OZONE DECREASES SPRING ROOT-GROWTH AND ROOT CARBOHYDRATE CONTENT IN PONDEROSA PINE THE YEAR FOLLOWING EXPOSURE

Storage carbohydrates are extremely important for new shoot and root development following dormancy or during periods of high stress. The hypothesis that ozone decreases carbohydrate storage and decreases new root growth during the year following exposure was investigated. Ponderosa pine seedlings (Pinus ponderosa Dougl. ex Laws.) exposed to ozone in 1989 were harvested prior to shoot elongation (nongrowing seedlings) and following a root growth test (growing seedlings) in the spring of 1990 to evaluate starch and soluble sugar concentrations. Seedlings exposed to the highest ozone level had 34% less lateral root biomass and 65% less new root biomass the following spring. Exposure to 122 and 169 ppm-h (sum of hourly means, 24 h/day, over the exposure period) during the 1989 growing season resulted in significant decreases of carbohydrate pools examined in both nongrowing and growing seedlings the following spring. Starch concentrations were most affected: at the highest ozone level, there were decreases of 43 and 44% in coarse and fine roots, respectively, of nongrowing seedlings, and 50, 65, and 62% in coarse, fine, and new roots, respectively, of growing seedlings. The results suggest that (i) ponderosa pine seedlings exposed to 122 and 169 ppm-h ozone for one season have significantly less root starch reserves available just prior to and during bud break the following year and (ii) spring root growth is decreased following ozone exposure. The carry-over effects of ozone stress may be important in long-lived perennial species that are annually subjected to ozone.