- Jiang, Mei
Graduate School, University of Maine
Chamber and field studies were established to determine the effects of acid fog on membrane associated calcium (mCa) and levels of oxidative stress in red spruce (Picea rubens Sarg.) needles. Five-year-old red spruce saplings were exposed to either acid fog, consisting of a mixture of H2SOa and HNO3 adjusted to pH 2.5, or distilled water fog at pH 5.6 during the tree growing season. A third group that received no fog served as the dry control.
Results from the chamber study provided evidence that acid fog significantly reduced mCa levels in red spruce mesophyll cells, indicating an acid-deposition induced calcium leaching from the membranes of photosynthetic mesophyll cells, and a disturbance to physiologically active calcium. Acid fog can lead to a reduction in mCa without effect on total foliar calcium. Distilled water fog exposure also led to reductions in mCa levels in young needles, suggesting that water films on needle surfaces can induce diffusional losses between needle interior and leaf surfaces. Red spruce trees from two field sites in Maine were also sampled. The results of the field study were consistent with those of the chamber studies in that lower mCa levels were associated with acid fog-exposed sites.
The second objective of this study was to test the hypothesis that acid fog increases oxidative stress levels as indicated by increased disulfide to thiol (SS/SH) ratios. The effects on total water-soluble thiol, disulfide, disulfide to thiol ratio, and photosynthetic pigments were investigated. In current-year needles, content of total thiol plus disulfide was significantly higher in acid fog-exposed needles than in dry control and distilled water fog-exposed needles. All of the increase was accounted for as disulfide, resulting in increased disulfide to thiol ratio. Treatment effects were less pronounced in one-year-old needles than in current-year ones. The results indicate that young red spruce needles exposed to acid fog experienced increased oxidative stress.