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

Rooted cuttings of hybrid Populus (DN34, Populus deltoides x nigra) we re grown outdoors in pots in open-top chambers at Ithaca, NY (74.5-deg rees-W, 42.5-degrees-N), during 1988 and 1989 (experiment 1) and durin g 1989 and 1990 (experiment 2). Ambient air was passed through charcoa l .filters to produce a 0.5 times ambient ozone treatment, and ozone g enerated from oxygen was added to produce one and two times ambient oz one treatments. In experiment 1, treatments were applied for 8-12 h ea ch day for 112 days of the 1988 growing season, then the plants were g rown 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. Chroni c exposure to ozone caused the following changes (statistically signif icant in one or both experiments at p < 0.05): (1) earlier leaf abscis sion, (2) decreased stem basal diameter, (3) decreased stem mass, (4) decreased internode length, (5) decreased shoot height (p = 0.005), an d (6) decreased leaf size in the growing season following ozone treatm ent. There was also strong evidence that ozone increased the number of leaves produced (p = 0.055). Finally, there was some evidence that oz one increased the ratio of shoot mass to root mass (p = 0.093).