Acute Effects of Acidic Fog on Photosynthetic Activity and Morphology of Phaseolus lunatus

Abstract

Acute effects of high-nitrate/low-sulfate acidic fogs with a pH of 2.5 and 3.0 were investigated on 3.5to 4-week-old Phaseolus lunatus L. in a series of replicated trials. After 24 hours, CO 2 assimilation rates of primary leaves were reduced by at least one-third by 3-hour fogs with a pH value of 2.5 as compared to control plants treated with a fog of pH 6.3. A 3-hour fog at pH 3.0 reduced C0 2 assimilation a minimum of 20%. Stomatal resistance increased in primary leaves of plants exposed to an acidic fog of pH 2.5 by > 37% compared to plants subjected to pH 6.3 fogs. StomataI resistances in leaves exposed to pH 3.0 fogs increased at least 27%. However, internal CO 2 concentrations were not significantly different between controland acidfogged plants at any pH. Standardizing plants for similar CO 2 assimilation rates allowed statistical separation of photosynthetically important variables as compared to unstandardized experimental designs with higher interplant variability. Methacrylate plastic sections of foliar lesions resulting from exposure to pH 2.0 fogs revealed that damage usually progressed vertically from the upper to lower epidermis. Xylem was less susceptible to damage than other tissues.