Effects of elevated O3 and CO2 on chlorophyll fluorescence and gas exchange in Scots pine during the third growing season Original Research Article

Naturally regenerated, 30-year-old Scots pines (Pinus Sylvestris L.) were grown in open-top chambers and exposed in situ to doubled ambient O3, doubled ambient CO2 and a combination of elevated O3 and CO2 from 15 April to 15 September for three growing seasons (1994–1996). To examine the effects of O3 and/or CO2 on photosynthesis, chlorophyll a fluorescence and gas exchange were measured simultaneously. Doubled ambient O3 significantly decreased the rates of photosynthesis at all levels of photon flux density. This was related mainly to a significant decrease in the photochemical efficiency of photosystem II (PS II) and the rate of whole electron transport, rather than to a decrease in stomatal conductance. When measurements were made at doubled ambient concentration of CO2 (700 μmol mol−1), doubled ambient CO2 treatment did not lead to a significant change in the intrinsic capacity of photosynthesis, as manifested by no changes in PS II, the rate of electron transport, the maximal rate of photosynthesis and the apparent quantum yield of CO2 assimilation. However, elevated CO2 increased the sensitivity of stomatal conductance to light and decreased maximal stomatal conductance. When O3 and CO2 were combined, the O3-induced decrease in photosynthesis rate was reduced significantly by a high concentration of CO2. This may be partly related to the decrease in stomatal conductance induced by the high concentration of CO2. The complete mechanism behind this interaction is, however, still unclear.