Adaptive changes in soybean and cotton to water stress I. Transpiration changes in relation to stomatal area and stomatal conductance

Inamulla and A. Isoda

The adaptive changes in soybean and cotton at various water scarcity levels were explored in terms of transpiration, staomatal role in transpiration, leaf temperature (TL) and CO2 assimilation rate (AN). Soybean showed lower flow rate of stem sap (FRSS), transpiration rate (E), stomatal conductance (gs), and AN, smaller size of stomatal area and stomatal density while larger leaf area, root dry matter weight and higher TL than those of cotton in all irrigation treatments. In water stress treatments, decrease in FRSS, E, gs, and AN of soybean and increase in its TL were larger than those of cotton. Stomatal area decreased in high water stress treatment non-significantly but stomatal density was not affected by the irrigation treatment in soybean. Stomatal area decreased significantly in cotton in water stress treatments. We concluded that soybean and cotton adapted to water stress differently. Soybean adapted to water stress by reducing its transpiration while cotton adapted to water stress by maintaining higher transpiration as compared with soybean in water stress treatments. Soybean reduced its transpiration rate by reducing gs. Reduction in gs of soybean was due to reduced FRSS, which might have been affected by lower root moisture absorption efficiency. Higher transpiration in cotton was due to higher gs, which was supported by higher FRSS, larger stomatal area, and probably the diaheliotropism. Higher gs and transpiration rate may have controlled increase in TL due to which the cotton may have avoided photoinhibitory damages and smaller decrease was observed in its AN in water stress treatments.