Ozone flux in Glycine max (L.) Merr.: Sites of regulation and relationship to leaf injury.

Hood and Dare cultivars of soybean, Glycine max (L.) Merr., vary in their foliar response to ozone. The physiological basis of this variation was investigated as a function of leaf age through an analysis of ozone flux data, leaf developmental morphology, and analogue modelling techniques. At all concentrations (0.25-0.58 μl l-1) and exposure times (1-4 h), resistance to O3 flux in the gas phase of the diffusive pathway (i.e., boundary layer and stomate) did not account fully for variation in pollutant uptake rates into the leaf interior. Ozone molecules experienced a residual resistance to diffusion that is not shared by effluxing water vapor molecules. Residual resistance to O3 flux increased with pollutant concentration and exposure time and was associated with age-dependent differences in foliar O3 response. Leaf morphology data, including stomatal frequency and the ratio of internal to external surface area, did not help explain cultivar or age-dependent differences in O3 flux. The extent of foliar injury was not consistently related to the magnitude of O3 flux into the leaf interior. An analysis of the residual resistance to O3 flux suggests that the gas and liquid phase pathways for O3, water vapor, and carbon dioxide are not identical.