Rate and Duration of Seed Component Accumulation in Water Stressed Soybeans.

Lack of adequate soil moisture during seed filling limits seed size in soybean (Glycine max L.) generally by shortening the duration of seed fill.  Seed growth rate typically remains largely unaffected. The smaller seeds often have increased seed protein concentration due to less detrimental effects on final protein accumulation relative to other major seed components. Such changes in seed composition despite stable rates of seed filling suggest compensation may occur among component accumulation rates.  Or the duration of accumulation of individual seed components is affected differentially by water stress. We examined these possibilities in two closely related soybean lines differing in protein concentration subjected to limited soil moisture during seed filling. The rates and durations of protein, oil, and residual accumulation in the seed were estimated at two canopy positions. The results indicate that both responses to water deficit can occur. The rates of accumulation of the different seed components often compensated resulting in a constant seed growth rate.  This compensation however, typically favored protein accumulation.  To our knowledge, this is the first demonstration of a differential response among rates and durations of seed component accumulation when soil moisture becomes limiting. The relative stability of protein accumulation likely reflects the availability of mobilizable N late in seed filling and the dependence of oil accumulation on current assimilate supply.