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Articles

Soybean Adaptation to Water Stress at Selected Stages of Growth ¹

United States Department of Agriculture, Agricultural Research Service, Auburn University, Auburn, Alabama 36849, Department of Agronomy and Soils, Auburn University, Auburn, Alabama 36849, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan, Department of Botany, Plant Pathology, and Microbiology, Auburn University, Auburn, Alabama 36849, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan

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.

¹ This study was supported in part by the United States Department of Agriculture, Agricultural Research Service, and by Auburn University Agricultural Experiment Station Projects Ala. 451 and 870. Additional support from the National Science Foundation under the United States-Japan Cooperative Science Program (Grant INT 8017281) and the Japanese Society for the Promotion of Science is gratefully acknowledged. Paper no. 3-82286 of the Alabama Agricultural Experiment Station.

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