Water use efficiency of rice plants under elevated CO2 and warming in two years with contrasting weather conditions

Water use efficiency (WUE) of rice (Oryza sativa L.) is a crucial factor to be considered in the management of water resources in paddy systems.This study investigated the changes in the WUE under elevated CO 2 concentration (e[CO 2 ]) and air temperature (eT air ) for two years with naturally varying weather conditions including solar radiation.Rice was cultivated under different [CO 2 ]-T air in 2019 -2020, with higher solar radiation during rice growing period in 2019 (2079 MJ m -2 ) than in 2020 (1929 MJ m -2 ).The WUE was calculated by analyzing the stable carbon isotope ratio ( 13 C) of rice plant tissues.e[CO 2 ] consistently increased WUE by > 80% in both years regardless of T air .Under any given [CO 2 ] and T air combination, the WUE of rice plants was consistently 10 -34% higher in 2020 than in 2019, suggesting that rice under lower solar radiation utilized water more efficiently by reducing transpiration.These results suggest that the weather variability needs to be taken into account in the assessment of water resources for developing irrigation schemes under warming with e[CO 2 ].