CHRONIC OZONE EXPOSURE ALTERS THE GROWTH OF LEAVES, STEMS AND ROOTS OF HYBRID POPULUS

Rooted cuttings of hybrid Populus (DN34, Populus deltoides x nigra) were grown outdoors in pots in open-top chambers at Ithaca, NY (74.5-degrees-W, 42.5-degrees-N), during 1988 and 1989 (experiment 1) and during 1989 and 1990 (experiment 2). Ambient air was passed through charcoal .filters to produce a 0.5 times ambient ozone treatment, and ozone generated from oxygen was added to produce one and two times ambient ozone treatments. In experiment 1, treatments were applied for 8-12 h each day for 112 days of the 1988 growing season, then the plants were grown outdoors with ambient ozone in 1989. In experiment 2, treatments were applied for 9 h each day for 98 days of the 1989 growing season, then the plants were grown outdoors with ambient ozone in 1990. Chronic exposure to ozone caused the following changes (statistically significant in one or both experiments at p < 0.05): (1) earlier leaf abscission, (2) decreased stem basal diameter, (3) decreased stem mass, (4) decreased internode length, (5) decreased shoot height (p = 0.005), and (6) decreased leaf size in the growing season following ozone treatment. There was also strong evidence that ozone increased the number of leaves produced (p = 0.055). Finally, there was some evidence that ozone increased the ratio of shoot mass to root mass (p = 0.093).