The rate of CO2 assimilation photosynthetic efficiency and assimilate accumulation were measured in 15 field-grown NS sunflower (Helianthus annuus L.) lines and hybrids. Transpiration and water use efficiency were also measured in sunflower leaves in order to evaluated the behavior of the examined genotypes under water stress. Photosynthetic efficiency, determined at saturating CO2 and at low light intensity, was high in sunflower leaves. At a high light intensity and ambient C02, however, the rate of carbon assimilation was 1/3 of the potential rate of photosynthesis at saturating COz. The tested sunflower genotypes differed in the efficiency of water use and transpiration at a high light intensity and ambient CO2. Our results on water use efficiency and/or transpiration efficiency can be used as indicators of hybrid sunflower adaptability to arid conditions.