Effects of different elevated CO₂ concentrations on chlorophyll contents, gas exchange, water use efficiency, and PSII activity on C₃ and C₄ cereal crops in a closed artificial ecosystem

Although terrestrial CO₂ concentrations [CO₂] are not expected to reach 1000 μmol 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 C₄ plants may respond to elevated CO₂, it is necessary to determine if leaves of closed artificial ecosystem grown plants have a fully developed C₄ photosynthetic apparatus, and whether or not photosynthesis in these leaves is more responsive to elevated [CO₂] than leaves of C₃ 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 C₃ plant and maize (Zea mays L., 'Susheng') of C₄ 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 C₃ plants of soybean was more sensitive to elevated [CO₂] below 3000 ppm than the C₄ 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 C₃ plants. The C₄ plant had the highest ETR, φPSII, 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.