GROWTH-RESPONSES OF YOUNG CUTTINGS OF POPULUS-DELTOIDES X NIGRA TO OZONE IN CONTROLLED ENVIRONMENTS

Controlled-environment facilities were used to determine the growth re sponse of hybrid poplar using concentrations and exposure frequencies of ozone (O3) that mimic ambient patterns. Young cuttings of Populus d eltoides x nigra were exposed to daily bell-shaped concentration profi les of O3 with four peak concentrations and two exposures per week for 1 to 4 weeks. Exposure to O3 initially stimulated height growth rate, but repeated exposure resulted in decreases in height growth rate. St em diameter growth rate decreased with increasing O3 concentration. Le af greenness decreased with repeated exposure. The rate of leaf produc tion declined with increasing O3 Concentration. Many leaves abscised i n the higher O3 treatments, resulting ultimately in stimulation of bud break. Leaf area, leaf dry weight. and root dry weight at harvest dec reased with increasing total O3 exposure regardless of number of expos ures. Stem dry weight was greater after limited initial O3 exposure bu t was less after repeated O3 exposures. Repeated exposures to O3 were uniformly detrimental to poplar growth parameters.