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Olszyk, D., D. Tingey, C. Wise and E. Davis. 2002. CO2 and O3 alter photosynthesis and water vapor exchange for Pinus ponderosa needles. Phyton 42:121-134. WED-02-003

Photosynthesis and water vapor exchange are key physiological processes that determine whole plant carbon uptake and water loss. CO2 and O3 affect these processes, but the combined effects of both pollutants are uncertain, especially for trees and over long periods of time. To evaluate the interactive effects of CO2 and 03 on needle photosynthesis, conductance, transpiration and water use efficiency (WUE, photosynthesis/transpiration) of tree seedlings, we grew Pinus ponderosa seedlings for three years in outdoor, sunlit, mesocosms under controlled climatic and edaphic conditions. CO2 alone (~270 Fl 1 -1 above ambient) increased photosynthetic rates, with only a slight decrease in stomatal conductance and transpiration. The net result of the CO2 effects was increased WUE. 03 alone (SUM O6 of 11.0 Fl 1 -1 h in 1999 and 26.1 Fl 1 -1 h in 2000) did not have any consistent effects on needle gas exchange on individual measurement dates. However, averaged over an entire summer exposure period in 2000, O3 tended to decrease WUE. There were significant CO2 x O3 interactions across the summer exposure period in 2000, with high O3 tending to decrease photosynthesis at ambient CO2; but increase transpiration and photosynthesis at elevated CO2. High O3 tended to reduce the increase in WUE with elevated CO2. These results reveal that possible interactive effects on photosynthesis and water vapor exchange must be considered when evaluating the effects of CO2 and O3 on plants.

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