LIGHT ENVIRONMENT ALTERS OZONE UPTAKE PER NET PHOTOSYNTHETIC RATE IN BLACK-CHERRY TREES

Foliar ozone uptake rates of different-sized black cherry (Prunus sero tina Ehrh.) trees were compared within a deciduous forest and adjacent openings in north-central Pennsylvania during one growing season. Stu dy trees included open-grown seedlings and saplings, forest understory seedlings and saplings, and sunlit and shaded portions of mature cano py tree crowns. Instantaneous ozone uptake rates were highest in high- light environments primarily because of higher stomatal conductances. Low ozone uptake rates of seedlings and saplings in the forest underst ory could be attributed partially to lower average ambient ozone conce ntrations compared to the canopy and open environments. Among the tree size and light combinations tested, ozone uptake rates were highest i n open-grown seedlings and lowest in forest-grown seedlings. Despite l ower ozone uptake rates of foliage in shaded environments, ozone uptak e per net photosynthesis of foliage in shaded environments was signifi cantly higher than that of foliage in sunlit environments because of w eaker coupling between net photosynthesis and stomatal conductance in shaded environments. The potential for greater ozone injury in shaded environments as a result of greater ozone uptake per net photosynthesi s is consistent with previous reports of greater ozone injury in shade d foliage than in sunlit foliage.