PLANT PHYSIOLOGY , Vol 103, Issue 1 251-256, Copyright © 1993 by American Society of Plant Biologists
|
ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Effect of Cations on Effective Permeability of Leaf Cuticles to Sulfuric Acid
S. J. Smalley, H. D. Hauser and V. S. Berg
Department of Biology, University of Northern Iowa, Cedar Falls, Iowa 50614-0421
Many plants are exposed to prolonged episodes of anthropogenic acid
precipitation with pH values of 4 or less, but there is little evidence of
widespread direct damage to the plant cells. Acids appear to permeate leaf
cuticle via charged pores, which act as a fixed buffer that delays but does
not stop acid movement. We investigated the effect of cations on the
movement of protons through astomatous isolated leaf cuticles of pear
(Pyrus communis L.) and rough lemon (Citrus limon [L.] Burm. fils cv
Ponderosa). Chloride salt solutions of Na, K, Ca, Cd, Mg, Gd, or Y in a
diffusion apparatus were applied to the morphological inner surface of the
cuticle, while the outer surface faced a large volume of pH 3 or 4 sulfuric
acid. Effective permeability was calculated from the change in the pH of
the inner solution as measured with a pH microelectrode. Monovalent cations
caused either no change (pear) or promotion (rough lemon) of proton
movement. Divalent cations reduced proton movement in a
concentration-dependent manner (both species), whereas trivalent cations
(rough lemon only) caused the effective permeability to decrease to near
zero. Inhibition by 10 mM CaCl2 was reversed with water. The effects of
these cations on the permeability of cuticles to protons was used to
elucidate mechanisms by which cations can protect leaves from acid
precipitation in nature.