Kinetics of exchange of ions between artificial precipitation and maple leaf surfaces

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Abstract
SUMMARY: Experiments were conducted to test the hypothesis that ion exchange between precipitation and leaves does not involve rapid transcuticular ion permeation. The kinetics of exchange of ions between artificial precipitation and sugar maple (Acer sacckarum Marsh.) leaf surfaces were evaluated by measuring ion concentrations in individual drops at pH 5.4 or 3.8 after time periods on the leaf ranging from 4 to 128 min. Similar experiments were conducted in which leaves, were immersed in artificial precipitation solutions. These experiments indicated that (1) mechanisms of exchange differed with ion species, (2) solution pH had significant effects on the rate of exchange, and (3) rates of ion diffusion through cuticles were small. Cu2+and Pb2+were rapidly adsorbed to the leaf surface at pH 5.4, but not at pH 3.8. Zn2−concentrations were unchanged by any treatment. K+, Ca2+, and Mg2+were released from the leaf surface, and this effect was greater at pH 3.8 than pH 5.4. Apparent permeability coefficients calculated from the kinetics of these experiments indicated net exchange was dominated by cuticle surface adsorption for Cu2+and Pb2+, and cuticular permeation for K+, Ca2+, and Mg2+. The magnitude of cuticular permeation, however, was small; calculated half‐times for cation efflux from foliage were of the order of 10 days, assuming continuous precipitation. These experiments support the hypothesis that permeation of ions through cuticles during precipitation events may not occur at biologically significant rates.