Seasonal changes in the temperature response of photosynthesis in canopy leaves of Quercus crispula in a cool-temperate forest

Understanding seasonal changes in photosynthetic characteristics of canopy leaves is indispensable for modeling the carbon balance in forests. We studied seasonal changes in gas exchange characteristics that are related to the temperature dependence of photosynthesis in canopy leaves of Quercus crispula Blume, one of the most abundant species in cool-temperate forests in Japan. Photosynthetic rate and ribulose-1,5-bisphosphate (RuBP) carboxylation capacity (V_cmax) at 20°C increased from June to August and then decreased in September. The activation energy of V_cmax, a measure of the temperature dependence of V_cmax, was highest in summer, indicating that V_cmax was most sensitive to leaf temperature at this time. The activation energy of V_cmax was significantly correlated with growth temperature. Other parameters related to the temperature dependence of photosynthesis, such as intercellular CO₂ partial pressure and temperature dependence of RuBP regeneration capacity, showed no clear seasonal trend. It was suggested that leaf senescence affected the balance between carboxylation and regeneration of RuBP. The model simulation showed that photosynthetic rate and its optimal temperature were highest in summer.