Can the stomatal changes caused by acute ozone exposure be predicted by changes occurring in the mesophyll? A simplification for models of vegetation response to the global increase in tropospheric elevated ozone episodes

The prediction of complex interactive effects of rising concentrations of ozone and CO₂ on vegetation will require robust models based on mechanistic understanding of how these two gases affect photosynthesis. This paper describes the development of a model of acute ozone exposure effects on wheat leaf photosynthesis, based on the mechanism of reactive oxygen scavenging processes. Based on experimental data, the dose of ozone to the leaf above a threshold flux, here termed the effective ozone dose, was found to be linearly related to the decline in the in vivo maximum rate of carboxylation. The proposed mechanism is that ozone damage to the photosynthetic apparatus will only occur above a critical rate of ozone delivery. By combining the model of the response of ribulose-1,5-bisphosphate-saturated and limited photosynthesis to ozone exposure with both a mechanistic biochemical model of leaf photosynthesis and a phenomenological model of stomatal conductance, it was possible to investigate the degree of dependency of ozone-induced stomatal closure on changes in the mesophyll. The stomatal conductance of the model simulation compared well with the magnitude of measured stomatal closure. The results indicate that the stomatal changes caused by acute ozone exposure can be predicted from changes in the mesophyll rather than directly on the stomata. The findings that the effects of ozone on photosynthesis can be predicted by an effective ozone dose to the leaf, and that the resulting reduction in CO₂ assimilation rate can, in turn, predict stomatal closure, greatly simplifies modelling the effects of elevated concentrations of ozone and CO₂ on wheat photosynthesis. Future work should determine whether the model can be adapted to predict chronic ozone exposure effects on photosynthesis, and whether it can be applied to other species by adjusting the values of threshold flux, related to the maximum scavenging capacity within the leaf, and the ozone slope coefficient, representing the inherent sensitivity of the photosynthetic apparatus to ozone.