No abstract

The redistribution of N from vegetative plant parts to the developing seed in soybeans [Glycine max (L.) Merrill] may influence the duration of seed filling and yield. The objective of this study was to investigate the N redistribution characteristics of soybean cultivars of varying maturities and growth habit. Eight cultivars ranging from Maturity Group II to V and including indeterminate, determinate, and semi‐dwarf growth habits were grown in the field in 1977 and 1978 at Lexington, Ky. using conventional cultural practices. The soil type was a Lanton silt loam (Cumulic Haplaquolls) in 1977 and an Eagam silt loam (Cumulic Hapludolls) in 1978. Nitrogen redistribution was estimated by harvesting plants at beginning seed growth (R5) and at maturity. The abscised leaf blades and petioles were also collected and the dry weight and total N was measured in all plant parts. The vegetative dry weight at RS increased in cultivars of later maturity. There were no consistent culvar differences in N concentration at R5. The proportion of seed N that came from redistribution varied from 30 to essentially 100% and there were significant cultivar differences. The cultivar differences were positively correlated with the amount of N in the plant at R5 which was determined primarily by the vegetative dry weight at R5. Late maturing cultivars got more of their seed N from redistribution than early maturing cultivars. Although there were significant cultivar differences in yield and the duration of seed fill, they were not related to the amount of seed N that came from redistribution. Nitrogen redistribution does not appear to be an important factor determining the duration of seed filling or yield in soybeans.

Comparisons were made of the accumulation of dry matter in the various plant components of a determinate (D66‐5566) strain had an indetermate (‘Kent’) soybean [Glycine max (L.) Merrill] cultivar using field‐grown plants for 2 years at Lexington, Ky. Kent and D66‐5566 are of similar maturity and yielding ability.At initial flowering D66‐5566 had reached 84% of its maximum height, compared with 64% for Kent. At this point the stem dry weight of D66‐5566 was 67% of the maximum compared with 30% for Kent. Although stem elongation of the determinate type was nearly complete when flowering began, it continued to increase in dry weight.At initial flowering Kent had produced 58% of its total vegetative material, compared to 78% for D66‐5566. Both types produced very little vegetative growth after a measurable amount of dry weight had accumulated in the pods and seeds. Kent produced more vegetative material during flowering and pod set, and the potential for competition for photosynthate between reproductive and vegetative growth was greater.The rate of accumulation of dry matter in the pods and seeds was greater for D66‐5566 than Kent in 1970, but there was no significant difference in 1971. The rates of both types were greater in 1970 than 1971, although grain yields were greater in 1971 than 1970. The length of the effective filling period, i.e., the grain yield divided by the rate of grain filling, was longer for both types in 1971 than 1970, and was thus positively associated with grain yields.