OZONE EXPOSURE RESPONSE RELATIONSHIPS FOR PHOTOSYNTHESIS IN GENETIC STRAINS OF LOBLOLLY-PINE SEEDLINGS

The effects of acidic precipitation and ozone exposure on gas exchange and chlorophyll content of three half-sibling families of loblolly pine (Pinus taeda L. ) seedlings were studied in the Piedmont of North Carolina, USA. Seedlings of families 8-80, 8-103 and 8-130 were grown in open-top field chambers and exposed to two acid precipitation treatments (pH 5.2 or 3.5) and five ozone exposures delivered in proportion to ambient concentrations (0.5 x to 3.0 x ambient). There were no statistically significant effects of acid precipitation on photosynthesis or chlorophyll and carotenoid content of the first-flush fascicles after one growing season. Elevated ozone significantly decreased photosynthesis rates and the content of chlorophyll and carotenoids. There was a strong inverse relationship between photosynthesis and the cumulative ozone exposure defined by the sigmoidal Weibull function. Photosynthetic rates were decreased after cumulative exposures of 100-150 ppm h (summation of the twelve 1-h means day-1). Photosynthesis rates were 90% inhibited by cumulative ozone exposures between 250 and 300 ppm h. The three half sibling families differed in sensitivity to ozone based on thresholds and steepness of the exposure-response relationships. The relative ozone sensitivity of the families corresponded to previous studies of foliar injury and growth of the same families. Chlorophyll and carotenoid content of the first-flush fascicles decreased linearly with increasing cumulative ozone exposure. Equivalent reductions in photosynthetic response to cumulative ozone exposure were found for either acute short-term exposures to high ozone concentrations or equivalent long-term chronic exposures to lower concentrations. This suggests that acute peaks of high ozone concentrations were not as important at our site as were cumulative exposure thresholds. Overall, these data also indicate that current ambient ozone concentrations in the southeastern USA may have a significant effect on the carbon fixation rates of some loblolly pine families.