RESPONSE OF GIANT SEQUOIA CANOPY FOLIAGE TO ELEVATED CONCENTRATIONS OF ATMOSPHERIC OZONE

We examined the physiological response of foliage in the upper third o f the canopy of 125-year-old giant sequoia (Sequoiadendron giganteum B uchholz.) trees to a 61-day exposure to 0.25x, 1x, 2x or 3x ambient oz one concentration. Four branch exposure chambers, one per ozone treatm ent, were installed on 1-m long secondary branches of each tree at a h eight of 34 m. No visible symptoms of foliar ozone damage were apparen t throughout the 61-day exposure period and none of the ozone treatmen ts affected branch growth. Despite the similarity in ozone concentrati ons in the branch chambers within a treatment, the trees exhibited dif ferent physiological responses to increasing ozone uptake. Differences in diurnal and seasonal patterns of g(s) among the trees led to a 2-f old greater ozone uptake in tree No. 2 compared with trees Nos. 1 and 3. Tree No. 3 had significantly higher CER and g(s) at 0.25x ambient o zone than trees Nos. I and 2, and g(s) and CER of tree No. 3 declined with increasing ozone uptake. The y-intercept of the regression for da rk respiration versus ozone uptake was significantly lower for tree No . 2 than for trees Nos. 1 and 3. In the 0.25x and 1x ozone treatments, the chlorophyll concentration of current-year foliage of trees Nos. 1 and 2 was significantly higher than that of current-year foliage of t ree No. 3. Chlorophyll concentration of current-year foliage on tree N o. 1 did not decline with increasing ozone uptake. In all trees, total needle water potential decreased with increasing ozone uptake, but tu rgor was constant. Although tree No. 2 had the greatest ozone uptake, g(s) was highest and foliar chlorophyll concentration was lowest in tr ee No. 3 in the 0.25x and 1x ambient atmospheric ozone treatments.