Photosynthetic acclimation and leaf traits of Stipa bungeana in response to elevated CO2 under five different watering conditions

Although plant performance under elevated CO2 (EC) and drought has been extensively studied, little is known about the leaf traits and photosynthetic performance of Stipa bungeana under EC and a water deficiency gradient. In order to investigate the effects of EC, watering, and their combination, S. bungeana seedlings were exposed to two CO2 regimes (ambient, CA: 390 ppm; elevated, EC: 550 ppm) and five levels of watering (-30%, -15%, control, +15%, +30%) from 1 June to 31 August in 2011, where the control water level was 240 mm. Gas exchange and leaf traits were measured after 90-d treatments. Gas-exchange characteristics, measured at the growth CA, indicated that EC significantly decreased the net photosynthetic rate (P (N)), water-use efficiency, nitrogen concentration based on mass, chlorophyll and malondialdehyde (MDA) content, while increased stomatal conductance (g (s)), intercellular CO2 concentration (C (i)), dark respiration, photorespiration, carbon concentration based on mass, C/N ratio, and leaf water potential. Compared to the effect of EC, watering showed an opposite trend only in case of P (N). The combination of both factors showed little influence on these physiological indicators, except for g (s), C (i), and MDA content. Photosynthetic acclimation to EC was attributed to the N limitation, C sink/source imbalance, and the decline of photosynthetic activity. The watering regulated photosynthesis through both stomatal and nonstomatal mechanisms. Our study also revealed that the effects of EC on photosynthesis were larger than those on respiration and did not compensate for the adverse effects of drought, suggesting that a future warm and dry climate might be unfavorable to S. bungeana. However, the depression of the growth of S. bungeana caused by EC was time-dependent at a smaller temporal scale.