Genotypic variation in yield under rainfed conditions and at a given level of evapotranspiration is mainly attributed to differences in dry matter produced per unit amount of water (Water Use Efficiency-WUE). WUE measured at the single leaf level by gas exchange techniques àrrelate well with that measured by the gravimetric method. WUE can be improved by either increasing the photosynthetic rate at a given stomâtal conductance or by decreasing the stomatal conductânce. Across species, photosynthetic rate is determined by the carboxylâtion efficiency at a given internal CO2 concentration. Carborylation efficiency in sunflower genotypes is mainly regulated by the efficiency of RuBisCO (A/RuBisCo). Selection of genotypes with efficient RuBisCO leads to increased photosynthetic rate and hence to higher WUE. We conclude that improvement of carboxylation efficiency could form an important strategy in increasing WUE and thereby the productivity both under rainfed and irrigated conditions.