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Keywords:

  • Acid rain;
  • leaf surfaces;
  • neutralization;
  • leaf chemistry;
  • particulate deposition

Summary

The relative abilities of four species to neutralize acidic rain droplets placed on their leaf surfaces were examined. Rain droplets (50 μl) of pH 4.7, 3.8, 3.0 and 2.4 were used. Determinations of droplet pH on the leaf surfaces and on a control parafilm surface were made at intervals until the leaves dried. Neutralizing abilities of leaves of sugar beet (Beta vulgaris), radish (Raphanus sativus), sunflower (Helianthus annuus) and the arctic herb (Artemisia tilesii) were compared. The plants were grown in sand culture with calcium supplied at a range of five concentrations, from 48 to 480 ppm to alter the calcium status of the foliage. While the Ca2+ treatments strongly influenced the Ca2+ content of leaves, they did not influence the neutralizing ability of the leaves. However, sugar beet, which showed the least ability to neutralize, had the lowest foliar Ca2+ content of the species examined. At raindrop pH values of 4.7, 3.8 and 3.0, the species were all able to increase droplet pH substantially before drying. The effect appeared to be two-phased, with a rapid initial neutralization probably due to solubilized surface particulates and evaporated leaf exudates, followed by a slower subsequent phase, probably due to droplet–cell wall exchange processes. At pH 2.4 only radish and sunflower were still able to neutralize, but this may have been due to leakage of the contents of damaged cells. In experiments comparing plants grown in filtered vs unfiltered air, and in sand vs soil in the greenhouse or outdoors, it was shown that charcoal filtering the air substantially reduced the neutralizing ability of the leaf surface. Surface wettability and the speed at which droplets dried out were important in determining the neutralizing ability of leaf surfaces.