The ability of an individual soybean [Glycine max (L.) Merr.] seed to utilize assimilate is an important part of the yield production process. Cotyledon cell number is a major determinant of genetic differences in seed growth rate. Experiments were conducted in the field and greenhouse with ‘McCall’, ‘Williams’, and ‘York’ cultivars to determine if source‐sink alterations would affect cotyledon cell number. Source‐sink alteration treatments (66% defoliation, 80% depodding, and 63% insolation reduction) were applied when tagged fruits were 20 to 25 mm long. Seeds from the tagged fruits were harvested at maturity and cotyledon cells were counted. Defoliation or shading reduced cotyledon cell number by 21 to 55% for McCall and York in the greenhouse and Williams in the field. Depodding increased cotyledon cell number by 26 to 102%. Cotyledon growth rates in an in vitro culture system increased with increasing sucrose concentrations and reached a maximum at 100 to 200 mM. Cotyledon growth rates in vitro with saturating levels of sucrose and asparagine were closely correlated with cotyledon cell number across treatments and genotypes. The data indicate that cotyledon cell number in soybean is influenced by the physiological environment during the cell division phase of seed development and that both cotyledon cell number and assimilate supply are important in determining seed growth rate.

Yield and yield components of ‘Braxton’ soybeans [Glycine max(L.) Merr.], Maturity Group VII, were measured for differing irrigation regimes and intrarow spacings. Soybeans were planted in 0.91 m rows in mid‐May, 1980 and 1981 on a Cecil sandy loam soil (clayey, kaolinitic, thermic Typic Hapludults). Prior to the V3 growth stage, plots were thinned to achieve intrarow spacings of 61,76,102, 152,305, and 457 mm in 1980 with spacings of 43 and 51 mm added in the 2nd year. Soil water regimes were full‐season irrigation (FSI), irrigation beginning at bloom (BI), and no irrigation (NI). Beginning at R4, plants from a 0.5‐m section of row were removed at 10‐ to 14‐day intervals for determination of seed growth rate and effective filling period. Yield components (pod number, seed number, seeds per pod, and single seed weight) were determined at R7 or maturity. Final seed yields were determined by harvesting bordered rows following end trimming. Seed yields for the 2 years, averaged over all spacings, were 3023,2876, and 1322 kg/ha for the FSI, BI, and NI higation treatments, respectively. While yields were increased significantly with irrigation, no significant difference in yield was measured between FSI and BI treatments. Yields among plant spacings were not significantly different for any irrigation treatment either year with the exception that the yield of the 457‐mm spacing of the BI treatment was significantly reduced in 1981. Seed number was highly correlated with seed yield, whereas single seed weight was not significantly correlated with yield. Increases in seed number under the irrigation treatments were due to increases in both pod number and seed per pod. Seed growth rate was increased and effective filling period was decreased by irrigation. We concluded that intrarow spacings up to 457 mm had little influence on yield. Even though rainfall was low during vegetative growth prior to bloom in both years, full‐season irrigation gave no yield advantage over bloom irrigation.

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Achievement of maximum crop yields necessitates the study of factors influencing yields. The influence of irrigation regime and plant population on growth of a Group VII determinate soybean [Glycine max (L.) Merr.] was measured to determine if a particular growth pattern would result in increased yield. In 1980 and 1981, ‘Braxton’ soybeans were grown in 0.91‐m rows on a Cecil sandy loam (clayey, kaolinitic, thermic Typic Hapludults) under full‐season irrigation (FSI), irrigation beginning at bloom (BI), and no irrigation (NI). Intrarow spacings ranged from 43 or 61 to 457 mm. At growth stage R2, FSI, BI, and NI plants had attained 72, 66, and 75%, respectively, of maximum height and 38, 35, and 55%, respectively, of maximum vegetative dry weight. Height, stem diameter, and dry weight were greater in the irrigated treatments than in NI. Fullseason irrigated plants were generally taller than BI and NI plants and had 1.7 times more dry weight at R2. With increasing spacing, height decreased while stem diameter increased. Vegetative weight was maximized at spacings of 76 mm or less. Development of canopy cover and LAI was more rapid with decreasing spacing and with irrigation. At least 90% canopy closure was obtained in all spacings of FSI and BI, whereas only 59% was achieved in NI. Yields were the same for FSI and BI, and, within irrigation treatments, there were no yield differences among spacings of 305 mm or less. We conclude that comparable yields can be obtained within a wide range of growth patterns, and that severe reductions in vegetative growth at R2 may have little influence on yield if water is supplied thereafter.

Nodal and other distribution patterns of seed yield and yield components are useful in modeling soybean [Glycine max(L.) Merr.] growth, in analyzing the factors responsible for yield differences between experimental treatments, and in predicting yield. The purpose of this study was to determine the influence of irrigation and intrarow spacing on the distribution of yield and yield components in a determinate soybean cultivar. ‘Braxton’ soybeans were grown in 0.91 m rows on a Cecil sandy loam soil (clayey, kaolinitic, thermic Typic Hapludults) at intrarow spacings of 43, 61, 102, and 305 mm between plants and under three irrigation treatments: full‐season irrigation (FSI), irrigation beginning at bloom (BI), and no irrigation (NI). At maturity, samples were collected to determine pod number, seeds per pod, seed number, yield, and single seed weight for the main stem positions at each node and for the branches originating at each node. Branches contributed more pods, seeds, and yield than did main stems in all treatments, while seeds per pod and single seed weight were similar on stems and branches. Increases in intrarow spacing increased seed number and yield on branches and decreased pod number, seed number, and yield on the stems. Irrigation had little influence on the branch/stem ratios of yield components. Full‐season irrigation and BI had greater numbers of pods, numbers of seeds, yield, and seeds per pod on stems and branches and greater single seed weight on stems than did NI. With the exception of stem seeds per pod which was greater for BI, there were no differences between the two irrigated treatments. Seed number, pod number, and yield were maximized in the lower half of the nodes whereas seeds per pod and single seed weight were greatest in the upper nodes. The upper six nodes contributed the least to yield in all treatments. Increases in intrarow spacing increased the contribution of the lower nodes to yield whereas irrigation decreased the relative contribution of the lowest nodal division. We concluded that the large contribution of lower nodes to yield was due to the large proportion of yield borne on branches produced from lower nodes. Increases in spacing resulted in greater contribution to yield of these large branches and thus to a greater concentration of yield at lower nodes. Irrigation influenced distribution of yield through influences on distribution of all yield components.