault et

al., 2001; Belanger et al., 2001; Long et al., 2004). Florida grows a wide range of early

to medium season potato cultivars, due to the short growing season, while majority of

growers in other states favor medium to late season cultivars (Pack et al., 2003).

‘Red LaSoda’ is one of the leading tablestock cultivars grown in Florida. It is a

mutant of the red variety ‘La Soda’ and was released by the USDA and Louisiana

Agricultural Experiment Station in 1953 (The Potato Association of America, 2010). ‘Red

LaSoda is considered an early to medium season cultivar, reaching maturity in 85 to 95

days (Hutchinson et al., 2008). The tubers are round or oblong shaped, with a deep red

skin color which fades with maturity. The flesh of the tuber is white, and the specific

gravity relatively low compared to other red-skinned cultivars. Although slightly

susceptible to hollow heart and growth cracks, it is reported to have good storability. It is

primarily used for boiling, baking, chipping and french frying (www.potatoes.wsu.edu).

The objective of this study was to determine the effect of N fertilizer application

method, rate, harvest time and storage on tuber yield and quality of surface-drip

irrigated ‘Red LaSoda’ potatoes. The underlying hypothesis was that by improving

fertilizer placement and timing, fertigation would produce higher yields and better tuber

quality compared to band-application with granular fertilizer. The second hypothesis

114

was, due to higher nutrient use efficiency, a lower nitrogen rate is required to produce

higher yields and tuber quality under fertigation.

Materials and Methods

Plant Material

‘Red LaSoda’ was grown in the spring season of 2013 (season 1) and 2014

(season 2) at the UF/IFAS Plant Science and Education Unit, Yelvington Farm in

Hastings, Florida. The planting dates were January 28, 2013 and January 29, 2014,

with the experimental layout following a completely randomized design with treatments

in a split plot design and four replications. The main plot was fertilizer treatment and the

sub-plot cultivar. Five fertilizer treatments, as those outlined in Chapter 4 for ‘Fabula’,

were applied (Fertigation treatments: R-0, R-112, R-224, R-336; granular treatment: G-

224). All production practices, harvest times, storage and tuber quality evaluations were

identical to those mentioned in Chapter 4.

Statistical Analysis

Experimental design and data analysis were done as described in Chapter 4.

Results

Weather Conditions

The average precipitation and temperatures varied over the two seasons, as

reported in Chapter 4 (Figure 4-1 and 4-2).

Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality

Nitrogen Uptake and Biomass Accumulation

Plant emergence was similar in both years, resulting in average plant population

of 49,624 plant ha-1. Therefore, plant emergence did not affect the evaluation of the

cultivar response to the different N treatments, in both seasons.

115

Fertilizer treatment had an effect on biomass accumulation, N uptake and

assimilation. Generally, both aboveground and tuber biomass increased with increased

N rate, with similar patterns being observed in both seasons (Table 5-1). For the

fertigation treatments, aboveground biomass was similar for N rates 112 kg ha-1 and

higher; with an average of 33.3 g plant-1.

However, the tuber biomass for R-224 and R-336 (average 73 g plant-1) were

significantly higher than R-112 (61.8 g plant-1). Tuber biomass of 38.5 g plant-1 for R-0

was the lowest among the fertigation treatments. The granular treatment G-224 had a

similar aboveground and tuber biomass to all fertigation rates, except R-0 which was

significantly lower. Meanwhile, R-112 had a significantly lower harvest index of 0.62,

while the other treatments averaged 0.69.

There was a significant interaction between fertilizer treatment and season on N

uptake and concentration in aboveground plant material and tubers. Aboveground N

uptake increased with N rate; the lowest N uptake of 0.25 g plant -1 and 0.27 g plant -1

was observed in R-0, in season 1 and 2, respectively (Table 5-2). Similar patterns were

observed in tuber N uptake. In both seasons, R-0 had the lowest tuber N uptake of 0.51

and 0.27 g N plant-1 for season 1 and 2, respectively. The other fertigation treatments

had similar contents which averaged 1.73 and 0.91 g N plant-1 for season 1 and 2,

respectively. Generally, seasonal differences were seen in tuber N uptake, with lower N

uptake being observed for all fertigation treatments in season 2. G-224 tuber N uptake

was similar to R-112 and R-224, but greater than R-0 and R-336 in season 1. In season

2, R-112 and R-224 tuber N uptakes (average 0.78 g N plant-1) were significantly lower

than G-224, which averaged 1.61 g N plant-1.

116

N treatment affected the total Kjeldahl N concentration in both years, with similar

patterns being observed between the aboveground and tuber N contents in each year

(Table 5-3). In season 1, R-336 had a significantly higher aboveground N content

(3.62%) than R-0 (1.70%) and R-112 (2.91%). The following season, R-336 (3.52%)

was significantly higher than R-0 (2.45%) only, and equal to the other treatments.

Meanwhile, G-224 was higher than R-0, but similar to the other fertigation treatments, in

both seasons. A similar pattern was observed in the tuber N concentration.

Tuber Yield and At Harvest Quality

Tuber total and marketable yields were affected by fertilizer treatment and

season; therefore each season was analyzed separately.

In season 1, R-0 had significantly lower total yields of 12,939 kg ha-1, while the

other fertigation treatments averaged 27,668 kg ha-1 (Table 5-4). There were no

significant differences in total yield between the granular treatment G-224 and all

fertigation treatments. A similar pattern was observed with the marketable yield, where

R-0 had the lowest proportion of the total yield (57%), while the other treatments ranged

between 76 to 84%. In addition, 30% of the total yield in R-0 was undersized tubers

(less than 4.78 cm diameter), while the other treatments averaged only 9 %

unmarketable yield. None of the treatments produced oversized tubers of 10.16 cm

diameter and higher.

Generally, total yield declined in all fertigation treatments in season 2; G-224

remained constant, averaging 20,942 kg ha-1. However, the effect of N fertigation rate

on yield followed similar patterns in both seasons. In season 2, G-224 had total yields of

21,445 kg ha-1, which were significantly higher than two lowest N fertigation , R-0 (6,833

kg ha-1 ) and R-112 (11,878 kg ha-1) (Table 5-5). However, there were no differences in

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marketable yields between the granular treatment and all fertigation treatments

(average 68% of total yield); except for R-0 which was significantly lower (44% of total

yield). There was an increase in proportion of the total yield allotted to undersized

tubers for all treatments in season 2. Irrespective of this, R-0 still maintained the highest

percentage unmarketable of 50% of total yield, while the other treatments ranged

between 20 to 32%.

N rate had an effect on tuber external disorders in both seasons, with tuber

decay accounting for the major differences observed. During season 1, R-112 had

significantly more tuber decay of 2,917 kg ha-1, compared to R-0 which averaged 1,071

kg ha-1 (Table 5-6), resulting in higher total physiological disorder yields (Table 5-4).

The incidence of physiological disorders declined in all treatments in season 2, with no

significant differences between fertigation treatments (Table 5-5). This was accounted

for by a decrease in tuber decay in all treatments, from an average of 1,777 kg ha-1 in

season 1, to 187 kg ha-1 the following year (Table 5-6). There were no differences

among application methods in both seasons.

Tuber specific gravity was affected by fertilizer treatment and season. In season

1, an increase in N rate resulted in a decrease in specific gravity; R-224 and R-336 had

lower averages of 1.051, while R-0 and R-112 averaged 1.054 (Figure 5-1 A). The

lowest specific gravity of 1.049 was observed in G-224. Meanwhile, in season 2,

specific gravity for R-224 (1.050) was significantly lower than the other fertigation

treatments, which averaged 1.053 (Figure 5-1 B). In addition, G-224, with a specific

gravity of 1.057, was significantly higher than all fertigation treatments.

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Effect of Nitrogen Application Method and Rate on Tuber Storage Quality

Initial Storage Quality

Fertilizer treatment had an effect on pulp dry matter content and pH; harvest time

affected peel dry matter content and firmness; while the interaction affected soluble

solids content of tuber initials (Table 5-7).

There was a significant interaction between harvest time and season on peel dry

matter content (DMC), with differences observed during season 1 only. H1 and H2

tubers had an average peel DMC of 14.9%, increasing to 17.5% at Harvest 3 (Figure 5-

2). On the other hand, fertilizer treatment had a greater effect on pulp DMC of tubers

grown in season 1. R-0 had the highest pulp DMC of 15.3%, while the other fertigation

treatments averaged 14%. G-224’s pulp DMC (13.13%) was similar to R-336 (13.6%),

but significantly lower than the other fertigation treatments, which averaged 14.56%

(Figure 5-3). There were no significant differences in both peel and pulp DMC among

treatments in season 2, with an average content of 15.0% and 15.8%, respectively.

Firmness of tuber initial storage quality was affected by harvest time and season.

Generally, a delay in tuber harvesting resulted in a decrease in firmness in season 1,

with H3 tubers having a lower average firmness of 11.9 N, compared to 12.8 N and

13.40 N for H1 and H2, respectively (Figure 5-4). No significant differences in firmness

were observed among treatments the in season 2, with an average firmness of 13.1 N.

Soluble solids content (SSC) was significantly affected by fertilizer treatment,

harvest time, and season. In season 1, R-0 had a significantly lower content of 2.9%

than the other treatments at H1, increasing to 3.8% at H3 (Table 5-8). The content in

the other treatments generally decreased with increased harvest time, with no

significant differences in content among all fertigation treatments by H3.

119

In season 2, R-0 SSC increased significantly from 3.4% at H1 to 3.8% at H2, with

no change thereafter (Table 5-9). No changes in SSC with increased harvest time were

observed in the other treatments. In addition, R-0, together with R-336, had the lowest

SSC compared to the other treatments, at all harvest times. The granular treatment, G-

224, maintained an equal or higher content than all fertigation treatments, at all harvest

times, in both seasons.

Meanwhile, fertilizer treatment had an effect on tissue pH, with similar patterns

being observed in both seasons. R-0 had the lowest reading of 3.63, while the other

fertigation treatments averaged 4.23 (Figure 5.5). G-224 average pH of 4.58 was

significantly higher than R-0 and R-112 readings, but similar to R-224 and R-336.

There were no treatment differences in ascorbic acid content (AAC) and total

titratable acidity (TTA), in both years. AAC for the tuber initials averaged 13.6 and 18.1

mg 100 g-1 for the peel and pulp, respectively; while TTA was 0.1%.

Storage Quality

In storage, the only parameters affected by increased storage time were tuber

fresh weight and peel dry matter content (Table 5-10). An increase in fresh weight loss

with increased storage time was observed in H1 tubers only, during season 1. The

weight loss increased from 1.6 % at 7 d to 2.2% at 14 d, while the other harvests

maintained their fresh weight throughout storage (Figure 5-6 A). In season 2, a

significant increase in weight loss with increased storage time was observed in both H1

and H2 tubers. This resulted in significantly higher losses of 2.7% in these earlier

harvests, compared to 1.2% for H3 tubers (Figure 5-6 B).

There was a significant interaction between season, harvest time, and storage

time on tuber peel dry matter content (Table 5-11). Significant differences in peel DMC

120

during were observed in season 1; where H3 had a significantly higher content of

17.54% at 0 d, compared to an average of 14.94% at the same time period for the

earlier harvest times. However, both H1 and H2 increased significantly in peel DMC at 7

d, giving an average of 17.27%. By the end of storage, H2 had the highest content on

19.24%, which was significantly higher than H1 and H3’s average of 16.51%.

In pulp DMC, although significant differences were observed at d 0 in season 2;

no further changes were observed with increased storage time in all fertilizer

treatments. By the end of storage, R-0 maintained the highest content of 14.89%, while

the other fertilizer treatments averaged 13.6%. Storage time also had no effect on pulp

dry matter content in season 1, with an average of 15.03% for all treatments.

Similar to pulp dry matter content, tuber firmness was maintained throughout

storage, resulting in H3 having the lowest firmness by the end of storage, as observed

in the initials. Storage time also had no significant effect on SSC; R-0 maintained the

lowest content of 3.54% throughout storage, while R-336 had the highest average

content of 4.41%. The granular treatment, G-224, maintained a higher content than all

fertigation treatments, except for R-336, which had a similar content, throughout

storage. The fertilizer treatment differences observed in the tuber pH were also

maintained throughout storage. Tuber storage had no significant effect on tuber

firmness (12.8 N), peel (average 13.2 mg 100g-1) and pulp ascorbic acid content

(average17.4 mg 100 g-1) or tissue titratable acidity (average 0.14%).

Discussion

There were no significant differences in plant populations between the two

seasons, indicating similar seed germination and plant establishment in both years.

121

N uptake was significantly affected by fertilizer rate and application method. N

was removed by potato plants at approximately the same rate in all fertigation

treatments, except for the zero N treatment (R-0), which was significantly. A decline in

tuber N recovery was observed in fertigation treatments during season 2, with no

significant seasonal changes in uptake for the granular treatment lower (Table 5-2). This

can be accounted for by heavy early season rainfalls coinciding with fertigation

applications, causing fertilizer to move beyond the root zone, thereby depleting

available soil N and reducing uptake (Fan and Mylavarapu, 2010, Kelling et al. 1997). A

sidedress in the granular treatment likely maintained fertilizer in the root zone, resulting

in higher N uptake.

Plant vines and tuber N content and biomass at full flower were also influenced

by fertilizer rate. Generally aboveground and tuber N content increased at higher N

rates, compared to the zero N treatment (Table 5-3), as observed in other studies

(Hutchinson et al., 2003). Zero N application also markedly reduced aboveground and

tuber biomass accumulation (Table 5-1). This is due to N deficiency which reduced leaf

area and canopy development, photosynthates production and translocation to the

tubers (Ojala et al., 1990). An increase in N rate generally increased both above ground

and tuber biomass, as observed in previous studies (Belanger et al., 2001; Ojala, et al.,

1990; Sharma and Dubey, 2000).

Availability of N to the growing plant also influenced tuber yields and size

distribution. A higher N fertilization had a positive effect on tuber total yield, with all N

treatments producing significantly higher yields than plants in the zero N treatment

(Tables 5-4 and 5-5), similar to previous studies (Belanger et al., 2002; Hutchinson et al,

122

2003; Sharma and Dubey, 2000). Higher crop growth at higher N rates resulted in more

synthesis and translocation of photosynthates to the tubers, and hence increased yields

(Kumar et al, 2007). However, increasing N rates beyond R-112 (112 kg ha-1) did not

significantly increase yield output. According to Rosen et al. (1993), this can be

attributed to a greater yield response by plants at the first fertilizer increment, followed

by a more gradual increase with succeeding increments. N uptake results agreed with

effects of N on total yields, where N unavailability (due to early season leaching rains in

season 2) during tuber initiation and bulking affected fresh tuber growth rate (Ojala et

al., 1990), which resulted in yield decline for all fertigation treatments during season 2

(Tables 5-2 and 5-5). Meanwhile, delayed N applications in the granular treatment

maintained a high N uptake and total yields in both seasons. Similar results of increased

yields with delayed N application (due to reduced leaching) were reported by Kelling

and Speth (1997). A lower harvest index for 112 kg N ha-1 treatment in season 2,

possibly affected yield, resulting in lower total yields than the granular treatment (Tables

5-1 and 5-5). This was likely due to nutrient stress during the vegetative growth stage,

particularly tuber bulking, leading to a reduction in dry matter accumulation (Yungen et

al., 1958; Hughes, 1974; Mackerron and Davies, 1986; Fabeiro at al., 2000).

Availability of N to the growing plant also influenced tuber size distribution,

incidence of physiological disorders and specific gravity. N stress at the zero N

treatment reduced tuber bulking rate, increased proportion of undersized tubers, and

decreased marketable yields (Tables 5-5 and 5-6) , which is agreement with other

studies at the same N rate (Dahlenburg et al., 1990; Hutchinson et al., 2003). Reduced

N uptake during season 2 significantly increased undersized tuber yield for N fertigation

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rates of 112 kg ha-1 and higher. On the other hand, higher levels of N were associated

with increased external defects during season 1 (Table 5-4) similar to findings by Long

et al. (2004). This was likely due to decreased deposition of suberin waxes with

fungicidal properties (Hochmuth and Hanlon, 2011) and/or decreased membrane

permeability (McGuire and Kelman, 1984) at the higher N rates, which increased

susceptibility to soft rot bacteria. Shekhawat et al. (1982) and Kumar et al. (1991) also

reported increased tuber decay with increased N rate.

In addition, as observed at higher N rates in other studies, excess N availability

during tuber bulking in the 224 and 336 kg ha-1 treatments likely delayed tuber maturity

(Porter and Sisson, 1993; Dahlenburg et al., 1990; Westermann et al., 1994;

Mohammed et al., 1999). On the other hand, a significantly higher N uptake rate in the

fertigation and granular N treatments of 336 kg ha-1 possibly resulted in higher specific

gravity (Figure 5-1), as reported in previous studies (Kunkel and Dow, 1961; Vakis,

1978; Papadopoulos, 1988).

N and harvest time were indirectly linked to potato storability through their effect

on tuber relative maturity and quality at harvest. Fresh weight loss in storage was

affected by harvest time, with tubers harvested one week after vine kill losing a

significantly higher percent in both seasons (Figure 5-6). In season 2, tubers harvested

two weeks after vine kill also lost more weight during storage than the final harvest time

(Figure 5-6 B).

Burton (1989) indicated that about 98% of tuber weight loss during storage was

due to water loss through immature periderms, which are not effective vapor barriers.

Delaying harvest time probably increased deposition of suberin waxes in the periderm,

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improving skin maturity as reported in other studies (Blenkinsop et al. 2002, Tyner,

1997; Makani, 2010; Nipa et al., 2013). On the other hand, N rate had no effect on

weight loss, contradicting some reports which indicated increased weight loss for higher

N rates (Jablonski, 2006; Wszelaczynska and Poberezny, 2011). This was likely due to

a cultivar effect, with an earlier tuber development associated with ‘Red LaSoda’

(Hutchinson et al., 2008), resulting in early skin maturation, irrespective of the N rate.

This is further supported by work done by Dahlenburg et al. (1990) who saw no

significant difference in weight loss for tubers fertilized with N rates ranging between 0

to 320 kg ha-1. Love and Pavek (1989) also reported lower weight loss in smooth-

skinned cultivars due to lower permeability, compared to net-skinned cultivars. ‘Red

LaSoda’ is classified as smooth-skinned (Hutchinson et al., 2008).

Weather conditions seemed to have an effect on dry matter content of tubers’

initial storage quality, with treatment effects being observed during the wetter season 1

only. The peel dry matter content depended on the harvest time, with a significantly

higher content being measured in tuber initial storage quality during the first two

harvests (Figure 5-2). High rainfall amounts just prior to the first harvest likely caused an

influx of water into the peel tissue, in order to maintain cell turgor pressure against

increased negative solute potential, resulting in a lower peel dry matter content

(Westermann et al., 1994). This water was later lost through transpiration during storage

(Lisinska and Leszczynski, 1989; Kaaber et al., 2001; Nipa et al., 2013), resulting in an

increase in peel dry matter content with increased storage time for H1 and H2 tubers

(Table 5-11).

125

On the other hand, fertilizer treatment had a greater effect on the pulp dry matter

content of tubers’ initial storage quality; a significantly higher pulp dry matter content

was observed in the zero N treatment (Figure 5-3). Pulp DMC decreased significantly at

112 kg ha-1, with no differences at higher rates. These results are in partial agreement

with other researchers who reported no significant increases in dry matter content after

a particular N rate (Lauer 1986; Westermann et al, 1994; Joern and Vitosh, 1995;

Kumar et al., 2007). There are no clear indications why pulp dry matter content in tuber

initials was higher in the zero N treatment, contradicting findings by Kumar et al. (2007)

and many other authors, who saw an increase in dry matter content with increased N

rate. The likely explanation could be less water uptake in the zero N treatment during

the wet harvest period in season 1, resulting in significantly higher DMC. A greater

weather effect is supported by the fact that this was only observed during one season.

Pulp dry matter content was maintained throughout storage, indicating that storage

conditions were ideal for limiting weight loss due to transpiration, as stated by

Blenkinsop et al. (2002).

Similar to peel dry matter content, firmness in tuber initials was significantly

affected by harvest time. The influx of water into tubers due to high rainfall at the

beginning of the harvest period in season 1 likely increased cell turgor, resulting in

significantly higher firmness at the earlier harvest times (Figure 5-4). According to Afek

et. al. (200), water loss in storage breaks down cell turgor, decreasing tuber firmness.

Cargill (1976) noted tuber loss in firmness is only realized at 5% or higher weight loss.

In this study, weight loss during storage did not exceed 3%, resulting in no change in

tuber firmness during storage, in both seasons.

126

N rate had no effect on ascorbic acid content (AAC) in tuber initial storage

quality, consistent with other findings (Baker et al., 1950). Similar AAC was also

observed at all harvest times, indicating early tuber development and maturity in ‘Red

LaSoda’ tubers, as indicated by Olson et al. (2010). In the present study, storage of

physiologically and compositionally mature tubers at 10°C, 85-85% RH for 14 d

minimized the oxidative and enzymatic degradation of AAC as has been reported in

long-term storage tests (Augustin 1975; Watada, 1987; Dale et al., 2003; Phillips at al.

2010).

An interaction of N rate and harvest time had an effect on soluble solids content

(SSC) in tubers’ initial storage quality. Although N fertigation rates of 0 and 336 kg ha-1

generally had significantly lower soluble solids at all harvest times, particularly in season

2 (Table 5-9), no further changes were observed during storage, indicating minimum

conversion to sugars in all treatments, during storage.

Conclusions

N fertilizer application method and rate affected tuber yield and harvest quality of

‘Red LaSoda’ tubers. Generally, insufficient N during critical growth periods in the zero

N fertigation treatment reduced tuber yields and size due to lower bulking rates.

Increasing N fertigation rate to 112 kg ha-1 fertigation was sufficient to promote higher

yields due to more synthesis and translocation of photosynthates to tubers. However, a

lower harvest index for this lower rate during season 2 reduced total yields. This was

more likely due to seasonal weather differences, particularly rainfall patterns, since this

was observed during season 2 only, which experienced heavy early seasonal rainfall.

Reduced N uptake during season 2 significantly increased undersized tuber yield for N

127

fertigation rates between 112 kg ha-1 and 336 kg ha-1. On the other hand, higher levels

of N were associated with increased external defects during season 1.

Results from this study also showed that the availability of N to the plant was

greatly determined by the application method and timing of application. Scheduling

fertigation applications, to meet projected crop N demands, produced comparable yields

at lower fertigation rates of 112 and 224 kg N ha-1 in season 1. However, heavy early

season rainfall in season 2 depleted soil N, resulting in significantly lower yields in the

fertigation treatments. This indicates that, in the event of high rainfalls at the beginning

of the season, delayed or supplemental N applications might still be required to optimize

yield in fertigation.

The initial and storage quality for most parameters of the tubers was highly

dependent on the harvest time. Any differences in quality of tuber initial storage quality

was generally observed in the less mature tubers harvested one week after vine kill.

Significant differences between fertilizer treatments in tuber initial storage quality were

generally accounted for by N stress at the lowest N rate of 0 kg ha-1. In storage, the only

parameters affected by increased storage time were tuber fresh weight and peel dry

matter content, with harvest time having a greater effect on storage quality losses.

Tuber quality was maintained throughout in all the other parameters, indicating good

storability for all N rates and application methods. This was likely due to a cultivar

effect, with an earlier tuber development associated with ‘Red LaSoda’ (Olson et al.,

2010), resulting in early maturation, irrespective of the N rate.

This study therefore concludes that fertigation with surface drip irrigation, using

224 kg N ha-1, is capable of producing comparable yields and tuber storage quality in

128

‘Red LaSoda’. There were indications of granular application being more advantageous

in the event of heavy early season. However, the rainfall distribution experienced in

season 1 was more representative of the Florida growing season, where higher rainfall

typically occurs towards the end of the season. In addition, ‘Red LaSoda’ tubers can be

harvested as early as one week after vine kill, because the cultivar matures earlier. It is

therefore, capable of maintaining acceptable quality when stored at 10°C, 80-85% RH,

irrespective of the fertilizer rate applied.

129

Table 5-1. Plant aboveground (AG) and tuber biomass, at full flower of ‘Red LaSoda’ potatoes grown under five nitrogen treatments (season 1 and 2).

Treatmentz

Dry Biomass Weight (g plant-1)

AG Tuber Total Harvest indexx

R-0 19.2 by 38.5 c 57.7 b 0.67 a R-112 38.1 a 61.8 b 99.9 a 0.62 b R-224 32.5 a 75.5 a 108.0 a 0.70 a R-336 29.4 a 70.5 a 99.9 a 0.71 a G-224 31.2 a 67.2 ab 98.4 a 0.68 a

zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level. xHarvest index: Tuber biomass/Total biomass

Table 5-2. Plant aboveground and tuber nitrogen uptake of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).

Nitrogen Uptake (gN plant-1)

Season 1 Season 2 Treatmentz AG Tuber Total AG Tuber Total

R-0 0.25 by 0.51 c 0.76 c 0.42 b 0.27 c 0.69 c R-112 0.96 a 1.67 ab 2.63 ab 0.81 a 0.71 b 1.52 b R-224 0.97 a 1.59 ab 2.56 ab 1.03 a 0.85 b 1.88 b R-336 0.95 a 1.93 a 2.88 a 1.20 a 1.18 a 2.38 a G-224 0.87 a 1.47 b 2.34 b 0.81 a 1.61 a 2.42 a

zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level

130

Table 5-3. Total Kjeldahl nitrogen of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).

Treatmentz

Total Kjeldahl Nitrogen (%)

Season 1 Season 2

Above ground Tuber Above ground Tuber

R-0 1.70 c 0.95 c 2.45 b 1.13 b R-112 2.91 b 1.66 b 3.30 a 1.50 ab R-224 3.16 ab 1.88 ab 2.71 ab 1.85 a R-336 3.62 a 2.11 a 3.52 a 1.73 a G-224 3.58 ab 2.01 ab 3.32 a 1.61 a

zFertilizer treatments of R-0, R-112, R-224, and r-336 represent fertigation rates of 0-336 kg ha

-1, and G-

224 is granular application at 224 kg ha-1

. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level

131

Table 5-4. Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 1)

Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu

Treatmenty (kg ha-1) (kg ha-1) (%x) (kg ha-1) (%) (kg ha-1) (%)

F-0 12,939 b 7,438 b 57.5 3,919 a 30.3 1,582 b 12.2 F-112 25,459 a 19,639 a 77.1 2,191 b 6.6 3,629 a 14.3 F-224 28,497 a 23,973 a 84.1 2,499 ab 8.8 2,026 ab 7.1 F-336 29,048 a 23,881 a 82.2 2,569 ab 8.8 2,598 ab 8.9 G-224 20,439 ab 15,599 ab 76.3 2,616 ab 12.8 2,224 ab 10.9 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

xPercent of total yield

wMarketable yield: tuber grades A1, A2, A3.

vUnmarketable grades A4, B, C.

uGreening, growth cracks, misshapen, decay

Table 5-5. Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 2)

Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu

Treatmenty (kg ha-1) (kg ha-1) (%x) (kg ha-1) (%) (kg ha-1) (%)

F-0 6,833 b 3,024 b 44.3 3,427 b 50.2 382 a 5.6 F-112 11,878 b 7,710 ab 64.9 3,750 ab 31.6 418 a 3.5 F-224 14,931 ab 9,445 ab 63.3 4,814 a 32.2 672 a 4.5 F-336 15,937 ab 11,053 ab 69.4 4,491 a 28.2 393 a 2.5 G-224 21,445 a 16,124 a 75.2 4,319 a 20.1 1,002 a 4.7 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

xPercent of total yield

wMarketable yield: tuber grades A1, A2, A3.

vUnmarketable grades A4, B, C.

uGreening, growth cracks, misshapen, decay

132

Table 5-6. Tuber external disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill in 2013 (season 1) and 2014 (season 2)

Treatmenty

External Physiological Disorders (kg ha-1)

Season 1 Season 2

Green GC Misshap Decay Green GC Misshap Decay

R-0 360 az 51 b 99 b 1,071 b 92 b 0 a 26 b 264 a R-112 444 a 99 b 169 ab 2,917 a 297 a 0 a 290 a 84 a R-224 44 b 0 b 88 b 1,893 ab 59 b 0 a 139 ab 220 a R-336 539 a 521 a 228 ab 1,310 b 154 ab 0 a 0 b 239 a G-224 173 ab 0 b 356 a 1,695 ab 532 a 0 a 334 a 136 a

z Means within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.

yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is granular application at 224 kg ha

-1.

Table 5-7. Analysis of variance of season, fertilizer treatment, and harvest time on tubers’ initial storage quality in ‘Red LaSoda’

Dry matter content (%)

Ascorbic acid content (mg 100 g-1)

Main Effects Peel Pulp Peel Pulp Firmness (N) SSC (%) TTA (%) pH

Year (Y) * * ns ns ns * ns ns Fertilizer (F) ns * ns ns ns * ns * Harvest (H) * ns ns ns * * ns ns Interactions:

Y x F ns * ns ns ns * ns ns Y x H * ns ns ns * * ns ns H x F ns ns ns ns ns * ns ns

Y x H x F ns ns ns ns ns * ns ns * Significant at p<0.05; ns: not significant

133

Table 5-8. Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 1).

Harvest Timex

Soluble Solids Content (%)

Fertilizer Treatmenty

R-0 R-112 R-224 R-336 G-224

H1 2.98 bBz 3.93 aA 3.65 aA 3.88 aA 3.90 aA H2 3.30 aB 3.33 bB 3.45 abAB 3.75 aA 3.55 aAB H3 3.14 abB 3.18 bB 3.20 bB 3.43 bAB 3.60 aA

z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital

letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-224

is granular application at 224 kg ha-1

.x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.

Table 5-9. Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 2).

Harvest Timex

Soluble Solids Content (%)

Fertilizer Treatmenty

R-0 R-112 R-224 R-336 G-224

H1 3.40 bCz 4.01 aB 4.07 aB 3.32 aC 4.55 aA H2 3.75 aC 4.18 aB 4.33 aAB 3.43 aC 4.62 aA H3 3.58 abC 4.09 aB 4.20 aAB 3.37 aC 4.56 aA

z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital

letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.

134

Table 5-10. Analysis of variance of fertilizer treatment, harvest time on tuber quality in ‘Red LaSoda’ tubers stored at 10°C, 80-85% RH for 14 d

Dry matter content (%)

Ascorbic acid content (mg 100 g-1)

Main Effects Peel Pulp Peel Pulp Weight loss (%)

Firmness (N)

SSC (%)

TTA (%)

pH

Year (Y) * * ns ns * ns ns ns ns Fertilizer (F) ns * ns ns ns ns * ns * Harvest (H) * ns ns ns * ns ns ns ns Storage (S) * ns ns ns * ns ns ns ns

Interactions:

Y x F * * ns ns ns ns ns ns ns Y x H ns ns ns ns * ns ns ns ns Y x S ns ns ns ns * ns ns ns ns F x H ns ns ns ns ns ns ns ns ns F x S ns ns ns ns ns ns ns ns ns H x S ns ns ns ns * ns ns ns ns

Y x F x H ns ns ns ns ns ns ns ns ns Y x F x S ns ns ns ns ns ns ns ns ns Y x H x S ns ns ns ns * ns ns ns ns F x H x S ns ns ns ns ns ns ns ns ns

Y x F x H x S ns ns ns ns ns ns ns ns ns * Significant at p<0.05; ns: not significant

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Table 5-11. Effect of harvest time on tuber peel dry matter content of ‘Red LaSoda’ stored for 14 d at 10°C, 80-85% RH

Storage timey

Peel dry matter content (%)

Harvest Timex

H1 H2 H3

0d 14.93 bBz 14.89 cB 17.54 aA 7d 17.49 aA 17.05 bA 16.39 aA

14d 16.59 aB 19.64 aA 16.43 aB z Means within a column followed by the same small letter do not differ significantly according to Tukey’s

Test at 5% level. yStorage time at 10ºC, 80-85% RH

xHarvest time – 1 week (H1), 2 weeks (H2), 3 weeks (H3) after vine kill.

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A)

B) Figure 5-1. Tuber specific gravity of ‘Red LaSoda’ potatoes grown under five fertilizer

treatments during A) season 1, and B) season 2

1.044

1.046

1.048

1.05

1.052

1.054

1.056

1.058

1.06

R-0 R-112 R-224 R-336 G-224

Sp

ecific

gra

vity

Fertilizer treatment

1.044

1.046

1.048

1.05

1.052

1.054

1.056

1.058

1.06

R-0 R-112 R-224 R-336 G-224

Sp

ecific

gra

vity

Fertilizer treatment

137

Figure 5-2. Peel dry matter content of ‘Red LaSoda’ tubers’ initial storage quality,

harvested 1-3 weeks after vine kill (season 1).

Figure 5-3. Pulp dry matter content of ‘Red LaSoda’ tubers’ initial storage quality grown

under five fertilizer treatments (season 1).

0

2

4

6

8

10

12

14

16

18

20

H1 H2 H3

Pe

el d

ry m

att

er

co

nte

nt (%

)

Harvest time after vine kill (weeks)

0

2

4

6

8

10

12

14

16

18

R-0 R-112 R-224 R-336 G-224

Pu

lp d

ry m

att

er

co

nte

nt (%

)

Fertilizer treatment

138

Figure 5-4. Firmness of ‘Red LaSoda’ tubers’ initial storage quality harvested 1-3 weeks

after vine kill (H1-3) (season 1)

Figure 5-5. Tissue pH of ‘Red LaSoda’ tubers’ initial storage quality, grown under five

fertilizer treatments during two seasons

10.5

11

11.5

12

12.5

13

13.5

14

H1 H2 H3

Bio

yie

ld F

orc

e (

N)

Harvest time after vine kill (weeks)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

R-0 R-112 R-224 R-336 G-224

Tis

su

e p

H

Fertilizer treatment

139

A)

B) Figure 5-6. Fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85%

RH during A) season 1, and B) season 2.

0

0.5

1

1.5

2

2.5

0d 7d 14d

Fre

sh

we

igh

t lo

ss (

%)

Storage time (days)

H1

H2

H3

0

0.5

1

1.5

2

2.5

3

3.5

0d 7d 14d

Fre

sh

we

igh

t lo

ss (

%)

Storage time (days)

H1

H2

H3

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CHAPTER 6 EVALUATION OF SKIN ADHESION STRENGTH AND ‘RAPID CURING’ AS A MEANS

OF MINIMIZING QUALITY LOSSES DURING STORAGE OF ‘NEW’ POTATOES.

Introduction

Removal of skin (skinning or scuffing) during harvesting and grading of Florida’s

new potatoes is very common (Chiputula, 2009). This is because the tubers are

harvested before complete physical maturation, giving them their characteristic thin

skin, which easily detaches from the underlying cortical tissue during harvest (Suslow

and Voss, 1996). In addition, the cylinder brush washers or studded rubber rolls

commonly used by growers during grading are abrasive and cause a high degree of

tuber skin and wound injury (Talburt and Smith, 1987). Harvesting physically mature

tubers reduces tuber skinning, susceptibility to decay and storage losses (Mohsenin,

1965).

The tuber periderm is comprised of three groups of cells: an outer phellem (skin),

phellogen (cork cambium), and underlying phelloderm. An immature periderm contains

a phellogen layer with thin radial cell walls which fracture easily during harvesting and

handling, resulting in the slipping of the phellem cells (Lulai and Freeman, 2001; Suslow

and Voss, 2000). Tuber physical maturity (skin-set) is attained when phellogen activity

slows sufficiently to permit strong adherence to underlying cortical tissue (Lulai and Orr,

1993; Wilcockson et al., 1980; Yagamuchi et al., 1966).

By affecting plant growth and tuber development, preharvest factors such as

nitrogen (N) rate have a great influence on tuber skin-set. It has been consistently

stated that increased vegetative growth, when fertilized with high N rates, significantly

reduces tuber compositional and physiological maturity (Kumar et al, 2004; Ojala et al.,

1990; Wszelaczynska and Poberezny, 2011). However, the unavailability of reliable

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objective measuring tools or methods has resulted in limited published work on how

preharvest factors affect tuber physical maturity.

To improve skin-set before harvest, plant vines of tablestock potatoes are killed

by spraying select herbicides when tubers reach horticultural maturity, a process termed

‘vine kill’ (Bohl, 2003; Plissey, 1993). Florida growers only harvest the tubers 14 to 21

days after vine kill, to ensure adequate skin-set (Mossler and Hutchinson, 2008).

However, high rainfalls can occur during the potato growing season in Florida, and

delaying harvest can cause losses due to tuber decay. Access to rapid and objective

devices to measure tuber skin-set would therefore help growers to best determine the

earliest possible harvest time, when tubers have reached adequate physical maturity

(Pavlista et al., 2002).

Halderson and Henning (1993) developed a hand-held device, later refined by

Lulai and Orr (1993), which uses a torque meter to measure tuber total resistance to

skinning injury in potato. Total resistance to skinning is a summation of the phellem

tensile component and phellogen shear component (Lulai, 2002). The device consists of

a rubber tip mounted on the end of a spring-loaded shaft which is connected to a

torquometer. The torsional force required to remove the skin from adjoining cells is

recorded when the rubber tip is pressed down on the tuber and twisted (Halderson and

Henning 1993; Lulai and Orr 1993).

Harvesting tubers with an immature skin not only affects the appearance, but

also increases quality losses during storage. Burton (1989) indicated that only 2% of

tuber weight loss during storage was due to respiration; the majority of water loss

occurred through immature periderms. A 250- to 1000- fold increase in dehydration

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further occurs in skinned or wounded tubers, compared to non-skinned tubers (Lulai,

2002). Curing tubers at 15-18°C and 80-95 % relative humidity (RH) for 2 weeks

promotes postharvest wound-healing and minimize weight loss (Hide et al., 1994;

Suslow and Voss, 2000). Under these conditions, suberized wound periderm develops

over the existing wound, preventing desiccation of tuber inner tissues during storage

(Lulai, 2001).

Due to high early season prices, Florida potatoes are only stored briefly, without

a dedicated curing step, before being distributed to their respective markets. This lack of

curing likely causes quality losses along the distribution chain because of the varying

storage conditions from producer to retail market to consumer. Previous research has

shown that early cultivars can also be rapidly cured for 8 days at elevated temperatures

up to 25°C, significantly increasing tuber shelf life (Lulai, 2007). ‘Rapid curing’ could be

a possible means of reducing quality loss of new potatoes in transit or during storage. It

involves storing tubers for a shorter duration under higher temperatures and RH

compared to standard curing conditions.

The first objective of this study was to determine the effect of N rate, plant vine

kill and harvest time on tuber physical maturity in two tablestock cultivars. The

hypothesis was that, by minimizing vine growth and facilitating early senescence of

vegetative tops, lower nitrogen rates would result in more mature tubers. Skin-set was

also expected to increase with increased delay to harvest time in all treatments. The

second objective was to evaluate rapid curing as a means of minimizing storage losses

in skinned tubers by promoting suberization and wound-healing. The hypothesis was

that tuber curing at 15°C to 25°C and 90 to 95% RH at the beginning of storage would

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increase the rate at which wound-periderm suberization occurred. Additionally, it was

hypothesized that rapid curing would minimize other quality losses during storage of

new potatoes, compared to non-cured tubers.

Materials and Methods

Experiment 1: Determination Of Tuber Resistance To Skinning Injury In Relation To Nitrogen Treatment and Harvest Time

Plant Material

Tablestock potato cultivars ‘Fabula’ and ‘Red LaSoda’ were grown at the

UF/IFAS Florida Partnership for Water Agriculture and Community Sustainability,

Cowpen Branch in Hastings, FL, during the spring seasons of 2013 (season 1) and

2014 (season 2). The planting dates were January 28, 2013 and January 29, 2014, with

the experimental layout following a completely randomized design with treatments in a

split plot design and four replications. The main plot was fertilizer treatment and the sub-

plot cultivar. The land was divided into 40 plots, each comprising of 4 rows, with the

cultivars and fertilizer treatments being randomly assigned. Potato seed pieces were

planted using 20 cm in-row spacing and 15 cm depth. There were four fertigation

treatments with N rates of 0, 112, 224, and 336 kg ha-1, and one granular treatment of

224 kg N ha-1, as outlined in Chapter 4. Plants were irrigated using surface drip

irrigation, with drip lines (16-mm inner diameter, 8-mm thickness, 30-cm emitter

spacing) with a flow rate of 0.5 L h-1100 m-1 (RO-DRIP, John Deere Water, Moline, IL,

USA) buried 5 cm above the seed piece.

Vine Kill and Tuber Harvesting

The two inner rows in each plot were divided into four sections, each 3-m long,

where tuber sampling was carried out. At maturity (91 and 98 DAP in seasons 1 and 2,

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respectively) one section in each plot was randomly selected to be the non-vine kill

treatment. Polythene mulch was used to completely cover the plants, just prior to the

desiccant spray, to avoid vine killing these sections. A single application of the chemical

desiccant glufosinate ammonium (Rely® 280 Herbicide; Bayer Crop Science, Research

Triangle Park, NC, USA), applied at a rate of 1535 ml ha-1, was sprayed throughout the

entire field trial. The plastic mulch covers were removed after the minimum re-entry time

(4 hours) using personal protective equipment had expired.

Harvest times were selected on the basis of number of weeks after vine kill. An

initial harvest (H0) was carried out by hand-harvesting tubers from two randomly

selected plants in each plot, prior to vine kill spray. Tubers were carefully placed in

paper bags, to avoid skinning injury while transporting to the laboratory for tests. The

same procedures were used for subsequent weekly harvests from the non-vine killed

(NVK) and vine killed (VK) sections. These were carried out 1-3 weeks after the vine

killing time point (H1 to H3). To quantify tuber decay after vine killing, a 3-m vine-killed

section in each plot was harvested at the same time points of 1-3 weeks after vine,

using a mechanical harvester.

Tuber Skin-set Tests

Upon arrival at the laboratory, loose sand was gently brushed off the tubers,

before sorting and randomization from each fertilizer treatment (n=4). Total resistance to

skinning was determined by recreating the skin-set testing device developed by J.L.

Halderson and R.C. Henning (1993), as described by Lulai and Orr (1993). The skin-set

testing device consisted of a Snap-on® “Torquometer”, model TQSO50FUA (Kenosha,

WI). The torquometer had a ‘main pointer’, which provided real-time readings, and a

‘follow-up pointer’, which showed the maximum torque reading reached during the skin-

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set tests. A #2 rubber stopper (20-mm diameter) was mounted on the end of a spring

loaded shaft which was connected to the torquometer. To minimize slippage during

tests, water resistant grit paper (#100, 20-mm diameter) was glued to the end of the

rubber tip (Weir, 2001), and the rubber stopper was replaced at each harvest time. To

take a torque reading, the rubber stopper was pressed against the skin on the

equatorial portion of each tuber, ensuring the main pointer remained at the zero point.

The torquometer was then twisted gradually until the skin sheared and a complete disk

of skin broke loose. The maximum torque reading (0 to 0.67 N•m range) was recorded

as the measured skin-set value and the follow-up pointer was re-set to zero for the

subsequent reading.

In season 1, the skin strength increased with succeeding harvests and by H3 the

device became difficult to use. The upper part of the device tended to detach; to avoid

this source of error, the following modifications were made to the device prior to season

2. The torquometer was rotated 90º in relation to the handle and fitted onto a drill press

stand. This permitted the torquometer to be pressed and secured against the tuber

surface with uniform force for each reading (Figure 6-1). Next, the handle was twisted

as, in the previous version, and shear strength was measured. The modified device

considerably reduced user fatigue and improved consistency in season 2.

Experiment 2: Evaluation Of Tuber Wound Periderm Suberization

Determination of ‘Rapid Curing’ Conditions

Freshly-harvested ‘Red LaSoda’ tubers were used to determine the most suitable

rapid curing conditions for new potatoes in spring 2012. Tubers were carefully washed

and fan-dried for 10 mins to remove all surface moisture, before wounding each tuber.

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A small circular area of periderm and underlying cortical tissue (20-mm diameter,

2-mm deep) was marked and carefully excised with a sharp blade from the equatorial

region of each tuber and discarded. The tubers were placed on trays in racks; to

maintain a high humidity (90-95%), humidification trays were placed in alternating order

with the trays containing the tuber samples (Figure 6-2). Humidification was

accomplished by soaking laboratory bench paper with deionized water. Loosely fitted

plastic covers were placed over the crates to maintain the high humidity, while still

allowing ventilation. Data loggers were placed in the racks to monitor temperature and

relative humidity.

The racks were placed in each of three temperature-controlled rooms. Three

curing temperatures of 15, 20, or 25ºC were selected, based on previous research.

Tubers (n=3) were evaluated for suberin deposition thickness on the wound site, using

histochemical analysis, at 1, 3, 5, and 7 d in storage. Whole tuber (n=4) fresh weight

was also evaluated at the beginning (0 d) and end of storage (7 d). The storage

temperature which promoted maximum suberin deposition in the shortest storage time,

while minimizing fresh weight loss, was then selected for further ‘rapid curing’ tests.

Effect of Storage Condition On Wound Periderm Suberization

In spring 2013 (season 1) and 2014 (season 2), ‘Fabula’ and ‘Red LaSoda’

tubers were grown using surface-drip irrigation and fertilized with granular N at 224 kg

ha-1. Tubers were hand-harvested three weeks after vine kill, and bagged in paper bags

to avoid skinning injury during transportation to the laboratory. Upon arrival, tubers were

washed and wounded using the method described in the tests to determine the rapid

curing conditions. After wounding, half of the tubers were placed in one of two storage

treatments. In the first treatment (T-20), tubers (n = 25) were placed in the humidity

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controlled racks (90-95% RH) and placed in the rapid curing storage room of 20ºC. After

5 d at the curing conditions, the T-20 tubers were transferred to the simulated

commercial storage conditions (10ºC, 80-85% RH) for the rest of the storage period.

The other half of the tubers wounded tubers (n=25) were stored in the second storage

treatment (T-10) of 10ºC, 80-85% RH, simulating commercial storage conditions, for

the entire 14 d storage duration. Evaluations for wound periderm suberization

(histochemical analysis), whole tuber fresh weight loss and dry matter content were

carried out at 0, 5, and 14 d.

Tissue Preparation And Histochemical Analysis

At each sampling time of 5 d and 14 d, tissue blocks (1 cm x 2 cm x 1 cm deep)

from the wounded area of each tuber sample (n=3) were excised and fixed in a solution

of formalin : acetic acid : 95% ethanol : deionized water (3 : 1 : 10 : 7, v/v/v/v). The

tissue blocks were sectioned into 10 µm sections using a sliding hand microtome

(model DK-10, Edmund Scientific, Barrington, N.J.). The sectioned tissues were

dehydrated in a graded ethanol series according to standard procedures (Ruzin, 1999).

Sections were mounted on subbed microscope slides and dried in a 58°C oven for 12 h,

followed by a full day at room temperature to ensure full adhesion to the microscope

slide.

Tissue was washed twice in xylene to remove paraffin, and then dehydrated

once more to 70% ethanol in a graded series. A 1% w/v Safranin O solution (Sigma-

Aldrich, Saint Louis, MO) was dissolved at room temperature. Tissue sections were

stained in the solution for 2 h, before the slides were rinsed in deionized water with

gentle agitation. The sections were counterstained in 0.15% w/v Fast Green FCF

solution (Sigma-Aldrich, Saint Louis, MO), according to standard procedures by

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Johansen (1940). The procedure allows analysis of suberized cells by staining them into

an intense red color.

The stained sections were examined using standard light microscopy with an

Olympus system microscope (model BX51, Olympus America, Inc., Melville, NY) using

10x magnification. The depth (µm) of suberized cell layers on the wound periderm was

measured with a caliber fixed in the microscope lens eye.

Statistical Analysis

The two cultivars, ‘Fabula’ and ‘Red LaSoda’, were analyzed separately using

Analysis of Variance (SAS Institute Inc. Version 9.3, Cary, NC, USA). A general linear

mixed model (PROC GLIMMIX) was performed to determine preharvest and

postharvest main and interaction effects on tuber skin maturity. The analysis was

implemented as a completely randomized design with split plot, where the main plots

were fertilizer treatment, harvest time and vine kill. Treatment means were separated

using the Tukey’s test and confidence limits of 95% calculated for each mean.

Results

Weather Conditions

Seasonal differences in rainfall occurrences just prior to vine kill and over the

following three week harvest period were observed (Figure 6-3). In season 1, a

cumulative rainfall amount of 23 mm occurred just prior to vine kill. An even higher

amount of 268 mm was experienced the following week between vine kill and the first

harvest (H1). Season 2 harvest period was drier, with only 36 mm cumulative rainfall

occurring between vine kill and H3.

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Tuber Decay Incidence

Tuber decay incidence in both potato cultivars was affected by harvest time and

season. Generally, high decay amounts of 2,756 kg ha-1 and 2,273 kg ha-1 for ‘Fabula’

and ‘Red LaSoda’, respectively, were observed in season 1. The following season, the

decay incidence was lower, with an average of only 592 kg ha-1 over the three week

harvest period, in both cultivars.

Decay incidence also increased with increased delay in tuber harvesting. During

season 1, decay in ‘Fabula’ increased from 1,104 kg ha-1 (4% of total yield) at H2 to

1,652 kg ha-1 (7% of total yield) at H3; there was no decay at H1. In season 2, tuber

decay was observed at all harvest times, although H3 had 466 kg ha-1 (11% of total

yield), which was significantly higher than the earlier harvests, which averaged 71 kg ha-

1 (2% of total yield) (Figure 6-4 A). In ‘Red LaSoda’, decay was observed at the final two

harvests, even under the drier conditions in season 2; average decay was 1,137 kg ha-1

(5% of total yield) and 287 kg ha-1(9% of total yield) at H2 and H3, respectively (Figure

6-4 B).

Experiment 1: Determination Of Tuber Resistance To Skinning Injury In Relation To Nitrogen Treatment and Harvest Time

There was a significant interaction between harvest time, fertilizer treatment and

season on the tuber skin strength of non-vine killed (NVK) and vine killed (VK) ‘Fabula’

tubers. In season 1, harvest time x fertilizer treatment had a significant effect on the skin

strength of NVK tubers (Table 6-1). There were no differences in skin strength among

the fertigation treatments just prior to vine kill time (H0), averaging 0.18 N•m. At H2, skin

strength for F-336, which was 0.15 N•m was significantly lower than F-112 and F-224,

which averaged 0.21 N•m. A significant increase in strength was observed in all

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fertigation treatments at H3, where F-0 (0.25 N•m) was significantly lower than F-112

and F-224, which both averaged 0.31 N•m. There were no differences in skin strength

between the granular and fertigation application methods in NVK tubers, at all harvest

times. Meanwhile, VK tubers were affected by harvest time only, with skin strength

increasing from an average of 0.18 N•m at the H0 and H1, to 0.26 at H2 (Table 6-2).

There were no differences in average skin-set readings between NVK and VK tubers at

all harvest points.

In season 2, harvest time had a greater effect on both NVK and VK in ‘Fabula’.

Significantly higher skin strength of 0.29 N•m was only observed at H3 for NVK tubers,

while the first three harvests averaged 0.25 N•m (Table 6-2). On the other hand, VK

tubers significantly increased from 0.24 N•m at H0, to 0.31 N•m at H1, with no further

significant increases. VK tubers harvested 1-3 weeks after vine kill had significantly

higher skin-set, average 0.32 N•m, compared to NVK tubers (average 0.26 N•m), during

the same time period.

In ‘Red LaSoda’, year x harvest time had a significant (p < 0.05) effect on the

tuber skin strength of both NVK and VK tubers (Table 6-3). In season 1, NVK ‘Red

LaSoda’ tubers at H2 had a significantly higher reading of 0.26 N•m, compared to an

average of 0.19 N•m for H0 and H1. A similar trend was observed in the VK tubers, with

no differences in skin-set readings at all harvests between NVK and VK treatments.

In season 2, the greatest skin strength in NVK was observed in H1 and H3

tubers, averaging 0.28 N•m, which was significantly higher than H0 and H2. In VK

tubers, H1 (0.29 N•m) had a higher reading than H0 and H2 (average 0.24 N•m); while

H3 the highest reading of 0.33. Differences in average skin-set between NVK and VK

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tubers was only observed at H3, where the NVK had a significantly lower reading of

0.27 N•m, compared to 0.33 N•m for VK tubers.

Experiment 2: Evaluation Of Tuber Wound Periderm Suberization

Determination of ‘Rapid Curing’ Conditions

To determine the rapid curing condition and duration, suberin deposition on

wounded tuber periderms was evaluated following 7 d storage at 15, 20, or 25ºC. A

significant interaction between storage temperature and storage time affected the

thickness of the suberized layer in the wound periderm. Suberization was observed at 3

d in all storage conditions, increasing with storage time for all temperatures (Table 6-4).

At 5 d, the thickest suberin layer of 26.7 µm was observed at 25ºC; while tubers

stored at 20ºC averaged 17.6 µm and 15ºC had the lowest reading of 13.7 µm. The

greatest thickness by the end of storage (7 d) was observed at 25ºC, with no significant

differences at the other two storage temperatures. On the other hand, the highest

cumulative weight loss during storage of 2.33% was observed at 25ºC, while the other

storage conditions averaged 0.97% (Figure 6-5). Storage temperature had no effect on

whole tuber dry matter content, with all treatments averaging 18.8% throughout storage.

Based on this data, it was therefore concluded that storage at 20ºC for 5 d would

facilitate adequate curing in the shortest time, without compromising tuber quality.

Effect of Storage Condition On Wound Periderm Suberization

Differences were observed in wound periderm suberization and fresh weight loss

between the two storage treatments of 10ºC, 80-85% (T-10) and 20ºC, 90-95% (T-20) in

‘Fabula’ tubers. At the end of the curing period (5 d), tubers stored at 20ºC, 90-95% (T-

20) had significantly higher suberization of 16.5 µm, compared to 12.5 µm in those at

10ºC, 80-85% (T-10) (Figure 6-6). Upon transfer to 10ºC, 80-85%, T-20’s suberization

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further increased to 24.0 µm during the remaining storage time, while no further

increases where seen in T-10 tubers. T-20 tubers also maintained significantly higher

weight loss of 3.55% throughout storage of 3.55%, compared to 2.87% in T-10 (Figure

6-8).

‘Red LaSoda’ followed a similar trend, showing more suberization in T-20 tubers

(18.4 µm) than T-10 (9.3 µm) at 5 d. By the end of storage, T-20 tubers had a

significantly thicker suberin layer of 26.8 µm, while T-10 was 13.6 µm (Figure 6-7).

There were no significant differences in weight loss between the treatments throughout

storage, averaging 1.51% loss at the end of storage. Whole tuber dry matter content

was not affected by storage condition or duration, averaging 13.25% and 14.93% in

‘Fabula’ and ‘Red LaSoda’, respectively.

Discussion

Experiment 1: Determination Of Tuber Resistance To Skinning Injury In Relation To Nitrogen Treatment and Harvest Time

Delaying tuber harvesting after vine kill by three weeks resulted in increased

‘Fabula’ tuber decay in both growing seasons. During season 1, tubers harvested two

weeks after vine kill also recorded a high decay incidence, added to the fact that the

overall decay was generally higher in this season too (Figure 6-4 A). This was attributed

to high rainfalls experienced during the harvest period in season 1. Meanwhile, ‘Red

LaSoda’ tubers harvested 2 and 3 weeks after vine kill had significantly higher decay

incidence, in both seasons (Figure 6-4 B).

During tuber growth, lenticels are the main ports of entry for decay pathogens. As

the tubers mature, suberin, a complex biopolyester, is deposited at the base of the

lenticel, forming a closing layer (Lulai, 2007). Waxes with fungicidal properties found in

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this suberized closing layer are believed to aid in keeping pathogens out (Lulai and

Corsini, 1998; Tyner, et al., 1997). By delaying tuber harvesting, given the high rainfall

amounts in season 1, lenticels cells were allowed to proliferate and rupture through the

protective suberized layer (Makani, 2010), predisposing the tuber to decay (Tyner, et

al., 1997). This resulted in high decay incidence as early as two weeks after vine kill, in

both cultivars.

According to Adams (1975), drier soil conditions promote lenticel suberization,

which would explain the decreased severity of decay during season 2 in both cultivars.

However, ‘Red LaSoda’ maintained a high decay incidence at both H2 and H3,

indicating the cultivar was more susceptible to decay. Genotypic differences in

resistance to bacterial infection have also been reported in previous studies (Bain and

Perombelon, 1988; Kumar et al, 1991).

Tuber periderm resistance generally increased with increased delay in harvest

time, in both cultivars. By affecting plant growth and development, fertilizer treatments

had an effect on periderm maturation in NVK ‘Fabula’ tubers. The highest level of N

fertigation treatment in this study (336 kg N ha-1) likely promoted vegetative growth over

tuber development (Hope et al., 1960; Shock, 2006; Wszelaczynska and Poberezny,

2011), thereby delaying tuber maturation at H1 (Table 6-1). The other extreme N

treatment of 0 kg N ha-1 also delayed skin-set with increased harvest time, resulting in

significantly lower periderm resistance than 112 and 224 kg N ha-1 at the final harvest.

This could possibly indicate that any form of plant stress, be it too high or too low N rate,

can affect phellogen activity and periderm maturation in NVK potatoes. However, these

results were only observed during season 1.

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In season 2, harvest time had a greater effect on the skin-set of NVK ‘Fabula’

tubers, with significantly higher readings at the final harvest (Table 6-2). The condition

of plants vines affects periderm maturation, increasing skinning resistance with

increased vine senescence (Bowen, 1996; Yagamuchi et al., 1966). With irrigation shut

off two weeks before the initial harvest (H0), and lower amounts of rainfall occurring

during the harvest period in season 2, natural vine senescence likely progressed

uniformly in all fertilizer treatments. However, significantly higher skin-sets were

observed at the final harvest only, indicating a delay in progression of plant senescence

for NVK tubers.

A delay in tuber harvesting also increased periderm maturation in VK ‘Fabula’

tubers (Table 6-2). Increased skin-set was observed two weeks after vine kill during

season 1, agreeing with previous studies (Halderson and Henning; 1993; Lulai and Orr

1993). In season 2, the drier weather during harvest period resulted in faster vine

senescence (Figure 6-3). Therefore, increased skin-set was observed as early as one

week after vine kill, agreeing with studies by Pavilsta (2002). Lack of significant rainfall

amounts during the harvest period in season 2 also increased the vine kill efficiency,

resulting in higher skin-set in tubers harvested 1-3 weeks after vine kill, compared to

NVK.

A slightly different pattern was observed in ‘Red LaSoda’ tubers. In season 1,

significantly higher skin-set was only observed at H2, with no differences in skin

resistance readings due to vine kill (Table 6-3). This indicates that high rainfalls

received just prior to vine kill and between H1 maintained growth and lush vines

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(Kempenaar and Struik, 2007; Lulai and Orr, 1994) in this cultivar, delaying senescence

and subsequent skin-set (Braue et al, 1983; Yagamuchi et al, 1966).

The drier conditions in season 2 accelerated vine senescence in both NVK and

VK tubers, resulting in significantly higher skin-sets by H1 (Table 6-3). The decrease in

skin resistance for both treatments at H2 is best explained by a temporary increase in

the fragile nature of the phellogen cell walls, as reported by Lulai (2002).

The similar skin-set readings between NVK and VK tubers until H2 during both

seasons can be explained by early periderm maturation and/or a general delay in

response to vine killing.

Experiment 2: Evaluation Of Tuber Wound Periderm Suberization

The main potato crop (storage potato) is commonly held at 15-18°C and 80-95 %

relative humidity (RH) for an average of 2 weeks (Hide et al., 1994; Suslow and Voss,

2000), before long-term storage. The results in this study show that increasing the

temperature to 20ºC while maintaining relative humidity at 90-95%, shortened the

required curing period in new potatoes (Table 6-4). Wound healing in tubers involves

the formation of closing layers of suberized cells, followed by suberization of newly

developed cells under the closing layer.

A quantifiable amount of wound periderm suberin was detected at 5 d in both the

curing (20ºC, 90-95%) and simulated commercial storage conditions (10ºC, 80-85%)

(Figures 6-6 and 6-7). This indicates increased stress-induced suberin deposition in

both storage treatments (Lulai, 2001). Additional stress at the high curing temperatures

increased suberization rate, while high humidity minimized cell desiccation at the wound

site (Burton, 1989; Kim and Lee, 1993; Lulai, 2007). Transferring tubers at 5 d from the

curing conditions to the lower storage temperature and humidity seemed to trigger more

156

stress-induced suberization. Lulai (2001) reported that any type of tuber stress

increased the rate of suberin deposition. Meanwhile, tubers of both cultivars kept at 10

ºC, 80-85% for the entire storage time maintained significantly lower suberization.

However, there were cultivar differences in weight loss response to storage

temperatures. The higher curing temperatures resulted in significantly more weight loss

in ‘Fabula’, compared to storage at the commercial conditions (Figure 6-8). According to

Lulai and Orr (1994), a delay in wound periderm formation and suberization leads to

higher weight loss at the beginning of storage. The 5-day time period for rapid curing

used in this study was based on rate of wound-healing in ‘Red LaSoda’, of which

quantifiable amounts were only detected at 3 days in storage. It is therefore possible

that ‘Fabula’ had a delayed response to wound-induced suberization, resulting in

significantly higher weight loss at the beginning of curing.

Another reason could be water loss through the native periderm. ‘Fabula’

generally has a longer seed dormancy period, thereby shortening the growing season.

This likely resulted in immature tubers, characterized by lower periderm suberization

and higher permeability (Lulai and Orr, 1994). This is further evidenced by the fact that

the earlier germinating ‘Red LaSoda’ had significantly lower weight loss throughout

storage, with no significant differences between storage conditions. No significant

differences were observed in whole tuber dry matter content in both cultivars, indicating

that storage conditions had a lesser effect on the physiological properties of the tubers.

Conclusions

Adequate skin-set reduces wounding and skinning of tubers during harvesting

and handling, thereby minimizing qualitative and quantitative losses during storage. It is

therefore important to use objective methods to measure tuber skin maturity before

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harvesting. The hand-held skin tester used in season 1 during this study proved difficult

to use as tuber skin resistance increased. The modified tester used in season 2 was

more efficient; being able to measure skin-set was at all harvest times. It also minimized

user fatigue. There were generally no differences in the range of skin-set readings

between the two devices.

The weather conditions experienced in season 1 were more representative of

Florida’s spring potato growing season, which is characterized by wet harvest periods

and increasing ambient temperatures, promoting high tuber decay. This study showed

that ‘Fabula’ tubers harvested three weeks after vine kill were more susceptible to

decay in both seasons. By delaying tuber harvesting, lenticels proliferated and ruptured

through the protective suberized layer, predisposing the tuber to decay. In ‘Red

LaSoda’, a higher decay incidence at both H2 and H3 is indicative of a lower resistance

to bacterial infection in the cultivar.

The effect of fertilizer rate, vine killing and harvest time also depended on the

prevailing weather conditions during harvest. High rainfall amounts during the harvest

period delayed skin-set in VK ‘Fabula’ tubers, with increased skin resistance observed

two weeks after vine kill. Drier weather during the harvest period resulted in faster vine

senescence; increased skin-set was observed as early as one week after vine kill. A

similar trend was observed in VK ‘Red LaSoda’ tubers; although a temporary increase

in the fragile nature of the phellogen cell walls likely caused a decrease in skin-set at

harvest 2 in season 2 only. There were also indications that the extreme N rates of 0

and 336 kg ha1 induced ‘Fabula’ plant stress, which affected tuber growth,

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development, and phellogen activity, resulting in a lower skin-set. However, this was

only observed in NVK plants in season 1.

The high rainfall in season 1 delayed vine desiccation after spraying, resulting in

no significant differences in skin-set readings between NVK and VK tubers in ‘Fabula’

tubers. Lower rainfall the following season increased the vine kill efficiency, resulting in

a higher skin-set in VK tubers. On the other hand, vine killing seemed to have less of an

effect on the earlier maturing ‘Red LaSoda’. This may have been due to early tuber

growth and development, resulting in a more mature periderm at vine kill time.

In storage, curing at 20ºC, 90-95% for 5 d increased stress-induced suberin

deposition, resulting in a thicker suberized wound-periderm in both cultivars, compared

to non-cured tubers. However, significantly lower fresh weight loss was observed in

‘Fabula’ tubers stored in simulated commercial storage conditions of 10ºC, 80-85% for

14 d. No differences in weight loss were observed between cured and non-cured ‘Red

LaSoda’ tubers. This indicated that quality of skinned or wounded new potato tubers is

better maintained at 10ºC, 80-85% RH.

In conclusion, this study showed that ‘Fabula’ and ‘Red LaSoda’ tubers

harvested two weeks after vine kill had increased skin-set, which can help minimize skin

injury during harvest and handling. However, in the event of drier harvest periods, with

no high rainfall occurring just prior to vine kill, tubers could be harvested as early as one

week after vine kill. The results also showed that rapid curing of wounded tubers after

harvest increased wound periderm suberization. However, the high curing temperatures

caused quality loss in ‘Fabula’; compared to storage at 10ºC, 80-85% RH. In ‘Red

LaSoda’, both storage treatments maintained tuber quality. This was attributed to a

159

more mature native periderm in the cultivar. It can therefore be concluded that, given

the short-term storage of new potatoes, storage at 10ºC, 80-85% RH is sufficient

enough to promote adequate wound healing, while minimizes storage losses. The

benefits of curing reported in for the fall crop, are likely due to the tubers having a more

mature native periderm at harvest, which minimizes weight loss during curing.

160

Table 6-1. Skin-set readings of non-vine killed ‘Fabula’ tubers grown under five nitrogen treatments and harvested over two weeks (91 to 105 days after planting), during season 1.

Harvest timez

Skin-Set Readings (Torque, N•m)

Fertilizer treatment

F-0y F-112 F-224 F-336 G-224

H0 0.17 bAx 0.18 bA 0.17 bA 0.19 bA 0.18 bA H1 0.18 bAB 0.21 bA 0.20 bA 0.15 bB 0.18 bAB H2 0.25 aB 0.31 aA 0.30 aA 0.29 aAB 0.28 aAB xMeans within a column (fertilizer treatment) followed by the same small letter or within a row (harvest

time) followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

. zHarvest times at maturity before vine kill (H0), 1 week (H1) and 2 weeks after vine kill (H2).

Table 6-2. Effect of vine kill on skin-set readings of ‘Fabula’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting).

Harvest timey

Skin-Set Readings (Torque, N•m)

Season 1 Season 2

Non-vine kill Vine kill Non-vine kill Vine kill

H0 0.18 bAx 0.18 bA 0.24 bA 0.24 bA H1 0.18 bA 0.19 bA 0.26 abB 0.31 aA H2 0.28 aA 0.26 aA 0.24 bB 0.32 aA H3 -z - 0.29 aB 0.34 aA xMeans within a column (vine kill treatment) followed by the same small letter or within a row (harvest

time), for each season, followed by the same capital letter, do not differ significantly according to Tukey’s

Test at 5% level. yHarvest times at maturity before vine kill (H0), 1 week (H1), 2 weeks (H2), and 3 weeks (H3) after vine

kill (H2). z- missing data point.

161

Table 6-3. Effect of vine kill on skin-set readings of ‘Red LaSoda’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting).

Harvest timey

Skin-set Readings (Torque, N•m)

Season 1 Season 2

Non-vine kill Vine kill Non-vine kill Vine kill

H0 0.20 bAx 0.20 bA 0.23 bA 0.23 cA H1 0.18 bA 0.19 bA 0.28 aA 0.29 bA H2 0.26 aA 0.28 aA 0.23 bA 0.24 cA H3 -z - 0.27 aB 0.33 aA xMeans within a column (vine kill treatment) followed by the same small letter or within a row (harvest

time), for each season, followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level. yHarvest times at maturity before vine kill (H0), 1 week (H1), 2 weeks (H2), and 3 weeks (H3) after vine

kill (H2). z- missing data point.

Table 6-4. Average thickness of suberized wound periderm layer of ‘Red LaSoda’ stored for 7 days at 15, 20, and 25ºC, 90-95% RH.

Storage time (days)

Suberized layer (µm)

Storage Temperature (ºC)

15 C 20 C 25 C

3 d 5.0 cAx 5.7 bA 10.0 bB 5 d 13.7 bC 17.6 aB 26.7 aA 7 d 18.6 aB 20.0 aB 27.5 aA xMeans within a column (fertilizer treatment) followed by the same small letter or within a row (harvest

time) followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level.

162

Figure 6-1. Modified skin-set tester used to measure resistance to periderm injury.

Figure 6-2. Layout of trays with tubers in racks for ‘rapid curing’ at 20ºC, 90-95% RH (left rack), and stimulated storage conditions at 10ºC, 80-85% (right rack).

163

Figure 6-3. Total rainfall experienced during tuber harvest period in season 1 and 2, where VK is vine kill and H1-3 is harvest 1-3 after vine kill.

0

20

40

60

80

100

120

VK

H1

H2

H3

2m

Rain

Tota

l (m

m)

Days after planting

Season 1

Season 2

164

A)

B)

Figure 6-4. Tuber decay (kg ha-1) in A) ‘Fabula and B) ‘Red LaSoda’ tubers harvested 1 to 3 weeks after vine kill (H1-H3) during two seasons.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

H1 H2 H3

Tuber

decay (

kg

/ha)

Harvest time after vine kill (weeks)

Season 1

Season 2

0

200

400

600

800

1000

1200

1400

1600

1800

H1 H2 H3

Tuber

decay (

kg

/ha)

Harvest time after vine kill (weeks)

Season 1

Season 2

165

Figure 6-5. Fresh weight loss in wounded ‘Red LaSoda’ tubers stored for 7 days at 15, 20, and 25ºC, 90-95% RH.

Figure 6-6. Wound periderm suberization of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)

0

0.5

1

1.5

2

2.5

3

15 C 20 C 25 C

Fre

sh w

eig

ht lo

ss (

%)

Storage Temperature (degree C)

0

5

10

15

20

25

30

0d 5d 14d

Suberin layer

(µm

)

Storage time (days)

T-10

T-20

Transfer to 10ºC, 80-85% RH

166

Figure 6-7. Wound periderm suberization of ‘Red LaSoda’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)

Figure 6-8. Fresh weight loss of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)

0

5

10

15

20

25

30

0d 5d 14d

Suberin layer

(µm

)

Storage time (days)

T-10

T-20

Transfer to 10ºC, 90-95% RH

0

0.5

1

1.5

2

2.5

3

3.5

4

0d 5d 14d

Fre

sh w

eig

ht lo

ss (

%)

Storage time (days)

T-10

T-20

Transfer to 10ºC, 80-85%

167

CHAPTER 7 SUMMARY AND FINAL CONCLUSIONS

Optimizing water and nitrogen (N) management is critical to potato yield and

tuber quality. Soil moisture and N stress during growth has generally been associated

with decreased yields, tuber harvest quality and storage ability. By applying water close

to the plant root zone, drip irrigation has the potential to minimize moisture stress, while

producing high yields and good tuber harvest quality. In addition, nutrient use efficiency

can be improved when fertilizer application timing and rate is better controlled to match

plant growth needs, using drip irrigation.

Many studies have shown comparable or improved yields and tuber quality with

drip irrigation. However, most of these studies been done on the fall crop, which are

generally medium- to late-maturing cultivars. The fall crop is usually cured to improve

skin-set, before the tubers are put into long-term storage at low temperatures. On the

other hand, Florida’s ‘new’ potatoes are early- to medium-season cultivars, which are

harvested with an immature periderm, making them more prone to postharvest quality

losses. The influence of preharvest factors on yield and storage quality of new potatoes

has not been fully explored, and applications of recommendations made for the fall crop

to the new potatoes could potentially cause yield and quality losses. Therefore, the

overall objective of this study was to determine how preharvest factors affect tuber yield,

harvest quality and storability of two tablestock potato cultivars, commonly grown in

Florida.

An interaction of irrigation method and harvest time on tuber yield and storage

quality was evaluated in spring 2011 and 2012. In ‘Fabula’ adequate water supply in

seepage and surface drip irrigation resulted in significantly higher yields than sub-

168

surface drip. However, significantly higher physiological disorders were observed in the

seepage irrigated tubers. In storage, harvest time had a greater effect on tuber quality;

less mature tubers, harvested one week after vine kill, experienced significant losses in

tuber quality with increased storage time. Meanwhile, in ‘Red LaSoda’, higher tuber

yields and at harvest quality was achieved with seepage irrigation. High decay was

observed in the drip irrigation methods, which was attributed to ‘Red LaSoda’ likely not

being well adapted to the soil wetting patterns of drip irrigation. In storage, similar to

‘Fabula’, significant losses were only found in the less mature tubers harvested one

week after vine kill, which also lost more weight and softened faster.

With surface drip irrigation showing good potential, particularly in ‘Fabula’, the

second objective was to determine how N fertilizer application method, rate and harvest

time affected tuber yield and storage quality of surface-drip irrigated potatoes. In

‘Fabula’, fertigation rates as low as 112 kg N ha-1 produced yields comparable to higher

N rates. However, fertigation rate of 224 kg N ha-1 consistently produced yields

comparable to the granular treatment, regardless of the varying seasonal weather. In

storage, differences in quality among the fertilizer treatments were generally observed

in tubers harvested one week after vine kill. The fertigation rate 224 kg N ha-1

maintained similar or better storage quality than the granular treatment, for most of the

quality parameters. Similar yield and tuber harvest quality results were observed in ‘Red

LaSoda’. In storage, the only parameters affected by increased storage time were tuber

fresh weight and peel dry matter content, in tubers harvested one week after vine kill.

This indicated that tuber maturity in ‘Red LaSoda’ was generally achieved as early as

harvest two, likely due to a longer growing season.

169

The third objective was to determine the effect of N rate, plant vine kill and

harvest time on tuber physical maturity. Total resistance to skinning was measured

using a modified skin-set testing device developed by J.L. Halderson and R.C. Henning

(1993). Seasonal variations in weather had a huge effect on the skin-set of both vine

killed and non-vine killed ‘Fabula’. The high rainfall in season 1 delayed vine desiccation

after spraying, resulting in no significant differences in skin-set readings between non-

and vine killed ‘Fabula’ tubers. Higher skin resistance in vine killed tubers was only

observed two weeks after vine kill. Lower rainfall the following season resulted in faster

vine senescence; increased skin-set was observed as early as one week after vine kill.

A similar trend was observed in vine killed ‘Red LaSoda’ tubers

The final objective was to evaluate rapid curing as a means of minimizing storage

losses in skinned tubers by promoting suberization and wound-healing. Rapid curing

conditions (20ºC, 90-95% RH) significantly increased wound periderm suberization.

However, the high temperatures compromised quality in ‘Fabula’; while no significant

differences were observed in storage quality between cured and non-cured ‘Red

LaSoda’ tubers. This indicated that storage at the simulated commercial conditions of

10ºC, 80-85% RH were ideal in minimizing quality loss in wounded tuber.

These results therefore showed that surface drip, using 224 kg N ha-1 has the

potential to produce high yields while maintaining tuber postharvest quality in ‘Fabula’.

In ‘Red LaSoda’ although drip irrigation produced comparable storage quality, seepage

resulted in higher yields. There is need for more research to determine if further

adjustments in the drip irrigation scheduling could produce yields comparable to

seepage irrigation.

170

The study also showed that tuber resistance to skin injury was highly dependent

on weather conditions. Given the characteristic high rainfalls experienced during the

growing season in Florida, which delay skin-set, the recommended harvest time to

avoid skin injury in both cultivars is two to three weeks after vine kill.

However, since skin-set is highly dependent on weather conditions, as reported

in this study, it therefore critical for growers to have access to objective measuring tools,

to better determine when to harvest. The modified device developed in this study is

portable, accurate, user friendly and is a viable option that can be used to measure

skin-set in new potatoes.

Lastly, based on observations in this study, 10°C, 80-85% RH is the

recommended storage condition for new potatoes, since it generally maintained

acceptable quality of tubers harvested two to three weeks after vine kill. These storage

conditions also proved to be effective in maintaining quality of wounded tubers, by

promoting adequate wound-healing while minimizing weight loss and storage decay.

171

APPENDIX SUPPORTING DATA

Table A-1. Tuber classification used to evaluate marketable and unmarketable yield after harvest, 3 weeks after vine kill.

Size identification

code

Diameter size range (cm) Classification

C 1.27 – 3.81 Unmarketable

B 3.81 – 4.78 Unmarketable

A1 4.78 – 6.35 Marketable

A2 6.35 – 8.26 Marketable

A3 8.26 – 10.16 Marketable

A4 ˃ 10.16 Unmarketable

Table A-2. Effect of interaction of fertilizer treatment and harvest time on tuber pulp dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d

Harvest Timex

Pulp Dry Matter Content (%)

Fertilizer Treatmenty

F-0 F-112 F-224 F-336 G-224

H1 16.1 aAz 14.9 aB 15.3 aAB 14.8 aB 14.6 aB

H2 14.6 bA 14.9 aA 14.8 aA 13.8 aB 14.2 aA

H3 13.8 bA 13.9 bA 14.4 aA 14.3 aA 13.8 aA z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital

letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.

Table A-3. Effect of an interaction of fertilizer treatment, harvest time and storage time on soluble solids content (SSC), and total titratable acidity (TTA) and pH of ‘Fabula’ tubers stored at 10°C, 80-85% RH for 14 d.

Fertilizer treatmenty

SSC TTA pH

% %

F-0 2.85 c 0.11 c 3.25 bz

F-112 3.48 b 0.13 b 3.52 b F-224 3.73 ab 0.14 ab 3.99 ab F-336 3.96 a 0.15 a 4.29 a G-224 3.95 a 0.15 a 4.27 a z Means within each column followed by the same small letter do not differ significantly according to

Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha

-1, and G-224 is

granular application at 224 kg ha-1

.

172

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BIOGRAPHICAL SKETCH

Mildred N. Makani was born in Masvingo, Zimbabwe, the last child in a family of

seven. She did her primary school education at Victoria High School, later proceeding to

do her Cambridge Ordinary Levels at St. Dominic’s High School, Chishawasha. She

completed her Advanced Levels at Victoria High School, before enrolling at the

University of Zimbabwe, where she graduated with a Bachelor of Science honors

degree in Plant Science.

Mildred then joined a private company which specialized in providing virus-free

plant material of drought tolerant crops such as sweet potato and cassava. She

specialized mostly in the postharvest area, with her work involving research and

extension work on the handling, storage and value-addition of the root crops after

harvest. In 2009, Mildred decided to pursue her graduate studies, joining Dr. Steven

Sargent’s Lab to pursue a Master of Science degree in Horticultural Sciences. Upon

completion of her master’s degree, she continued with a Doctor of Philosophy in the

same area, graduating in December 2014.