Needle chlorosis in Sitka spruce following a three-year exposure to low concentrations of ozone: changes in mineral...

Two-year-old seedlings of Sitka spruce were exposed to 70 nl 1^-1 ozone or to filtered air over three successive summers in outdoor large-scale fumigation chambers (Solardomes). Seven months after the last period of exposure to the pollutant and just prior to budburst, upper-surface chlorisis affecting only the older needles of ozone-exposed trees was observed. In many respects, the symptoms appeared to be similar to those characteristic of type 1 spruce damage occurring in parts of mainland Europe. Chlorophyll pigments were reduced in the ozone-exposed older foliage, but no change in the ratio of chlorophylls to carotene was observed. The content of ascorbic acid was clearly related to the amount of foliar damage observed on the trees exposed to ozone and the largest increases were seen in those trees which were most visibly damaged. Although none of the foliage examined was deficient in any of the nutrient cations which were measured, the concentration of Mg in the older needles was significantly reduced by exposure to ozone, irrespective of damage symptoms. Exposure to ozone also resulted in increases in the ratios of K: Mg and Ca: Mg. In the older needles, leaching of Mg^2- and K^- by 0.5 mM H₂ SO₄ , pH 3.0, was enhanced by prior exposure to ozone, but the amounts removed were small (< 6%). It is suggested that long-term exposure to ozone has a cumulative effect on plant tissue and that the observed chlorosis was the result of accelerated senescence.