Red spruce (Picea rubens Sarg.) seedlings were exposed to elevated levels of ozone during a growing season and into the period of early winter frosts. Samples were taken for histological analysis, chlorophyll content, and carotenoid content. Ozone treatments increased mesophyll cell disruption after frosts in early winter. Cell injury increased with increasing ozone concentrations up to 2 × ambient. In the spring, injury was assayed by ranking trees based on overall appearance and by counting the number of shoots with brown needles. Ozone treatments had no effect on winter injury as scored by the ranking method. There was also no effect on the number of injured flushes when all trees were considered. However, when only those trees exhibiting winter injury were considered, there was a small, but statistically significant effect of ozone on the number of injured shoots. Thus, there is evidence that ozone increases injury from freezing temperatures in the autumn. Ozone slightly increases browning in the spring in sensitive trees. There were no correlations between histological injury in the autumn and appearance in the spring, indicating that there may be different aspects of the winter injury syndrome.
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