Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2004
Publication Date: 10/15/2004
Citation: Zhu, X., Ort, D.R., Whitmarsh, C.J., Long, S.P. 2004. 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. Journal of Experimental Botany. 55:1167-1175. Interpretive Summary: Although light is the source of energy for photosynthesis and, for most crops, there is a direct relationship between the amount of light absorbed over the growing season and economic yield of the crop, for long periods of sunny days plants receive more light than they can use. In fact, plants have very sophisticated mechanisms to avoid damage during those periods when there is too much light. This is a process known as photoprotection. However, when plants enter a photoprotected state, photosynthesis is much less efficient. The work presented here investigates the fact that the photoprotected state is very slow to reverse when conditions change and light is no longer in excess thus resulting in a situation where plants are operating at low photosynthetic efficiency when there is no adaptive reason to do so. We show that more rapid reversal of the photoprotected state to the high efficiency state would substantially improve crop performance and yield. This work is important as it identifies a previously unrecognized agronomic trait that may exhibit genetic variation among different cultivars and plants varieties and thus be a useful breeding trait to improve productivity.
Technical Abstract: 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 ('CO2), and decreases the convexity of the non-rectangular hyperbola describing the response of leaf CO2 assimilation to photon flux ('). At high light, a decrease in '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 'CO2 and ' when light drops below excess levels. Low 'CO2 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 '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 104 µm 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 'CO2 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° N, 120th day of the year). The delay in recovery of photoprotection was predicted to decrease A'c by 17% at 30 °C and 32% at 10 °C for a chilling-susceptible species, and by 12.8% at 30 °C and 24% at 10 °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.