THE INFLUENCE OF OZONE CHEMISTRY AND METEOROLOGY ON PLANT-EXPOSURE TOPHOTOOXIDANTS

Concentrations of ozone have approximately doubled over the past centu ry. In polluted air, photochemical reactions may produce concentration s of ozone up to 100 ppbv or more, even in areas relatively remote fro m sources of industrial pollution. The large ozone concentrations obse rved in these episodes, which may persist for several days, are marker s for the presence of intense photochemical activity in the atmosphere . Experiments designed to investigate the effects of ozone on plants r arely, if ever, reproduce the chemical conditions which are always ass ociated with ozone episodes. Exposure of plants to ozone is strongly i nfluenced by topography and meterological conditions. Windy sites whic h are well coupled to the atmosphere experience only a small diurnal v ariation in ozone concentration, and peak concentrations during episod es are generally much larger than at sheltered sites. Stomatal opening effectively controls the amount and rate at which ozone enters leaves . Factors which affect stomatal opening therefore determine the dose o f ozone for a given concentration in air. Extrapolation of results fro m controlled experiments to the held must assess whether growing condi tions, including stomatal conductance, are comparable between experime nt and held. The Critical Levels approach has shown that crop plants a nd natural vegetation are likely to be exposed to potentially damaging ozone concentrations in most industrialised countries.