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 coordination of plant responses to drought; however, the effect of ABA on g(m) is not welldefined. 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.