The use of the ratio between the photosynthesis parameters P-ml and V-cmaxfor scaling up photosynthesis of C-3 plants 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 carboxylat ion and the potential rate of RuBP (ribulose-1,5-bisphosphate) regeneration , at a reference temperature of 20 degrees 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 regen eration, 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 analy sis, scaling up photosynthesis from the leaf to the canopy level, is applie d in the present paper. No changes are predicted to occur for sunlit leaves , which receive both direct and diffuse radiation, as long as incident radi ation does not cause carboxylation to shift from RuBP saturation to RuBP li mitation, which, depending on incident radiation and canopy structure, migh t occur deeper down in the canopy. Carboxylation of shaded leaves, which re ceive solely diffuse radiation, is generally limited by the regeneration of RuBP, and is thus prone to be affected by changes in the temperature respo nse 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 temper ature response is negligible at high leaf temperatures, but may become sign ificant when leaf temperatures are low and photosynthetically active radiat ion incident on shaded leaves is comparably high, as in the upper canopy la yers. The largest effects on whole canopy photosynthesis will therefore occ ur under cool conditions and a completely overcast sky, when all leaves rec eive diffuse radiation only. (C) 1999 Academic Press.