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Keywords:

  • Amino acids;
  • glutamate dehydrogenase;
  • glutamine synthetase;
  • ozone;
  • Pinus taeda;
  • soluble protein

summary

Dose-response relationship of ozone effects on foliar nitrogen metabolism of two half-sib families of Pinus taeda L. was studied. Trees were exposed to six ozone concentrations, ranging from 0.2 to 3 times the ambient, for two consecutive growing seasons (1988 and 1989) in open-top chambers. Content of total chlorophyll, soluble protein and soluble amino acids, and activities of glutamine synthetase and glutamate dehydrogenase, were measured in second-flush needles of 1989, harvested in November of 1989. Root collar diameter growth rate was also determined. Variation in depth to soil mottling, an indicator of changing redox conditions, significantly affected tree growth and nitrogen metabolism. Therefore depth to soil mottling was used as a covariate and dose-response curves were provided with adjusted means. A decline of root collar diameter growth rate and foliar chlorophyll content was found at the highest ozone level. However, both these parameters were affected much more by soil conditions than by ozone. Glutamine synthetase, glutamate dehydrogenase and amino acids, especially glutamate and glutamine, increased by approx. 2-fold and 3-fold, respectively, at two and three times ambient ozone. However, soluble protein content was only affected at the highest ozone concentration, showing a 25-fold increase. Nitrogen metabolism was more influenced by ozone than by the variation in edaphic conditions. No consistent ozone × family interaction on nitrogen metabolism could be found, but the two half-sib families significantly differed in glutamate dehydrogenase activity and the contents of glutamate and glycine, indicating a regulatory function of this enzyme between nitrogen and carbon metabolism. The increase of amino-N turnover and the following rise in soluble protein content are explained by the strategy of the plant to reallocate the nutrients from needles subjected to accelerated senescence by ozone. Possible implications of this process in carbohydrate metabolism and carbon partitioning are discussed.