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.

An understanding of the physiological changes that have occurred in cultivar development in the past would be helpful in the continued production of high yielding cultivars of the various crop species. Field experiments using conventional cultural practices with supplemental irrigation were conducted in 1976 to determine the physiological basis for yield differences between old, low yielding soybean [Glycine max (L.) Merr.] cultivars (Lincoln' and ‘Dorman’) and new high yielding cultivars (‘Williams’ and ‘Essex’). Mechanical support was used to prevent lodging for comparison to control plots with no mechanical support. There was no lodging in Lincoln and Williams (Maturity Group III); whereas, in maturity group V, Dorman lodged significantly more than Essex in the control plots. However, there was no relationship between lodging and yield. Williams produced 34% more yield than Lincoln, primarily because of larger seed. Essex produced 88% more yield than Dorman, primarily because of more seed per unit area. Yield differences were not correlated with total shoot weights or CO2 uptake rates of a single leaf during vegetative or early reproductive growth. Rates of acetylene reduction were similar for Williams and Lincoln, but Essex had a higher rate than Dorman. The yield advantage of Williams resulted partially from a longer filling period (12%), which was associated with higher CO2 uptake and acetylene reduction rates late in the filling period. Essex and Dorman produced similar total shoot weights late in the filling period, so the yield advantage for Essex resulted from partitioning more photosynthate to the seed. The data suggest that yield improvement in the future may be possible by lengthening the filling period. It may not be possible to increase seed yields by increasing the partitioning of photosynthate to the seed if the partitioning in current cultivars is approaching the maximum.