Regulated thermal dissipation of absorbed light energy within the photosystem II antenna system helps protect photosystem II from damage in excess light. This reversible photoprotective process decreases the maximum quantum yield of photosystem II (Fv)/Fm) and CO2 assimilation (phiCO2), and decreases the convexity of the non-rectangular hyperbola describing the response of leaf CO2 assimilation to photon flux (theta). At high light, a decrease in phiCO2 has minimal impact on carbon gain, while high thermal energy dissipation protects PSII against oxidative damage. Light in leaf canopies in the field is continually fluctuating and a finite period of time is required for recovery of phiCO2 and when light drops below excess levels. Low phiCO2) and can limit the rate of photosynthetic carbon assimilation on transfer to low light, an effect prolonged by low temperature. What is the cost of this delayed reversal of thermal energy dissipation and phiCO2 recovery to potential CO2 uptake by a canopy in the field? To address this question a reverse ray-tracing algorithm for predicting the light dynamics of 120 randomly selected individual points in a model canopy was used to describe the discontinuity and heterogeneity of light flux within the canopy. Because photoprotection is at the level of the cell, not the leaf, light was simulated for small points of 10(4) micro m rather than as an average for a leaf. The predicted light dynamics were combined with empirical equations simulating the dynamics of the light-dependent decrease and recovery of phiCO2 and and their effects on the integrated daily canopy carbon uptake (A'c). The simulation was for a model canopy of leaf area index 3 with random inclination and orientation of foliage, on a clear sky day (latitude 44 degrees N, 120th day of the year). The delay in recovery of photoprotection was predicted to decrease A'c by 17% at 30 degrees C and 32% at 10 degrees C for a chilling-susceptible species, and by 12.8% at 30 degrees C and 24% at 10 degrees C for a chilling-tolerant species. These predictions suggest that the selection, or engineering, of genotypes capable of more rapid recovery from the photoprotected state would substantially increase carbon uptake by crop canopies in the field. Copyright 2004 Society for Experimental Biology
