THE FLUX AND DISTRIBUTION OF XYLEM SAP CALCIUM TO ADAXIAL AND ABAXIAL EPIDERMAL TISSUE IN RELATION TO STOMATAL BEHAVIOR

Inherent differences in the responses of stomata on abaxial and adaxial epidermal surfaces of leaves of Commelina communis have previously been suggested to be due to differences in the concentrations of apoplastic Ca. Adaxial stomata have also been reported to be more sensitive than abaxial stomata to applied abscisic acid (ABA). The aims of these experiments were to determine the validity of these conclusions and to see if xylem sap Ca has a role in determining the response of stomata to ABA. It can be shown from measurements of relative stomatal resistance (determined with a viscous flow porometer) and stomatal conductance that stomata were more open in plants grown on 8.0 mol m-3 Ca, than with those grown on 2.0 mol m-3 Ca. When attached leaves were fed with ABA via the transpiration stream neither the extent nor the rate with which conductance declined was dependent on Ca nutrition. The extent of Ca accumulation within both epidermes was related to the concentration of Ca in the rhizosphere and in the xylem sap. It did not, however, appear to reflect the apparent differences in the flux of the transpiration stream between the two epidermes. Plants growing at the lower Ca concentration accumulated proportionally more epidermal Ca relative to Ca in xylem sap. The evidence presented suggests that Ca movement from the xylem to the epidermis cannot be simply described by a mass flow model, and that the distribution of Ca is not an adequate explanation of the differences in the behaviour of adaxial and abaxial stomata. The potential role for changes in xylem sap Ca to act as a regulator of stomatal behaviour are discussed.