Light dependence of carboxylation capacity for C<sub>3</sub> photosynthesis models

Abstract

C3 photosynthesis at high light is often modeled by assuming limitation by the maximum capacity of Rubisco carboxylation (VCmax) at low CO2 concentrations, by electron transport capacity (Jmax) at higher CO2 concentrations, and sometimes by triose-phosphate utilization rate at the highest CO2 concentrations. Net photosynthetic rate (P N) at lower light is often modeled simply by assuming that it becomes limited by electron transport (J). However, it is known that Rubisco can become deactivated at less than saturating light, and it is possible that P N at low light could be limited by the rate of Rubisco carboxylation (V C) rather than J. This could have important consequences for responses of P N to CO2 and temperature at low light. In this work, P N responses to CO2 concentration of common bean, quinoa, and soybean leaves measured over a wide range of temperatures and PPFDs were compared with rates modeled assuming either VC or J limitation at limiting light. In all cases, observed rates of P N were better predicted by assuming limitation by VC rather than J at limiting light both below and above the current ambient CO2. One manifestation of this plant response was that the relative stimulation of P N with increasing the ambient CO2 concentration from 380 to 570 µmol mol-1 did not decrease at less than saturating PPFDs. The ratio of VC to VCmax at each lower PPFD varied linearly with the ratio of P N at low PPFD to P N at high PPFD measured at 380 µmol(CO2) mol-1 in all cases. This modification of the standard C3 biochemical model was much better at reproducing observed responses of light-limited P N to CO2 concentrations from pre-industrial to projected future atmospheric concentrations.