Seasonal change in the balance between capacities of RuBP carboxylation and RuBP regeneration affects CO₂ response of photosynthesis in Polygonum cuspidatum

The balance between the capacities of RuBP (ribulose-1,5-bisphosphate) carboxylation (V_cmax) and RuBP regeneration (expressed as the maximum electron transport rate, J_max) determines the CO₂ dependence of the photosynthetic rate. As it has been suggested that this balance changes depending on the growth temperature, the hypothesis that the seasonal change in air temperature affects the balance and modulates the CO ₂ response of photosynthesis was tested. V_cmax and J _max were determined in summer and autumn for young and old leaves of Polygonum cuspidatum grown at two CO₂ concentrations (370 and 700 μmol mol^-1). Elevated CO₂ concentration tended to reduce both V_cmax and J_max without changing the J _max:V_cmax ratio. The seasonal environment, on the other hand, altered the ratio such that the J_max:V_cmax ratio was higher in autumn leaves than summer leaves. This alternation made the photosynthetic rate more dependent on CO₂ concentration in autumn. Therefore, when photosynthetic rates were compared at growth CO₂ concentration, the stimulation in photosynthetic rate was higher in young-autumn than in young-summer leaves. In old-autumn leaves, the stimulation of photosynthesis brought by a change in the J_max:V_cmax ratio was partly offset by accelerated leaf senescence under elevated CO₂. Across the two seasons and the two CO₂ concentrations, V _cmax was strongly correlated with Rubisco and J_max with cytochrome f content. These results suggest that seasonal change in climate affects the relative amounts of photosynthetic proteins, which in turn affect the CO₂ response of photosynthesis.