COMPARATIVE ABILITIES OF LEAF SURFACES TO NEUTRALIZE ACIDIC RAINDROPS

II. THE INFLUENCE OF LEAF WETTABILITY, LEAF AGE AND RAIN DURATION ON CHANGES IN DROPLET pH AND CHEMISTRY ON LEAF SURFACES

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SUMMARY

A study was made of the ability of leaf surfaces of sunflower, radish, cabbage and sugar beet to neutralize acid rains (pH 30) of varying duration. Sprayings with simulated rain of 5, 15, 45 and 90 min duration were made weekly. The species were selected for their contrasting leaf surfaces (e.g. wettability, droplet retention, hairiness and thickness of epicuticular waxes). Following acidic sprays, the pH of raindrops on leaves was measured at intervals until drops dried. Leaves of cabbage and sugar beet, which had smaller areas of contact with raindrops than those of radish and sunflower, were markedly less able to neutralize acidic droplets. Droplets on the cotyledons of all species had higher pHs than droplets on the true leaves. Differences in age of true leaves, however, did not significantly affect droplet pH. The duration of rain spraying had a significant effect on leaf surface neutralization. In 2- and 3-week-old plants, long exposures to rain (i.e. 45 and 90 min) markedly increased neutralization in comparison with short exposures. In 6-week-old plants, the effect was the reverse; sprays of 45 and 90 min significantly decreased foliar neutralization compared with sprays of 5 and 15 min. Significant decreases in both leaf and root dry weights were found for cabbage and sugar beet treated with rain of pH 30 for 45 and 90 min, compared with both unsprayed plants and those treated with the same pH for shorter periods. Radish showed a similar but non-significant trend.

Chemical analysis of raindrops (initially pH 3–0) left on foliage for 75 min showed an increase in the concentrations of Ca2+, Mg2+ and K+, sometimes up to 50-fold. The elemental composition of drying droplets collected on 4-week-old plants was not affected by the duration of previous rain exposures. In contrast, the Ca2+ and Mg2+ concentrations of drying droplets on 7-week-old plants exposed to 45 and 90 min rains were significantly lower than on those exposed to 5 and 15 min rains. Drying droplets differed sharply in their cation concentrations in different species. There was, however, a poor correlation between the degree of droplet neutralization and its acquired cation status. Results from a subsequent experiment using cabbage, radish, broccoli, brussel sprout and tomato, in which droplet pHs were measured over acid rain-induced lesions and over uninjured leaf tissue, showed significantly higher pHs in the droplets over the foliar lesions.

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