Effect of ozone on plants

Ozone enters plant leaves via stomata, tiny valved pores on the leaf surface that regulate the exchange of gas between plant and air. During the day, the stomata are normally open to permit entry of CO₂ for photosynthesis. Unfortunately, at this time O₃ levels are highest.

Once inside the leaf, O₃ oxidizes molecules in cell membranes, causing the membranes to break down. Because O₃ occurs naturally in the atmosphere, plants have evolved some protective mechanisms, including "antioxidants" like vitamins C and E, and specialized proteins (enzymes) that repair injury from O₃. But at higher O₃ levels, these protective mechanisms are inadequate to prevent injury to tissues.

Ozone can cause direct damage to leaf tissue, often visible as flecking, bronzing, water-soaked spotting, and premature aging of leaves. Furthermore, high O₃ concentrations may cause the stomata to close, which cuts the flow of CO₂ and shuts down photosynthesis. As a result of the direct damage and the reduced photosynthesis, yields of some plants can be dramatically reduced by long-term exposure to elevated O₃ levels.

Although scientists have studied the effects of O₃ on various crops in Canada and elsewhere for more than 40 years, fluctuations in O₃ concentrations in polluted air pose major difficulties in providing reliable estimates of the damage caused to crops.