Effects of simulated acid rain on leaf wettability, rain retention and uptake of some inorganic ions

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Abstract
Leaves of Phaseolus vulgaris L., Vicia faba L., Pisum sativum L. and Brassica napus L. were exposed from emergence to full expansion to seven treatments of simulated acid rain at pH values between 5.6 and 2.6. Droplet leaf contact angles in all species decreased on leaves exposed to simulated acid rain at pH .ltoreq. 4.6 relative to those exposed at pH 5.6. Retention of rain containing fluorescein increased on P. vulgaris leaves exposed to simulated acid rain at pH 4.6 and at pH .ltoreq. 3.8. Retention by B. napus leaves was increased at pH .ltoreq. 4.6. Uptake of the three ions studied was in the order 86Rb+ > 35SO42- > 63Ni2+. Uptake of 86Rb+ increased into B. napus leaves and decreased into P. vulgaris leves exposed at pH .ltoreq. 3.4. Uptake of 35SO42- and 63Ni2+ by B. napus leaves increased after exposure to simulated acid rain at pH 2.6 but it was unaffected in P. vulgaris. Up to 7% of applied 35SO42- was found in the epicuticular wax layer in B. napus leaves 48h after application, most being found after exposure at pH 2.6. The order of lateral movement of ions within leaves paralleled the order of uptake. Movement of all three ions was increased in B. napus leaves exposed previously to simulated acid rain at pH 2.6. The increased retention of fluorescein on leaves exposed to simulated acid rain and the reduced contact angles for water are attributed to decreases in surface roughness. The altered ion uptake pattern could also be related to changes in surface wax structure or could be associated with observed changes in properties of cuticular membranes. These results demonstrate that the interaction of plants with their atmospheric environment could possibly be affected by pre-exposure to acid rain in amounts and at pH values that occur in ambient rainfall.