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-cmxax) 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 CO2 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 mumol 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.