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CO2 AND O3 ALTER PHOTOSYNTHESIS AND WATER VAPOR EXCHANGE FOR PINUS PONDEROSA NEEDLES
Olszyk, D M., D T. Tingey, C M. Wise, AND E. S. Davis. CO2 AND O3 ALTER PHOTOSYNTHESIS AND WATER VAPOR EXCHANGE FOR PINUS PONDEROSA NEEDLES. PHYTON. PHYTON-Redactio, Austria, 42(3):121-134, (2002).
1. Effects of CO2 and O3 were determined for a key component of ecosystem carbon and water cycling: needle gas exchange (photosynthesis, conductance, transpiration and water use efficiency). The measurements were made on Pinus ponderosa seedlings grown in outdoor, sunlit, mesocosms under controlled climatic conditions.
2. Elevated CO2 alone (~280 ppm above ambient) increased instantaneous net photosynthetic rates, with only a tendency to reduce stomatal conductance and transpiration. The net result of these effects was increased water use efficiency.
3. High O3 alone (moderate SUM06 of 10.4 ppm . hr in 1999 or greater SUM O6 of 26.1 ppm . hr in 2000) did not have any consistent effects on gas exchange on individual measurement dates. However, over the summer exposure period in 2000, high O3 tended to decrease WUE.
4. There were significant CO2 x O3 interactions across the summer exposure period in 2000, with high O3 tending to decrease stomatal conductance and photosynthesis at ambient CO2; but increase stomatal conductance, transpiration and photosynthesis at elevated CO2. High O3 tended to reduce the increase in WUE with elevated CO2 but not at ambient CO2.
5. These results indicate that gas exchange in response to elevated CO2 or high O3 must consider possible confounding effects due to concurrent exposure with the reciprocal pollutant.