Elevated CO2 alleviates the impact of drought on barley improving water status by lowering stomatal conductance and delaying its effects on photosynthesis

Abstract We analysed the impact of elevated CO 2 on water relations, water use efficiency and photosynthetic gas exchange in barley ( Hordeum vulgare L.) under wet and drying soil conditions. Soil moisture was less depleted under elevated compared to ambient [CO 2 ]. Elevated CO 2 had no significant effect on the water relations of irrigated plants, except on whole plant hydraulic conductance, which was markedly decreased at elevated compared to ambient CO 2 concentrations. The values of relative water content, water potential and osmotic potential were higher under elevated CO 2 during the entire drought period. The better water status of water-limited plants grown at elevated CO 2 was the result of stomatal control rather than of osmotic adjustment. Despite the low stomatal conductance produced by elevated CO 2 , net photosynthesis was higher under elevated than ambient CO 2 concentrations. With water shortage, photosynthesis was maintained for longer at higher rates under elevated CO 2 . The reduction of stomatal conductance and therefore transpiration, and the enhancement of carbon assimilation by elevated CO 2 , increased instantaneous and whole plant water use efficiency in both irrigated and droughted plants. Thus, the metabolism of barley plants grown under elevated CO 2 and moderate or mild water deficit conditions is benefited by increased photosynthesis and lower transpiration. The reduction in plant water use results in a marked increase in soil water content which delays the onset and severity of water deficit.