DIURNAL TIME-COURSE OF LEAF GAS-EXCHANGE RATES AND RELATED CHARACTERSIN DROUGHT-ACCLIMATED AND IRRIGATED TRIFOLIUM-SUBTERRANEUM

The effects of drought acclimation on the diurnal time courses of phot osynthesis and related characters were studied in Trifolium subterrane um L. leaves during two consecutive late spring days. Leaf CO2 assimil ation rate and transpiration rate followed irradiance variations in ir rigated plants. Under drought, a bimodal pattern of leaf CO2 assimilat ion rate developed although stomatal conductance remained uniform and low. Instantaneous water-use efficiency was much higher in droughted p lants during the early morning and late evening, while during the midd le of the day it was close to the value of irrigated plants. Net carbo n gain in plants under drought reached 40% of the carbon gain in irrig ated plants with a significant saving of water (80%). Average data der ived from midday values of leaf CO2 assimilation rates and instantaneo us water-use efficiency did not provide good estimates of the daily ca rbon gain and water-use efficiency for droughted leaves. Coupled with the morphological. changes as a result of acclimation to progressive d rought, modifications of diurnal patterns of leaf gas exchange rates e ffectively contribute to a sustained carbon gain during drought. These modifications significantly improve water-use efficiency, mainly by e nabling the plant to take advantage of morning and evening hours with high air humidity.