CHANGES IN GROWTH, LEAF ABSCISSION, AND BIOMASS ASSOCIATED WITH SEASONAL TROPOSPHERIC OZONE EXPOSURES OF POPULUS-TREMULOIDES CLONES AND SEEDLINGS

The effects of single-season tropospheric ozone (O-3) exposures on-gro wth, leaf abscission, and biomass of trembling aspen (Populus tremuloi des Michx.) rooted cuttings and seedlings were studied. Plants were gr own in the Upper Peninsula of Michigan in open-top chambers with O-3 e xposures that ranged from 7 to 92 ppm-h. Depending on the genotype, to tal seasonal O-3 exposure in the range of 50-92 ppm-h had negative imp acts on stem, retained leaf, and root biomass accumulation and on diam eter growth. Leaf abscission generally increased with increasing O-3 e xposure and was the principal cause of the decrease in leaf biomass of the O-3-treated plants. Considerable genetic variation in O-3 respons es occurred, as shown by 3 differences in sensitivities among clones a nd among seedlings. However, the responses to O-3 of rooted cuttings a nd seedlings were similar when seedling means were compared with clona l means for leaf abscission, diameter growth, retained leaf biomass, a nd root biomass. Comparison of a single square-wave treatment (52 ppm- h) with 70 and 92 ppm-h episodic exposures suggested that the plant re sponse to the square-wave exposure was similar to the response to the highest episodic exposure even though the 92 ppm-h episodic exposure w as almost twice the square-wave exposure. Our results are consistent w ith previous studies that show that P. tremuloides is highly responsiv e to O-3 exposure and this response has a strong genetic component.