Abstract: Exploring the impacts of CO₂ and soil water availability on the photosynthetic performance and water use efficiency of three green manure plants could provide theoretical basis for the adaptive management of grassland ecosystems under future climate change. An experiment was conducted in an artificial climate chamber with precisely controled CO₂ concentrations of 400 (natural atmospheric) and 800 μmol·mol^-1 (doubled), and four water treatments, 80% field water holding capacity (FC) (full irrigation control group), 55%-60% FC (mild water deficit), 35%-40% FC (moderate water deficit), <35% FC (severe water deficit) to investigate the impacts of increasing CO₂ concentration and water deficit on chlorophyll content, gas exchange variables, and water use efficiency (WUE) of oilseed rape (Brassica napus), white clover (Trifolium repens), and alfalfa (Medicago sativa). The results showed that under the same CO₂ concentration, when soil moisture was less than 40% FC, the chlorophyll content and gas exchange parameters of three plants were significantly decreased. The treatment of 55%-60% FC did not alter the total chlorophyll content of three species, but reduced the photosynthetic rate (P_n) and transpiration rate (T_r) of white clover and alfalfa by 6%-25% and did not affect their WUE. Compared with atmospheric CO₂ concentration, the doubled CO₂ concentration significantly decreased the P_n of oilseed rape by 21.5% under the full irrigation treatment, increased the P_n of three species under mild water deficit, increased the P_n of oilseed rape and alfalfa under moderate water deficit, but only improved the P_n of alfalfa under severe water deficit. The doubled CO₂ concentration significantly increased WUE of white clover and alfalfa under all water deficit conditions, but only increased WUE of oilseed rape under mild water deficit. Increasing CO₂ concentration and water deficit significantly interacted to affect P_n of three species and the WUE of oilseed rape. In summary, the three species differed in their responses to doubled atmospheric CO₂ concentration and different levels of water deficit. Our results suggested that elevated CO₂ concentration could improve the adverse effects of mild water deficit on photosynthetic performance and WUE of three species, but only improve the photosynthetic performance of alfalfa under severe water deficit.

Key words: doubled CO₂ concentration, water deficit, gas exchange parameter, water use efficiency