Effect of exposure to simulated acid rain and ozone on the visible injury of several weed and crop plants

An experiment was carried out to analyse the combined effect of ozone (O3) and acid rain on the visible injury of weed and crop plants. Four species of both weeds and crops, which are relatively sensitive to acid rain, were exposed to 0.0, 0.05, 0.10 or 0.15ppm of O3 for 8hr day-1 for three days and then 10ml day-1 of simulated acid rain at pH 5.6, 3.5, 3.0, 2.5 or 2.0 were sprayed for three days. The visible injury rate of these leaves was determined. The threshold concentration of O3 for the appearance of visible injury ranged between 0.10 and 0.15ppm (Table 1). Although the injured leaf area expanded by the exposure to acid rain, the interaction between O3 and rain pH was not significant (Table 2). In all the plants the percentage of injured leaf area (ILA) of the leaves that had not experienced O3 injury increased markedly after the exposure to acid rain at pH 2.0 (Fig. 1). Analysis of variance showed that the interaction between O3 and pH was not significant expcept for Arthraxon hispidus and Brassica spp. though the effects of both O3 and rain pH were significant in most plants (Table 3). ILA of a few plants such as Oplismenus undulatifolius and Perilla ocymoides increased after the exposure to acid rain at pH 2.5 due to the previous exposure to O3, indicating that O3 stress affected the induction of visible injury by acid rain to some extent. These results suggest that it is unlikely that plants will be injured by short-term exposure to O3 and acid rain at the current ambient levels.