Effects of instantaneous and growth <scp>CO</scp>₂ levels and abscisic acid on stomatal and mesophyll conductances

Abstract

Abstract C 3 photosynthesis is often limited by CO 2 diffusivity or stomatal ( g s ) and mesophyll ( g m ) conductances. To characterize effects of stomatal closure induced by either high CO 2 or abscisic acid (ABA) application on g m , we examined g s and g m in the wild type (Col‐0) and ost1 and slac1‐2 mutants of Arabidopsis thaliana grown at 390 or 780 μmol mol −1 CO 2 . Stomata of these mutants were reported to be insensitive to both high CO 2 and ABA. When the ambient CO 2 increased instantaneously, g m decreased in all these plants, whereas g s in ost1 and slac1‐2 was unchanged. Therefore, the decrease in g m in response to high CO 2 occurred irrespective of the responses of g s . g m was mainly determined by the instantaneous CO 2 concentration during the measurement and not markedly by the CO 2 concentration during the growth. Exogenous application of ABA to Col‐0 caused the decrease in the intercellular CO 2 concentration ( C i ). With the decrease in C i , g m did not increase but decreased, indicating that the response of g m to CO 2 and that to ABA are differently regulated and that ABA content in the leaves plays an important role in the regulation of g m .