Responses of Nitrate-Fed and Nitrogen-Fixing Soybeans to Progressive Water Stress

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Abstract
Soybean plants, receiving nitrate-N or dependent on nitrogen fixation, were subjected to progressive water deprivation under controlled environmental conditions in a growth room. Comparative studies were carried out with respect to well-watered, control plants. Stressed, nitrate-fed plants had relatively greater root development, higher transpiration rates per unit leaf area, lower threshold values of leaf water potential for stomatal closure and osmotic adjustment in upper leaves. Therefore, these plants were adopting mechanisms to achieve stress tolerance. In contrast, stressed, nitrogen-fixing plants adopted mechanisms orientated to avoid stress: lower transpiration rates, stomatal closure at higher leaf water potentials, and delayed onset of osmotic adjustment. Root development in these plants stopped under more severe stress. Nitrogen-fixing plants were more conservative in terms of water use than nitrate-fed plants.