Effects of temperature on CO2 dependence of gas exchanges in C3 and C4 crop plants

The effects of elevated CO2 in the atmosphere and the accompanied temperature rise predicted for the future on gas exchanges of two summer C3 (rice, soybean) and two C4 Japanese millet, finger millet) crop plants were examined. Plants were grown in artificially illuminated growth cabinets under 350 and 500μ mol mol-1 ambient CO2 (Ca) and were measured for rates of CO2 exchange (CER) and transpiration (E) of leaves at 23, 28 and 33°C in terms of Ca (0-500μ mol mol-1). The responses of CER to Ca were slightly lower in plants grown in high Ca than those in normal Ca and were largely influenced by temperature. The promotive effect of elevating Ca on CER was larger at higher temperatures, especially in C4 crop plants. With the rise of Ca, the E in C4 crop plants decreased more than in C3 crop plants and it was correlated with the decrease in stomatal conductance to CO2 transfer. The water use efficiency (WUE) of leaves increased with the rise in Ca but the effect of temperature on WUE was unclear. It is concluded that, whthin limits, under high Ca, C4 crop plants expand their photosynthetic capacity in an environment of high temperature.