EFFECT OF CATIONS ON EFFECTIVE PERMEABILITY OF LEAF CUTICLES TO SULFURIC-ACID

Many plants are exposed to prolonged episodes of anthropogenic acid pr ecipitation with pH values of 4 or less, but there is little evidence of widespread direct damage to the plant cells. Acids appear to permea te leaf cuticle via charged pores, which act as a fixed buffer that de lays but does not stop acid movement. We investigated the effect of ca tions on the movement of protons through astomatous isolated leaf cuti cles of pear (Pyrus communis L.) and rough lemon (Citrus limon [L.] Bu rm. fils cv Ponderosa). Chloride salt solutions of Na, K, Ca, Cd, Mg, Gd, or Y in a diffusion apparatus were applied to the morphological in ner surface of the cuticle, while the outer surface faced a large volu me of pH 3 or 4 sulfuric acid. Effective permeability was calculated f rom the change in the pH of the inner solution as measured with a pH m icroelectrode. Monovalent cations caused either no change (pear) or pr omotion (rough lemon) of proton movement. Divalent cations reduced pro ton movement in a concentration-dependent manner (both species), where as trivalent cations (rough lemon only) caused the effective permeabil ity to decrease to near zero. Inhibition by 10 mm CaCl2 was reversed w ith water. The effects of these cations on the permeability of cuticle s to protons was used to elucidate mechanisms by which cations can pro tect leaves from acid precipitation in nature.