Effects of watering and CO₂ on leaf photosynthesis and water use efficiency of maize

Elevated atmospheric CO₂ concentration and soil water deficiency have posed some impacts on the plant growth, leaf gas exchange and biochemical characteristics of maize ( Zea mays L.). It is a high demand to explore the physiological and ecological responses of agricultural ecosystems to future climate change. This study aims to further understand the key processes and potential mechanisms of elevated atmospheric CO₂ concentration and soil water deficiency on the growth, physiological and biochemical characteristics of maize. Eight environmental growth chambers were employed to examine the changes in plant biomass, stomatal morphology and distribution, leaf gas exchange, and chlorophyll fluorescence of maize with the ambient CO₂ concentration (400 µmol/mol) and elevated CO₂ concentration (800 µmol/mol) along a soil water gradient, including full irrigation (75%~85% FC), mild water deficiency (65%~75% FC), moderate water deficiency (55%~65% FC), and severe water deficiency (45%~55% FC). A split-plot experiment was designed with two factors of CO₂ concentration and watering, where CO₂ concentration was the main plot with two levels, and watering was the subplot with four water treatments. Environmental growth chambers were selected to control the CO₂ treatments with high-purity CO₂ source from a CO₂ bottle tank. Specifically, the CO₂ concentration was maintained at an ambient CO₂ concentration of 400 µmol/mol in four environmental growth chambers, whereas, the target CO₂ concentration in the other four chambers was supplied with the elevated CO₂ concentration (800 µmol/mol). The results showed that the soil water deficiency significantly decreased the aboveground biomass (P<0.05), but the elevated CO₂ concentration increased the aboveground biomass and the total biomass of maize plants under mild water deficiency. Meanwhile, the contents of nitrogen and nonstructural carbohydrates in leaves were also outstandingly enhanced by the elevated CO₂ concentration. Moreover, the elevated CO₂ concentration substantially increased by 15.8% (P<0.05) and 25.7% (P=0.001), respectively, in the net photosynthetic rates (P_n) of maize plants under mild and moderate water deficiency, indicating the strong CO₂ fertilization effect on maize plants. However, the leaf transpiration rate ( T_r) and stomatal conductance (G _s) were significantly declined by elevated CO₂ concentration. Thus, the leaf-level water use efficiency (WUEI) was drastically enhanced under elevated CO₂ concentration (P<0.001). Furthermore, the elevated CO₂ concentration resulted in the decrease of chlorophyll under mild water deficiency. There was a significant increase in the photosynthetic electron transport rate (ETR) and photochemical quenching coefficient (qP) of maize plants subjected to mild water deficiency. Additionally, the soil water deficiency also dominated the stomatal density and morphology of maize leaves. But the most regular pattern of stomatal distribution was found on maize leaves under mild water deficiency. Therefore, the maize plants under mild water deficiency can benefit from the higher CO₂ concentration with “CO₂ fertilization effect” via plant biomass accumulation, leaf photosynthesis, and water use efficiency of maize. Thus, the deficiency irrigation during the growth of maize plants can be expected to serve as “CO₂ fertilization effect” from the future higher atmospheric CO₂ concentration. The finding can also provide scientific evidence to determine the physiological and ecological mechanisms of maize, in response to the elevated atmospheric CO₂ and soil water deficiency under future climate change.