The Use of the Ratio between the Photosynthesis Parameters Pmland Vcmaxfor Scaling up Photosynthesis of C3Plants from Leaves to Canopies: A Critical Examination of Different Modelling Approaches

Recent models of photosynthesis have adopted the close correlation between the main photosynthetic component processes, the maximum rate of carboxylation and the potential rate of RuBP (ribulose-1,5-bisphosphate) regeneration, at a reference temperature of 20°C. When using the ratio between these two processes in models of photosynthesis, assumptions though have to be made about the temperature response of the potential rate of RuBP regeneration, which varies with growth conditions and among species. In order to assess the effects of deviations from the real temperature response of the potential rate of RuBP regeneration on photosynthesis, a sensitivity analysis, scaling up photosynthesis from the leaf to the canopy level, is applied in the present paper. No changes are predicted to occur for sunlit leaves, which receive both direct and diffuse radiation, as long as incident radiation does not cause carboxylation to shift from RuBP saturation to RuBP limitation, which, depending on incident radiation and canopy structure, might occur deeper down in the canopy. Carboxylation of shaded leaves, which receive solely diffuse radiation, is generally limited by the regeneration of RuBP, and is thus prone to be affected by changes in the temperature response of the potential rate of RuBP regeneration. Due to the saturation type response of the RuBP-limited rate of carboxylation to temperature at light intensities below saturation, the impact of deviations from the real temperature response is negligible at high leaf temperatures, but may become significant when leaf temperatures are low and photosynthetically active radiation incident on shaded leaves is comparably high, as in the upper canopy layers. The largest effects on whole canopy photosynthesis will therefore occur under cool conditions and a completely overcast sky, when all leaves receive diffuse radiation only.