Response and acclimation of photosynthesis in rice leaves to free-air CO2 enrichment (FACE)]

The net photosynthetic rate (Pn), water use efficiency (WUE), and apparent quantum yield of carbon assimilation of rice leaves were boserved contrastively under ambient air (380 mumol.mol-1 CO2) and FACE (580 mumol.mol-1CO2). The results showed that the observed index under FACE were significantly higher than those under ambient air. Nevertheless, along with the time of high CO2 treatment prolonged, the enhancement effect of high CO2 on net photosynthetic rate declined gradually. At the same CO2 concentration, Pn and carboxylation efficiency (CE) in rice leaves under FACE were lower than those under ambient air. Although the stomatal conductances in FACE leaves was obviously lower than that in ambient leaves, their intercellular CO2 concentrations were not significantly different, which implied that the photosynthetic down-regulation in rice leaves grown under FACE was not caused by the decrease of stomatal conductance.