Adaptive Responses of Soybean and Cotton to Water Stress: I. Transpiration Changes in Relation to Stomatal Area and Stomatal Conductance

The adaptive responses of soybean and cotton to various irrigation levels were explored in terms of transpiration, stomatal role in transpiration, leaf temperature (Tl) and CO2 assimilation rate (An). Compared with cotton, soybean showed a lower flow rate of stem sap (FRSS), transpiration rate (E), stomatal conductance (gs), stomatal density and An and had a smaller stomatal area but larger leaf area, heavier root dry matter and higher Tl at all irrigation levels. Under water stress conditions, FRSS, E, gs, and An decreased and Tl increased more in soybean than in cotton. Stomatal area decreased in response to water stress though nonsignificantly but stomatal density was not affected by water stress in soybean. Stomatal area decreased significantly in response to water stress in cotton. We concluded that soybean and cotton adapted to water stress differently. Soybean adapted to water stress by reducing transpiration while cotton adapted to water stress by maintaining higher transpiration as compared with soybean. Soybean reduced the transpiration rate by reducing gs. Reduction of gs in soybean was due to reduced FRSS, which might have resulted from the lower root moisture absorption efficiency. The higher transpiration in cotton was due to a higher gs, which was supported by a higher FRSS, larger stomatal area, and probably the diaheliotropism. The higher gs and transpiration rate suppressed the increase in Tl thus preventing the decrease of An in response to water stress

Adaptive Responses of Soybean and Cotton to Water Stress II. Changes in CO2 Assimilation Rate, Chlorophyll Fluorescence and Photochemical Reflectance Index in Relation to Leaf Temperature

Adaptive changes were studied comparatively in soybean and cotton grown in pots under four irrigation conditions i.e. normal irrigation (equal to the evapotranspiration of the crop), and 50%, 25% and 10% of the normal irrigation. In soybean, the maximum quantum yield of PSII (Fv/Fm) was generally higher while the actual quantum yield of PSII (∆F/Fm’) and CO2 assimilation rate (An) were lower than in cotton.The intensity of the decrease in Fv/Fm, ∆F/Fm’ and An by water-stress treatments was larger in soybean than in cotton. The decrease in ∆F/Fm’ in soybean under water stress was accompanied by a significant increase in non-photochemical quenching (NPQ) and significant decrease in photochemical reflectance index (PRI). Chlorophyll content decreased significantly under severe water stress only in soybean. The increase in leaf temperature (Tl) in response to water stress was significantly larger in soybean than in cotton. Tl was highly and negatively correlated with Fv/Fm, An, PRI and ∆F/Fm’ while it was highly and positively correlated with NPQ of both crops. Especially in soybean, the correlations of Tl with An, Fv/Fm and PRI were significant. It was concluded that soybean adapted to water stress by dissipating the excess excitation energy thermally with the down-regulation of PSII activity to protect its photosynthetic apparatus from the photodamaging effect of water stress and high Tl. This photoprotective mechanism might be supported by the paraheliotropic leaf movement of the crop. Cotton adapted to water stress by keeping Tl lower to protect the photosynthetic apparatus from photodamage. Probably higher transpiration kept Tl of the crop lower under drought stress