Effects of increased atmospheric CO₂ concentration and temperature on carbon and nitrogen metabolism in maize at the grain filling stage

Future climate change will bring considerable challenges to agricultural production and food security. Presently, research on the effects of elevated CO₂ concentration and increased temperature on crops is mostly focused on C₃ crops, while research on C₄ crops is rare. Maize is the most widely planted C₄ crop in the world, it is of great significance to explore the response of maize to elevated CO₂ concentration, increased temperature, and their combination to assess the impacts of future climate change on C₄ crops. The maize variety ‘Xianyu-335’ was used. Four treatments were set up in controlled chambers: CK (CO₂ concentration 400 μmol·mol⁻¹, ambient temperature), EC (CO₂ concentration 600 μmol·mol⁻¹, ambient temperature), ET (CO₂ concentration 400 μmol·mol⁻¹, 2 ℃ higher than ambient temperature), and ECT (CO₂ concentration 600 μmol·mol⁻¹, 2 ℃ higher than ambient temperature). The related indices of photosynthetic physiology, glucose metabolism, and nitrogen metabolism of maize leaves were measured at the grain-filling stage, and the biomass of maize was measured after ripening. The results showed that: 1) under elevated CO₂ concentrations, the chlorophyll content, sucrose content, net photosynthetic rate, sucrose synthase activity, pyruvate kinase activity, and α-ketoglutarate dehydrogenase activity in leaves were significantly increased (P<0.05), while glutamate synthase activity was significantly decreased (P<0.05). Additionally, aboveground biomass and spike mass were significantly increased by 35.8% and 170.2%, respectively (P<0.05). 2) At increased temperatures, the net photosynthetic rate, and activities of sucrose synthase and pyruvate kinase of leaves were significantly increased (P<0.05), while α-ketoglutarate dehydrogenase and glutamate synthase activities were significantly decreased (P<0.05), and the above-ground biomass and the biomasses of leaf, stem, and spike were significantly decreased by 37.0%, 28.7%, 32.3%, and 62.2%, respectively (P<0.05). 3) Under the combination of elevated CO₂ concentration and increased temperature, the net photosynthetic rate and pyruvate kinase activity of leaves were significantly increased (P<0.05), whereas the chlorophyll content, and activities of α-ketoglutarate dehydrogenase and glutamate synthase were significantly decreased (P<0.05), and the leaf biomass was significantly decreased by 23.4% (P<0.05). In conclusion, elevated CO₂ concentration could alleviate the negative impact of increased temperatures on maize biomass by increasing photosynthesis and the activity of enzymes related to glucose metabolism and photosynthetic metabolites. Under elevated CO₂, increased temperature, or their combination, nitrogen metabolism in maize is inhibited; thus, leaves are subjected to nitrogen stress, which harms maize quality.