Effects of different elevated CO2 concentrations on chlorophyll contents, gas exchange, water use efficiency, and PSII activity on C-3 and C-4 cereal crops in a closed artificial ecosystem

摘要

Although terrestrial CO2 concentrations [CO2] are not expected to reach 1000 mu mol mol(-1) (or ppm) for many decades, CO2 levels in closed systems such as growth chambers and greenhouses can easily exceed this concentration. CO2 levels in life support systems (LSS) in space can exceed 10,000 ppm (1 %). In order to understand how photosynthesis in C-4 plants may respond to elevated CO2, it is necessary to determine if leaves of closed artificial ecosystem grown plants have a fully developed C-4 photosynthetic apparatus, and whether or not photosynthesis in these leaves is more responsive to elevated [CO2] than leaves of C-3 plants. To address this issue, we evaluated the response of gas exchange, water use efficiency, and photosynthetic efficiency of PSII by soybean (Glycine max (L.) Merr., 'Heihe35') of a typical C-3 plant and maize (Zea mays L., 'Susheng') of C-4 plant under four CO2 concentrations (500, 1000, 3000, and 5000 ppm), which were grown under controlled environmental conditions of Lunar Palace 1. The results showed that photosynthetic pigment by the C-3 plants of soybean was more sensitive to elevated [CO2] below 3000 ppm than the C-4 plants of maize. Elevated [CO2] to 1000 ppm induced a higher initial photosynthetic rate, while super-elevated [CO2] appeared to negate such initial growth promotion for C-3 plants. The C-4 plant had the highest ETR, phi PSII, and qP under 500-3000 ppm [CO2], but then decreased substantially at 5000 ppm [CO2] for both species. Therefore, photosynthetic down-regulation and a decrease in photosynthetic electron transport occurred by both species in response to super-elevated [CO2] at 3000 and 5000 ppm. Accordingly, plants can be selected for and adapt to the efficient use of elevated CO2 concentration in LSS.