Effects of Elevated CO2 Concentration and Temperature on the Growth and Photosynthetic Characteristics of Populus simonii x P. nigra '1307' Leaves

This study aimed to establish the effects of elevated CO2 concentration and temperature on the photosystem II (PSII) performance and photosynthetic characteristics of Populus simonii x P. nigra '1307' leaves. Different CO2 concentrations (400 and 800 mu mol<middle dot>mol(-1)) and temperatures (room temperature and room temperature +2 degrees C) were set in artificial climate change simulation and control chambers, and the rapid chlorophyll fluorescence induction kinetics curve (OJIP curve) of Populus simonii x P. nigra '1307' was determined. The generated OJIP curve was used to analyze the change characteristics in photosynthetic performance. The results revealed that under elevated temperature conditions, the concentrations of chlorophyll a, b in Populus simonii x P. nigra '1307' leaves were significantly increased. At the same time, there were no significant changes in the chlorophyll concentration under the superimposed effect of elevated CO2 concentration and temperature. The PSII comprehensive performance index (PIABS) of Populus simonii x P. nigra '1307' was significantly inhibited under elevated temperatures due to the increased closure degree (V-j) of the PSII reaction center and the damage of the receptor side. This reduced the electron transfer capacity per unit reaction center (ETo/RC) and unit cross-sectional area, which decreased the quantum yield of the electron transfer. Under the elevated CO2 concentration, ETo/RC was also inhibited. Still, PIABS was enhanced owing to the increased number of active PSII per unit area and the low reduction rate of the primary quinone receptor (Q(A)). Under the superimposed effect of the two factors, the electron transfer performance of the donor and receptor sides of PSII was improved compared to the treatments only subjected to elevated temperature; thus, PIABS was not significantly reduced compared to the control. Therefore, the continuous increase in temperature by 2 degrees C significantly inhibits the electron transfer capacity of the photosynthetic system of Populus simonii x P. nigra '1307' leaves. On the other hand, an increase in CO2 concentration expands the PSII reaction center, while enhancing the electron transfer capacity of the donor and receptor sides, which alleviates the photosynthetic inhibition caused by the elevated temperature.