Leaf Gas Exchange and Water Relations of Lupins and Wheat. III. Abscisic Acid and Drought-Induced Stomatal Closure

Abstract

Changes in the content of endogenous abscisic acid (ABA) were followed in glasshouse experiments during stomatal closure induced by drought in leaves of lupin (Lupinus cosentinii Guss. cv. Eregulla) and wheat (Triticum aestivum L. cvv. Gamenya and Warigal), species which differ in stomatal sensitivity to changes in leaf water potential. Increases in bulk leaf ABA concentration were closely correlated with decreases in leaf conductance in both species. In lupin, substantial increases in ABA and decreases in conductance occurred over a very narrow range of leaf water potential. ABA concentrations in wheat leaves were highly negatively correlated with bulk leaf turgor, but there was no significant relationship between ABA and turgor in lupin. However, ABA accumulated progressively in the leaves of both species as soil water content decreased. Stomatal closure in lupin could be induced by supplying exogenous ABA to detached leaves via the transpiration stream at concentrations of 10-4 to 10-2 mol m-3 of (+)-ABA. Abaxial stomata closed more readily than those on the adaxial surface in response to both drought and applied ABA. Stomatal response to ABA was not affected by the presence of the cytokinin zeatin, and zeatin by itself had no effect on conductance. When treatments designed to reduce endogenous cytokinin concentrations were imposed (prolonged leaf detachment or prior drought), stomatal response to low concentrations of ABA was enhanced. However, such treatments did not significantly change the stomatal response to high ABA concentrations, nor affect the stomatal conductance of leaves supplied with water alone. It is concluded that drought-induced stomatal closure could be mediated by ABA in both wheat and lupin, despite the initially small change in leaf water status in the latter species.