The Significance of Light-limiting Photosynthesis to Crop Canopy Carbon Gain and Productivity—A Theoretical Analysis

Abstract

Analyses of the photosynthetic rates of CO₂ uptake (A) by crop leaves have centred on light-saturated rates (A_sat). Little attention has been given to the apparent quantum yield (ø), which determines the initial slope of the response of A to incident photon flux (Q), or to the convexity coefficient (Θ), which determines the duration of the transition from light-limited to light-saturated photosynthesis as light is increased. To assess the quantitative significance of these parameters of the leaf photosynthetic response to photosynthesis at the crop level, a mechanistic mathematical model was constructed which relates the individual leaf light response to the daily crop CO₂ uptake. Computer simulations of the model were conducted for canopy sizes, architectures, and light levels typical for some major tropical cereals. The simulations suggest ø rather than A_sat to be the major determinant of crop CO₂ uptake, under a majority of conditions. Sensitivity analysis shows that crop CO₂ uptake is only more sensitive to A.a! than ø under the highest light levels, and then only when leaf area index is relatively low. The simulations also suggest that if ø and Θ are decreased in parallel, as has been suggested for photoinhibition, then canopy CO₂ uptake is strongly decreased for all combinations of leaf area index and photon flux.