[Responses of diurnal variation of flag-leaf photosynthesis and photosynthetic pigment content to elevated atmospheric CO2 concentration and temperature of Japonica rice during late growth stage: A FACE study.]

A local popular Japonica rice (Oryza sating L.) cultivar, Nanjing 9108, was tested with free air controlled enrichment (FACE) approach to study the responses of photosynthetic pigment content and diurnal variation of flag-leaf photosynthesis to elevated atmospheric CO2 concentration and temperature. Four alternative treatments were designed with two CO2 concentration levels (ambient and elevated 200 μmol·mol-1) and two air temperature regimes (ambient and elevated 1-2 ℃). Diurnal variation of flag-leaf photosynthesis was measured in the middle full stage and the late full stage, and photosynthetic pigment of the leaf was analyzed afterward. Results showed that diurnal variation of net photosynthetic rate (Pn) in each treatment followed a double-peak curve with midday depression feature during late growth stage. Compared to Pn under ambient condition, Pn under elevated CO2 concentration increased by 47.6% and 39.1% on average at middle full stage and late full stage, respectively. There was a negative correlation between temperature and Pn with no significance. Both elevated CO2 concentration and temperature had a significant negative effect on stomatal conductance (gs), decreased by 17.0% and 11.8% on average, respectively. Elevated CO2 concentration significantly reduced transpiration rate (Tr), chlorophyll a (Chl a), chlorophyll b (Chl b), carotene (Car), total chlorophyll (Chl t) and chlorophyll a/b ratio (Chl a/b) during late growth stage of rice by 5.9%, 50.4%, 21.3%, 41.4%, 39.4% and 21.4% on average, respectively, whereas water use efficiency (WUE) increased by 47.9%. However, there were opposite effects on Tr, WUE and photosynthetic pigment content under elevated temperature, with Tr increased by 10.2% and WUE decreased by 20.4%. It could be concluded that elevated CO2 concentration had a greater effect on Pn, gs and photosynthetic pigment content of rice leaf than elevated temperature did during late growth stage. Therefore, it should be paid more attention to the colligate effects of elevated CO2 concentration and high temperature on photosynthesis and photosynthetic pigment content to reduce negative effect of high air temperature.