Short periods of water stress during seed filling, leaf senescence, and yield of soybean

The effects of short periods of water stress during seed-filling on leaf senescence, seed-fill duration, and yield of soybean [Glycine mar (L.) Merrill] are not well understood. Short stress periods were investigated in two greenhouse experiments with cultivar Elgin 87 grown in soil-filled pots. All pots received adequate water until the beginning of growth stage R6 when a continuous water-stress treatment (pots received 40% of the water needed to bring controls to pot capacity) was initiated and maintained until maturity. Water stress was relieved in other pots (watered as the control) after 5 or 13 d in Exp. 1 and 3 or 6 d in Exp. 2. Each treatment was replicated six to eight times in a completely randomized design. The carbon exchange rate was rapidly reduced by continuous water stress resulting in earlier maturity, significantly lower yield (39%), and smaller seeds (25-33%). The carbon exchange rate rapidly increased to near control levels in the early stress-relief treatment, but it was always less than the control for the rest of seed filling. These plants matured sooner and produced significantly lower yields (10-23%) and smaller seeds (9-17%) than control plants. Late stress relief also reduced yield and seed size relative to the control. Yield and seed size of both stress relief treatments, however, were greater than the continuous stress treatment. Water stress-induced acceleration of senescence could not be stopped by eliminating the stress after a short period. Short periods of water stress during seed filling may, therefore, have larger than expected effects on yield.