OZONE EFFECTS ON SEEDLING SUGAR MAPLE (ACER-SACCHARUM) AND YELLOW-POPLAR (LIRIODENDRON-TULIPIFERA) - GAS-EXCHANGE

The effects of one to two seasons of ozone (O-3) exposure ranging from subambient to 1.7 times ambient on the gas exchange of sugar maple (A cer saccharum Marsh.) and yellow-poplar (Liriodendron tulipifera L.) w ere studied in standard 3 m diameter open-top chambers. Cumulative O-3 exposures based on SUM00 index ranged from 16 to 107 ppm.h in 1990 an d 31 to 197 ppm.h in 1991. During the 120-day exposure in 1990, rates of net photosynthesis and stomatal conductance of sugar maple foliage were not affected by O-3 Based on estimates from light-response curves , net photosynthesis at saturating light, dark respiration, compensati on irradiance, and quantum efficiency were not affected in sugar maple leaves exposed to elevated 03 Gas exchange rates of leaves of yellow- poplar grown in elevated O-3 were significantly reduced in 1990 and 19 91, when compared with foliage of seedlings grown in charcoal-filtered air (CF air). In 1990, seasonal monthly net photosynthesis of yellow- poplar leaves was reduced 11.7% in 1.5 times ambient O-3 air compared with leaves of seedlings grown in CF air. The light response of yellow -poplar foliage generally was not affected by exposure to O-3 during e ither growing season; with the exception in 1991 of net photosynthesis at saturating light bring reduced 13 to 42% in leaves of seedlings ex posed to 1.7 times ambient O-3 air compared with leaves of seedlings g rown in CF air. Stomatal conductance rates were generally not affected by exposure to O-3 during either exposure season. In 1991, the respon se of newly mature (node 3 to 5 from apex) and older leaves (node 8 to 12 from apex) of ozone-exposed yellow-poplar were compared and respon ded similarly to elevated O-3. Ozone effects on seasonal diurnal photo synthetic response of newly mature and older yellow-poplar foliage wer e observed in 1991. Reductions (21-42%) in net photosynthetic rates of leaves of yellow-poprar exposed to elevated O-3 were first observed a fter 42 days of treatment (113 ppm.h exposure). Predawn and post-sunse t respiration and stomatal conductance were not affected by exposure t o elevated O-3. Despite significant reductions of net photosynthesis o f O-3-exposed yellow-poplar during both seasons, significant negative growth effects were not observed until the final harvest in June 1992. Root/shoot ratios of yellow-poplar seedlings exposed to 1.7 times amb ient O-3 were reduced 30% compared with those seedlings exposed to CF air.