Seed yield and agronomic traits of old and modern soybean cultivars under irrigation and soil water-deficit

Field Crops Research, 27 ( 1991 ) 71-82 Elsevier Science Publishers B.V., Amsterdam

71

Seed yield and agronomic traits of old and modern soybean cultivars under irrigation and soil water-deficit* J.R. Frederick a, J.T. Woolley b, J.D. Hesketh b and D.B. Peters b apee Dee Research and Education Center, Route 1, Box 531, Florence, SC 29501, USA bUSDA-Agricultural Research Service, S-215 Turner Hall, 1102 S. Goodwin Ave., Urbana, IL 61801, USA (Accepted 2 August 1990)

ABSTRACT Frederick, J.R., Woolley, J.T., Hesketh, J.D. and Peters, D.B., 1991. Seed yield and agronomic traits of old and modem soybean cultivars under irrigation and soil water-deficit. Field Crops Res., 27: 71-82. An understanding of how soybean [ Glycine max (L.) Merr. ] cultivars of diverse yield potential respond to soil water-deficit would be valuable to soybean breeders in planning future selections. Our objectives were to evaluate differences in seed yield and agronomic trait performance under irrigation and soil water-deficit of soybean cultivars that vary in seed-yield potential. Two older, lower-yielding ('Manchu" and 'Dunfield') and two modern, higher-yielding ('Williams 82' and 'Clark 63") cultivars were drought-stressed between first flowering and harvest maturity in 1984 and 1985 on a Flanagan (Aquic Argiudoll) silt loam soil at Urbana, Illinois. Averaged over years, seed yields of Williams 82 and Clark 63 were 31 and 9% higher than those of Manchu and Dunfield under irrigation and drought stress, respectively. Relative to the irrigated conditions, the smaller seed-yield differences between the old and modern cultivars under drought stress resulted from smaller differences in pod number per branch. The old and modern cultivars did not differ in their apparent harvest index, plant number, branch number per plant, seed number per pod, and seed weight responses to soil water treatment. Leaf-area index of Williams 82 changed the most, and Manchu the least, in relation to soil water. Leaf-area duration in the lower portion of the crop canopy was.increased by drought stress to a similar extent for each cultivar, while the period of reproductive development was shortened by severe soil water-deficit more for Manchu and Dunfield than for the modem cultivars. The results of this study show greater seed-yield increases with irrigation for Williams 82 and Clark 63 than for the older cult ivars which were a function of a greater number of pods and increased vegetative biomass.

INTRODUCTION

Because soybean (Glycine m a x (L.) Merr. ) are often produced in areas of limited rainfall, further yield increases may be achieved by selecting for higher plant productivity under low soil water conditions (Boyer, 1982 ). Yield de*Contribution from the Department of Agronomy, Illinois Agriculture Experiment Station, Univ. of Illinois, and USDA-Agricultural Research Service, Urbana, IL 61801, U.S.A.

0378-4290/91/$03.50

© 1991 Elsevier Science Publishers B.V. All rights reserved.

72

J.R. FREDERICK ET AL.

creases resulting from drought stress depend both on the phenological timing of the stress and on the degree of yield-component compensation (Korte et al., 1983b). Drought stress applied during the flowering, pod formation, or seed-filling stages of development reduces seed yield due to decreases in the number of pods per plant, in the number of seeds per plant, or in individual seed weight, respectively (Korte et al., 1983b; Kadhem et al., 1985b). Cultivar differences in yield response to soil water treatment (Kadhem et al., 1985a), however, have been found to depend not only on differences in individual yield component responses, but also on differences in yield-component compensation (Kadhem et al., 1985b). The gradual development of soil water-deficit allows slow and continual changes of physiological processes in crop plants, often resulting in reduced crop growth and development (Jordan, 1983 ). The total leaf area of a crop is an important determinant of final seed yield (Sinclair et al., 1981 ), and is one of the components of growth that is most sensitive to drought stress (Jordan, 1983 ). Soil water-deficit can reduce canopy leaf area in soybean by decreasing individual leaf areas, reducing leaf numbers, and by increasing the rate of leaf senescence and abscission. Reproductive growth is also altered by drought stress, with soil water-deficit reported to decrease the duration of reproductive development (Korte et al., 1983a; Kadhem et al., 1985a) and the length of the seed-filling period (Sionit and Kramer, 1977 ). To fully optimize soybean seed yield, future breeding programs may be directed towards matching specific genotypes to specific production environments, such as areas of limited rainfall (Specht et al., 1986). When grown together over a large number of diverse environments, older soybean cultivars have been reported to have lower absolute yields than modem cultivars, but to have equal stability for yield performance (Wilcox et al., 1979). Examining only the effects of drought stress on yield performance, Mederski and Jeffers ( 1973 ) also found older soybean cultivars to have lower absolute yields than modern cultivars, but to be less sensitive to limited soil water. In contrast, Boyer et al. (1980) proposed that older soybean cultivars experience yield-inhibiting plant water-deficits at midday under rainfed conditions, and suggested that yield increases in new cultivars were associated with a more favorable shoot water status. The different observations of Boyer et al. (1980), Mederski and Jeffers (1973), and Wilcox et al. (1979) may have resulted from differences among studies in soil water conditions, although soil waterpotentials were not measured in any of those experiments. Information concerning the yield and agronomic-trait responses of lowerand higher-yielding cultivars to soil water treatment would be valuable to soybean breeders in planning future selections. Elucidating the physiological and/ or morphological factors that contribute to genotypic differences in soybean yield response to soil water would also aid in the selection of genotypes with high yields under drought stress (Specht et al., 1986). Therefore, our objec-

SOYBEAN UNDER IRRIGATION AND SOIL WATER-DEFICIT

73

rive was to determine whether widely adapted soybean cultivars of high seedyield potential were more sensitive to drought stress than widely adapted cultivars of lower seed-yield potential. An additional objective of this study was to determine how soil water-deficit modified the yield components and vegetative and reproductive traits that were associated with high yields under favorable soil moisture conditions. MATERIALS AND METHODS

Experimental design and methods of treatment application have been described in detail previously (Frederick et al., 1990). Four soybean cultivars were grown on a Flanagan (Aquic Argiudoll) silt loam soil at Urbana, Illinois in 1984 and 1985. Cultivars evaluated were 'Manchu' (released in 1911 ), 'Dunfield' (1927), 'Clark 63' (1963), and 'Williams 82' (1982). These cultivars were selected for this experiment based on their seed-yield stability over diverse environments, and because of their wide adaptation to regions of soybean production in Illinois during their respective era of production, as described by Frederick et al. (1990). Manchu and Dunfield were also selected for this experiment because they represent two of a limited number of soybean introductions that form the genetic base of a large number of recently developed cultivars (Specht and Williams, 1984), including Williams 82 and Clark 63 (Allen and Bhardwaj, 1987 ). Seeds were hand-sown 29 May and 30 May in 1984 and 1985, respectively. Treatments were assigned in a split-plot factorial arrangement in a randomized complete-block design with the two soil water treatments (soil water-deficit and natural rainfall plus supplemental irrigation) representing whole plots and cultivars as sub-plots. The soil water-deficit treatment was initiated two weeks before first flowering and was continued until harvest maturity. To obtain a drought stress in this treatment, rainfall was prevented from entering the soil by placing polyethylene film between the soybean rows, as described by Frederick et al. (1990). The film was rolled up during the day to allow for evaporation of soil water. Mainstem vegetative and reproductive development were monitored once per week, except one week before and after the termination of vegetative mainstem growth and the R5 and R7 growth stages (Ritchie et al., 1985), when plant development was evaluated twice a week. The seed-filling period was estimated to be the time between stages R5 and R7. Reproductive development and dates of leaf appearance were monitored on two sets often plants from each cultivar. In 1985, leaf-area duration was estimated as the period between leaf appearance and leaf senescence. On the plants used to monitor crop development, the first main-stem node (starting at the soil surface ) containing a live (non-senescent) trifoliate leaf was determined once per week. A trifoliate leaf was considered to be senescent if less than 50% of the leaf was green. At growth stage RS, a 1-m-long section of crop row was hand-harvested

74

J.R. FREDERICK ET AL.

from each experimental plot for measurement of leaf-area index, using a portable leaf-area meter (LI-3000, LI-COR Inc., Lincoln, Nebraska). Because of visual differences in canopy development observed among the cultivars in 1984, leaflet number m -2 crop row was counted and area per leaflet was calculated in 1985. At harvest maturity, two l-m-long sections of row were hand-harvested from each cultivar and used to determine seed yield components (plants m2; branches plant-l; pods branch-l; seeds pod-l; and individual seed weight). The sections were randomly selected from the center row (s) of each cultivar. The number of seed-bearing plants and the number of branches greater than 20 m m in length (including mainstem) were counted, and the samples were bagged to prevent pod or seed loss during handling and storage. The remainder of the center rows was hand-harvested for seed-yield determination. Each seed-yield and yield-component sample was oven-dried at 70°C for 3 days. The dried samples were weighed to determine the biological yield, and the number of pods per sample was counted. The seeds were cleaned, and total seed weight and the 200-seed weight were determined for each sample. These data were used to calculate the number of branches per plant, pods per branch, seeds per pod, apparent harvest index (HI; seed yield/biological yield at harvest maturity), and straw yield. Seed yield, straw yield, and plant number data were converted to an area basis, and seed yields and seed weights to a 13% moisture basis. Number of seeds m - 2 w a s calculated as the product of the number of plants m -2, branches plant- 1, pods branch- l, and seeds pod- ~. Data from each year were analyzed as a fixed-model, split-plot factorial. A Fisher's protected LSDo.o5was calculated to compare cultivars within each soil water treatment for those variables in which a significant cultivar effect was found. A combined analysis of the data, with years considered fixed, was conducted as described by McIntosh ( 1983 ). RESULTS AND DISCUSSION

Climatic and soil moisture conditions Greater amounts of precipitation were received during the growing season in 1985 than in 1984, with more overcast days and lower average monthly temperatures occurring in 1985 (Table 1 ). In both years, large differences in soil water-potentials were found between the irrigated and drought-stressed plots, with greater soil water-potential differences between treatments occurring in 1984 than in 1985 (Frederick et al., 1990).

Seed yield and agronomic traits Significance levels of the various treatment effects calculated from the combined analysis of the data are shown in Table 2 for each of the variables

SOYBEAN UNDER

IRRIGATION

75

AND SOIL WATER-DEFICIT

TABLE l S e a s o n a l c h a r a c t e r i s t i c s a t U r b a n a I L in 1 9 8 4 a n d 1 9 8 5

Month

June July

August September

Rainfall (mm)

Average air temp.

Average irradiation

(°C)

( M J m - 2 d a y -1 )

1984

1985

1984

1985

1984

1985

25 106 60 87

119 110 124 18

23.6 22.5 23.2 18.0

20.7 22.9 21.5 19.0

23.0 22.5 20.9 15.1

20.5 20.6 17.5 16.5

TABLE 2 S i g n i f i c a n c e levels a o f y e a r ( Y ) , soil w a t e r ( W ) , c u l t i v a r ( C ) , a n d interaction effects from c o m b i n e d a n a l y s i s ( 1 9 8 4 a n d 1 9 8 5 ) c o n d u c t e d o n s e e d y i e l d , a p p a r e n t h a r v e s t i n d e x , s t r a w y i e l d , s e e d yield c o m p o n e n t s , l e a f - a r e a i n d e x ( L ) , a n d d a y s t o p h y s i o l o g i c a l m a t u r i t y (PM)

Variable

Source of variation Y

W

Y× W

C

W× C

Y× W× C

Seed yield Harvest index Straw y i e l d Plants m- 2 Branches plant- 1 Pods branch- ~ Seeds pod- ~ Seeds m- 2

* ** ** ** ** ** * **

** * ** * ** ** n.s **

** n.s. * ** n.s. * n.s. •

** ** ** * ** ** ** **

** n.s. ** n.s. n.s. ** n.s. **

n.s. n.s. * * n.s. * n.s. •

Seed weight L

n.s. *

** **

n.s. *

** *

n.s. n.s.

** n.s.

PM

**

**

*

**

*

**

a . a n d ** i n d i c a t e s i g n i f i c a n c e a t t h e 0 . 0 5 a n d 0.01 levels, r e s p e c t i v e l y ; n.s. a c t i o n w a s n o t s i g n i f i c a n t a t 0 . 0 5 level.

indicates effect or i n t e r -

measured. A significant year effect occurred for each of the variables except individual seed weights (Table 2). Seed yields in 1984 generally were higher under irrigated conditions and lower under drought-stressed conditions than in 1985 (Table 3 ). This year× soil-water-treatment interaction (Table 2 ) may have resulted from the greater severity of the drought-stress treatment in 1984 compared with 1985 (Frederick et al., 1990) and the more frequent clear days in 1984 (Table 1 ), which probably contributed to higher leaf photosynthetic rates under the irrigated conditions. Under irrigation in 1984, average seed yield of the modern cultivars (Williams 82 and Clark 63 ) was 43% higher than the average yield of the old cultivars (Dunfield and Manchu), but there were no significant differences in yield among cultivars under drought stress.

76

J.R. FREDERICKETAL.

TABLE 3 Effect o f soil water t r e a t m e n t ( d r o u g h t ( D ) versus irrigated ( I ) ) on seed yield, a p p a r e n t harvest index, a n d straw yield o f the m o d e r n ( W i l l i a m s 82 a n d Clark 63 ) a n d older ( D u n f i e l d a n d M a n c h u ) s o y b e a n cultivars in 1984 a n d 1985 Cultivar/Treatment

G r a i n yield ( k g h a -1 )

Apparent harvest index ( k g k g - l )

Straw yield (gm -2)

1984

1985

1984

1985

1984

1985

D I

2152 3657

2536 2875

0.40 0.41

0.54 0.52

288 461

185 256

Clark 63 D I

1927 3241

2488 2775

0.40 0.39

0.51 0.48

257 504

214 289

1875 2413

2137 2217

0.39 0.35

0.48 0.45

279 396

226 247

D I LSDa

1937 2398 430

2425 2533 98

0.40 0.40 0.02

0.52 0.49 0.04

266 332 51

195 244 n.s.

Effects b Soil water Cultivar Interaction

** ** **

** ** **

* * **

* ** n.s.

** ** **

* n.s. n.s.

Williams82

Dunfield D I

Manchu

aLSD for c o m p a r i s o n o f cultivars within a soil water t r e a t m e n t . b • a n d ** indicate significance at the 0.05 a n d 0.01 levels, respectively, n.s., n o t significant at the 0.05 level.

In 1985, the modern cultivars yielded 19% more than Manchu and Dunfield under irrigation, and had significantly greater yields than Dunfield under soil water-deficit. The apparent harvest indices (HIS) were lower in 1984 than 1985 under both soil water treatments with only small increases in HI found due to drought stress (Table 3). The old and modern cultivars did not differ in their HI responses to drought stress, because differences in seed-yield responses among the old and modern cultivars due to soil water-deficit were associated with similar differences in straw-yield responses (Table 3 ). Averaged over cultivars, straw yields in 1984 were 33 and 63% higher under drought stress and irrigation, respectively, than those in 1985. The higher straw yields in 1984 may have partially resulted from the greater irradiation in that year (Table 1). Under drought stress, the old and modern cultivars did not differ in number of seed-bearing plants and branches per plant (Table 4). Only in 1984

SOYBEAN UNDER IRRIGATION AND SOIL WATER-DEFICIT

77

TABLE 4

Effect of soil water treatment (drought ( D ) versus irrigated ( I ) ) on seed yield components of the modern (Williams 82 and Clark 63) and older (Dunfield and Manchu) soybean cultivars in 1984 and 1985 Cultivar/Treatment

Plantsm -2

Branches plant - ~

Pod a

Seeds

branchNo. p o d - ~

No.m

2

Weight ( m g )

1984

1985

1984

1985

1984

1985

1984

1985

1984

1985

1984

1985

32.4 34.1

33.9 33.9

1.8 1.3

2.2 1.9

10.4 17.8

8.4 11.1

2.3 2.4

2.4 2.4

1395 1894

1504 1716

167 186

168 173

31.5 33.0

33.9 34.1

1.5 1,2

2.1 2.0

12.0 24.7

9.3 12.5

2.3 2.5

2.4 2.3

1304 2445

1589 1961

148 151

151 151

31.9 31.5

33.4 33.0

1.5 1,3

1.8 1.8

11.9 16.6

11.4 12.2

2.1 2.1

1.9 1.9

1196 1428

1302 1377

160 164

157 171

D I LSDb

27.8 32.4 2.6

33.0 31.9 n.s.

1.7 1.3 n.s.

2.4 2.2 0.4

12.7 15.2 2.4

9.0 10.9 1.3

2.2 2.2 0.1

1.9 2.0 0.2

1320 1408 217

1354 1530 103

154 182 16

177 178 6

Effects c Soil water Cultivar Interaction

** * n.s.

n.s. n.s. n.s.

** n.s. n.s.

n.s. * n.s.

** ** **

** ** *

n.s. ** n.s.

n.s. ** n.s.

** ** **

** ** *

** ** n.s.

** ** **

Williams 82 D I

Clark 63 D I

Dunfleld D I

Manchu

aNumber includes main stem. bLSDfor comparison ofcultivars within a soil water treatment. c • a n d ** indicate significance at the 0.05 and 0.01 levels, respectively. n.s.. not significant at the 0.05 level.

was the number of seed-bearing plants decreased by soil water-deficit, with smaller plants in the crop row observed to produce no pods and/or to die prematurely. In contrast, soil water-deficit increased the number of branches per plant in 1984 (Table 4), with smaller increases found in 1985 ( P < 0.08). In 1984, the modern cultivars had a greater number of pods per branch under irrigation than the older cultivars, but in 1985 the number of pods per branch produced by the modern cultivars under irrigation was similar to that of the older cultivars (Table 4 ). In comparison to 1984, the fewer number of pods per branch in 1985 for the irrigated Williams 82 and Clark 63 plants contributed to their lower yields in 1985. The pod number per branch responses of the cultivars to soil water-deficit were similar to those of the seed yields. In both years, the modern cultivars had greater decreases in pod number per branch as a result of drought stress than did the older cultivars, resulting in a similar number of pods per branch among cultivars under soil waterdeficit. Because the old and modern cultivars did not differ in branch number

78

J.R. FREDERICK ET AL.

per plant responses to drought stress, pod number per plant differences between the old and modern cultivars in response to drought stress were similar to those of the pod number per branch (data not shown). Soil water treatment did not affect the number of seeds per pod in either year (Table 4). However, over both years and both water treatments the modern cultivars produced 20% more seeds per pod than the older cultivars. The modern cultivars generally had a greater number of seeds m-2 than the older cultivars under irrigation, with smaller differences between the old and modern cultivars found under drought stress (Table 4). Because the old and modern cultivars did not differ in their plant number m-2, branch number per plant, and seed number pod-1 responses to drought stress, the smaller percentage decrease in seed number m -2 for the older cultivars as a result of soil water-deficit was primarily due to their smaller percentage reductions in pod number per branch. Comparing old and new cultivars, Gay et al. (1980) proposed that the greater seed number per unit area for the newer cultivar they examined was due to a more efficient partitioning of photosynthate to the seed. Our HI data, on the other hand, suggest that the old and modern cultivars did not differ in their partitioning of photosynthate between vegetative and reproductive structures. In the drier year of 1984, seed weights decreased an average of 8% by drought stress, but in 1985 the seed weights were reduced only 3% (Table 4). Over all cultivars, little association was found between individual seed weight under irrigation and the size of the reduction in individual seed weight that resulted from drought stress. The old and modern cultivar did not differ in their individual seed-weight responses to soil water deficit. Averaged over cultivars, leaf-area index (L) was reduced 38% by soil waterdeficit in the drier growing season of 1984, but decreased only 8% (nonsignificant) in 1985 (Table 5). In both years, Williams 82 had the largest percentage decreases in L due to drought stress, and Manchu the smallest. No consistent differences in L were found between the old and modern cultivars under drought stress. Leaf-area indices generally were larger in 1985 under both soil water treatments, which reflected the higher soil water-potentials in that year (Frederick et al., 1990). Although drought stress reduced the area per leaflet of each cultivar, it had little effect on their leaflet number (Table 5 ). These responses indicate that leaf expansion was more sensitive to soil water-deficit than was leaf initiation, as reported by others (Muchow et al., 1986 ). Although the modern cultivars had a greater number of leaflets than the older cultivars under irrigation, the cultivars did not differ in leaflet number under drought stress. Senescence of lower leaves in the crop canopy was delayed in the droughtstressed plots to a similar extent for each cultivar (Fig. 1 ). However, the rate of leaf senescence was enhanced or unaffected for leaves in the upper portion of the canopy. Although the rapid development of drought stress has previously been reported (Muchow et al., 1986 ) to increase the rate of leaf senes-

SOYBEAN UNDER IRRIGATION AND SOIL WATER-DEFICIT

79

TABLE 5

Effect of soil water treatment (drought ( D ) versus irrigated ( I ) ) on leaf-area index ( L ) , area per leaflet, leaflet number, days to physiological maturity, and seed-filling period of the modem (Williams 82 and Clark 63) and older (Dunfield and Manchu) soybean cultivars in 1984 and/or 1985 Cultivar/Treatment

L

Leaflets ( 1985 )

(m 2 m-2)

Days to

Seed-filling period

maturity

(days)

1984

1985

Area (cm 2)

No. m -2

1984

1985

1984

1985

2.52 4.88

4.42 5.37

34.2 42.0

1288 1278

105 111

112 114

29 34

38 40

3.10 4.51

4.51 5.29

34.2 40.0

1315 1328

105 ll0

112 114

28 29

35 35

3.22 5.01

4.70 5.06

39.6 49.7

1197 1024

95 105

105 107

24 28

30 31

D I LSDa

2.91 4.18 0.57

4.26 3.99 n.s.

31.3 34.7 5.6

1372 1157 127

92 104 4

106 108 2

23 30 4

35 38 5

Effects b Soil water Cultivar Interaction

** * *

n.s. n.s. n.s.

** *. n.s.

Williams82 D 1

Clark 63 D I

Dnnfield D I

Manchu

.

.

n.s. . n.s.

.

** . *

.

** . n.s.

** .

n.s. .

n.s.

n.s.

aLSD for comparison of cultivars within a soil water treatment. b * and ** indicate significance at the 0.05 and 0.01 levels, respectively. n.s., not significant at the 0.05 level.

cence in soybean, Jordan (1983 ) proposed that leaf-area duration may not be seriously altered when soil water-deficit develops gradually during vegetative development. Our data support this concept, because the plants responded to drought stress by producing smaller leaflets without increasing the rate of leaf senescence or reducing leaflet number. In the drier year of 1984, average date of physiological maturity of the cultivars was 8 days earlier under drought stress than under irrigation (Table 5 ). The number of days to maturity was shortened by soil water-deficit more for the older cultivars than for the modem cultivars. In 1985, the number of days to maturity was only slightly reduced by drought stress for each cultivar. Only in 1984 was there a significant reduction in the length of the seed-filling period due to soil water-deficit (Table 5 ). In 1984, the number of days that the seed-filling period was decreased by drought stress only partially accounted for the number of days that the time to maturity was shortened, although similar reductions occurred in both variables in 1985.

80

J.R. F R E D E R I C K ET AL. 6(3

60

~

• " • Irrigated

~

~°

50

~

40 ¸

40

~ 30m u~ 20-

D-II I r r i g a t e d

ght

d'

4

30

~'/~"•"l"

WILLIAMS82

CLARK63

20

10

10 5

10

15

0

5

i0

15

60

60I-I

Irrigoted

Al~ Drought

50-

m-ell Irrigated

50

~Droughf

40

40-

3o

,.

20

[XlNRN_D

MANO-U

10

10 5

10 NODE I ' I ~ I ~ R

15

.-.

0

. . . . . . . . . . . . . . . . . 5 10 15 NODE NUMBER

Fig. 1. Leaf-area duration oftrifoliate leaf on each mainstem node for the irrigated and droughtstress treated Williams 82, Clark 63, Dunfield and Manchu soybean plants in 1985. Node number 1 indicates node containing unifoliate leaf. Asterisk indicates that difference between soil water treatments at that node was significant at the 0.05 probability level.

Even though the older cultivars attained physiological maturity earlier under irrigation than the modem cultivars, only the Dunfield plants in 1985 had a significantly shorter seed-filling period than the modem cultivars. Williams 82 had the longest duration of seed-fill in both years, but Clark 63 had a shorter filling period than Manchu. Therefore, a consistent relationship between seedyield potential and duration of seed-fill was not demonstrated for the cultivars we examined. The reported data have shown agronomic-trait differences between the old and modem soybean cultivars in response to soil water treatment. Most of those differences between the old and modern cultivars were found in both 1984 and 1985, which differed in severity of the drought-stress treatment. In comparison to Dunfield and Manchu, the higher yields of the two modern cultivars under irrigation were associated with a greater number of seeds per pod. Williams 82 and Clark 63 were more yield-responsive to soil water treatment than were the older cultivars, primarily because of greater changes in pod number per branch. These results support the trends observed by Mederski and Jeffers ( 1973 ), but differ from those responses reported for maize (Zea mays L. ), where lower-yielding hybrids were more responsive to soil water treatment than higher-yielding hybrids (Frederick et al., 1989). In comparison with the modem soybean cultivars in the study reported here, the smaller difference in yield of the older soybean cultivars between the soil water

SOYBEANUNDERIRRIGATIONAND SOILWATER-DEFICIT

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treatments may have partially been due to the older cultivars greater stomatal closure under soil water-deficit (Frederick et al., 1990). In terms of leaf-area index, Williams 82 was the most responsive cultivar to increased soil water, and Manchu the least. Although drought stress delayed the rate of leaf senescence in the lower portion of the canopy for each cultivar, the old and m o d e m cultivars did not differ in the length of the delay. Apparent HI responses to soil water treatment were similar among the cultivars, indicating that the old and m o d e m cultivars used in this study do not differ in their partitioning of dry-matter between vegetative and reproductive structures. In addition, the seed and vegetative biomasses of the cultivars were reduced to a similar extent by drought stress. These results suggest either that the processes which limit vegetative growth under drought stress also reduce seed development, or that decreases in vegetative growth under soil waterdeficit directly limit reproductive development. Specht et al. (1986) suggested that increased yields under favorable soil water conditions may be achieved by selecting for increased vegetative dry-matter. Our data for the cultivars we examined support this hypothesis since seed-yield differences between the older and m o d e m cultivars were associated with similar differences in straw yield.

REFERENCES Allen, F.L. and Bhardwaj, H.L., 1987. Genetic relationships and selected pedigree diagrams of North American soybean cultivars. Univ. Tenn. Agric. Exp. Sm., Knoxville, Tech. Bull. 652. Boyer, J.S., 1982. Plant productivity and environment. Science, 218: 443-448. Boyer, J.S., Johnson, R.R. and Saupe, S.G., 1980. Afternoon water deficits and grain yields in old and new soybean cultivars. Agron. J., 72:981-986. Frederick, J.R., Hesketh, J.D., Peters, D.B. and Below, F.E., 1989. Yield and reproductive trait responses of maize hybrids to drought stress. Maydica, 34:319-328. Frederick, J.R., Woolley, J.T., Hesketh, J.D. and Peters, D.B., 1990. Water deficit development in old and new soybean cultivars. Agron. J., 82:76-81. Gay, S., Egli, D.B. and Reicosky, D.A., 1980. Physiological aspects of yield improvement in soybeans. Agron. J., 72:387-391. Jordan, W.R., 1983. Whole plant responses to water deficits: an overview. In: H.M. Taylor, W.R. Jordan and T.R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Plants. American Society of Agronomy, Madison, WI, pp. 289-317. Kadhem, F.A., Specht, J.E. and Williams, J.H., 1985a. Soybean irrigation serially timed during stages R 1 to R6. I. Agronomic responses. Agron. J., 77:291-298. Kadhem, F.A., Specht, J.E. and Williams, J.H., 1985b. Soybean irrigation serially timed during stages R1 to R6. II. Yield component responses. Agron. J., 77: 299-304. Kone, L.L., Williams, J.H., Specht, J.E. and Sorensen, R.C., 1983a. Irrigation of soybean genotypes during reproductive ontogeny. I. Agronomic responses. Crop Sci., 23:521- 527. Korte, L.L., Specht, J.E., Williams, J.H. and Sorensen, R.C., 1983b. Irrigation of soybean genotypes during reproductive ontogeny. II. Yield component responses. Crop Sci., 23: 528-533. Mclntosh, M.S., 1983. Analysis of combined experiments. Agron. J., 75:153-155.

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