Objectives The objective of the study was to evaluate effects of doubled CO₂ concentration and drought stress on photosynthesis characteristics and non-structural carbohydrates (NSC) ofBothriochloa ischaemum, and provide scientific basis for the growth ofB. ischaemum under future elevated CO₂ concentration in arid and semi-arid areas.

Methods Using the split plot design, the effects of doubled CO₂ concentration and drought stress on the photosynthesis and NSC contents inB. ischaemum were investigated. The main plot was two CO₂ concentrations (ambient CO₂ concentration, 400 μmol/mol, and doubled CO₂ concentration, 800 μmol/mol), and the split-plot was three water treatments (severe drought stress, 35%–40% field capacity (FC), moderate drought stress, 55%–60% FC, and control, 75%–80% FC).

Results The results showed that the parameters of photosynthesis-light response curves ofB. ischaemum and the NSC contents were significantly influenced by the doubled atmospheric CO₂ concentration and drought stress, while there were no significant synergetic effects between them. The doubled CO₂ concentration significantly increased the maximum photosynthetic rate (P_max ), apparent quantum yield (AQY), light saturation point (LSP) and light compensation point (LCP) (P < 0.01), while the drought stress significantly decreasedP_max ,AQY andLSP (P < 0.01). The doubled CO₂ concentration and drought stress increased the soluble sugar content in the aboveground part ofB. ischaemum. Under ambient CO₂ concentration, the moderate drought stress and severe drought stress significantly decreased the starch contents in the aboveground and root parts ofB. ischaemum. The doubled CO₂ concentration increased the starch contents in the aboveground part ofB. ischaemum by 17.4%, 44.2%, and 18.7%, respectively under the control, moderate drought stress and severe drought stress, and in the root part by 17.3%, 88.4%, and 54.4%, respectively. Under ambient CO₂ concentration, the moderate drought stress and severe drought stress significantly decreased the NSC contents in the root part ofB. ischaemum. Under the doubled CO₂ concentration, the NSC contents in the aboveground part were significantly increased under the control and moderate drought stress treatments, and the NSC contents in the root part were significantly increased under the moderate drought stress and severe drought stress treatments. Under ambient CO₂ concentration, the moderate drought stress and severe drought stress significantly increased the ratios of soluble sugar contents and NSC contents in the aboveground and root parts ofB. ischaemum. Under the doubled CO₂ concentration, the severe drought stress significantly increased the ratio of soluble sugar content and NSC content in the aboveground part ofB. ischaemum, and the ratios under the control and moderate drought stress had no significant difference.

Conclusions Drought stress facilitated the transfer of starch into soluble sugar, and increased the soluble sugar content and decreased the starch content. The doubled CO₂ concentration increased the starch contents and NSC contents in the aboveground part and root part, and provided the source of soluble sugar that maintained metabolic activities and survival during the drought event. Therefore, it is speculated that in arid and semi-arid areas, elevated CO₂ concentration could increase plant photosynthesis, increase NSC accumulation, alleviate the adverse effect induced by drought stress, and improve the drought resistance. Our findings provided new insights into the underlying mechanisms and responses of plant species to future climate changes.