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

The prediction of complex interactive effects of rising concentrations of o zone and CO2 on vegetation will require robust models based on mechanistic understanding of how these two gases affect photosynthesis. This paper desc ribes the development of a model of acute ozone exposure effects on wheat l eaf photosynthesis, based on the mechanism of reactive oxygen scavenging pr ocesses. 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 linea rly related to the decline in the in vivo maximum rate of carboxylation. Th e proposed mechanism is that ozone damage to the photosynthetic apparatus w ill only occur above a critical rate of ozone delivery. By combining the model of the response of ribulose-1,5-bisphosphate-saturat ed and limited photosynthesis to ozone exposure with both a mechanistic bio chemical model of leaf photosynthesis and a phenomenological model of stoma tal conductance, it was possible to investigate the degree of dependency of ozone-induced stomatal closure on changes in the mesophyll. The stomatal c onductance of the model simulation compared well with the magnitude of meas ured stomatal closure. The results indicate that the stomatal changes cause d by acute ozone exposure can be predicted from changes in the mesophyll ra ther than directly on the stomata. The findings that the effects of ozone on photosynthesis can be predicted b y an effective ozone dose to the leaf, and that the resulting reduction in CO2 assimilation rate can, in turn, predict stomatal closure, greatly simpl ifies 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 w hether it can be applied to other species by adjusting the values of thresh old flux, related to the maximum scavenging capacity within the leaf, and t he ozone slope coefficient, representing the inherent sensitivity of the ph otosynthetic apparatus to ozone.