Soybean adaptation to water stress at selected stages of growth.

Soybean (Glycine max [L.] Merr. cv Braxton) plants were grown in sandy soil with only natural rainfall (N) or with supplemental irrigation (I). Water-stressed plants grew more extensive root systems, whereas irrigated plants developed larger shoots and smaller root systems. Maximum stomatal apertures were observed at the beginning of each photoperiod. Partial stomatal closure occurred each afternoon, but stomata of I plants remained open longer than those of N plants. Significant reductions in net carbon fixation rate generally accompanied decreases in stomatal aperture, which coincided with periods of high temperature, low relative humidity, maximum solar radiation, and water stress. Leaf water potential decreased from morning to afternoon, with a greater decrease observed for N plants. Midafternoon stomatal closure did not occur in N plants with very large root systems following a heavy rain which saturated the soil profile. With smaller root systems and greater evaporative demand from larger shoots, the I plants continued to show midafternoon stress following the heavy rain. The large root systems of the N plants absorbed sufficient water to meet shoot evaporative demand for several days following the rain. Root soil system resistance apparently contributed to the afternoon water stress in the I plants.