| Abstract: | Inherent differences in the responses of stomata on abaxialand adaxial epidermal surfaces of leaves of Commelina communishave previously been suggested to be due to differences in theconcentrations of apoplastic Ca. Adaxial stomata have also beenreported to be more sensitive than abaxial stomata to appliedabscisic acid (ABA). The aims of these experiments were to determinethe validity of these conclusions and to see if xylem sap Cahas 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 conductancethat stomata were more open in plants grown on 8-0 mol m–3Ca, than with those grown on 2-0 mol m–3 Ca. When attachedleaves were fed with ABA via the transpiration stream neitherthe extent nor the rate with which conductance declined wasdependent on Ca nutrition. The extent of Ca accumulation within both epidermes was relatedto the concentration of Ca in the rhizosphere and in the xylemsap. It did not, however, appear to reflect the apparent differencesin the flux of the transpiration stream between the two epidermes.Plants growing at the lower Ca concentration accumulated proportionallymore epidermal Ca relative to Ca in xylem sap. The evidencepresented suggests that Ca movement from the xylem to the epidermiscannot be simply described by a mass flow model, and that thedistribution of Ca is not an adequate explanation of the differencesin the behaviour of adaxial and abaxial stomata. The potentialrole for changes in xylem sap Ca to act as a regulator of stomatalbehaviour are discussed. Key words: Abscisic acid, calcium, Commelina communis L., stomatal conductance |
