Abscisic Acid Induces Rapid Reductions in Mesophyll Conductance to Carbon Dioxide

The rate of photosynthesis (A) of plants exposed to water deficit is a function of stomatal (g_s) and mesophyll (g_m) conductance determining the availability of CO₂ at the site of carboxylation within the chloroplast. Mesophyll conductance often represents the greatest impediment to photosynthetic uptake of CO₂, and a crucial determinant of the photosynthetic effects of drought. Abscisic acid (ABA) plays a fundamental role in signalling and co-ordination of plant responses to drought; however, the effect of ABA on g_m is not well-defined. Rose, cherry, olive and poplar were exposed to exogenous ABA and their leaf gas exchange parameters recorded over a four hour period. Application with ABA induced reductions in values of A, g_s and g_m in all four species. Reduced g_m occurred within one hour of ABA treatment in three of the four analysed species; indicating that the effect of ABA on g_m occurs on a shorter timescale than previously considered. These declines in g_m values associated with ABA were not the result of physical changes in leaf properties due to altered turgor affecting movement of CO₂, or caused by a reduction in the sub-stomatal concentration of CO₂ (C_i). Increased [ABA] likely induces biochemical changes in the properties of the interface between the sub-stomatal air-space and mesophyll layer through the actions of cooporins to regulate the transport of CO₂. The results of this study provide further evidence that g_m is highly responsive to fluctuations in the external environment, and stress signals such as ABA induce co-ordinated modifications of both g_s and g_m in the regulation of photosynthesis.