Plant Cuticle as a Barrier to Acid Rain Penetration

Abstract

Exposure of leaf cells to acid precipitation is minimized by the presence of the plant cuticle, a waxy layer that covers the surface of leaves. The movement of substances through the cuticle is controlled by the conductance of the cuticle to the substance, and by the ease with which drops of precipitation wet the surface. When present, crystalline epicuticular waxes on the surface of the cuticle greatly reduce the potential for exchange of ions and molecules between drop and leaf by not allowing contact between the drop and the leaf surface. Some foliar leaching is normal, but increased leaching caused by acid precipitation has been observed both in the laboratory and in the field. Rates of leaching are typically low unless there has been damage to cells. Damage has been observed even in the absence of obvious disruption of the cuticle. Erosion of epicuticular waxes by acidic Solutions has been observed for only a few species. Erosion of the cuticle would increase leaching due to the associated increased wetting of the leaf surface, and would alter conditions for microbial populations, including pathogens. For some species under some conditions, direct damage due to elevated acidity may prove important. Under most conditions, however, the direct effects of acidity on foliage are likely to be minor compared with the responses to gaseous pollutants, such as ozone.