Osmotic Stress Temporarily Reverses the Inhibitions of Photosynthesis and Stomatal Conductance by Abscisic Acid—Evidence that Abscisic Acid Induces a Localized Closure of Stomata in Intact, Detached Leaves

Abstract

Ward, D. A. and Drake, B. G. 1988. Osmotic stress temporarily reverses the inhibitions of photosynthesis and stomatal conductance by abscisic acid—evidence that abscisic acid induces a localized closure of stomata in intact, detached leaves.—J. exp. Bot 39: 147–155. The influence of osmotic stress on whole leaf gas exchange was monitored in detached leaves of Glycine max supplied with an exogenous concentration (10⁻⁵ mol dm⁻³) of ±abscisic acid (ABA) sufficient to inhibit net photosynthesis and stomatal conductance by 60% and 70%, respectively, under a saturating irradiance and normal air. Raising the osmotic (sorbitol) concentration of the ABA solutions feeding leaves elicited rapid and synchronous reversals of the ABA-dependent inhibitions of net photosynthesis and conductance. These reversals reached a peak simultaneously, after which photosynthesis and conductance declined. The magnitude of the transient stimulations at peak height was dependent upon the sorbitol concentration of the ABA feeding solution, although the time-course of the transients (half time, 4–6 min) was similar for the different osmotic concentrations applied. Irrespective of transient size the relative changes of photosynthesis and conductance were comparable; consequently the calculated partial pressure of CO₂ in the substomatal space (Ci) remained relatively constant during the transient phase. In contrast to the ABA-treated leaves, elevating the osmotic concentration of the distilled water supply feeding control leaves stimulated conductance to a much greater relative extent than photosynthesis. The co-stimulations of photosynthesis and conductance induced in ABA-treated leaves by osmotic shock were not due to a restriction in the transpirational uptake of ABA and occurred irrespective of the source osmoticum applied. These data are consistent with the hypothesis that the ABA-dependent inhibition of photosynthesis at constant Ci is an artifact caused by the spatially heterogeneous closure of stomata in response to ABA. Alternative explanations for the responses are, however, considered.