Interactive effect of elevated CO₂ concentration and drought on photosynthetic and physiological indexes of foxtail millet

There is a lack of knowledge on the interactive effects of elevated atmospheric carbon dioxide (CO₂) concentrations (CO₂) and drought on the photosynthesis and physiological processes underlying foxtail millet drought resistance. An experiment was conducted in an open-top chamber with two CO₂ treatments, ambient CO₂ (CK, 400 μmol·mol^-1) and elevated CO₂ (ECO₂, 600 μmol·mol^-1), and two water treatments, normal water (relative water content was 75%-85% soil capacity) and drought (relative water content was 35%-45% soil capacity). We quantified the interactive effects of elevated CO₂ and drought on the gas exchange parameters, fluorescence parameters, and drought-resistant physiological indicators in millet. The results showed that elevated CO₂ could reduce the content of photosynthetic pigment under drought conditions. Elevated CO₂ aggravated millet stomatal closure at the booting stage, alleviated the negative effects of drought on the net photosynthetic rate at the filling stage, and increased the water utilization efficiency. During the booting stage, elevated CO₂ resulted in a 66.7% reduction in stomatal conductance under normal water conditions and a 77.7% reduction under drought conditions. During the grouting period, under normal water conditions, elevated CO₂ led to a 19.0% increase in the grain net photosynthetic rate and a 37.1% increase in water use efficiency; under drought conditions, it led to an 87.7% increase in the grain net photosynthetic rate and a 39.2% increase in water use efficiency, respectively compared with that of ambient CO₂. Drought significantly reduced all of the millet fluorescence kinetic parameters, except non-photochemical quenching (NPQ), and elevated CO₂ alleviated this effect in the grouting period. At the booting stage, ECO₂ and drought showed significant interactive effects on the intrinsic efficiency of photosystem Ⅱ (PSⅡ) (F_v/F_m'), PSⅡ photochemistry (ΦPSⅡ), and the highest photosynthetic electron transport (ETR) and photochemical quenching coefficient (qP). Elevated CO₂ significantly reduced the content of cellulose and activity of peroxidase (POD) under normal water conditions. POD activity (under ECO₂), and contents of proline, soluble total sugar, starch content (under CK), and the cellulose content (under ECO₂) were significantly increased under drought conditions. We conclude that elevated CO₂ can enhance the drought resistance of C₄ foxtail millet by improving the photochemical efficiency of photosystem Ⅱ and the activity of antioxidant enzymes.