THE INFLUENCE OF INCREASING RHIZOSPHERIC CALCIUM ON THE ABILITY OF LUPINUS-LUTEUS L TO CONTROL WATER-USE EFFICIENCY

The stomatal behaviour of Lupinus luteus L. was examined to determine the ability of a calcifuge to regulate water loss against a background of different rhizospheric Ca concentrations. A previous study had suggested that a cultivar of this species had limited stomatal sensitivity to ABA, and so the physiological events which enable L. luteus to adjust its water use efficiency (WUE) in response to changes in both atmospheric and soil water availability were explored in a number of ways. L. luteus did not lack the ability to control water loss and there was no evidence to suggest that ABA was unable to regulate stomatal behaviour during water deficits. Stomatal closure was shown to occur in a linear manner when different concentrations of ABA were applied. The presence of different concentrations of Ca within xylem sap did not influence the degree to which stomata closed in response to applied ABA. The patterns of stomatal behaviour in response to changes in atmospheric humidity, declining soil water potentials and applied ABA, were generally similar to those of other species. The presence of high rhizospheric Ca (8 mol m-3) did not apparently induce stomatal closure when gas exchange measurements were made at a DELTA-W of 10 mmol mol-1, when compared with plants grown on 1 mol m-3 Ca. However, when measurements of gas exchange were made over a range of DELTA-W values, the relationship between assimilation and stomatal conductance suggested that the degree of coupling between these two variables was less pronounced with plants grown at 8 mol m-3 compared to those at 1 mol m-3 Ca. The effect of this on the regulation of leaf conductance was to perturb WUE. There was little evidence here to support a role for changes in the flux of xylem sap Ca as a means of root-to-shoot signalling.