The significance of light-limiting photosynthesis to crop canopy carbon gain and productivity--a theoretical analysis
1993
Long, S.P.
Analyses of the photosynthetic rates of CO2 uptake (A) by crop leaves have centred on light-saturated rates (Asat). Little attention has been given to the apparent quantum yield, which determines the initial slope of the response of A to incident photon flux (Q), or to the convexity coefficient, 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 apparent quantum yield rather than Asat 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 Asat than apparent quantum yield under the highest light levels, and then only when leaf area index is relatively low. The simulations also suggest that if apparent quantum yield and convexity coefficient 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.
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