INTERACTION OF OZONE POLLUTION AND LIGHT EFFECTS ON PHOTOSYNTHESIS INA FOREST CANOPY EXPERIMENT

Ozone pollution may reduce net carbon gain in forests, yet data from m ature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested, We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of s ugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95 nmol mol(-1), 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45 nmol mol(-1)) over 3 months, The branch pairs were selected along a gradient from relatively high irradiance (PPFD 1 4.5 mol m(-2) d(-1)) to deep shade (0.7 mol m(-2) d(-1)). Ozone reduce d light-saturated rates of net photosynthesis (A(sat)) and increased d ark respiration by as much as 56 acid 40%, respectively, Compared to s un leaves, shade leaves exhibited greater proportional reductions in A (sat) and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. Wi th increasing ozone dose over time, A(sat) became uncoupled from stoma tal conductance as ratios of internal to external concentrations of ca rbon dioxide increased, reducing water-use efficiency. Ozone reduced n et photosynthesis and impaired stomatal function, with these effects d epending on the irradiance environment of the canopy leaves, Increased ozone sensitivity of shade leaves compared to sun leaves has conseque nces for net carbon gain in canopies.