The slow reversibility of photosystem II thermal energy dissipation on transfer from high to low light may cause large losses in carbon gain by crop canopies: a theoretical analysis

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 (F-v/F-m) and CO2 assimilation (Phi(CO2)), 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 Phi(CO2) 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 Phi(CO2) and theta when light drops below excess levels. Low Phi(CO2) and theta 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 Phi(CO2) 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) mum 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 Phi(CO2) and theta 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 44degrees N, 120th day of the year). The delay in recovery of photoprotection was predicted to decrease A'(c) by 17% at 30degreesC and 32% at 10degreesC for a chilling-susceptible species, and by 12.8% at 30 fdegreesC and 24% at 10degreesC 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.