Soybean Seed Growth. I. Timing of Growth of Individual Seeds 1

The seed‐filling duration for whole soybean [ Glycine mar (L.) Merr .] plants of indeterminate cultivars is not well defined, because the initiation and termination of rapid dry matter accumulation by individual seeds are sequential, not synchronous. Arbitrary selection of events to delimit duration may not identify durations which are physiologically important. To relate the contribution of individual seeds to whole‐plant seed‐filling duration, the times of initiation and termination of rapid growth were determined for individual seeds at all morphological positions of individual 'Chippewa 64' plants grown under field conditions in Ithaca, N.Y. The soil type was a Hudson silty clay loam (fine, illitic, mesic Glossaquic Hapludalf). Initiation was determined directly by determining the date when the ovule silhouette within intact pods reached a size of 2.5 ✕ 2.0 mm. Termination was defined as the time when cotyledon peripheries changed from green to yellow. Duration of growth of individual seeds was calculated as the time between these two events. Initiation of seed growth progressed sequentially up the stem of plants over periods of more than 21 days in some larger plants. Morphological characteristics other than node number influenced the time of initiation slightly. Seeds on secondary racemes initiated rapid growth 6 to 9 days later than seeds at the same nodes which were located on primary racemes. Seeds ceased growth in a pattern similar to that of initiation but over shorter periods. The long periods for initiation indicate that arbitrarily selected single events are less likely to identify physiologically well‐defined measures of seed‐filling duration.