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SPAD-CHLOROPHYLL RESPONSE TO NITROGENFERTILIZATION AND EVALUATION OF NITROGEN STATUSIN DRYLAND AND IRRIGATED PUMPKINS*John. M. Swiader a & Ame Moore aa Dept. of Natural Resources & Environmental Sciences , University of Illinois , Urbana, IL61801Published online: 14 Feb 2007.

To cite this article: John. M. Swiader & Ame Moore (2002) SPAD-CHLOROPHYLL RESPONSE TO NITROGEN FERTILIZATION ANDEVALUATION OF NITROGEN STATUS IN DRYLAND AND IRRIGATED PUMPKINS*, Journal of Plant Nutrition, 25:5, 1089-1100, DOI:10.1081/PLN-120003941

To link to this article: http://dx.doi.org/10.1081/PLN-120003941

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SPAD-CHLOROPHYLL RESPONSE TO

NITROGEN FERTILIZATION ANDEVALUATION OF NITROGEN STATUS INDRYLAND AND IRRIGATED PUMPKINS*

John M. Swiader** and Ame Moore

Dept. of Natural Resources & Environmental Sciences,

University of Illinois, Urbana, IL 61801

ABSTRACT

Field experiments were conducted on a dryland Flanagan loam

and on an irrigated Plainfield sand to assess N status in proces-

sing pumpkins (Cucurbita moschata) using the Minolta SPAD-

502 chlorophyll meter. At each location, five rates of nitrogen

(N) (0, 84, 168, 252, and 336 kg ha�1) were applied to three

pumpkin cultivars (hybrid ‘698’, hybrid ‘401’, and ‘Libby-

Select’), with leaf SPAD (Soil Plant Analysis Development)

readings and total N measured at anthesis, early-fruiting, and

mid-fruiting stages. Fertilizer N requirements for 90% and 100%

yield were estimated at 61 and 191 kg N ha�1 for dryland pump-

kins and 148 and 245 kg N ha�1 for irrigated pumpkins, respec-

tively. Cultivar and N rate affected leaf SPAD readings and

N concentrations at both locations, with greatest effects on the

JOURNAL OF PLANT NUTRITION, 25(5), 1089–1100 (2002)

1089

Copyright # 2002 by Marcel Dekker, Inc. www.dekker.com

*This research was conducted under Hatch Project 65-333, University of Illinois, College

of Agricultural, Consumer, and Environmental Sciences.

**Corresponding author. E-mail: jswiader@uiuc.edu

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irrigated sand. For both dryland and irrigated pumpkins, SPAD

leaf readings at each sampling date were significantly correlated

with both leaf N concentrations and fruit yield, although the

relationships were markedly weaker on the dryland loam than

on the irrigated sand. The results suggest that maximum or

near-maximum yields of both dryland and irrigated pumpkins

can be expected when leaf SPAD readings are �56.7–59.0 units

at anthesis, �55.1–57.6 units at early-fruiting, and 52.2–54.3

units at mid-fruiting. The results also showed that while normal-

ized leaf SPAD readings derived from high-N in-field reference

plots can readily identify the critical threshold (i.e. 10% yield

reduction) for N deficiency in pumpkins, they would be less pre-

cise predicting optimum N status (i.e. maximum yield). The

potential usefulness for the SPAD 502 chlorophyll meter as a N

management tool in estimating plant N status was demonstrated

in irrigated pumpkins, and to a lesser degree in dryland produc-

tion. However, because pumpkin N requirements could vary

with cultivar and growing conditions, a range of SPAD values is

proposed.

INTRODUCTION

For processing pumpkins (Cucurbita moschata), it is generally recognized

that large amounts of N are required for high yields. Fertilization recommenda-

tions for N in pumpkins commonly range from �112 kg N ha�1 on some heavy

dryland loams to over 196 kg N ha�1 on irrigated sand (1), although in actual

practice N rates may be considerably higher (Douglas Scheirer, personal

communication). However, while too little N fertilizer can result in reduced fruit

yield, and therefore reduce profits, an excess of applied N may lead to

contamination of ground and surface water supplies. In addition, excessive or

mis-timed N application can markedly reduce yields in these crops by causing

delayed flowering, resulting in insufficient time for fruit ripening and large

amounts of unusable immature fruits at harvest (2).

Subsequently, for processing pumpkins it is important that growers have an

accurate assessment of real-time plant N status. Traditional laboratory tissue

testing is too inconvenient and time-consuming to be used this way. In recent

years, a rapid, non-destructive method for leaf ‘chlorophyll’ analysis using a

hand-held spectrophotometer (SPAD-502 chlorophyll meter) to measure the

relative ‘greenness’ of leaves has received much attention to assess N status in

plants. In the procedure, the difference in light attenuation at 650 (peak

chlorophyll measurement) and 940 nm (non chlorophyll absorbance) is measured.

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A microprocessor calculates a ‘SPAD’ (Soil Plant Analysis Development) value,

ranging from 0 to 80, which is proportional to the relative optical density between

the two wavelengths. Leaf greenness, which is closely related chlorophyll, is then

related to leaf N.

Originally developed for N management in rice production (3), SPAD

technology has been used in corn (4–7), cotton (8), pepper (9), potato (10), and

greenhouse tomatoes (11), with varying degrees of success. Although the

technique is quick and relatively easy, factors other than available N may affect

leaf chlorophyll content, including cultivar, growth stage, and environment

(5,10). To address this concern and standardize readings, some researchers

suggest a ‘normalization’ procedure that compares SPAD readings of a crop with

those of a site-specific in-field reference of known N sufficiency (6,12).

The SPAD-chlorophyll meter has not been evaluated for pumpkins.

Production environments with different moisture, temperature, light, and soil

regimes may affect test results and effectiveness. This situation is especially

appropriate to pumpkin production in Illinois, where the commercial acreage

(�4000 ha) of processing pumpkins is evenly split between irrigated production

on sandy soils, used primarily for the early crop, and dryland production on heavy

soils, used for the main-season and late crops. The objectives of this present study

were to determine the feasibility of using field chlorophyll measurements to

assess N status in pumpkins, and to what extent do different production

environments (i.e. dryland vs. irrigated production) and cultivars influence

results. In the process, fruit yield response to applied N rate was characterized in

both production systems.

MATERIALS AND METHODS

Two field experiments were conducted: one on a dryland Flanagan silty

loam (fine, montmorillonitic, mesic Aquic Argiudolls) at the University of

Illinois Vegetable Crops South Farm in Urbana, IL, and the other on an irrigated

Plainfield loamy fine sand (sandy, mixed, mesic Typic Udipsamment) at the

University of Illinois Sand Research Field (UISRF) in Havana, IL. The Flanagan

loam was characterized by relatively high organic matter content (4.0%) and

high nutrient holding capacity (CEC 20.2 meq 100 g�1), while the Plainfield

sand was moderate to low in organic matter content (1.2%), with a CEC of

4.8 meq 100 g�1. At each location, five rates of N (0, 84, 168, 252, and

336 kg N ha�1) were applied. On the dryland loam, all N was applied as dry-

blend NH4NO3, broadcast preplant, and disked-in (0–15 cm), while on the

Plainfield sand, N was applied as urea-ammonium nitrate solution (28-0-0), two-

thirds broadcast just before planting, and one-third sidedressed when vines

began to run. The experimental design was a split-plot, with N rates as

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main-plots and pumpkin cultivars as subplots. Individual N plots comprised

142 m2 (10.6 m613.4 m) arranged in a CR design, and replicated four times.

Additional overhead sprinkler irrigation was applied to all plots at UISRF to

provide a total (including rainfall) of 38 mm of water per week. Seasonal rainfall

(May 1 to September 15) totaled 384 mm on the dryland loam and 307 mm on

the fertigated sand. Rainfall distribution was generally consistent at both sites,

with no extended periods without rain. At both locations, preplant soil

phosphorus (P) tested high, so no additional P was applied. Preplant potassium

(K) as potassium chloride was applied at 84 kg K=ha at UISRF; no K fertilizer

was applied at Urbana.

In late May, open-pollinated pumpkin cultivar Libby-Select and hybrids

‘698’ and ‘401’ were seeded at 0.46-m within-row intervals in double rows 1.5 m

apart and 7.3 m long in the center of each plot. When seedlings developed two

true leaves, hills were thinned to one plant, resulting in a stand of 32 plants per

plot, or �9500 plants ha�1 (note: this approximation is based on an area of

4.6 m67.4 m, and disregards a border of 3.0 m on each side and each end, which

was used to limit the effects of plants in neighboring plots).

At each location, leaf ‘greenness’ measurements, using the Minolta

SPAD-502 chlorophyll meter (Minolta Corp, Ramsey, NJ), were made on the

most recent fully-expanded leaf closest to the growing tip from 30 plants in

each N fertility plot at first female flower anthesis, early-fruiting (anthesis date

plus 14 days), and mid-fruiting (anthesis date plus 30 days) stages. Due to

differences in flower development, anthesis dates ranged from 44 days after

seeding in ‘698’ on the irrigated sand to 52 days after seeding in both ‘401’

and ‘Libby-Select’ on the dryland loam. Leaf SPAD measurements were made

about midday (�1 h), and were taken approximately 10 cm in from the distal

leaf margin and 7 cm outwards from the midrib. After recording SPAD

measurements, 20 of the same leaves used for the SPAD readings were

harvested, and later assayed for total N concentration using a micro-Kjeldahl

technique (13). In the assay, total N was determined using leaf disks (�1.5 cm

diameter) taken from approximately the same leaf location as used in the SPAD

measurements.

Fruits were harvested in a once-over operation when it was estimated that

80% of the main fruits had reached commercial maturity, as indicated by a change

in rind color from green to tan. Yield data was recorded for fruit fresh weight; as

is standard industry practice, fruits smaller than 15 cm in diameter were

considered unusable and were not harvested.

Analysis of variance was used to test for main and interactive effects of N

fertilizer rate, location, and cultivar on fruit yield, leaf SPAD readings, and

leaf total N concentrations. Regression analysis was used to calculate best-

fit functions between leaf SPAD readings and leaf N concentrations and fruit

yield.

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RESULTS AND DISCUSSION

Mean fruit yields were higher in hybrid ‘401’ than in either ‘Libby-Select’

or hybrid ‘698’ (Table 1). In each of the cultivars, pumpkin fruit yield response to

N fertilization rate was markedly greater on the irrigated sand than on the dryland

loam. In dryland pumpkins, yields increased 29% with increasing N rate up to

84 kg N ha�1, while in irrigated pumpkins, yields increased over 2-fold with

increasing N rate up to 252 kg N ha�1. At both locations, fruit yields tended to

level-off or decrease at 336 kg N ha�1, as reported by previous researchers (14).

From the response functions generated from the data, maximum fruit yields of

74.4 and 82.0 Mg ha�1 were estimated at 191 and 245 kg N ha�1 in dryland and

irrigated pumpkins, respectively. Similarly, 90% maximum fruit yield was

estimated at 61 kg N ha�1 in dryland pumpkins and 148 kg N ha�1 on the

irrigated sand.

At each sampling date, leaf SPAD readings were affected by main effects

for location, cultivar, and N rate (Table 1). Additionally, significant interactions

between location and N rate, and between location and cultivar affected SPAD

values at both the early- and mid-fruiting stages. In general, leaf SPAD readings

were higher in dryland pumpkins than in irrigated pumpkins, and increased with

increasing rates of applied N. Cultivar effects, although significant, were

numerically small, with readings highest in either ‘Libby-Select’ or ‘401’, and

lowest in ‘698’. At early- and mid-fruiting, effects of N rate and cultivar on leaf

SPAD response were greater in irrigated pumpkins than in dryland crops. There

was no cultivar6N rate interaction affecting leaf SPAD readings at any sampling

date, indicating that throughout the season the relative effect of N fertilization on

leaf SPAD values was consistent across cultivars. Pooled over cultivars and N

rates, SPAD values at anthesis, early-fruiting, and mid-fruiting averaged 57.3,

55.4, and 52.9 units in dryland pumpkins, and 54.1, 52.3, and 47.8 units in

irrigated pumpkins, respectively.

Similar to the response in leaf SPAD readings, leaf N concentrations

increased with increasing rates of applied N, and were higher in dryland plants

than in irrigated crops (Table 1). At each sampling date, there were significant

cultivars effects, with leaf N concentrations highest in ‘401’, intermediate in

‘Libby-Select’, and lowest in ‘698’. Significant interactions between location and

N rate affected leaf N concentrations at both the early- and mid-fruiting stages, as

effects of N fertilization on leaf N content were greater on the irrigated sand than

on the dryland loam. These interactions, as well as those affecting leaf SPAD

readings, were not unexpected since differences in leaf greenness among N

treatments were clearly visible throughout fruit production on the irrigated sand,

but not on the dryland loam.

For both dryland and irrigated pumpkins, SPAD leaf readings at each

sampling date were significantly correlated with leaf N concentrations, although

SPAD-CHLOROPHYLL RESPONSE 1093

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SPAD-CHLOROPHYLL RESPONSE 1095

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the relationships were much weaker on the dryland loam than on the irrigated sand

(Fig. 1). In dryland pumpkins, the relationship between SPAD readings and leaf N

was curvilinear at the early sampling date, and linear for the latter two dates. This

pattern was reversed in the irrigated crop, where there was a linear relationship

between leaf SPAD readings and leaf N at anthesis, and quadratic relationships at

both early-fruiting and mid-fruiting. In each case, correlations were higher at the

early-fruiting and mid-fruiting stages than earlier in the season at anthesis.

Significant linear relationships between leaf SPAD readings and pumpkin

fruit yield occurred at both locations (Fig. 2). This response was consistent across

sampling dates and cultivars. However, similar to the relationship between leaf

SPAD and tissue N, the correlations between leaf SPAD readings and pumpkin

yield were considerably lower on the dryland loam than on the irrigated sand.

Figure 1. Relationship between leaf total N concentrations and leaf SPAD-chlorophyll

readings in dryland and irrigated pumpkins at: (‘a’) anthesis, (‘b’) early-fruiting, and

(‘c’) mid-fruiting. Regressions equations for dryland pumpkins were: y¼ 10.2þ

1.84x7 0.017x2 at anthesis; y¼ 40.5þ 0.37x at early-fruiting; y¼ 35.1þ 0.48x at mid-

fruiting. Regression equations for irrigated pumpkins on sand were: y¼ 38.3þ 0.44x at

anthesis; y¼ 19.6þ 1.32x7 0.011x2 at early-fruiting; y¼�5.5þ 2.40x7 0.024x2 at

mid-fruiting.

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This effect was most evident at the early sampling date, where the linear

regression between leaf SPAD readings and fruit yield in dryland pumpkins was

significant, but very low (r2¼ 0.12).

Using the significant relationship between pumpkin fruit yield and leaf

SPAD readings, SPAD values at anthesis, early-fruiting, and mid-fruiting

associated with maximum yields were estimated at 62.5, 59.4, and 57.4 units

in dryland pumpkins, and 59.0, 57.6, and 54.3 units in irrigated pumpkins,

respectively (Table 2). Similarly, leaf SPAD values at anthesis, early-fruiting, and

mid-fruiting for 90% maximum yields were estimated at 56.5, 54.7, and 52.2

units in dryland pumpkins, and 56.7, 55.1, and 51.2 units in irrigated pumpkins,

respectively. Comparison of these values with those of an in-field sufficiency

reference (i.e. SPAD reading in the N treatment giving maximum yield) showed

that at each sampling date the normalized readings were within 1 SPAD unit of

the estimated SPAD values associated with 90% maximum yield, but differed

Figure 2. Relationship between leaf SPAD-chlorophyll readings at (‘a’) anthesis,

(‘b’) early-fruiting, and (‘c’) mid-fruiting and final fruit yield in dryland and irrigated

pumpkins. Regressions equations for dryland pumpkins were: y¼�2.63þ 1.23x at

anthesis; y¼�20.5þ 1.60x at early-fruiting; y¼�7.07þ 1.42x at mid-fruiting. Regres-

sion equations for irrigated pumpkins were: y¼�126.1þ 3.53x at anthesis;

y¼�105.82þ 3.26x at early-fruiting; y¼�64.0þ 2.69x at mid-fruiting.

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by �6 units in dryland pumpkins and �3 units in irrigated pumpkins with the

estimated SPAD values associated with maximum yield.

Critical SPAD values have been derived for a number of crops, including 52

to 56 units at the one-quarter milk stage in corn (5), 50 units at heading in rice (3),

and 49 units at first bloom in cotton (8). In this current study, we considered the

leaf SPAD values associated with 90% maximum yield as the critical threshold

level, above which yield response to further increase in leaf SPAD-chlorophyll

would be relatively small. In potato, Minotti et al. (10) concluded that a treatment

was fertilized sufficiently when the SPAD values were within 1 to 2 units of those

of an in-field reference of known sufficiency. Normalizing leaf SPAD readings by

comparison with a site-specific high-N reference plot is commonly used to

improve the accuracy of the test (5,6). However, this practice can be cumbersome

and inefficient. Our results suggest that while leaf SPAD readings from high-N

reference plots can accurately identify the critical threshold level (i.e. 10% yield

reduction) for N deficiency in pumpkins, they would be less precise predicting the

optimum N requirement (i.e. maximum yield).

Based on the results in this current study, maximum or near-maximum fruit

yields of either dryland or irrigated pumpkins can be expected when leaf SPAD

readings are �56.7–59.0 units at anthesis, �55.1–57.6 units at early-fruiting, and

Table 2. Estimated SPAD Values for Maximum Yield, 90% Maximum Yield, and Field

Sufficiency Reference at Three Growth Stages in Dryland and Irrigated Pumpkins

Leaf SPAD Units

Site Growth Stage

Maximum

Yieldz

90% Maximum

Yieldy

Sufficiency

Referencex

Dryland Anthesis 62.5 (þ6.6)w 56.6 (þ0.7) 55.9

Early-fruiting 59.4 (þ5.6) 54.7 (þ0.9) 53.8

Mid-fruiting 57.4 (þ6.2) 52.2 (þ1.0) 51.2

Irrigated Anthesis 59.0 (þ3.4) 56.7 (þ0.7) 56.0

Early-fruiting 57.6 (þ2.1) 55.1 (�0.4) 55.5

Mid-fruiting 54.3 (þ2.9) 51.2 (�0.2) 51.4

wNumber in parentheses represents the differential between estimated leaf SPAD reading

and sufficiency reference SPAD value.xAssociated with N fertilizer treatment giving highest yield; 84 kg N ha�1 in dryland

pumpkins, 252 kg N ha� 1 in irrigated pumpkins.yBased on an estimated yield of 67.0 Mg ha�1 in dryland pumpkins, and 73.8 Mg ha�1 in

irrigated pumpkins.zBased on an estimated yield of 74.4 Mg ha�1 in dryland pumpkins, and 82.0 Mg ha�1 in

irrigated pumpkins.

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52.2–54.3 units during mid-fruiting. These values appear to be reasonable

approximations for both dryland and irrigated pumpkins, although with a higher

degree of accuracy on the irrigated sand. Since SPAD values may change

markedly during crop growth, SPAD testing should be done at a specific maturity

stage. The optimum time of plant sampling would be at early-fruiting (shortly

after fruit set), which is late enough for meaningful correlation between fruit yield

and leaf SPAD readings, and early enough to sidedress with N before vines fill in

between the rows.

CONCLUSIONS

The high yield response to N rate on the irrigated sand, in combination with

the relatively low yields on the dryland loam, allowed for evaluation of SPAD

‘chlorophyll’ measurement over a range of production environments. The high

yields of both dryland and irrigated pumpkins at optimum N rate in this study

were representative of commercial production in Illinois, where top yields of

processing pumpkins commonly exceed 70 Mg ha�1 (Douglas Scheirer, personal

communication). In total, these results suggest the potential usefulness for the

SPAD-502 chlorophyll meter as a N management tool in estimating plant N status

in irrigated pumpkins, and to a lesser degree in dryland production. The SPAD

values derived in this study should hold true over a wide range of N fertility

conditions, and may be applicable for other pumpkin and winter squash types.

However, because pumpkin N requirements could vary with cultivar and growing

conditions, a range of SPAD values is proposed.

REFERENCES

1. Gerber, J.M.; Swiader, J.M. Fertilizer Guide for Commercial Vegetable

Growers, Coop. Ext. Ser. Cir. 1185; Univ. of Ill.: Urbana, IL, 1985.

2. Swiader, J.M.; Sipp, S.K.; Brown, R.E. Pumpkin Growth, Flowering, and

Fruiting Response to Nitrogen and Potassium Sprinkler-Fertigation in

Sandy Soil. J. Am. Soc. Hortic. Sci. 1994, 119, 414–419.

3. Takebe, M.; Yoneyama, T. Measurement of Leaf Color Scores and its

Implication to Nitrogen Nutrition of Rice Plants. Jpn. Agr. Res. Quart.

1989, 23, 86–93.

4. Bullock, D.G.; Anderson, D.S. Evaluation of the Minolta SPAD-502

Chlorophyll Meter for Nitrogen Management in Corn. J. Plant Nutr. 1998,

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5. Piekielek, W.P.; Fox, R.H.; Tooth, J.D.; Macneal, K.E. Use of Chlorophyll

Meter at the Early Dent Stage of Corn to Evaluate Nitrogen sufficiency.

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9. Hartz, T.K.; LeStrange, M.E.; May, D.M. Nitrogen Requirements of Drip-

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10. Minotti, P.L.; Halseth, D.E.; Sieczka, J.B. Field Chlorophyll Measurements

to Assess the Nitrogen Status of Potato Varieties. HortScience 1994, 29,

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11. Sandoval-Villa, M.; Guertal, E.A.; Wood, C.W. Tomato Leaf Chlorophyll

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13. Nelson, D.W.; Sommers, L.E. Total Nitrogen Analysis of Soil and Plant

Tissues. J. Assn. Offc. Anal. Chem. 1980, 63, 770–778.

14. Swiader, J.M.; Al-Redhaiman, K. Petiole-Sap Nitrate Response and

Sufficiency Ranges in Dryland and Sprinkler-Fertigated Pumpkins. J. Veg.

Crop Prod. 1999, 4, 45–56.

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