Seedlings of red spruce (Picearubens Sarg.) were exposed repeatedly, either in the field or in the greenhouse, to sulfuric acid mist, nitric acid mist (with or without ammonium ions), or a combination of sulfuric and nitric acid mist (with or without ammonium ions) for 10 to 39% of the time over periods of 6 to 19 weeks. Sulfuric acid mist produced visible foliar injury after repeated overnight exposures to pH 3 or less. Foliar symptoms were significantly less or absent after exposures to nitric acid or combined sulphuric–nitric acid mist at the same pH values. Abscission of needles during the subsequent winter also was increased by sulfuric acid mist. Intermittent exposures that contained fewer hours of treatment with acidic mist and longer drying periods than continuous exposures caused greater amounts of foliar injury. Wind during the drying periods enhanced foliar injury, especially with sulfuric acid mist. Significant reductions in shoot elongation occurred with sulfuric acid mist in those experiments where seedlings continued to grow throughout the treatment period. Different provenances responded similarly to acidic mist. The results indicate that red spruce is particularly vulnerable to injury from acidic mist when sulfate concentrations are high and when mist exposures are intermittent with repeated periods of drying of liquid on foliage.

SUMMARYCucumis sativus L. plants were grown in the greenhouse and exposed 21 times to simulated acidic rain over a period of 6 weeks to compare the responses of vegetative and reproductive tissues. Chemical composition and temporal characteristics of rain were patterned according to rainfall in the growing season in the eastern US. Flowers were hand-pollinated to promote fruit set. Foliar symptoms were the most sensitive response of all the variables measured. Necrosis was visible after exposures to simulated acidic rain at pH 2-6, 30, and 3-4 but not at 3*8, 4-2, 46, 5"0, or 5*4. Despite these injury symptoms, there were no significant reductions in most measures of vegetative or reproductive tissues, even at pH 2-6. When durations of exposure were increased from 80 to 160 min, there was an increase in the severity of foliar injury, a significant reduction in dry mass of stems, numbers of female flowers produced and dry mass of flowers and immature fruit. However, there still were no significant reductions in numbers or weight of fruit, even though the plants were exposed to as much as a 20-fold greater deposition of acidity than ambient rainfall (average pH 4-0 to 4-3) in the north-eastern US. Evidently, cucumber plants have the capacity to recover and compensate for initial decreases in foliage and flower production, and the reproductive stage is not particularly susceptible to simulated acidic rain. These findings support previous results indicating that considerable increases in acidity of ambient rain would have to occur before the yield of crops would be measurably reduced.