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Open‐top field chambers are the most widely used technology for evaluating the impacts of air pollutants on vegetation. This study was conducted to evaluate the long‐term effects of chambers on Valencia orange trees (Citrus sinensis [L.] Osbeck). The trees were exposed to ambient ozone (O3) for 51 months in large (4.3‐m diam. by 2.9‐m high) nonfiltered open‐top chambers (NF) and in ambient air without chambers (AA). During this period, the 12‐h daytime (M12) average O3 concentration was approximately 8% lower, and the cumulative O3 exposure for hourly average concentrations >0.1 µL L−1 (SUM10) was 29% lower than for NF than for AA trees. By December 1984 (after eight months in chambers), canopy volume for NF trees was 67% greater than for AA trees; and by March 1988 canopy volume for NF trees was 104% greater than for AA trees (P < 0.001). Specific leaf areas (cm2 g−1) were larger for NF than AA trees (P < 0.05). Total harvestable fruit weight combined for years 1986, 1987 and 1988 was 98% higher for NF than for AA trees (P < 0.01). The higher yield for NF trees was associated with lower immature fruit drop, especially during the growing seasons prior to the 1986 and 1987 harvests (P < 0.01). Warm season (April–October) rates of stomatal conductance and transpiration were 14 and 12% lower, respectively, for NF than AA trees; which coincided with 29% more negative leaf water potentials for NF than AA trees (P < 0.05) . In contrast, cool season net photosynthetic rates were 18% lower for NF than for AA trees, and starch concentrations were 31% higher in leaves from NF compared to AA trees in February 1988 (P < 0.05). Overall, the yield increases for NF compared to AA trees could, in part, be accounted for by decreased flux of O3 into leaves (based on decreased O3 exposure and leaf conductance). However, other factors, i.e., increased tree growth, altered leaf C allocation, and lack of wind stress occurring only in chambers, likely contributed to higher NF tree yields.

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