Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature-dependent susceptibility to photoinhibition




To determine the effects of elevated CO2 concentration ([CO2]) on the temperature-dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1-year-old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO2] levels (ambient: 400 µmol mol−1 and elevated: 800 µmol mol−1) and limited N availability (90 mg N plant−1). Plants grown under elevated [CO2] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated-CO2-grown plants exceeded those grown under ambient [CO2] when compared at their growth [CO2]. Electron transport rates were significantly lower in elevated-CO2-grown plants than ambient-CO2-grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 − qP)× Fv′/Fm′] between CO2 treatments across the temperature range. The quantum yield of regulated non-photochemical energy loss was significantly higher in elevated-CO2-grown plants than ambient, when compared at their respective growth [CO2] below 25°C. These results suggest that elevated-CO2-induced downregulation might not exacerbate the temperature-dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.