Effects of Ambient and Acute Partial Pressures of Ozone on Leaf Net CO(2) Assimilation of Field-Grown Vitis vinifera L

Mature, field-grown Vitis vinifera L. grapevines grown in open-top chambers were exposed to either charcoal-filtered air or ambient ozone partial pressures throughout the growing season. Individual leaves also were exposed to ozone partial pressures of 0.2, 0.4, or 0.6 micropascals per pascal for 5 hours. No visual ozone damage was found on leaves exposed to any of the treatments. Chronic exposure to ambient O₃ partial pressures reduced net CO₂ assimilation rate (A) between 5 and 13% at various times throughout the season when compared to the filtered treatment. Exposure of leaves to 0.2 micropascals per pascal O₃ for 5 hours had no significant effect on A; however, A was reduced 84% for leaves exposed to 0.6 micropascals per pascal O₃ when compared to the controls after 5 hours. Intercellular CO₂ partial pressure (c_i) was lower for leaves exposed to 0.2 micropascals per pascal O₃ when compared to the controls, while c_i of the leaves treated with 0.6 micropascals per pascal of 0₃ increased during the fumigation. The long-term effects of ambient O₃ and short-term exposure to acute levels of O₃ reduced grape leaf photosynthesis due to a reduction in both stomatal and mesophyll conductances.