LIGHT ENVIRONMENT ALTERS RESPONSE TO OZONE STRESS IN SEEDLINGS OF ACER-SACCHARUM MARSH AND HYBRID POPULUS L .2. DIAGNOSTIC GAS-EXCHANGE ANDLEAF CHEMISTRY

Diagnostic gas exchange measurements and foliar chemical assays were c onducted on hybrid poplar (Populus tristis Fisch. x P. balsamifera L. cv. Tristis) and sugar maple (Acer saccharum Marsh.) seedlings grown u nder contrasting light and ozone treatments. Seedlings were grown in l ow irradiance (c. 2.5 mol m-2 d-1) and six-fold greater irradiance (c. 16.6 mol m-2 d-1) in combination with low (< 10 nl l-1) and elevated (99-115 nl l-1) ozone. Analysis of light response curves showed ozone- induced reductions in photosynthetic capacity and quantum yield for un shaded poplar and shaded sugar maple, but not the contrasting light tr eatments. Photosynthesis at saturating CO2 concentrations was decrease d in the elevated ozone treatment in both the unshaded and shaded popl ar and in shaded sugar maple. Poplar had significant reductions in chl orophyll concentration due to ozone exposure in both unshaded and shad ed treatments. Older leaves of unshaded poplar plants had significantl y greater reductions in chlorophyll levels due to ozone than older lea ves of shaded plants. In maple, only shade-grown leaves had significan t decreases in chlorophyll concentration due to ozone exposure. The di agnostic gas exchange measurements in conjunction with chlorophyll mea surements indicate that in hybrid poplar, unshaded leaves may be more sensitive to ozone than shade leaves, while in sugar maple, shade leav es are more sensitive to ozone. For hybrid poplar a decrease in photos ynthetic capacity, quantum yield and chlorophyll concentration in the unshaded, moderately high light environment due to elevated ozone is c onsistent with prior studies. The results indicating that sugar maple seedlings may be more detrimentally affected by elevated ozone in the lower light environment may have serious implications for this and oth er shade-adapted species with respect to their performance in an under storey environment.