Neutralization of acid droplets on plant leaf surfaces

Sulfuric acid mist exposure of bush bean leaves at a low rate of precipitat ion suggested that acid on the leaf surface was neutralized by cations leac hed from leaf tissues and that Ca-S compounds were accumulated on the leaf surface (Kohno, 1994). This report summarizes visual observations of the ne utralization process of acid on leaf surfaces as determined by a pH-imaging microscope. Small droplets of sulfuric acid were placed on the adaxial lea f surface and allowed to air dry under laboratory conditions. Droplets (0.1 mul) of sulfuric acid took about 7-8 minutes to dry. Leaf samples were cut at various times after the acid droplets dried. The adaxial leaf surface w as placed on the pH-adjusted agar film layer on the pH-imaging sensor of th e microscope. Hydrogen ions dispersed into the film layer and resulting pH distributions were visualized as pH distribution patterns. The size of the acidic area generated became smaller with time after the acid was added and allowed to dry. Results indicate that leaves could neutralize the surface acid probably by ion exchange with cations from their surface tissues and c ould recover from strong temporary acid stress imposed by acid rain or acid fog in a relatively short period of time. Our findings indicate that acidi c precipitation at current acidity levels does not pose a direct threat to plants.