Effects of simulated acid fog on leaf acidification and injury development of pinto beans

Abstract

Pinto beans (Phaseolus vulgaris L. cv. UI 111) were exposed for 8 h to simulated fog solutions made up of 2:1 nitric acid: sulfuric acid plus background ions, acidified to pH 3.2, 2.8, 2.4, and 2.0. Rate of fog deposition was ca. 1 mm h−1. Examination of visible foliar injury development, scanning electron microscopy (SEM) examination, and measurement of leaf extract pH were performed every 2 h. Both visual and SEM observations during exposure showed no change in leaf surface characteristics for plants exposed to pH 3.2 and 2.8 fog solutions but 1 week after pH 2.8 exposures, injury was seen on primary leaves and young trifoliate leaves. The first indications of injury at pH 2.4 were seen under SEM after 8 h of exposure. Foliar injury was apparent after only 2 h of exposure at pH 2.0 and severe acid necrosis developed after 24 h. Changes in acidity of leaf extracts were closely correlated with subsequent injury development. No changes were observed in extracts of plants exposed to pH 3.2 simulated fog compared with controls. After 8 h of exposure, leaf extracts of plants exposed to pH 2.8 fog had dropped 0.05 pH units. Plants exposed to pH 2.4 and pH 2.0 for 8 h had leaf extracts that were 0.12 and 0.18 pH units lower, respectively. Comparison of the three measured parameters of acid fog effects (visible injury development, SEM examination of leaf surfaces, and determination of leaf extract pH) showed extract pH to be a simple, quantifiable, and sensitive indicator of the negative effects of acid precipitation on plants.