Modeling the Temperature Dependence of C3 Photosynthesis

The steady-state C3 model of photosynthesis originally developed by Graham Farquhar et al. (1980) and subsequently modified by others describes responses of leaf carbon assimilation to environmental variation. This mechanistic model states that photosynthesis will be limited by the slowest of three biochemical processes: (1) the maximum rate of Rubisco-catalyzed carboxylation, (2) the rate of ribulose 1,5-bisphosphate (RuBP) regeneration via electron transport (J), or (3) the rate of RuBP regeneration via triose phosphate utilization (TPU). Each of these processes is modeled with parameters that have different responses to temperature; therefore accurate temperature functions are vital to model photosyn-thetic responses accurately, and they are critical to predict plant ecosystem responses to future predicted increases in temperature and CO2. Temperature functions used for modeling are frequently derived from Arrhenius equations, which describe changes in rate constants with temperature. Rubisco-limited photosynthesis is modeled using five parameters: two describe enzyme kinetics of carboxylation (V cmaxand K c), one accounts for photorespiration (K o), one for Rubisco specificity for CO2 vs. O2 (Γ*) and one for mitochondrial respiration (R d). At light saturation and current atmospheric CO2 concentration, photosynthesis is usually carboxylation-limited. Two of the above parameters, Γ * and R d, are also used to model the rate of RuBP regeneration and follow the same temperature functions. The maximum rate of electron transport (J max) is needed to model RuBP regeneration, which is particularly important at sub-saturating irradiance, higher temperatures, or supra ambient CO2 concentrations, and is estimated by fitting the response of potential electron transport to light. Unlike the parameters that are dependent on the relatively conserved kinetic properties of Rubisco enzyme, electron transport is sensitive to environmental variation and can vary widely even within species. Triose phosphate limitation can occur at high CO2, low O2, high irradiance, or low temperature and is generally difficult to anticipate in field based experiments. Finally, consideration should be given to the supply of CO2 to the site of carboxylation mediated by mesophyll conductance (g m) when modeling photosynthetic responses to temperature, as g m has been shown to vary with temperature, among species and growth conditions.