:Lo'\>'q1

LEAF GAS EXCHANGE AS INFLUENCED BYENVIRONMENTAL FACTORS IN MANGO

CULTIVARS (MANGTFERA IIVDICA ¿), GRO\ryN INTHE SEMI ARID TROPICS

BY

PETER ROBERT JOIINSON

ADELAIDE LTNIVERSITY DEPARTMENT OFHORTICULTURE, VITICULTURE AND OENOLOGY

Masters of Agricultural Science Degree

Submitted For Examination

August 1998

This work contains no material which has been accepted for the award of any other

degree or diploma in any university or other tertiary institution and, to the best of my

knowledge and belief, contains no material previously published or written by another

person, except where due reference has been made in the text.

I give consent to this copy of my thesis, when deposited in the Universþ Library,

being available for loan and photocopying.

Peter R Johnson

(.2417198\

Acknowledgments

I would like to acknowledge my supervisor and friend the late Dr Elias Chacko who

provided the initial inspiration for the study as well as the follow up encouragement

and support. Tragically Dr Chacko passed away before my thesis was completed, I

wish to dedicate this thesis in memory of him.

I thank Dr Don Aspinall for being able to review my drafts and offer helpful

suggestions.

I would also like to thank Dr Kesi Kesavan for providing local support and helpful

discussions.

TABLE OF CONTENTS

LIST OF TABLES

I.IST OF FIGURES

ABBREVIATIONS

ABSTRACT

1. INTRODUCTION

2. MATERIALS AND METHODS

2.1 Location

2.t.1

2.1.2

2.t.3

2.t.4

2.2

2.3

3. RESULTS

Methods

2.2.1 Gas exchange measurements

2.2.2 Monitoring soil moisture

Experiments on factors influencing leaf gas exchange and related

parameters.

2.3.1 Bxperiment 1 Iradiance

2.3.2 Experiment 2 Leaf age

2.3.3 Experiment 3 Vapour pressure def,rcit

2.3.4 Experiment 4 Fruiting

2.3.5 Experiment 5 Soil moisture

Experimental site

Climate

Soils

Experimental trees

Effect of iradiance on gas exchange

Gas exchange as influenced by leaf emergence

Seasonal and diurnal variations in leaf gas

exchange in three mango cultivars

Effect offruiting on leafgas exchange

Effect of soil moisture on leaf gas exchange

PageI

1

J

4

6

t7

3.1

3.2

J.J

I7

t7

t7

18

18

2I

2l

2l

28

JJ

24

24

24

25

26

26

28

Experiment 1

Experiment 2

Experiment 3

Experiment 4

Experiment 5

3.4

3.5

36

47

5l

TABLE OF CONTENTS cont.

4. DISCUSSION

5. BIBLIOGRAPHY

58

69

Table

List of tables

Title Page

Table 1

Table2.l

Table2.2

Table 3.1

Table 3.2

Table 3.3

Table 4

Table 5

Figure

Average monthly temperatures, rainfall, relative humidity and evaporation 17

at the Ord river irrigation area

Yield for cultivars Kensington, Irwin and Tommy Atkins the Ord river 19

irrigation area

Date for full anthesis for cultivars Kensington, Irwin and Tommy Atkins. 19

Light and leaf gas exchange parameters of cultivar Kensington 29

Light and leaf gas exchange parameters of cultivar Irwin 29

Light and leaf gas exchange parameters of cultivar Tommy Atkins 30

Effect ofdrought on internal CO, concentrations 56

Effect ofdrought on fruit productivity and size 56

List of figures

Title Page

Figure I

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure l0

Figure 11

Figure 12

Growth rate of cultivars Kensington, Irwin and Tommy Atkins.

Relationship between volumetric moisture content and neutron moisture

probe readings.

Net photosynthesis as a function of photosynthetic photon flux density.

Transpiration as a function of photosynthetic photon flux density.

Stomatal conductance as a function of photosynthetic photon flux density.

Internal CO, concentration as a function of photosynthetic photon flux

density.

Development of leaf length.

Relationship between leaf age and net photosynthesis.

Relationship between leaf age and transpiration.

Relationship between leaf age and stomatal conductance.

Diurnal variations in net photosynthesis under conditions of high vapour

pressure deficit.

Diurnal variations in net photosynthesis under conditions of low vapour

pressure deficit.

Diurnal variations in stomatal conductance under conditions of high vapotìr

pressure deficit.

3l

3l

.tz

32

34

34

34

35

38

20

23

38

I

Figure l3 39

Figure Title Fage

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 2l

Figure22

Figure 23

Figure 24

Figure 25

Figure 26

Figlure2T

Figure 28

Figure 29

Figure 30

Figure 31

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

Diurnal variations in stomatal conductance under conditions of low vapour

pressure deficit.

Diurnal variations in transpiration under conditions of high vapour pressure

deficit.

Diurnal variations in transpiration under conditions of low vapour pressure

deficit.

Diurnal variations in internal CO, concentration under conditions of high

vapour pressure deficit.

Diurnal variations in internal CO, concentration under conditions of low

vapour pressure deficit.

Diurnal variations in vapour pressure deficit.

Relationship between conductance and net photosynthesis.

Relationship between conductance and vapour pressure deficit.

Relationship between conductance and leaf temperature.

Relationship between net photosynthesis and leaf temperature.

Relationship between cultivar conductance and seasonal changes in vapour

pressure deficit.

Seasonal variations in leafgas exchange parameters.

Seasonal changes in vapour pressure deficit.

Effect offruiting on net photosynthesis

Effect offruiting on net photosynthesis over a diurnal cycle.

Effect of fruiting on stomatal conductance.

Effect of fruiting on stomatal conductance over a diurnal cycle'

Effect of fruiting on transpiration conductance.

Effect offruiting on transpiration conductance over a diurnal cycle'

Effect of drought on net photosynthesis.

Effect of drought on stomatal conductance.

Effect of drought on transpiration conductance.

Soil moisture content during drought period.

39

40

40

4l

4t

42

42

42

43

43

43

45

46

48

48

49

49

50

50

53

54

55

57

2

ABA

ci

Dil\{

E1

fd

fl

8.

ha

Irr

IRGA

T1

LGE

MT

Nir

NMP

ORIA

P,ru*

Pn

PPF

PPFD

RH

SE

SMC

VPD

VMC

Vr

ABBREVIATIONS

Abscisic Acid

Internal carbon dioxide concentration

DayA{ight

Transpiration

Fruit development

Flowering

Stomatal Conductance

Harvest

Irrigated

Infra Red Gas Analyser

Leaf temperature

Leaf Gas Exchange

Metric Tonne

No Irrigation

Neutron Moisture Probe

Ord River Irrigation Area

Maximum Net Photosynthesis

Net Photosynthesis

Photosynthetic Photon Flux

Photosynthetic Photon Flux Density

Relative Humidity

Standard Error

Soil Moisture Content

Vapour Pressure Deficit

Volumetric Moisture Content

Leaf 'Water Potential

PL L-'

mmol m-' s-t

mmol m-' s-t

oc

¡.rmol m-2 s-t

pmol m-' s-r

pmol quanta m-t s-t

pmol m-t s-1

%

MPa

cm

kPa

J

ABSTRACT

Leaf gas exchange (LGE) of mango cultivars Kensington, Irwin and Tommy Atkins was

investigated in a series of field experiments under varying environmental and

physiological conditions. These experiments were conducted in the Ord River Irrigation

Area Kununuffa'Western Australia

1. The effect of photosynthetic photon flux density (PPFD) at the leaf surface was

studied. Light saturation of photosynthesis of mature leaves occurred between 1250 -1500 pmol m-' s-' on Kensington, Tommy Atkins and Irwin between 1250-1750 pmol

m-' s-'. There was no significance (P<0.05) between cultivars.

2. Mango leaves reached their full size at four weeks after emergence. Maximum Pn,

and gs was reached 5 weeks after leaf emergence. Cultivar had no influence on the

results.

3. Pronounced diurnal variations in Pn, g., Er and C, were observed as a result of

fluctuating vapour pressure deficit. Pn, g., and E, were significantly lower in cultivar

Kensington during periods of high VPD (Dry Season), However little cultivar difference

occurred during periods of low VPD (V/et season). Since high VPDs are commonly

experienced during the period of fruit development, carbon assimilation may become

limited by stressful atmospheric conditions as the day proceeds. An inverse relationship

existed between g. and VPD in all cultivars. Seasonal Pn, g., and E, variations were

more pronounced than the variations between cultivars.

4. A significant reduction in Pn was observed on leaves adjacent to developing fruit

with both Kensington and Irwin cultivars. It is assumed that the fruit may primarily

affect stomatal aperture with subsequent effects on Pn and Er of the leaves. Possible

mechanisms leading to such effects are discussed.

4

5. Non irrigated trees had a significant reduction in g, and Later areduction in Pn, when

compared to irrigated trees although no significant differences in LGE were observed

between cultivars. Kensington responded with a reduction in fnrit size whereas Irwin

was observed to have greater fruit drop and leaf abscission.

5

u3t.ì

5)-

,1

l:yER.(;

INTRODUCTION

Mango (lt[angifera indica I) is the fourth most important fruit crop in the world behind

apples, oranges and bananas. Production is estimated at 18 million tonnes per annum

(FAO, 1995). Over the last four years production has been increasing annually by 25%

compared to the last decade where the increase was l.5Yo pet annuln, representing an

increase in planting area and market demand. India produces 550lo of world production

and a further 2IYo is produced in Asia. Currently Australia produces some 40,000

tonnes or 0.2%o of world production. However production in Australia is increasing at a

rapid rate and is expected to be around 80,000 tonnes within the next five years.

Cultivation of mango has occurred for an estimated 4000 years in India, early travellers

to India reported the importance of this fruit (Mukherjee 1972). Mango was first

brought to the attention of the outside world by Hwen T'sang (632-645 AD) (Mukherjee

1953). The Portuguese spread the mango into East Africa and Brazil from there it spread

over most of tropical Africa and the Americas (Popenoe 1927). With relatively recent

introductions, 100 years ago into countries such as Australia, mango is now grown in

most tropical and subtropical regions of the world with some production spreading into

the warmer temperate regions of the world.

Although mangoes have been grown in Australia for in excess of 100 years they are a

very recent introduction into Kununurra, the oldest commercial plantation being only 20

years of age. The area planted has increased by some 400% over the last 10 years

consisting mostly of cultivars Kensington and Irwin. Production is expected to increase

from the current 500 tonnes to 4000 tonnes over the next few years. The region with its

very distinct dry season, abundant water supply and early production time (October and

November) is rapidly becoming the centre of mango production in Western Australia'

However in spite of this, it is becoming increasingly apparent that there are some

production constraints on the important cultivar Kensington due to its erratic

performance under the local environmental conditions.

6

Botany of Mangfera indica

Botanically mango is a member of the family Anacardiaceae which also includes

cashew (Anacardium occidentale L.), pistachio (Pistacia vera L.) and imbu (Spondias

tuberose Anuda). The genus Mangifera contains some 68 species found from

Madagascar, India and South East Asia to New Guinea. However apart from Mangifera

indica and to a much lesser extent Mangiftra odorata, little or no cultivation of

members of this genus has occurred. The centre of origin for Mangifera indica is

reputed to be the subtropical, northeastern Indo-Burmese region, where it is found

growing in forests (Mukherjee 1953).

Char act er i s t i c s of Mangifer a indi c a

The mango is an evergreen tree of the tropics typically grown in climates with a

pronounced dry season (seasonally wet - dry tropics). The best areas for commercial

production are those with a cool and/or dry period prior to flowering, abundant soil

moisture (from rain and/or irrigation), and moderately hot temperatures (30 -330C)

during fruit development (Chacko 1986).

The trees are deep rooted, dome shaped growing up to 40 m tall (Whiley 1980), the

growth and form is determined by an orthotropic, periodically active, terminal meristem

which produces an indeterminate trunk bearing tiers of branches (Schaffer and Andersen

1994). Branching is a result of terminal flowering. The leaves are simple, entire, 8-40 X

2-10 cm, narrowly elliptic or lanceolate and produced in flushes. Vegetative growth

flushes occur throughout the year, the number and frequency of each flush is determinecl

by the cultivar, temperature conditions, tree maturity, current fruit load, and previous

cropping history (Whiley et al. 1989: Chacko 1986: Issarakraisila et al. l99l). In deep,

well-drained soils, mango trees develop a large tap root with an extensive fibrous root

system (Singh 1978). The feeder roots are relatively shallow with more than 80%

occurring within 600 mm of the surface (Bojappa and Singh I975). Mangoes grow over

7

a wide range of moisture regimes being quite drought tolerant as well as capable of

standing up to heavy rainfall and withstanding short periods of flooding (Schaffer et al.

1992). Mature trees are renowned for their drought tolerance being able to survive in

excess of eight months without rain (Gandhi 1955), they belong to a group of plants

which possess lacticifers and are also known for drought tolerance, eg., Musa sp.,

Nerium oleander and Hevea brasiliensis.

The inflorescence is a widely branched terminal panicle, 10-60 cm in length with 1000-

6000 flowers. It is polygamous with male and hermaphrodite flowers in the same

inflorescence (Whiley 19S0). Trees may produce a few to thousands of panicles

depending upon genetic and climatic factors and cultural practices þrevious and

current). The number of perfect (hermaphrodite) flowers per panicle varies from year to

year, depending on the location of the panicle in the tree, and by cultivar ranging from

2-75% (Singh lg7ï,Popenoe 1927 and Young and Sauls 1981).

Maximum yields for mango (33 MTlha) are very low compared to other fruit crops such

as apples (112 MT/ha), orange (80 MT/ha) and peach (56 MT/ha) (Schaffer and

Andersen 1994). Yields in the Semi-arid tropical regions of Australia average only (8-

12 MTlha) for Kensington Pride cultivar and (12-15 MT/ha) for Irwin. Inconsistent

production is a major problem for the industry, successive annual yields can fluctuate up

to 150%. As a greater understanding of the effects of environmental factors on

responses of mango trees is gained, opportunities to increase sustainable yield should

improve (Schaffer and Andersen 1994).

Effect of environmental influences on Mangifera indica

The environment exerts a profound influence on the growth and flowering behaviour of

mango trees (Chacko 1989). Cull (1987) proposed that to develop consistent "normal"

phenological growth patterns that predispose towards high cropping performance of

mango, the tree requires strong and consistent climatic stimuli. These growth and

8

flowering patterns are easily affected by seasonal irregularities in temperature and

rainfall. High temperatures (>301250C -DayÀtright), high relative humidity and soil

moisture levels nearing field capacity generally lead to erratic flowering and excessive

vegetative growth (Whiley et al. 1989). This can be observed on trees grown in the wet

tropics where these climatic stimuli prevail; under these conditions flushing, fruiting

and flowering will often occur simultaneously on different branches of the same tree.

It appears that climatic requirements for commercial mango production are critical and

severe yield reduction may be linked to these requirements not being met (Chacko

1986). In spite of this, the response of mango trees to climatological factors has received

little attention. Two important aspects of the environment which influence plant growth

considerably are light and temperature regimes yet their influence on mango is poorly

understood.

Shading of leaves under natural conditions has been shown to prevent or delay the

formation of flower buds. (Singh 1971, Toohill 1984). Light perception by mature

leaves is essential for induction of floral primordia in mango buds (Schaffer et al. 1992).

However the relationship between light and flower induction is not clearly understood

due to the influences of other environmental factors involved in flowering such as

temperature and water stress.

Early studies of the photosynthetic response of mango leaves to incident light energy,

grown in containers (Schaffer and Gaye 1989), field grown during winter (Whiley,

personnel communication) and grown in poor soils in Florida (Schaffer and Gaye 1989)

revealed that the light saturation point in mango was generally low at around 630 pmol

quanta m-ts-t. However during the summer light saturation occurred at a PPF of about

1200 ¡rmol quanta m-rs-r (Whiley, personnel communication). Restricted root growth

and partial photoinhibition during the winter are possible as causes for these

discrepancies in light saturation level, both factors have the effect of reducing the

photosynthetic capacity of the plant (Arp l99I: Pongsomtroon et al. 1992).

9

Mangoes readily grow in all tropical and subtropical climates with a mean temperature

between 24400C, appearing to do best in the regions know as the dry tropics

(Mukherjee 1953). Although mango can grow well in areas with registered temperatures

up to 480C (Mukherjee 1953), cold temperatures limit crop production. Considerable

difference among mango cultivars have been observed in their flowering response to

temperature (Chacko 1939). Cultivar Irwin can initiate flowers at fairly high

temperatures (30/20 "C DayA{ight) when compared to Kensington, Carabao and many

other Indian cultivars (Whiley et al. 1989). Observations in Northern Australia where

the occurrence of low temperature can be erratic have shown that Irwin initiates flower

buds under higher DA{ temperatures than those required for Kensington (Chacko 1989).

Mango production occurs in areas of both high and low humidity. However there have

been few studies on the effects of relative humidity (RH) and vapour pressure deficit

(VPD) on physiology or growth and development of mango (Schaffer and Andersen

ree4).

All the various environmental factors known to have a positive influence on floral

initiation such as water stress, low temperature and atmospheric stress appeat to check

the growth of roots and shoots of mango trees, possibly through reduced carbon

assimilation in the leaves (Chacko 1989).

Leaf gas exchange in Mangifera indica

Leaf gas exchange (LGE) variables which include net photosynthesis (Pn), stomatal

conductance (g,), transpiration (E,) and internal CO, concentration (C') are largely

unknown for mango. LGE provides a basis for understanding limitations to

productivity, especially the impact of drought, and as mango is grown in climates

ranging from the wet and the dry tropics to the subtropics and warm temperate regions

of the world it is to be expected that these parameters will vary widely.

10

Basic photosynthesis studies on mango began in 1934 on the cultivar Carabao (Agati,

1937). This initial study determined that leaves did not begin photosynthetic activity

until they were at least 15 days of age and assimilation rates were greatest during the

morning whilst declining during the afternoon probably due to external factors.

However it is only recently, with the advent of portable infra red gas analysers (IRGA),

that more intensive leaf gas exchange studies have become possible. Detailed studies of

mango photosynthesis has only happened in the last ten years. It is now possible to

report with confidence maximum photosynthesis rates (P,nu*, pmol m-' s-t) of single,

well-lit leaves in various environments. With potted trees and to some extent with trees

growït in calcareous soils "which restricts root growth" in southern Florida typical P,ou*

values of 7 pmol m-' s-' have been reported (Schaffer et al. 1986, Schaffer and Andersen

1994).In a low stress environment such as Southern Queensland, however, a higher

average P,nu* of I4-I5 pmol m-2 s-t in summer has been found \t/ith P.u* in winter falling

to 7-8 ¡rmol m-2 s-' presumable due to partial photoinhibition when the temperature

drops below 10-120C (Wolstenholme and Whiley 1995). In the monsoonal tropics of

Northern Australia wet season rates reach l4-I5 ¡rmol m-2 s-r dropping below 10 ¡.rmol

m-' s-t during the hot dry season (Chacko unpublished data). Some recent studies

(Schaffer et al 1994) have shown that Mango has a photosynthetic potential similar to

that of citrus, avocado and macadamia "for single well lit leaves". The leaves also have

a high stomatal density which under conditions of high aperture increases the potential

for rapid gas exchange. However, in reality, on a canopy basis the actual rate of

photosynthesis will be greatly decreased by the high proportion of shaded leaves as well

as by the presence ofa high proportion ofolder, less efficient leaves as a consequence of

the longevity of leaves (Wolstenholme and Whiley 1995). During the long dry season in

the monsoonal tropics monthly average daytime temperatures in the 24300C range are

accompanied by high night time temperatures of 15 -180C. As a result both elevated

photorespiration and dark respiration drastically reduce net carbon gain. Possibilities of

photoinhibition in stressful environments may also exist (V/hiley et al. 1996). Carbon

gain during dry season conditions may therefore be restricted by environmental stress.

1l

Understanding leaf gas exchange and the influence of various external environmental

factors influence is important to the understanding of the ecophysiology and the

formation of yield in any crop. Limitations to the growth and yield of mango trees

brought about by the atmospheric environment and its interaction with soil water status

are poorly understood. The effect of the environment on leaf gas exchange will be

examined in order to define limitations to Pn, which will help in defining limitations to

productivity.

Influence of Water

Humidity

Alphalo and Jarvis (1991) consider VPD to be the best measure of humidity as it is

more closely related to evaporative demand thano/o relative humidity (RH) which has a

temperature dependent relationship with evaporative demand. In most cases, when VPD

is kept constant g, shows an optimum response curve to temperature. (Pearcy, 1977;

Roessler and Monson 1985). Pn and gs response to leaf temperature are essentially

identical, however in some cases Pn and g, differ in their response at high temperatures

(Pearcy, 1977; Roessler and Monson 1985). Differences in response apparently due to

Pn being independent of VPD but declining as leaf temperature increases and g, being

independent of leaf temperature but declining as VPD increases have been observed in

Yucca glauca (Roessler and Monson 1985). Different expressions for humidity and

different mathematical functions have been used to describe the response of stomata to

humidity, including RH at the leaf surface (Ball et al., 1987), or the external

environment (Harley et al., 1992) and vapour pressure difference between leaf

temperature and vapour pressure in the air (Lloyd et aL.,1991).

Chacko (1986) found transpiration to be correlated with both RH and VPD for mature

mango trees. Increases in VPD (1 .5-2.5 kPa) and temperature (29-340C) were observed

to cause increases in g" on both fully expanded young and mature leaves under field

Idt:,Û

i

t2

'lil,,lj

,t

conditions (Reich and Borchet 1988). In contrast in containerised trees Pongsomboon et

al. (1992) found that g, was inversely corelated with VPD at both low temperatures

(15/100C, daylnight) and higher temperatures (30/200C). At similar levels for both VPD

and RH, g, and Pn levels were found to be greater for leaves exposed to 301200C than to

15/100C. It was therefore concluded that VPD and temperature independently influence

g. and Pn in mango.

Diurnal leaf gas exchange patterns within mango canopies have not been extensively

studied. Chacko (unpublished data) found during the dry season (high VPD) that Pn was

significantly lower than during the wet season (low VPD) in freld studies on Irwin,

Tommy Atkins, Strawberry and Kensington cultivars. Presumably this was due to the

effects of water stress. Studies on potted Nam Dok Mai trees found that g, responded

more to temperature-induced changes in VPD than to PPF (Pongsomboon et al. 1992)'

As PPF increased ì.¡1r was relatively constant regardless of temperature. At lower

temperatures (15/100C, daylnight) compared to higher temperatures (30/200C ) reduced

Pn rates was related to decreased g, at each PPF. This was partially due to decreased

VPD at the lower temperature (Pongsomboon ¿/ al. 1992)'

Soil moisture

The major limitation to photosynthesis caused by plant water deficits above a relative

water content of 70 o/o is due to stomatal limitation of the conductance of CO, to the

mesophyll cells and this limitation is readily reversible when plant water status

increases (Kaiser 19S7). He further surmises that at a relative water content between 30

and 70Yo the reduction in photosynthesis is due to inhibition of enzymes involved in

photosynthesis because of the increased concentration of ions within the cell (Kaiser

1987).

By increasing CO, concentrations to 1900 pL L-r no difference in Pn between droughted

and irrigated sunflower plants was found although gs was lower in the droughted plants

tI

l

!

13

I

(Wise et at. 1990), whereas Pn of droughted plants at ambient CO, concentrations of

340 ¡tL L-t was in all cases lower (Quick et al. 1992).Thus it was concluded that

limitations of Pn in droughted plants imposed by the stomata could be overcome by an

increased ambient CO, concentration (Wise et al. 1990). Similar observations on Pn

have been made on droughted Lupinus albus, Helianthus annuus artd Eucalyptus

globulus when exposed to high CO, concentrations, however lower Pn was observed in

Vitis vinifera (Quick et al. 1992). In severe water stress, when leaf water potential (y')

was below -1.8 MPa it was observed that the decline in Pn could not be overcome by

increasing CO, (Boyer I97l).

Methods used to calculate stomatal limitations to Pn are discussed by Jones (1985)

These methods usually rely on the relationship between Pn and internal COt

concentrations (Ci), that is lower Pn at a particular Ci value is assumed to be due to a

direct limitation on Pn. However the problem with this approach is the assumptions in

the calculation of Ci . C, is proportional to the ratio between Pn and g. (C, : Ca-l.6PrV g,

X Ca: ambient CO, concentration.) but the relationship between Pn and g, in non linear

(Terashima et al. 1988). Two values of Pn and g, (Pn1, gsl and Pn2, gs2) will

correspond to Ci values of C,l and Cl2. This range of values can occur in the same leaf

cuvette if there is non-homogenous stomatal opening. If they are averaged before Ci is

calculated, eg Ci3 :(Pn1+Pn2)/(gs1+gs2), then Ci3 is not necessarily equal to (Cil +

Ci2)12 (Bunce 1988). If some stomata in the cuvette are open and some closed then Ci3

will be much lower than (Ci1+Ci2)12. This will lead to a decline in the apparent

relationship between Pn and Ci such that in droughted plants the Pn at any Ci will

appear lower suggesting a direct limitation of Fn. Applications of ABA via the Xylem

stream have been shown to cause non-homogenous stomatal opening (DownÍon et al.

1988: Terashima et al. 1988) thus putting some doubt to the arguments that changes in

the relationship between Pn and Ci due to a consequence of soil drought as proof that

drought directly limits Pn. It has yet to be established as to whether non-homogenous

stomatal opening occurs when plants are droughted as a result of ABA production,

guard cell water status, or as a result of high VPD. Although Sharkey (1990) suggests

!

14

low air humidity and low soil water potential can cause non-homogenous stomatal

opening.

ABA production may result from non hydraulic signals originating from the roots of

droughted plants (Incoll and Jewer 1987) other signals include cytokinins and changes

in the mineral content of xylem sap (Gollan et al. 1992). The role of signals originating

from the roots of droughted plants in controlling LGE has recently been reviewed

(Incoll and Jewer 1987: Davies et al. 1990: Davies and Zhang I99l), indicating that non

hydraulic signals may be involved in stomatal closure observed in droughted plants.

This field of study has not been conducted on Mango.

Studies on the soil water relations of potted mangoes have revealed a number of

adaptive characteristics. A loss of turgor of potted Kensington trees occurred at a Vr of -

1.75MPa (Pongsomboon 1992), with permanent damage to leaves occurring at ry, of -

3.45 MPa. This indicates that mango leaves tolerate less internal leaf water stress than

other fruit trees such as orange, macadamia and custard apple where permanent wilting

occurs at -6.6,-5.0 and -3.8 MPa respectively (Fereres et al. l9l9; Stephenson et al.

1989; and George and Nissen 1992). Wolstenholme and Whiley (1995) hypothesised

that mango leaves maintain turgor through a much slower development of internal water

stress using stomatal regulation and probable osmotic adjustment. The osmotic

adjustment possibly being significantly influenced by the network of resin ducts

permeating the tree. From the studies it appears ry, is not a good indicator of the onset

of water stress in mango as with many other species (Kaufmamt 1976: Steme et al.

1977). Atthough Pongsomboon (1992) found a linear correlation between g, and r.¡r, it

was a much slower response than in other tree crops. Reich and Borchet (1988)

conhrmed under field conditions that stomatal regulation in mango signif,rcantly

reduced the rate of development of internal water deficit compared to other species. In

containerised mango trees g, was also sensitive to water deficit and declined as ryt

decreased (Yan and Chen 1980). Measurable changes in g, may therefore be useful in

determining the early onset of moisture stress in mango.

15

Conclusions

Controversy exists as to how plants sense water deficits in the atmosphere and in the

soil and how best to measure the early onset of water deficits in plants. Pn limitations

can occur as a result of low supply of CO, as a result of stomatal closure. High VPD and

temperature increase stomatal limitations, this limitation increased with soil drought.

Mangifera indica appears to have considerable genotypic diversity in its response to

environmental variables on physiology, growth and production. Mango trees are

considered drought tolerant due to the maintenance of high water status rather than

resistance of tissue to damage under conditions of water stress. The maintenance of Pn

as stress develops, through gradual reduction of g,, is likely to contribute to the

productivity and survival of mango dwing periods of drought (Schaffer et al. 1994).

Objectives

In order to find ways of better exploiting the existing genetic potential for yield which,

in most crops is largely unrealised (Boyer 1982), this study is concentrating on the

photosynthetic response of the leaves of several mango cultivars to changing

environmental conditions, during different periods of tree growth. Defining the impact

of the atmospheric environment on LGE in Kununurra should help elucidate some

limitations to productivity. Also, understanding the response of LGE of mango leaves to

soil drought will assist in determining its likely effect on yield.

l6

,, MATERIALS AND METHODS

2.1 Location

2.1.1 Experimental site:

The experiments were conducted at Kununurra, 'Western Australia on the Ord River

Irrigation Area (ORIA), (latitude 15"42'5, longitude 128"36'E).The site is on the

horticultural research block, Agriculture 'Western Australia King location 318. The

orchard consists of a range of mango cultivars planted at a 10x 9 m spacing in a

randomised block design. The trees were planted in 1984 and have reached

approximately 4 metres in height and a diameter of 5 metres.

2.1.2 Climate

Kununurra is part of the seasonally wet - dry northern Australian semi arid tropics with

an average annual rainfall of 787 mm. This is somewhat lower than in most monsoonal

areas of the world and temperatures are somewhat higher (Table 1).

Table I Average monthly temperatures, rainfall, relative humidity and evaporation on

the Ord River Irrigation Area.

Feb Mar Apr Mqy Jun Sep Oct Nov Dec

Mean Max o

Mean Mino C

Rainfall (mm)

Humidity (9.00

am)

Evaporation (mm)

24.4

190 5

65

242

201.9

68.5

35.5

23.4

114.8

606

35.1

20.1

44 I

44.3

l',t.9

12.0

36.2

155

0.6

32.5

19 0

36

342

38 5

22.8

23.3

39.0

24.8

123.8

55.8

787

15.3

3.2

357

t 4.t

6.5

31.9

38.9

24.5

63.1

47.7

230 116 195 217 2ll 203 209 241 294 330 287 275 2869

The wet season extends from early December to the end of March. The distinct long dry

season extends from April until November. Temperatures rise sharply in October

t7

(beyond 40" C) whilst often the humidity is extremely low thus creating a very high

vaporü pressure deficit. Some rainfall occurs in October and November and these

months are considered the build up to the wet season. October and November also

coincide with the fruiting period of Mangoes on the ORIA.

2.t.3 Soils

The soils at the experimental site are a deep sandy red earth known as Cockatoo sands.

These have been deposited adjacent to sandstone ridges. The depth of the sand over a

gravel bed varies; however in this site it would average 1.2 to 1.5 meters. These soils are

characterised by a very low water holding capacity and very low nutrient status with a

pH around 7 .0 to 7 .5.

2.1.4 Experimental trees

The trees used for all experiments are part of an Agriculture Western Australia

replicated cultivar trial, which was planted in 1984. Detailed yield, flowering and

growth data have been recorded from these trees over the past 1l years (Table

2.1,2.2),(Fig 1). The following three cultivars were selected for the present experiment

I Kensington;lhe most important commercial cultivar in Australia.

2lrwín; a consistently high yielding and regular bearing cultivar in this environment.

3 Tommy Atkìns; an important cultivar in many countries which has performed very

well in the ORIA.

Yields of these varieties where comparable with similar yields recorded in the Northern

Tenitory under similar environmental conditions. However Kensington yields are

somewhat lower than trees of comparable ages in Queensland and Carnarvon WA.

l8

V/ith the extensive yield, growth and flowering data (Table 2.1,2.2), (Fig 1) aheady

available on the individual trees selection of treatment trees \¡/as on uniform

performance. All trees were grafted onto cultivar Sabre as a rootstock.

Table2.I Mean yield (kg) for cultivars Kensington, Irwin and Tommy Atkins at

Agriculture Vy'estern Australia's trial site OzuA on cockatoo sand, planted in 1984.

Means of 4 replicates and LSD (P:0.05) are shown.

* Data Agriculture Western Australia.

Table2j Date of full anthesis for cultivars Kensington, Irwin and Tommy Atkins

at Agriculture Western Australia's trial site ORIA on cockatoo sand, planted in 1984.

Means (tse) of 4 replicates and LSD (P:0.05) are shown

1-Sep t 3.8N/A3-Aug t 2

23-Jul t 0.013-Aug t 0.028-Jul t 0.0N/A6-Aug t 0.025-Jult 7.66-Aug9.9

13-Sep t 0.0N/A1-Aug t 0.023-Jul t3.713-Aug t 0.04-Aug t 0.0N/A23-Jul !9.230-Jul t 0.07-Aug13.1

7-Sep t 3.1

N/A'1-Aug t 2.925-Jul ¡ 1.7

13-Aug t 0.728-Jult 0.0N/A17-Jul x 4.1

29-Julx2.63-Aug7.8

19871988198919901991

'1992199319941995Grand MeanLSD P=0.05 (days)

Tommy Atkins (days)Irwin (days)Kensington (days)Year

Data Agriculture Western Australia.a

198719881989199019911992199319941995Grand Mean (92-95)LSD P=0.05

29 t 10.262 r 18.976 t25.4116 !23.497 ! 14.178 !9.7130 i 32.991 !27.4155 r 52.8110.270.7

14 !2.000064 ! 4.376 r 10.964 r 9.1

43 !6.298 r 14.07033.6

22 ! 5.258 ! 14.140 r 16.393 ù 20.378 t 13.372 ! 19.168 r 16.6121!32.7165 È 50.9101

63.0

Kensington (Kg)Year Irwin (Kg) Tommy Atkins (Kg)

t9

oooÊ-t)

oO

1200

I 000

800

600

400

200

+Tommy Atkins

-+ Kensington+-lMin

84 85 86 87 88 89 90 9f 92 93 94 95

Year

Figure 1 Growth rate (trunk circumference mm) of cultivars Kensington, Irwinand Tommy Atkins at Agriculture'Western Australia's trial site ORIA on cockatoo sand.

A total of six trees (two trees of each cultivar) were used for the experiments. All trees

were of the same age and trees within each cultivar were of comparable height and

diameter. Due to the time required to conduct each measurement it was determined that

it was not possible to use 3 reps of each cultivar as originally proposed. This would have

created a time difference of 1.5 hours between the first and last measurement in each set

creating large variation.

The trees were fertilised regularly with a once a year application of Phosphorus at (320)

g P and gypsum totalling Qaq e Ca per tree. Twelve applications of KNO, totalling 480

g N and 250 gK, six applications of Zn and Cu totalling 40gZn and 40 g Cu were also

applied per annum.

The trees were irrigated from July till December and from March till April totalling 37

kilolitres per tree per annum (Inigation was scheduled using evaporation X crop factor

of 0.9). Trees are droughted during May and June to minimise the risk of a vegetative

flush during these months.

20

)', Methods

2.2.1 Gas exchange measurements

Net photosynthesis (P"), transpiration (E,) and stomatal conductance (9,) wele

monitored on clear days using a portable photosynthesis measuring system ADC LCA3

(Analytical Developments Company, Hoddeston, Herts, England) Infra-Red Gas

Analyser (IRGA) fitted with a Parkinson leaf chamber.

Air and leaf temperature, photosynthetic photon flux density ( PPFD) and humidrty of

the air, were recorded simultaneously with the IRGA during each gas exchange

measurement. Air temperatures and humidities were used for calculating vapour

pressure deficit.

Use of the ADC-LCA3 in high temperatures created some problems, a distortion giving

higher internal and external temperatures would occur. To minimise this problem the

handle of the Parkinson leafchamber was insulated with polyurethane foam to reduce

excessive temperature rises whilst the handle was exposed to the sunlight during

readings. Flow rate of air through the ADC LCA3 was increased from 300 ml/min to

500 ml/min. This was considered necessary as the internal temperature of the ADC

LCA3 would rise too quickly and influence the readings.

2.2.2 Monitoring soil moisture

Aluminium access tubes were installed at two sites located approximately 60 cm from

the trunk of the each tree. These tubes were installed to a depth of 1.8m. The soil

moisture content was monitored using a neutron moisture probe (NMP). The data

2t

collected from the probe counts per 16 seconds was converted into volumetric moisture

content (VMC) in (%). Readings were taken at the soil depths of

20,30,40,50,60,80,100,120 cm. Atthough gravel beds were encountered whilst drilling

several of the access holes, these were at a depth of 140 cm and should not have

influenced the readings at 120 cm. There is however some evidence of permanent soil

moisture at depths in excess of 2.0 metres from test holes drilled in close vicinity to this

trial.

Calibrqtion of Neutron probe readings vs. soil moisture content

Core soil samples from both wet and dry sites were taken using the soil which had been

removed for the installation of access tubes. The soil samples were taken at 20, 30, 40,

50, 60, 70, 80, 100, and 120 cm depth a soil moisture reading was taken at each depth

immediately after the soil sample was collected. Samples were weighed to determine

wet and dry weights, the latter after drying the samples at 700C for 48 hours. The

volumetric moisture content (VMC) of the sample was calculated as the difference

between wet and dry weights as related to the volume of the sample. The results were

then plotted against the corresponding neutron probe reading and analysed using

regression analysis.

The water holding capacity of the top soil layer 0 - 40 cm was lower than at depths

greater than 50 cm as indicated by the slopes ofthe corresponding regression equation

(Fig 2). This is due to the more sandy texture in the upper horizons compares to the

more clayey texture of the sub soil.

22

0-40cm

0.16

o.14

o.'12

0.'l

0.08

006004o.o2

0

0.50.46

o.40.35

0_3

o.25o.2

0.'t 50.1

0.050

R2 = o72

Y=-00O5+0.000026X

1000 2000

R'= 0 58

Y=-1.13+0.00026X

4700

ao

a

4000 5000 6000

5700 6200

aa

a

(J¿

a

0 3000

Probe count

50 - 120 cm

aoa

a

aa a

a(J a

a

4200 5200

Probe count

Figure 2 Relationship between volumetric moisture content (VMC) and neutron

moisture probe readings on Cockatoo sand.

23

2.3 Environmental factors influencing leaf gas exchange and related

parameters.

2.3.1 Experiment I Effect of irradiance on leaf gas exchange

The effect of irradiance levels on gas exchange (Pn, g., E, and C') of the mango cultivars

Tommy Atkins, Irwin and Kensington was measured over three days. Ten recently

emerged leaves approximately three months old were used on each cultivar.

The experimental leaves were numbered and measurements were taken at each

irradiance level in a consecutive sequence starting with full sun then reducing. The

leaves were exposed perpendicular to full sunlight. Reduced irradiance levels were

achieved by placing a muslin cloth over the Parkinson leafchamber. By placing

additional layers of cloth the light levels reaching the leaf were altered to reduce PPFD

by 0.15 pmol m-2s-r for each layer. Approximately 2- 3 minutes were allowed for the

leaf to equilibrate before the data was collected. Readings were taken at each light level

of all leaves before reducing the level. Typically these measurements were taken over a

30 minute period. The experiment was conducted in July 1995 and repeated several

times. Although these measurements were taken during months with a typically high

VPD no attempt was made to collect data under standard VPD conditions. Results were

then plotted as (Pn, g,, Er and C,) vs PPFD using the software minitab for regression

analysis.

2.3.2 Experiment 2 Effect of leaf age on gas exchange

This study was conducted in February and March 1995, which corresponded with a

period of strong vegetative tree growth in Kununurra. Sixty leaves were tagged on the

experimental trees of the three cultivars. On each tree 5 older leaves of approximaÍely 4

months of age were compared with five newly emerging leaves to determine leaf gas

exchange on older and newly developing leaves. Mature leaves selected were exposed to

24

full sun , whilst terminals were selected for the newly emerging leaves that were in a

similar position on the tree and would be exposed to fuIl sunlight during development

all leaves were on the north eastern side of the tree and would have experienced some

mutual shading from approximately 1500 hrs onwards. Leaf length and maximum

width were recorded on the newly emerging leaves corresponding to leaf area, as an

indicator of the rate of leaf expansion. The third newly emerged leaf from the tip of the

terminal bud was sampled for measurements.

To minimise the effect from diurnal variation in Pn, g. and E', all measurements were

taken between 0800 hr - 0930 hrs. Readings continued weekly until there \ryas no

significant difference between the readings from the old leaves and the newly emerged

leaves.

2.3.3 Experiment 3 Effect of seasonal and diurnal variations in Vapour Pressure

defrcit on gas exchange of three mango cultivars

To assess the effect of VPD on leaf gas exchange, measurements were recorded

seasonally and diurnally on the experimental trees from each cultivar.

1. Seasonal measurements of Pn, g. and E, were conducted each month over a

twelve month period. Each data point represents the mean value of a set of

measurements made on ten leaves (5 leaves on each of the two trees). The first fully

expanded leaf from the end of the terminal bud was selected for the measurements.

Leaves were selected only if they \¡/ere exposed to full sunlight. The number of

replicates was deliberately minimised, so a set of readings could be completed in 10

minutes, with recordings on all trees being conducted between 0800 - 0900 hrs each

morning to reduce the effect from dirunal variation in Pn, g, and Er. During periods

when shoots were flowering, fruiting or flushing careful attention was made to collect

data from shoots where these activities were not occurring.

25

2. Diurnal measurements of Pn g. and E, were conducted at times where VPD was

at its seasonal highest during early October and when it was at its seasonal lowest

during February. Additional measurements were taken when the VPD was between

these two seasons during July. Measurements were conducted at 0600, 0800, 1000,

1200,1400 and 1600 hours. The first fully expanded leaf from the end of the terminal

bud was selected for each measurement. These diurnal measurements were conducted

on non-fruiting shoots, even during the fruiting period (October).

2.3.4 Experiment 4 Effect of fruiting on leaf gas exchange

Cultivars Kensington Pride and Irwin alone were used for this experiment. Ten shoots

of each cultivar were tagged at the beginning of floral bud emergence, each shoot bore a

single terminal panicle. Panicles were removed from five of the terminals as flowering

commenced (non-fruiting) and the other five were retained as the fruiting treatment. Gas

exchange was measured on the fulty expanded mature leaf closest to the panicle,

normally leaf position 4 - 6 from the tip, thereby ensuring that the experimental leaves

were located close to the fruit. As the fruit developed it was necessary to support the

fruiting branches in order to maintain the experimental leaves in full sun. This was

conducted in order that the exposure of leaves to sun was comparable between

treatments.

Measurements continued weekly through the flowering and fruit development period.

All measurements were conducted between 0800 - 0900 hrs. At fruit maturity diurnal

measurements were also recorded.

2.3.5 Experiment 5 Effect of soil moisture on gas exchange

The effect of irrigation on gas exchange was studied in 1995 on the experimental trees.

Three of the trees, one of each cultivar, were not irrigated during flowering to harvest.

All the six trees in the experiment were droughted during May and June as this is

26

coÍìmon commercial practice to reduce the risk of vegetative flushes during these

months. Irrigation commenced on the irrigated trees in late July at the approximate tate

of 1500 litres per week per tree was applied in two applications during the week this

would continue until mid/late December. The moisture stressed treatment trees were not

irrigated at a\l. V/eekly measurements of Pn, 8., E, and C, were conducted once the

treatment was imposed, prior to that monthly measurements were taken. All

measgrements were conducted at 0800 - 0930 hrs each time. The first fully expanded

leaf from the end of the terminal bud was selected for each measurement. All

measurements were conducted on non flowering/fruiting shoots to minimise any

influence fruiting may have had on gas exchange. Measurement of soil moisture was

also conducted on the same day with the neutron probe.

27

3 RESULTS

3.1 Experiment 1 Effect of irradiance on leaf gas exchange

The relationship between net photosynthesis (Pn) of the mango leaves and

photosynthetic flux density (PPFD) was significant in all the cultivars examined (Fig 3)'

All the cultivars displayed a typical light saturation curve with no significant (P<0.05)

cultivar differences. In Kensington, Tommy Atkins and Irwin light saturation occurred

at a PPFD between 1250-1750 ¡rmol m-2 s-'1Fig 3). Both on Kensington and Tommy

Atkins there appears to be a decline in Pn once PPFD exceeds 1600 - 1700 pmol m-' s-t

(Fig 3) this however was found not to be significant (P10.05) (Table 3.1, 3.3).

Pn and gs levels measured were extremely low this was particularly evident in

Kensington. As the experiment was conducted during July VPD \Mas reasonable high it

is quite feasible that the stomata on Kensington were partially closed accounting for the

very low readings.

At a PPFD of 500 pmol m-2 s-'Pn was significantly reduced (P<0.05) by approximately

50%o inall cultivars, at light intensities of 100 ¡rmol m-2 s-rPn was close to zero. Log Pn

values were used in the regression curve so as to reduce some of the variation.

Kensington has the lowest Ff value of 0.54. As the Pn levels were extremely low at the

time of the experiment the difference between the lowest value and the highest value

was very small. Similarly Irwin recorded the highest Pn values, and the FÉ value of 0.64

was the highest recorded, as there was a greater difference between minimum and

maximum Pn measurements

E, and g, generally increased as PPFD increased in all cultivars (Fig 4,5) although due

to the high standard error on many of the data points the correlation was very low with

most Ff <0.05. Ci significantly decreased (P<0.05) from approximately 230 (¡rL L-t) to

about 96(¡rL L-') as PPFD increased from 0 to 750 (¡rmol m-t s-t) in Kensington (Table

3.1). All cultivars exhibited a decline in C, as PPFD increased with the minimum Ci

28

being reached at 1500-1700 and 1250-1500 ¡rmol m-2 s-t for Irwin and Tommy Atkins

respectively. Tommy Atkins showed a significant increase (PS0.05) in Ci when PPFD

exceeded 1750 mmol m-2 s-r. Both Irwin and Kensington exhibited a similar trend but

this was not significant (PsO.05).

Table 3.1 Range of photosynthetic photon flux densities and calculated mean

values of data sets for Pn, g,, E, and C, on Kensington cultivar. Means (tse) of replicates

and LSD (P:0 are shown.

Table3.2 Range of photosynthetic photon flux densities and calculated mean

values of data sets for Pn, g,, E, and C, on Irwin cultivar. Means (tse) of replicates and

LSD (P:0.05) are shown.

1750-20001 500-1 7501 250-1 5001 000-1 250750-l 000500-7500-500LSD P= 0.05

0.27 !0.10.20 r 0.00.43 r 0.1

0.33 r 0.1

0.20 r 0.1

0.18 r 0.00.10 r 0.00.21

3.3 r 1.9

5.0 r 3.57.5 r 0.64.3 ! 1.1

3.3 I 1.9

2.5 x 1.00.0 t 0.07.3

1.07 r 0.30.95 r 0.31.39 t 0.6I .13 t 0.61.00 r 0.50.4010.20.40 r 0.00.58

111 + 72137 x52152 ! 18

117 r.149618177 t30237 x 1773.4

Range of photosyntheticphoton flux density (pmolm-'s-t)

Transpiration(mmol m-'s-t)

Stomatalconductance(mmol m-'s-t)

Calculated netphotosynthesis(¡rmol m-2 s-t)

Internal COtconcentration(pL L-')

157 + 6487r58150 t71116+69151x78121 + 108197 x10295.7

0.67 r 0.1

0.8 r 0.20.67 r 0.00.8 t 0.1

0.65 r 0.1

0.6 + 0.20.27 x0.10.33

35 r 8.530 r 10.028 t 8.318 r 4.816 x4.115 r 6.531 1.817.3

3.3 r 0.63.9 I 1.42.6 r 0.62.6 r 0.62.0 r 0.31.4 L0.40.7 !0.21.38

1750-20001 500-17501 250-1 5001 000-1 250750-1000500-7500-500LSD P= 0.05

Internal CO.-concentration(pL L-')

Transpiration(mmol m-'s-')

Stomatalconductance(mmol m-'s-')

Calculated netphotosynthesis(pmol m-2 s-')

Range of photosyntheticphoton flux density (pmolm-'s-t)

29

Table 3.3 Range of photosynthetic photon flux densities and calculated mean

values of data sets for Pr, g., E, and C, on Tommy Atkins cultivar. Means (tse) ofreplicates and LSD .05) are shown.

3.0 t-0.52.6 !.0.44.1 10.92.4 t0.42.1 xO.41.4 +.0.50.07 t 0.31.7

1 750-20001 500-1 7501250-1 5001 000-1 250750-1 000500-7500-500LSD P= 0.05

0.7 r 0.1

0.5 r 0.1

0.5 r 0.20.6 r 0.1

0.5 r 0.1

0.6 r 0.20.2 !0.10.29

31.4 x718.0 r 627.5 ! 10

13.3 r 3.312.9 x36.0 !2.41.7 x1.615.7

201.51 13.5107.21 38.1128.4159.8153.478.7

+13x26t29130x21x32t20

Range of photosyntheticphoton flux density (pmolm-'s-t)

Transpiration(mmol m-'s-')

Stomatalconductance(mmol m-'s-')

Netphotosynthesis(¡rmol m-2 s-')

Internal CO.-concentration(pL L-')

30

A

É=o5a

B

É=o6a

t

lm

nfP(¡rUm¿s)

a

OO

a

.O

a

a

l@

mo(¡umts)

aa

oa

aoa

aa

o807

1061- o.5È

õ(I4Eo.a- 02

aa'

aAo4

^031- 021- ot{oE {,1ë azci ¿¡ä 4.4J ¡.5

oa aa

aa

aa

aa aa

l5m m

2m

aa

1¡¡D

a

oa

2m

0

o

a

aao a

a

aa

0

a

ao

a olo0

5æ oãD&6æm1m12014@1m1mwo6rum¿;)

0

Ð

a

aao

o8

04

02

0

42

46

ê

Ða

a ' oa

a

o

a

1.4

1.2-b 1.0

E.8E.e=.4!l

.2

_o

aao

I o'toloo

.l 0a a

a o

a

aao

o

o

a

a

a

Ð0

a

aa0

100 1m

mnqnu m¿s)

lo

cc

08060402

0

{44.64.8

-l

E

Ê

ÊÀ

J

É=05/ ,a aa .l

12

.. 1o

o' oBEEoôÉ9ol6oz

o0

a,a

a

a

aa

oaa0

aa

a.l

tm

mO<ptUni")

aa aa

o

0a

50

I a

0a

1cI)

m6røm¿;)

ñt50 m 0

Figure 3 Log Net photosynthesis (Pn) as a

function of photosynthetic photon flux densþ(PPFD) in varieties A)Kensington, B)Irwin and

C)Tommy Atkins. R2 values are shown.

Figure 4 Transpiration (Er) as a function ofphotosynthetic photon flux density (PPFD) in

varieties A)Kensington, B)Irwin and C)Tommy

Atkins.

3l

A

O'oo

0oA

n

îlsE¡loE

;5

0

Ijo1-cã'E'0EræE8_ 1t8æFoÉ

ajo96É

€2ÐEræ8ræ8o3EOI

-}¡gJë6É

ã2mItEræÉo:ræoos!o

aaa

ata a a ao aa

tm ín 2m

mopum¿í)

o

aa a

t¡a'¡ a

O

0

0:m lm tÐ M

mo6runf s'¡

ooo t -1.

B7D

^æ-r" 50

É¡þ!æ,>n*ro

0

€0

^sl.loiæE9nú

10

aa

oa

aao

a

Ia oa

aa

a

ao oa

Oa

o aaaa a

mo6"tu"f s'¡

aaa

lm

1@

mD6rrl m¿ir¡

o

a

ooaa

aa

1mÐ N Ð

a OO

lm 1Ð ñn

IfTo6ndmtír)

c ca

a

a

a aa

aoa aO

aaaa o a

oo

a

00

a

¡a

ta a Iaa aa

5@ l@ 1m N

mo(l¡r¡m¿i\

Figure 6 Internal carbon dioxide concentration

(pL L-') as a function of photosynthetic photon

flux density (mmol m-t s-t) for cultivars

A)Kensington, B)Irwin and C)Tommy Atkins.

aa

m

0ao

Figure 5 Stomatal conductance (g,) as a

function of photosynthetic photon flux densþ(PPFD) in varieties A)Kensington, B)Irwin and

C)Tommy Atkins.

tg) 2m

ùtt

32

.,{lttl,ì

3.2 Experiment 2 Effect of leaf age on gas exchange

The trees were closely observed prior to flushing for swelling of the terminal buds and

week 0 was chosen as that time when the buds had emerged and where approximately 2

to 3 cm in length, at this stage the new leaves had not begun to separate'

The leaf size increased rapidly in the second and third week after emergence achieving

full expansion by week 4 (Fig 7). By week five all leaves were fully expanded and no

significant differences was recorded between old and new leaves in any of the parameters

measured.

All cultivars exhibited similar gas exchange pattems during leaf development' Net

photosynthesis measurements from week I till \Ã/eek 3 were close to zeto in newly

emerged leaves. Between week 3 and week 6 Pn values increased rapidly reaching

maximum values 7 weeks after leaf emergence and approximately 3-4 weeks after full

leaf expansion (Fig 8). However, by week 5 there was no significant difference between

the old and new leaves with alt cultivars. As the measurements were taken during

February an upward slope in Pn of the older leaves was observed and maximum values

were measured in week 7. Cultivar appeared to have little influence on levels of Pn

recorded during this exPeriment.

Transpiration and stomatal conductance values were close to zero during week I and 2

for cultivars Kensington and Irwin and began to increase from week 3 (Fig 9 & l0)'

cultivar Tommy Atkins remained at or close to zero until week 4. Maximum values were

recorded in week 7 however there was no significant difference of gs and E, between old

and new leaves from week 5 with Kensington and week 6 with Irwin and Tommy Atkins'

I

!

33

250

E zatÉ

t5n rso

É,9

3 loo

=69so¿

, a

+-KPTA

]rgn

\I¡eekl Week2 Week3 Week4 Week5 Week6 WeekT

Leaf age

Figure 7 Development of leaf length using the means of five leaves

A AT

EoEE

rrl-

456

0

A

E

Éâ.

642

T

ITI¡x

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234 567I

1234Week

x

3

B

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B^301- 25lE 20

õ 1.5

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ëo

È

I

È

tr

¡

.{',IIII

I

r2

0 r23

¡x

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c

Week

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40

^ 35

ca

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ÉzoEtt.:a lor¡- os

00

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06 234567

I

Figure 8 Relationship between leaf age and

net photosynthesis of cultivars A)Kensington, B) Irwin and C) Tommy Atkins

Venical lines indicate SE * indicate

significance at P=g.gt.

Week

Figure 9 Relationship benveen leaf age and

transpiration (E,) of cultivars A) Kensington

B) lnryin and C) Tommy Atkins. Vertical

lines indicate SE. * indicate significance at

p=0.05

r

'r1

34

A

Ð

troÉÉ

à0

IÉ

oÉts

ÐI

Btn

:D

ttlr u

IJ

1234561

¡I

01234567

Week Week

c

-ûfr'E l$o

ts

bf)

-,-ctd....... tÈ

xxx

1234561

x

Week

Figure 10 Relationship between leaf age and stomatal conductance of cultivars A) Kensington B) Irwin

unã ci Tommy Atkins. Venical lines indicate SE. * indicate significance at p:0.05

35

3.3 Experiment 3 Seasonal and diurnal variations in leaf gas exchange on three

mango cultivars

Gas exchange measurements carried out between 06:00 hr and 16:00 hr showed an

increasing net photosynthesis (Pn) in the morning conesponding with an increase in

photon flux density (PPFD), this was exaggerated during the wet season due to early

morning clouds at the time of the 06:00 hr recordings'

During the dry season a mean maximum Pn of 5.4,11.5, and 8.9 pmol m-2 s-' was

measured for Kensington, Irwin and Tommy Atkins respectively whilst in the wet season

1I.2, 11.5, and 11.5 pmot m-2 s-rwas recorded (Fig ll &' 12). The Pn value for

Kensington was significantly lower than that for Irwin and Tommy Atkins during the dry

season, however similar high values for Pn \¡úere measured for all cultivars during the wet

season. The maximum Pn values during the wet season were maintained throughout most

of the day whereas during the dry season a high Pn maximum was maintained for a

period of only three to four hours between 06:0.0 hr and 10:00 hr.

A significant (P<0.05) decline in Pn in Kensington was recorded on day 3 of the wet

season measurements. This coincided with VPD peaking at 2-5 kPa for a short period

around midday. High VPD during the wet season is common due to the absence of cloud

cover. All cultivars in the dry season exhibited a decline in Pn during the diurnal cycle

with minimum values occurring around midday. A decline in Pn at the end of the day is

most likely the result of a decline in PPFD levels this also resulted in a high standard

error for the later measurements which can be attributed to increased shadowing of the

Parkinson leaf chamber as the measurements were taken. However decreases in Pn

observed after 0800 hr during both seasons suggests the influence of environmental

factors other than PPFD such as humidity and temperature on the CO, assimilation of the

mango leaves.

Stomatal conductance follows a similar diumal curve to that of Pn (Fig 13 e A) There is

a strong conelation between Pn and gs in both the wet season and the dry season

36

measurements. (Fig 20). Cultivar Irwin recorded similar gs values during the wet and dry

season however there was a greater diurnal decline in gs in the dry season than in the wet

corresponding to a drop in vPD. Tommy Atkins and Kensington showed a rapid decline

in conductance during the diurnal cycle in the dry season however Kensington

demonstrated a slight rise in the afternoon. No significant difference in gs was recorded

between cultivars during the wet season with the exception of day 3 where Kensington

recorded significantly (P<0.05) lower conductance than Irwin and Tommy Atkins during

the time of high vPD. Interestingly during periods of low vPD (early morning)

Kensington had higher gs than Irwin.

Typical dry season maximum values of E, for Irwin, Kensington and Tommy Atkins were

35, 15 and26mmol m' s-t respectively. Irwin constantly exhibited similar E, pattems for

both the wet and dry season (Fig 15 &, 16), reaching maximum values at 8:00 h and

remaining constant throughout the diurnal cycle. However Kensington' whilst displaying

a similar pattern, had significantly lower E, than Irwin during the dry season but not

during the wet season. Tommy Atkins also recorded lower E, values which became

significantly different (Pf0.05) from Irwin during the later measurements, both Irwin and

Kensington peaked later in the afternoon whilst Tommy Atkins peaked mid morning'

Ci of all cultivars during the dry season generally 220 ¡;Jr-La at 6:00 (Fig 17)' Tommy

Atkins and Kensington both declined during the day 8:00- l4:00 to between 190-160 pL

L-r . However Irwin Ci was stable throughout the dav, significant differences (Pf0'05)

existed between Irwin and Kensington on several occasions' During the wet season

Kensington ci was significantly higher (Ps0.05) than both Irwin and Tommy Atkins (Fig

lg) on many occasions. Kensington Ci was relatively stable throughout the diurnal cycle

between 220-250 pL L-t, Irwin and Tommy Atkins tended to fluctuate more during the

cycle but generallY had lowcr Ci'

37

Þ/1

ððð

ðð

DySeæm

TmHs

ïläwr

.l: : : Ì.'.'. rr.*-.'3'.' r. .... | ì:.¡

Þ/112 l2

l0

I

6

4

EoE

trÈ

6E

IaÈ

ox0

0

2

0

xo

6CO 8(D 10cD 12û 14@ 1ô@

br2

o

6ü 8S lO0 l2m l4:0 160

xxxoo0

14@ 16æ

p

10

A

e.

¡10onEBo

å6Ee

Þ/2

6(D 8CD 10@ 12(I) 14(D lô(D

A Þ/3

ø

EocaEG

6

4

xxo

2ðx

0

a

EoE

È

a@ 8@ lqo 12f! ',14@ 16@

12 q3

a

Ito

ts

oÈ

cG4

+le-r- ls¡r-.r-TA

x fÊlM/o NFftA 4

ðððð xo

'12@ 14@ ',lâ(D

x

ô@ 8@ 10cD 6@ 8q)

Figure 12 Diurnal variation in Net

photosynthesis of cultivars Kensington 'Irwin, and Tommy Atkins for wet season

(Low Vapour pressure deficit (VPD) at

ORIA on cockatoo sand. x indicates

significance between Kensington and Irwin

at p:6.9t ,o indicates significance between

Kensington and Tommy Atkins at p=g'gt'Vertical bars rePresent SE

lo@ 12(lJ

TÍstls

Figure I I Diurnal variation in Net

photosynthesis of cultivars Kensington ,

Irwin, and Tommy Atkins for dry season

(High VPD.) at ORIA on cockatoo sand' xindicates significance between Kensington

and Irwin at p:g.gt ,o indicates significance

between Kensington and Tommy Atkins at

p:0.05. Vertical bars rePresent SE

38

DySæût \lESmÞ/1

I

-{-XP-+Lsin-{-TA

x l(/|ruowlh

rb,l

60 8@

æ

1@

È

II I IT-t' {c

T

8ü lo0 l2m l4:0 lóco

'l\..Il

'i í.s) r

0

0ð

6.C0

I

úmt4ml2m

õ

loæ80

bl2

6ü

br2

È

Ð

1Ð

'fæ

1æ

1C0

s)

e

€0

@

4n

Ð x

xo0

xo

xo

xo

6@

B'3

6

1ÐÈ

È

Ð

8æ 10@ 12@ 14@ 1âû)

100 12rlJ

Tìmll¡

14(D 1ôq)

zÐ

6cD 8@ 10@ 12rÐ 14@ f6@

È/3

14æ 16C0

x

xo

xo

8C06æ0

10cD 12@

Ilmlls

Figure 13 Diurnal variation in Stomatal

conductance of cultivars Kensington, Irwin, and

Tommy Atkins for dry season (High VPD.) at

ORIA on cockatoo sand. x indicates significance

between Kensington and Irwin at p:0'05 ,oindicates significance between Kensin gton and

Tommy Atkins at p=g.gt. Vertical bars

represent sE

Figure 14 Diurnal variation in Stomatal

conductance of cultivars Kensington , Irwin, and

Tommy Atkins for wet season (Low Vapour

pressure deficit (VPD) at ORIA on cockatoo

sand. x indicates significance between

Kensington and Irwin at p:g.gt ,o indicates

significance between Kensington and Tommy

Atkins at p:0.05. Vertical bars represent SE

39

ïtëSeãûtDySeæùt

w1 ry140

leo'E z5

E.oc.9 1.5

tii1.0

o5

00

Èt240

35

^30t25Eõ20É,

815ti r.o

05

00

r l.I\

I.¡-

30

125nE

20

Í-

Ii{

¡'À

'.1

-II

ù" õEE

xrxo00 to

'¡i 10

o5

00

o

I

0 o

6@ 8CO 'loo 12@ 14@ 1ôq)6(D

w2

8@ 10(I) 12@ 14l' 16æ

8co 10(D 12(¡ 14(D 16@

t

EoEE

t¡J

25

20

1.O

05

35

30

1.5

't.0

xo

xo

âcD 8æ 10co 12@ 14@ 16@ 6@

q/3

x0

30

125nE

20

oE 1.5gü 1.0

05

00

Þ/3

eq)

-{-19-r-tuh{-TAx l9two t{'fa

ø

EõEE

uxx

xo

X o

00

80 10@ 12@

T[ÊHs

l4(D 16C0 6m 8@ 10c0 12(l)

TTTEBE

14(D 16U)

Figure 15 Diurnal variation inTranspiration of cultivars Kensington , Irwin,

and Tommy Atkins for dry season (High

VPD.) at ORIA on cockatoo sand. x indicates

significance between Kensington and Irwin at

p:0.05 ,o indicates significance between

Kensington and Tommy Atkins at p:0.05'

Vertical bars rePresent SE

Figure 16 Diurnal variation inTranspiration of cultivars Kensington, Irwin,and Tommy Atkins for wet season (Low

Vapour pressure deficit (VPD) at ORIA on

cockatoo sand. x indicates significance

between Kensington and Irwin at p:0.05 ,oindicates signifi cance between Kensington

and Tommy Atkins at p:0.05. Vertical bars

represent SE

40

ûySeæm

âûl 8CO loco 12@ 14@ 16CD

Èr2

lllÊtSûr

Dq/13D JJ

to

ecoocooooõE

tr

uÐ

JJ1

og

ooÊoo

oofEos

x

0

IJJaa.9

EtrooÊoo

ooG

EoE

æJJ1trodEootoo

o(,)

rEo

Èr'lJJ

co

eÊoocoo

ooõEotr

n1tD

m

1Ð

1@

1@

Ð 0x0

xo o

xÐ

U

3CD

6CO ACO ß@ 12(JJ 14C0 16c)

w2

âo 8@ 10(I) e@ É(D 1ê@

ry3

xxo

X

0

x

0

t@

s

x0o

06@

q3o

8@ lo@ '12@ 14@ 16(D

14C0 16q)

1æ

JJ

cod

oooo

o(,)

GEo

-{-NP-r-li,m-+-TA

x f8tw

-{-NP{-luh-N-TA

x 19tuo {/Ia

0x

0

Figure l? Diurnal variation in Internal

CO2 concentration in cultivars Kensington ,

Irwin, and Tommy Atkins for dry season

(High VPD.) at ORIA on cockatoo sand. xindicates signifi cance between Kensington

and Irwin at p=9.95 'o indicates significance

between Kensington and Tommy Atkins at

p:0.05. Vertical bars rePresent SE

6@ 8cD

Figure l8 Diurnal variation in Internal

CO2 concentration in cultivars Kensington ,

Irwin, and Tommy Atkins for wet season

(Low Vapour pressure deficit (VPD) at ORIA

on cockatoo sand. x indicates significance

between Kensington and Irwin at p=0.05 'oindicates significance between Kensington

and Tommy Atkins at p:0.05. Vertical bars

represent SE

10@ 12@

TrElts

14q) 16@ACD 8cD 10@ 12@

TreÌls

41

The diurnal vPD levels for both wet and dry season follow a similar curve with the

lowest recording early in the morning with a steady rise in vPD until midday followed by

a decline towards evening (Fig 19). The highest reading recorded during the diurnal

studies was 3.1 kPa during the dry season and2.2kPa during the wet. The VPD measured

was still relatively high during the wet as measurements were only made on dry days'

A correlation R2:0.63 exists between conductance and net photosynthesis. A negative

correlation R2:0.50 exists between conductance and VPD (Fig 2l).Leaf temperature (Tt)

is strongly correlated with gs and Pn R2: 0.9 and 0.73 respectively (Fig 22,23). T, or

VpD influenced the decline of LGE on all cultivars at PPFD greater than 1000 mmol m-2

s-r, although the strong relationship between T, and vPD makes it diffrcult to distinguish

between relationships of Tr and VPD with LGE'

âÈ

!

4

3.53

2.52

1.51

0.50

Dey t

@ @

Figure 19 Diurnal differences in VPD for dry season and wet season

*=08

oæ4Ðæ1(D13l1'0gt (rd m''1g)

Figure 20 Relationship between conductance and

net photosYnthesis

005115225335\lÐ(lrh)

Figure 2l Relationship between conductance and

vapour Pressure deficit.

*Wet+Dry

çN

Day 2

@o@o

Dsy 3

1æ

o

a

a

a

I+

+

+

9

I7

6

4o

E

È

I

i'f

-i,

a^ænE@

E.&29

0

ô

*=oÐ a

aa

aa

2

I

42

*=os a É=oæa1Ð

@a

EoÊ

aa

a

EoEç9È

aa

4

5=- ilt+94¡X-1 75)Ë

15nÉæ5ûLdlrpaüæt

Figure 22 Relationship between conductance and

leaf temperature. Each point is the mean of five

replicate leaves

f =oE)

n=-slg-ocpo*+¿ox

lsaBÉ2833S36t¡*sre*æ0q

Figure 23 Relationship between net

photosynthesis and leaf temperature. Each point

is the mean of five replicate leaves

TN óo*=o¡¡

B a

a

2 3 4

Ð

Ðo

m

ña

A

E

E

a

lmoE

o

aaa

0

l0

50

\¡Ð0dà)

aa

aa

oo o

a

05

lvID(dà)

a^ 1501ø

E

El0É

Ð50

É=oat

c

a

0

a

50

\iID(Hà)

Figure 24 Correlation between conductance of A Kensington, B Tommy Atkins, C Irwin and seasonal

changes in VPD

All cultivars exhibited an inverse linear relationship between conductance and VPD (Fig

24). Kensington has the highest linear relationship at RÍ : 0.59 however both Irwin and

Tommy Atkins had only a slight correlation of RÍ :0.47 and F: 0'33 respectively'

43

Seasonal variation

Pronounced seasonal variations in Pn, gs and E, of all the mango cultivars were observed

(Fig 25). Seasonal Net photosynthesis recordings varied greatly between cultivars' All

cultivars peaked during March, Kensington recorded the highest Pn at 12'6 pmol m-' s-t'

and was significantly higher than Irwin with 10.7 pmol m-2 s-r lFig 25), Kensington

however exhibited a rapid decline in net photosynthesis from April to May' A decline in

Pn was also observed in Irwin and Tommy Atkins but not to the same extent as to that

observed with Kensington. From the month of May through to October' Kensington had a

significantly lower (Pf0.05) Pn than Irwin and Tommy Atkins' with an average Pn of

around 4.0 ¡rmol m-t s-' this represented a three fold decline in Pn from March' Irwin

whilst declining in Pn from April to October was still photosynthesising at an average

rate of 8 pmol m-' s-', a decline of only two units from its March peak and remained

consistent throughout the year. Tommy Atkins Pn declined continuously from April to

September reaching a low of 4.0 pmor m-2 s-t which was still significantly higher

(p<0.05) than Kensington's lowest rate at 2.8 ¡rmol m-2 s-r also occurring in September'

All cultivars experienced an -increase in Pn from October through to December'

Kensington having the greatest increase from 2'8 to 5.3 ¡rmol m-2 s-t '

Seasonal variations in gs for all cultivars displayed a similar pattern to seasonal Pn'

Kensington reached its maximum gs during March at 250 mmol/ m-2 s-l significantly

higher (Pf0.05) than Irwin and Tommy Atkins both at approximately 200 mmol/ m-' s-t'

All cultivars experienced a decline in gs during April and May although from May till

August Kensington had a significantly lower (P<0'05) gs than Irwin and Tommy Atkins'

Lowest gs values occurred for all cultivars during september.

E, peaked during March for Kensington and Irwin at approximately 3'5 mmol m-2 s-r and

April for Tommy Atkins at 4 mmol m-r s-r. Kensington E, rate declined dramatically

during the Dry season to a mean level of approximately l'mmol m-t s-t before slightly

recovering in october to December. Irwin however maintained a significantly higher

(Pf0.05) E, level of 2 mmol m-' s-t during the dry season'

44

¡.

ots

É

A14

't2

l0

I

6

4

2

0

¡

- . :i""'':1- .:: I. . ..

-¡- K€nsington

- ô - lrwin- -,r. -.T.Atkins

¡ KP/IRWo KP/TA

ã.-T ,l

-.tT.¡.¡ E

xðx ðððð

EEËã9Ëåäå8å8o

?i..

fd

a.'t'

hafl300

250

200

B

oE¿

Ð

150

100

50

0

'¡- -.&---r-.8 "*'E -'"!'::

t,'4:

õ

Ê!bh!o>ÐoEEËåsE=ãa

Nirlrr

oz

ãx

a.

!r'

'I-

ooâ

cIrr45

4.0

3.5

3.0

2.5

2.O

1.5

1.0

0.5

0.0

TI.I

-r.ìÉ

oE

f¡

r?,¡-tI\

III

c\

xoxo ððððð

cÐ=å>o>èPÊ!à!EEËo*ÊEEãöoza

Figure 25 Seasonal variations in A) Net photosynthesis (Pn); B) Stomatal conductance (gÐ; C)

Täspiration (E,); of mango leaves , of cultivars Kensington, Irwin and Tommy Atkins, at oRlA on

cockatoo sand. x indicates signiftcance between Kensington and Irwin at p:6'gt 'O

indicates significance

between Kensington and Tommy Atkins at p:g'gt'

In = irrigated, Nir : No Inigation. fl : flowering, fd = fruit development, ha : harvest' Vertical bars

represent SE

45

dÈ-va

5

4

3

2

I

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Figure 26 Seasonal changes in vapour pressure deficit' Measurements calculated from seasonal data using

the mean VPD at 9.00 am for each month'

An obvious low in gas exchange in september coincides with a peak in vPD (Fig 26) A

sharp decline in April also coincides with an increase in VPD, although Et in Tommy

Atkins actually peaked in April at a time of high vPD. Both Kensington and Irwin

showed declining E, at this time. Although inigation was switched off in April the soil

moisture levels were still high initially, due to seasonal rainfall' No increase in LGE was

observed when inigation commenced in July'

46

3.4 Experiment 4 Effect of Fruiting on Pn, gs and Et

The gas exchange of the fruiting shoot leaves tended to decline as the sink demand of the

fruits presumably increased with rapid fruit growth'

Figure 27 indtcates that Pn rates of leaves on fruiting shoots were significantly lower than

those on non fruiting shoots during the later season (corresponding with the period of

rapid fruit frll) from week 9 onwards. Prior to this, from the deblossoming treatment

being apptied in week 0 till week 7, Pn rates were also lower in fruiting shoots but this

was not signif,rcant. There was no difference in Pn, gs, and Et prior to deblossoming' Pn

rates of fruiting shoots were significantly lower over the diurnal cycle just prior to harvest

(Fig 28), although the observation for Kensington was not significantly different at 16:00

hrs this was most likety due to lower PPFD levels. An overall decline in the Pn rate was

observed in Kensington with the maximum rate of 4'6 pmol m'2 s-t being observed in

week I whereas in Irwin the maximum rate of 5.7 ¡rmol m-2 S-t was not observed until

weeks 9 and 13

Stomatal conductance levels showed a correlation with the Pn rates in the fruiting and

non fruiting shoots of both Kensington and Irwin (Fig 29 ). Lower levels of conductance

were observed in fruiting shoots. However the difference was not significant' During the

study period vPD was steadily increasing towards the end of the dry season, this

contributed to very low gs levels. The high standard effor recorded during this period can

be attributed to the degree of accuracy of the IRGA when recording very low gs values'

this was particularly evident in Kensington. No significant difference could be found

between gs of fruiting and non-fruiting shoots over the diurnal cycle (Fig 30)' Although

gs levels were slightly lower for both varieties early in the morning'

E, on fruiting and non-fruiting shoots of both Kensington and Irwin appeared to be at

similar levels (Fig 31). E, levels in Kensington was lower on fruiting shoots during week

7 andg however this difference was not significant. Fruiting shoots of Irwin showed a

decline in E, from week 9 and was significantly lower in week l1' However during week

47

13 E, levels showed a rise to be similar to those of non-fruiting shoots' Kensington also

experienced a rise in E, of fruiting shoots in week 1 l ' At fruit maturity the E' levels

appeared very similar in both the fruiting and non-fruiting shoots of each variety over the

diurnal cycle (Fig 32). Although at 14.00 hrs when vPD was at its greatest a noticeable

decline in E, on fruiting shoots was observed on both varieties however the difference

was not significant.

During the experiment a progressively yellowing of the leaves was observed on the

fruiting shoots but not on the non-fruiting shoots this was most evident with Kensington'

A

0E

4

À2

-+-lttfilþ

-r-RilþItl

l3579ll13

trI

135791113

ìWeek \IËek

Figure 27 Net photosynthesis Pn of mango leaves of cultivars A) Kensington and B) Irwin' between

fruiting and non fruiting shoots Vertical barslndicate standard eror [ * ] indicate significant difference (P

f 0.0s).

8:00

t.I

I l:00

[-]

l4:00 I 6:00

t.l

oË

É

6

5

4

3

2

['] IKP Non Fruiting

lKp Fruiting

EIrwin Non Fruiting

Etlrwin Fruiting

0

Figure 28 Net photosynthesis (Pn) of lango leaves fruiting and non fruiting shoots for cultivars

Kensington and Irwin over a diurnal cycle. Vertlcal bars indicate standard error [ * ] indicates signifrcance

( p < 0.05)

48

BA

'É

E

Ð

l) Eo

E

Ð

I + lùúiti!-+ Fnilig

l3s79ll l3579llIlrù'þek\ùr€ek

Figure 29 Stomatal conductance (gs) of mango leaves of cultivars A) Kensington and B) Irwin' between

a,îitirrg and non fruiting shoots vertiãal bars indicate standard effor [ + ] indicate significant difference

G < 0.0s).

45

40

351- 30qÉ t<

E20.:? 15øÞ0 lo

5

0

IKP Non Fruiting

lKp Fruiting

tr Irwin Non Fruiting

@Irwin Fruiting

8:00 I l:00 14:00 16:00

Figure 30 Stomatal conductance (gs) of mango leaves fruiting and non fruiting shoots for cultivars

Kãnsington and Irwin over a diurnal cycle. Bars indicate standard error.

49

AB

o

Þ

lll

08

--

õE

ú

20

IJ

05

{-Nülftjli!-ùFrilirg

135't

V'bek

9ll13 1357rv\êek

9ll13

Figure 3l Transpiration E, of mango leaves of cultivars A)Kensington and B) Irwin, between fruiting

anã non fruiting shoots Verticàt b.r indicate standard error [*] indicate significant difference (p < 0.05)'

Ø

o

r!

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

8:00 I l:00 l4:00 l6:00

Figure 32 Transpiration (E,) of mango leaves fruiting and non fruiting shoots for cultivars Kensington

and Irwin over a diurnal cycle. Vertical bars indicate standard error .

lKPNon Fruiting

lKp Fruiting

trIrwinNon Fruiting

@Irwin Fruiting

50

t/ßB

v\ tl :RSi ./.

$'l

3.5 Experiment 5 Effect of soil moisture on Pn, gs and Et.

Irrigation ceased from April until early July in line with standard commercial practice'

During the second week of July when inigation commences on coÍtmercial properties the

treatments were imposed. In treatment 1 irrigation commenced in line with commercial

practice and continued throughout fruit development. In treatment 2 no irrigation was

given and trees were water stressed throughout the fruiting period.

There were no significant differences in gas exchange between treatments prior to

inigation commencing in July (Fig 33,34,35)'

A gradual decline in Pn was observed in Kensington over the study period' Once the

treatment commenced a sharp decline in Pn was observed in Kensington on week 3 but

this was not maintained and a steady decline continued in both treatments until week 13'

From this point a decline continued in the non irrigated treatment and a significant

increase in Pn occurred in the inigated treatment (Fig 33). A sharp decline in Pn for

Irwin was observed in July when inigation commenced, however this was not related to

the inigation as it was observed in both treatments. A significant difference between

treatments was observed in Pn for Irwin from week 11, and Kensington from week 13' A

slight increase in Pn for the inigated treatment (Kensington) was observed at this time

which was not apparent with lrwin. Pn in Irwin appeared to decline at a slightly greater

rate in the droughted treatment from week 11 onwards than in the irrigated treatment.

From week 3 onwards after the treatment was applied stomatal conductance levels were

significantly lower in the stressed treatments for both Kensington and lrwin. Although the

response from the treatment appeared to be relatively quick the difference between

inigated and non inigated treatments appeared to remain constant throughout the trial

even whilst the water volume in the soil was steadily decreasing in the non irrigated

treatment.(Fig 36)

5l

A significant consistent decrease in transpiration was observed from week 9 in

Kensington and from week 1l in lrwin. Although this was maintained over the

observation period the reduction in transpiration was only slight in the non irrigated

treatment for both cultivars and did not appear to increase until week l5 when Irwin had a

decline in transpiration in excess of 0.5 mmol m-' s't'

Internal concentrations of CO, were slightly lower on the stressed trees however this was

not significant p:0.05 (Table 3) apart from week l5 when CO, concentrations fluctuated

between 167 - gg pL L-' and222-147 p,LL-t on droughted and inigated Kensington trees

respectively. whilst Irwin fluctuated between 221-135 pL L-' añ 235-169 pL L-t on

droughted and irri gated treatments respectively'

productivity of non inigated trees was observed to be much lower than irrigated trees

(Table 4) although it is not possible to determine if these values are significant. Non

inigated Irwin yielding 45% less than the inigated treatment and Kensington some 25olo

less. Fruit size on the Kensington was noticeably lower on the non-irrigated treatment

however there was little difference between treatments in Irwin fruit size.

52

iii,

ütI

A

xxx

I7654321

0

1-

EoEl-ÉÈ

Month

c?)

Week

o

Week

Þ- O\ cl lô

B12 +Inigàd

*Nonlrrigatedx SignificancB

o\

x x

10

o

or

I6

4

2

0 r-

x

=àÈså>(.)

a

Month

Figure 33 Net photosynthesis pn of mango leaves of cultivars Kensington and Irwin, between inigated

and non inigated trees. úertical bars indicate standard error, * indicate significant difference (p<0'05)'

I

53

A

90

80

-70-- 60'E

50

E409308o 20

l00

xxÞ- o\

xxxxx

Month

+Inigated*Nonlrrigatedx Sigrificance

Week

1ñ120

ÇræI EsooEooro 40

20

0xxxx

cî l.-

xx()

âEÀ>l6

>'

Month Week

Figure 34 Stomatal conductance gs of mango leaves of cultivars Kensington and lrwin, between irrigated

onã no' inigated trees, * indicate significant difference (p<0.05).

r¡

54

3.5

3.01-

2.5I! z.o

oÉ t.s

oÍ 1'o

0.5

0.0

otr

f¡l

nfrr rrr

Week

cô

x

o\

x

r-

x x x

oÈ ó

z

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Month

=à9àLGHÉÈ=.=Ã

Month

+Inigated*Non inigated

x Significance

xx xx

aô

Week

Þ- o\

Figure 35 Transpiration E, of mango leaves of cultivars Kensington and Irwin, between inigated and

non irrigated trees. veftical bis indicãte standard error * indicate significant difference (p<0'05)' nlln :

Both treatments were not irrigated' In: treatment I inigated'

II

r

55

I

Table 3 The effect of droughting on Ci for Kensington and Irwin cultivar. Means

(tse of and LSD (P:0 are shown. * indicates si

Table 4 Effect of Inigation on productivity and fruit size.

The effect of irrigation reflected in marked differences in SMC between irrigated and

non-irrigated treatments (Fig 36). The SMC of the uninigated treatments did not decline

from approximately week 7 onwards as the soil was presumably depleted of plant

available soil moisture. In spite of this depletion Pn and E' rates in the droughted

treatment remained relatively high in comparison with the control. A possible explanation

for this is that the droughted trees were extracting the water from an a¡ea outside of the

experimental zone possible at a greater depth'

LSD (P= 0.05) 95.6 75.8V/eek l5 222 x58.4 102 t38.7 *' 198 t 45.8 l5l x29.7Week 13 209 ¡40 116 + 17 .6 203 t 30.1 180 t 75.4Week 11 147 ¡35.1 144 x 18.2 218 ¡ 12.6 148 t 38.6Week 9 189 x22.3 156 t 41.5 169 t 18.6 135 t 18.7Week 7 162 t 17.6 l2l x14.3 196 x 47.2 186 x 41.2Week 6 220 ¡46.2 173 t 33.8 208 x33.4 178 t 35.9Week 5 186 t 39.5 98 x21.9 189 t 21.3 168 t 9.9Week 3 205 x19.9 167 x28.5 235 x39.5 183 t 14.8Week I 182 ¡31.2 124 x19.6 209 x41.2 221x21.3July 133 x47.0 81.4 t 9.9 207 x35.1 215 t 61.8June 140 t 28.3 100 + 1.1 200 ¡6.9 142 ¡26.8May 236 ¡22.4 221x6.3 217 x6.l 170 t 8.5April 162 ¡ 17.2 148 x21.5 158.8 t 15.9 156 x9.9

Time KensingtonIrrigated InternalCO, concentration(pL L'' )

KensingtonDroughted lnternalCO, concentration(pL L-')

Irwin IrrigatedInternal CO,concentration(pL L-')

Irwin DroughtedInternal CO,concentration(pL L-')

Non Inigated 177 751 235inigated 320 r286 249

Non 151 573 263Inigated 201 633 318

Total weight Total Fruit Averagetments fruit Size

(Ke) Number (Grams)

56

I

I

j

IL

I

I

It was observed that during the trial the droughted Irwin suffered excessive leaf drop

which was not observed to the same extent on the droughted Kensington. However no

data was collected to quantiff this.

Kensington ++Non líigåtsd,IB

'17

16

15

'14

't3

12

11

10

É

(J

Ø

Apr MaY Jun Jul I 3 579111315

I f1 13 16

Month lrVeek

lrwin

'18

17

l6?rsõ14Ø

12

11

'10

Apr May Jun Jul I 3 5 7

Month Week

Figure 36 Soil moisture content (SMC) at 0 - 120 cm soil depth on cockatoo sand'

57

4 DISCUSSION

The results of this study show that Mango leaves have a moderate photosynthetic

capacity, however environmental and cultivar differences play an important role in leaf

gas exchange of mango.

Irradiance

The inadiance and net photosynthesis (Pn) relationship showed a typical response of

leaves to light displayed in a hyperbolic curve (Lakso and Seeley 1978)' Light saturation

occurred at an irradiance level of 1200 ¡rmol m2 s-r, similar levels have been observed in

cashews (Shaper 1991), macadamia (Allen and De Jager 1979) and mango (cultivar

Tommy Atkins, Whiley et al. unpublished data). These levels are somewhat higher than

has been observed in other fruit tree species such as apple which are reported to saturate

at 600-800 pmol m2 s-t llakso and Seeley 1978), olives at 400-500 pmol m2 s-t (Bongi er

al. 1987)and sour cherry leaves at 800-1200 pmol m's-' lsams and Flore 1982)'

The shapes of the photosynthesis light response curves and the maximum CO, uptake

finally reached differ depending on genetics, developmental and seasonal stage, and

whether the leaf is adapted to sun or shade (Larcher 1969). Cultivar can play a role in

determining the inadiance level at which light saturation occurs, observations of cultivar

influence on light saturation levels have been made with tea, (Smith et al- 1994), apple

(Looney 196S) and lowbush blueberry @orsyth and Hatl 1965). Indicating that some

cultivars are more readily adaptable to higher levels of light intensity' Difference in

cultivar pn rates has been attributed to genetic traits such as greater leaf thickness,

specific leaf weight, and chlorophyll content (Looney 1963). Kensington, Irwin and

Tommy Atkins did not have significantly different light saturation levels. Further studies

of a wider range of cultivars would be necessary to determine if this applied to all mango

cultivars

58

Saturation occurred at ppFD levels between 1250-1700 ¡-rmol m-2 s-' however PPFD

levels at this time of year often exceed 2000 pmol m-' s-'' oversaturation with light has

been realised after prolonged exposure of samples to vary high inadiances (Kozlowski

lg57: Jarvis 1g64). It is difficult however to estimate to what extent oversaturation with

light may be a limiting factor under natural conditions. Obviously in concern with effect

of radiation-dependent parameters such as leaf temperature or transpiration, very high

irradiation will frequently produce a reduction in net photosynthesis which may be partly

due to photoinhibition, the inhibition of photosynthesis caused by surplus light energy

trapped by chlorophyll to yield active oxygen toxic to organs (Yamada et al' 1996)'

Photoinhibition due to prolonged exposure to high photon flux densities has been

observed ín vitis vinifera and Nerium oleander (correia et aI. 1990; Powles and

Björkman l9g2). A slight reduction in Pn was observed in Kensington as light intensity

increased possibly indicating some level of oversaturation. However further studies

would be necessary to more accurately assess the potential for photoinhibition in this

cultivar.

ci declined with PPFD as gs showed a weaker response to PPFD than Pn' This could also

have reduced the pn response to PPFD at higher levels of irradiance through the lower Ci

concentration. This could occur if the consumption of co, by Pn was faster than its

supply by increased gs thus resulting in a lower substrate supply for Pn' Stomatal

aperture is known to be responsive to CO, but the relationship differs considerably

between plant species and within the same species under different growth conditions

(Morison 1987).

Commercial mango plantations are rarely pruned selectively, partially because of a lack

of information about the optimal canopy design for light use within the canopy (schaffer

and Gaye l9S9). In an orchard environment Pn limiting light conditions occur as a result

of mutual shading of leaves. As the crop canopy saturates at higher irradiation levels than

single leaves (Fisher 1984), canopy structure as well as adequate spacing to prevent

excessive intermingling of branches is important to reduce the detrimental effect of

59

shading. Differential light penetration has been shown to influence vegetative growth

(Jackson 1980), flower initiation (cain lgTl), fruit set (Jackson and Palmer 1977) and

fruit colour (Jackson, et al. 1977). In the mature mango orchard there is generally a high

proportion of shade leaves to sun leaves, thus the majority of the leaves receive well

below the saturation point of approx ppFD 1200 pmol m-2 s-t. Because sun leaves of

mango have a greater quantum use efficiency than shade leaves (Schaffer and Gaye 1989)

selective pruning would improve photosynthetic effrciency.

The incident light regime in which leaves develop affects leaf gas exchange

characteristics (Schaffer and Gaye 1989). This has also been shown to occur for other

fruit crops such as apple (Barden lg74), and citrus (Syvertsen 1985). Adequate light

interception is essential for recently matured leaves as these leaves exhibit the highest Pn

in the field (Chacko unpublished data), and are the closest to the fruits which develop

from terminal inflorescence on current seasons shoots' This has been shown to be

important in other tree crops such as peach (Crews et al. 1975)' As mango leaves

developed under high light intensities have a greater quantum use efficient than those

grown under lower light intensities. (Schaffer and Gaye 1989), newly developed leaves

need to be positioned to have adequate light interception'

Leaf age

Leaf age is an important factor in determining leaf photosynthesis. As the mango leaf

expands during flushing Pn rate rises rapidly. Maximum Pn and gs were reached well

after full leaf expansion, which is consistent with other tropical crops and subtropical

crops such as coffee (Yamaguchi and Friend lg79), olive (Bongi et al' 1987) and cashew

(Schaper 1991) and has previously been observed in mango (chacko and

Ananthanarayanan 1935). However in apple (Kennedy and Johnson l98l)' citrus

(syvertsen 1985), sour cherry (sams and Flore l9s2) and rose (Bozarth et al' 1982),

maximum Pn and gs values have been observed to peak before or right at full leaf

expansion.

60

Mango leaves expand rapidly reaching full expansion in four weeks. However their

photosynthetic capacity develops at a slower rate compared to a temperate fruit such as

apple, which has been reported to begin photosynthesising at ten days of age (Watson and

Landsberg Lg79). pn did not occur in the mango leaves until they were approximately 4

weeks of age. In apple immature stomates and therefore high stomatal resistance have

been attributed to the inability of the leaf to reach maximum Pn levels during

development (Slack 1974). Development of the conductance curve in relation to leaf age

observed in the present experiment would support these findings. Slow development of

the photosynthetic capacity of the mango leaf in relation to temperate fruits may be

related to the longevity of the leaf (Sestak 1935). Mango leaves may be retained on the

tree for 4-5 years (Chacko pers. comm) and still remain active if not damaged by diseases

or insects. In other evergreen tree crops leaf longevity varies from approximately one year

in cashew (Schaper 1991); to around two years in olive (Bongi et al. 1987). The new

leaves in an orchard are likely to experience a decline in photosynthetic productivity after

approximately 6 to 10 months due to shading from new growth flushes.

Fruiting

During the dry season (May to October) vegetative growth rates decrease, coinciding with

the period of flowering and fruit development. The leaves closest to the developing

panicle are most likely to be the main contributors of carbohydrates to the developing

fruits (Chacko pers. comm). The effect of fruiting on such leaves was investigated in

experiment 4. The results showed that the presence of the developing mango fruit has a

significant inhibitory effect on Pn by the leaves in close proximity to the fruit. Whilst

transpiration and conductance also appeared to be reduced these effects were not

significant, probably due to lack of accuracy of the ADC in measuring the low values

recorded at this time of year. Similar inhibitory effects of fruit on nearby leaves have been

observed in cashew (Schaper 1991). However this effect is the opposite to that found in

many other fruit crops. In general fruits are strong sinks for assimilates and increase leaf

photosynthesis in adjacent leaves, although this response is not always simple' The

presence offruit in apple has been reported to increase Pn and accelerate the translocation

6l

of photosynthates from the leaves (Hansen 1969), however the presence of fruit did not

affect spur leaf photosynthesis (Ferree and Palmer 1982) or leaf photosynthesis in young

apple trees (Proct or et aL 1976). Significant increases in Pn on fruiting shoots have also

been reported in pistachio, pecan, strawberry and grapevine (Vemmos 1994; Wood 1988;

Chomaetal.|982;andLoveysandKriedemann|974),aneffectgenerallypresumedto

be due to the increased sink demand. However schaffer et al' (1986) found the influence

of fruit on Pn in strawberry to be variable. As the level of fruit production on mango is

low it is possible that Pn over the whole tree is in large potential excess and translocation

of more remote sources of assimilates is important'

The data indicated that time of day did not appear to influence the results as an inhibitory

effect was observed throughout the diurnal cycle. In apple, however, fruiting was shown

to have little effect on Pn in the morning but have large influences in the afternoon

(Hansen 1970).

In non-fruiting shoots lower Pn rates are often observed, an effect, which is commonly

attributed to an accumulation of assimilates which then suppresses Pn (Schaffer er a/'

1936). Mango fruits develop rapidly over a period of 110 to 120 days, this rapid growth

of a nearby sink may compete with the leaves for a limited supply of nitrogen' leading to

senescence of the leaf mobilisation and loss of leaf nitrogen and a consequent drop in its

photosynthetic capacity. Yellowing of the leaves adjacent to the fruit was observed in this

experiment. Studies on peach and citrus have indicated a high conelation between leaf

nitrogen content and CO, assimilation rate (Dejong and Doyle 1985' Syvertsen 1984)'

However, when a similar observation was made on fruiting cashew terminals (schaper

lggl) chlorophyll loss and nitrogen depletion appeared after a decline in Pn and the

addition of nitrogen had no effect on Pn of fruiting shoots. Further studies are needed to

establish the role of rapid sink growth and its link to a reduction in Pn'

Stomatal regulation appears to be influenced by the developing fruits and may contribute

to the reduced CO2 uptake. Stomatal conductance has been shown to contribute to the

increase in Pn in apple under simila¡ circumstances (Proctor 1979)' However in

62

strawberry the increase in Pn could not be attributed to an increases in stomatal

conductance of fruiting plants (Schaffer et aI' 1986)'

Although it is uncertain how fruit development influences stomatal opening it may be

related to hormone production by the fruit. Fruits produce a wide range of hormones

including abscisic acid (Crane 1969), which is known for its association with stomatal

closure (Loveys and Kriedemann lg74). Studies on glasshouse grapevines showed that

the removal of fruit resulted in increasing levels of endogenous abscisic acid which had

an effect on stomatal conductance and influenced Pn ( Loveys and Kriedemann 1974).

Mango fruits in Northern Australia develop under highty stressful atmospheric conditions

(.,high vapour pressure deficit"), possibly causing the fruit and/or leaves to produce an

excess of abscisic acid thus inducing partial stomatal closure. Further studies would be

required to determine the effect environmental conditions have on the production of

abscisic acid in mango fruits'

Vapour Pressure Deficit

High Vapour pressure deficit (VPD) can occur periodically throughout the year in

Kununurra. However values in excess of 4 kPa (4.0 kPa) are often experienced during the

months of September, october and April. The most obvious direct effect of high VPD is

the control of the rate of transpiration from the leaf resulting in water loss. Initially

transpiration losses are proportional to a rise in VPD, however once the VPD exceeds 2'5

kpa transpirational water loss slows, this is most likely due to stomatal closure (Swalls

and O'Leary lg71).VPD has a direct effect on stomatal aperture (Lange et al' l97l)'The

stomatal response to atmospheric conditions is one of the key factors responsible for the

development of plant water deficits (Kaufmann 1981). A weak negative correlation with

VpD and conductance was observed in the present study (RÍ:0.32) which supports

similar findings on Nam Dok Mai cultivar of mango (Pongsomboon ef aI' l99l)' A

correlation between Pn and conductance was also found. Similar correlations have been

reported for olive (Bongi et at. 1987), and apple (Flore et al. 1985)'

63

It is not unusual for Pn and gs to change in unison. A depression in leaf gas exchange

over the course of the day has been demonstrated at midday in Arbutus unedo (Raschke

and Resemann 1986) and during the afternoon in Yucca glauca (Roessler and Monson

l9g5), in both cases these falls were caused by increased VPD and leaf temperature.

Roessler and Monson (19S5) hypothesised that an increase in leaf temperature caused Pn

to decline and the increase in VPD caused gs to decline. However Raschke and Resemann

(1986) concluded that the depression was due solely to VPD. Schulze et al. (1972) also

proposed that VPD was the cause of midday stomatal closure in desert growing species.

In the present study, conductance of the fully expanded leaves increased with increasing

VpD from 1.8 to 2.5kPa. Similar increases have been observed previously in mango

with VpD between 1.5 and 2.5 Y,Pa (Reich and Borchert 1988), indicating that mango

leaves are relatively tolerant to atmospheric stress. In contrast to this apple leaves were

severely stressed at a VPD of 2.0 kPa (Flore et al. 1985). However, in mango also' VPD

in excess of 2.5 kPa resulted in partial stomatal closure and reduced gas exchange, this

being more evident in Kensington than in lrwin.

photosynthetic differences between cultivars of a species exist (Larcher 1969 and Looney

1963) and may result from the capacity of each cultivar for sucrose synthesis which may

ultimately limit the maximum Pn (Stitt 19s6) this hypothesis however is unlikely. Since

seasonal observations indicate that all the cultivars studied here have the capacity for

relatively high rates of Pn lO-I2 pmol m2 s-' during a period of low VPD 1.8 -2.0 kPa, it

may be concluded that the cultivar has only a minimal or no effect on maximum Pn'

Whilst there was no evidence of cultivar difference in P** significant cultivar differences

in Pn were observed. Irwin and Tommy Atkins had a higher Pn than Kensington

throughout most of the year suggesting that these cultivars are able to maintain greater

photosynthetic efficiency than Kensington in this environment. Significant clonal

differences in pn were observed during the diurnal and seasonal cycle corresponding to

changes in VpD. Cultivar response to changing atmospheric conditions appears to be

significant. This is most likely due to stomatal regulation. Stomatal closure in response to

64

high VpD and soil drought is well known (Schulze 1986; Kaufmann 19Sl). Stomatal

sensitivity to diurnal changes in inadiance, temperature and VPD involving interactions

between abscisic acid production and the sensitivity of guard cells to abscisic acid is

probably meditated by changes in leaf epidermal water potential (Tardieu and Davies

l9g2). Stomatal sensitivity to VPD and other environmental variables varies with species

(El-Sharkawi et a|.1984), variation has also been shown to exist between some cultivars

of tea (Smith et at. 1994). The apparent sensitivity of Kensington to high VPD may be

related to its genetic origin as it is a polyembryonic cultivar originating in an area (South

East Asia) of high humidity all year. Tommy Atkins and Irwin however afe

monoembryonic cultivars of Indian origin (Mukherjee 1953), which evolved in areas with

pronounced periods of low humidity and therefore high VPD'

Under the conditions of high VPD experienced in Northern Australia Irwin and Tommy

Atkins have significantly higher productivity and regular bearing characteristics than

Kensington (Johnson and Robinson 1997), and appear to have a greater carbon uptake

during the seasonal cycle. However screening mango cultivars for differences in carbon

uptake may not prove productive since the heritability of Pn is low (Ozbun 1978) and

does not necessarily correlate with productivity (Evans 1975, Ozbun 1978)' The

relationship between Pn, partitioning and yield is an area that warrants fuither study as

correlation between Pn and productivity appear to exist during certain growing periods of

plants rather than during the whole growing season (ozbun 1978).

Soil moisture

Water stress is the single most important factor limiting productivity and yield as it

occurs frequently even under moist conditions (Roberts 1987).V/ater deficit affects

almost every process in plants (Kramer l9S7) and it is of frequent occurrence in the semi

arid tropics mostly due to lack of suffrcient rainfall and its irregular distribution. In many

tropical species water balance is the main climatic factor determining the length of the

growing season rather than cold temperatures and/or day length as in temperate climates

65

(Goldsworthy l9S4). In Mango, the growing season in the semi arid tropics could be

considered continuous as flowering and fruiting take place during the dry season, and

vigorous vegetative flushing occurs in the wet season

Gas exchange rates during the dry season were well below the maximum observed during

the wet season even on the irrigated trees. This is partly due to the influence of fruiting

(Experiment 4) and high VPD (Experiment 3) affecting stomatal aperture but other

factors such as cool night temperatures may also contribute to reduced gas exchange'

Leaves normally recover from stressful atmospheric conditions of the day during the

night, however this recovery can be reduced by soil moisture stress (Elfuing and

Kaufmann lg72). Reduced gas exchange in leaves subjected to soil moisture stress has

been observed in peach, almond, pistachio and cashew (Xiloyannis et al. 1980; Castel and

Fereres L982;Behboudian et al.1986; Shaper 1991)'

partial recovery of LGE parameters of the pre-treatment droughted trees occurred on

week I for Kensington and week 3 for Irwin after irrigation. Failure to fully recover is

most likely due to other external factors such as high VPD contribute to reduced LGE,

although there is a possibility that plant water status has not recovered or there was

internal damage to the leaf structure.

In the inigated and early in the drought treatment it appeared that gs can maintarn

adequate Ci for moderate Pn and that the Pn rate may be limited by reduced Ci when gs

declines. As the treatment progressed Ci remained relatively stable however gs and Pn

showed some levels of decline. The levels of Ci were maintained presumably by the

stomata or an inhibition of Pn which resulted in less CO, reduction and consequently

higher Ci. Calculations of Ci are less reliable, when gs and Pn a¡e low particularly when

non-homogenous stomatal opening occurs, causing Ci to be overestimated (Downtoî et

a/. lggg). Non homogeneity in stomatal opening can occur when plants are droughted

(Kaiser 1987; Sharkey and Seemann' 1989)'

66

Below 1g0 cm depth root density may have been too low to maintain the balance between

water absorption by the roots and evaporation demand by the leaves (Kaufmann and

Fiscus 1985). E, is normally driven by the evaporative demand of the atmosphere to

which leaves are more or less coupled (Jarvis 1985). As the flow of water through the

plant was inhibited due to low soil water availability, Stomatal aperture was reduced

which was followed by a reduced E,. Interestingly during this study Et was not reduced to

the same extent as gs, whereas in most fruit crops water deficit often reduces E, most'

(Lakso 1985). Stomatal regulation in mango appears to prevent leaves from losing water

faster than their roots can replace it by absorption. Although stomatal closure limits

carbon uptake and dry matter production (Kaufmann 1931) and may fully account for

inhibition of photosynthesis in a water stress situation (Downton et aI' 1988), it prevents

leaves from dehydration and a total loss of turgor. The relatively quick recovery of

stomatal conductance of the leaves in the inigated treatment after the initial preflowering

droughting may be connected with their ability to control water efflux in water stress

situations

A reduction in productivity and fruit size due to water stress in mango is well

documented (Singh and Arora 1965; Azzouz et al., 1977; Pongsomboon 1991; and

Larson et at. l99l), and this study showed a reduction in productivþ from non irrigated

trees. What is interesting is an apparent difference in cultivar productivity response to

drought, whilst no apparent differences were observed in leaf gas exchange' Non irrigated

Kensington appeared to respond by reduced fruit size whereas non irrigated Irwin

responded with greater fruit drop. This suggests that Kensington may be more drought

tolerant than Irwin, however further study would be required to substantiate this. It was

noted that Irwin had considerably greater leaf drop during drought than was observed for

Kensington .Leafabscission is a mechanism to limit water loss (Kramer 1987). Reduced

Pn on its own is often poorly related to dry matter production (Ozbun 1978)' however

when combined with reductions in leaî area, yield losses in the form of fruit drop may be

inevitable (Lakso I 985)'

67

yield potential of tropical fruit crops is highly dependent on current assimilation and little

is derived from accumulated reserves of dry matter (Goldsworthy 1984), therefore

maintaining high levels of Pn during fruit development would be required to reach full

yield potential. The maintenance of Pn as water stress develops, through a gradual

reduction of gs is likely to contribute to the productivity and survival of mango during

periods ofdrought.

Conclusions

The environmental influences on mango during fruit development (SeptOct) are extreme

producing high VPD in excess of 4.0 kPa, and PPFD around 2000 pmol m-' s-t '' This

combined with the internal stresses (assimilate partitioning) associated with crop load and

water Stress, have a profound influence on leaf gas exchange' Diurnal changes in

atmospheric and leaf temperature are accompanied by changes in VPD in the field' which

may independently affect stomatal behaviour as in other tropical and temperate crops (El-

Sharkawy et al. 1984; Smith, 1989).Since VPD, illuminance and conductance do not vary

independently in the field it is difficult to completely discriminate between their effects.

Differences between cultivars in LGE response to changing environmental conditions

were significant. With Kensington cultivar appearing to be the most sensitive to extreme

atmospheric conditions. There appeared to be no observable cultivar differences in LGE

with changing soil moisture status, however'

It appears from the results that a broad genotypic variation within Mangifera cultivars in

response to most environmental factors provides an opportunity to select cultivars

adapted to particular climatic regions based on responses to a variety of environmental

conditions rather than a single environmental factor'

68

6. BIBLIOGRAPHY

Agati, J.A. (1937). The rate of photosynthesis of carabao mango leaves (Mangifera indica

Z.)underfreldconditions'Phil.J.ofAgric.S:|2|-|43.

Allan, P. and De Jager, J. (lg7g). Net photosynthesis in macadamia and pawpaw and

possiblealleviationofheatstress.ActaHort.L02:23-29.

Alphalo, P.J. and Jarvis, P.G. (1991). Do stomata respond to relative humidity? Plant'

Celt and Environment' 14:127-132'

Arp, w.J. (1991). Effects of source-sink relations on photosynthetic acclamation to

elevated COr. Plant, CeII and Environment' 14 869-878

Azzottz, s., El-Nokrashy, M.A. and Dahshan, I.M. (1977). Effect of frequency of

irrigation on tree production and fruit qualrty of mango' Agric'Res'Rev' 55: 59-65'

Ball, J.T., 'Woodrow, I.E. and Berry, J. A. (1987). -A model predicting stomatal

conductance and its contribution to the control of photosynthesis under different

environmental conditions. In progress in photosynthesis research. (J. Biggens'

Ed) Martinus Publishers, Dordrecht ' pp22l-224'

Barden, J.A. (1974). Net photosynthesis, dark respiration, specific leaf weight' ffid

growth of young apple trees as influenced by light regime' J'Amer'soc'Hort'sci'

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