SEASONAL PATTERNS OF LIGHT-SATURATED PHOTOSYNTHESIS AND LEAF CONDUCTANCE FOR MATURE AND SEEDLING QUERCUS-RUBRA L FOLIAGE - DIFFERENTIAL SENSITIVITY TO OZONE EXPOSURE

Extrapolation of the effects of ozone on seedlings to large trees and forest stands is a common objective of current assessment activities, but few studies have examined whether seedlings are useful surrogates for understanding how mature trees respond to ozone. This two-year stu dy utilized a replicated open-top chamber facility to test the effects of subambient, ambient and twice ambient ozone concentrations on ligh t-saturated net photosynthesis (P(max)) and leaf conductance (g1) of l eaves from mature trees and genetically related seedlings of northern red oak (Quercus rubra L.). Gas exchange measurements were collected f our times during the 1992 and 1993 growing seasons. Both P(max) and g1 of all foliage followed normal seasonal patterns of ontogeny, but mat ure tree foliage had greater P(max) and g1 than seedling foliage at ph ysiological maturity. At the end of the growing season, P(max) and g1 of the mature tree foliage exposed to ambient (almost-equal-to 80-100 ppm-h) and twice ambient (almost-equal-to 150-190 ppm-h) exposures of ozone were reduced 25 and 50%, respectively, compared with the values for foliage in the subambient ozone treatment (almost-equal-to 35 ppm- h). In seedling leaves, P(max) and g1 were less affected by ozone expo sure than in mature leaves. Extrapolations of the results of seedling exposure studies to foliar responses of mature forests without conside ring differences in foliar anatomy and stomatal response between juven ile and mature foliage may introduce large errors into projections of the response of mature trees to ozone.