Effects of CO2 concentration and temperature on leaf photosynthesis and water use efficiency in maize

Elevated atmospheric CO2 has resulted in climate warming since the end of 21th century due to its greenhouse effect. Global warming coupled with elevated CO2 concentration could have a drastic consequence for physiological processes of maize (Zea mays L.), a stable plant with C4 photosynthetic pathway. Understanding photosynthetic processes, transpiration and water use efficiency of maize under different CO2 concentration and temperature would shed insight into how maize would respond to climate change. Using growing chambers, we measured growth, photosynthesis and water use efficiency of maize grown in six chambers with day/night temperature controlled at 25/19℃to 37/31℃ at ambient CO2 concentration (400 μmol/mol) and elevated CO2 concentration (800 μmol/mol), respectively. Other factors in the chambers were kept the same, with the relative humidity being 55% - 65%, photosynthetic photon flux density (PPFD) being 1000 μmol/m2·s, and daily 12 h photoperiod for 60 days. In each treatment, we measured the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), leaf water use efficiency (WUE), using a portable photosynthesis system (Licor-6400). In addition, we also measured plant biomass, leaf area, total carbon and nitrogen contents, and soluble sugars (glucose, fructose, and sucrose) in the plant. The results showed that the temperature had a dramatic impact on growth of the maize, whereas the biomass and height of the plants grown under the elevated CO2 were almost the same as those grown under the ambient CO2. It was also found that the response of Pn to temperature was not affected by CO2 when temperature was 25/19℃ and 37/31℃, while the elevated CO2 increased Pn by 16.4% (P<0.05) when temperature was risen to 37/31℃, suggesting that CO2 elevation might have improved the tolerance of maize to thermal stress thereby reducing thermal damage to maize leaves. Warming significantly increased soluble sugars concentrations, especially the concentration of fructose, when temperature was risen from 25/19℃ to 31/25℃, but followed by a decline when temperature was further risen to 37/31℃, alluding that thermal stress might have suppressed the photosynthesis leading to a decrease in soluble sugars content of the leaves. At temperature 37/31℃, the elevated CO2 boosted soluble sugars in the leaves due to the increased photosynthetic rates, while a further temperature rise reduced the water use efficiency (WUE) of the leaves. The elevated CO2 enhanced WUE by ameliorating the adverse effect of temperature on WUE, but under different mechanisms. When temperature was 25/19℃ and 31/25℃, the elevated CO2 improved WUE due to the reduction in Tr, while when the temperature was 37/25℃, the elevated CO2 alleviated the adverse effect of temperature on photosynthesis to improve WUE. The results suggested that climate warming may reduce Pn and WUE of maize, but the associated elevated atmospheric CO2 could alleviate the adverse impact of the warming on Pn thereby improving the WUE. These findings are helpful for adequately assessing the consequence of climate change for growth and water use efficiency of maize and have important implication for ecosystem management in response to climate change.