EFFECTS OF OZONE ON GROWTH OF FOUR WOODY ORNAMENTAL PLANTS

Ozone exposure of coniferous plants during their second growing season for 4 consecutive days per week for 6 or 7 wk resulted in stimulation of shoot elongation in three of four species. No visible injury symptoms were noted. Compared to control plants, Pfitzer juniper and dense yew elongated significantly more at a low level (10 pphm for 4 wk and 20 pphm from week five to 6 or 7 wk) while white cedar elongated more at a high level (20 pphm for 4 wk and 40 pphm from week five to 6 or 7 wk). Elongation of tamarix juniper with no ozone exceeded that at either the low or the high level of exposure. Shoot elongation at the high level was equal to or less than that with no ozone for all species but white cedar. Plants exposed to the high level of ozone had generally lower chlorophyll contents, while the low level of ozone significantly increased chlorophyll a and b in Pfitzer juniper.