Ozone is an air pollutant which can lead to plant tissue injury and reductions in growth and productivity. Plants respond to ozone by compensatory or defensive reactions such as avoidance by stomatal closure, detoxification of ozone by chemical reaction, adjustment by alteration of metabolic pathways or repair of injured tissue. The response is dependent on intensity of the ozone exposure, environmental factors influencing both plant photosynthesis and ozone deposition to plant surfaces, and defensive mechanisms inherent within plants. Hence the exposure of the plant to ozone is critical in the plant response to and damage from ozone.

The current air quality standards for ozone are based on the measured eight hour average ambient concentration. However, some biologists have recommended a cumulative ozone exposure parameter where the ozone concentrations over a set period of time are used to protect vegetation. Musselman and Massman (1999) describe a third alternative which uses quantifiable flux-based numerical parameters as a replacement for measured ambient ozone concentration.

This alternative contains parameters that are much more closely related to plant response than are the current ambient ozone concentrations. Massman et al. (2000) develop the dose-based standard further by using physical reasoning based on plant defenses and general resistance concepts of dry deposition. The model is developed in terms of plant injury, but could be extended to include damage, which historically has been the focus of air quality standards. With this dose-based approach, Massman et al. (2000) clarify some issues concerning the plant response to ozone and the use of fluxes to determine an ozone standard. They demonstrate that using a dose-based standard is likely to prove more useful in protecting vegetation from ozone, to be more precise and more discriminating than a cumulative exposure-based standard. In addition, they explored the interactions of the daily cycles of ozone concentration and plant stomatal conductance. In a related paper, Musselman and Minnick (2000) focused on the importance of night time conductance to ozone air quality standards. These research findings should help improve the standards associated with ozone for vegetation.

Massman, William J.; Musselman, Robert C.; Lefohn, Allen S. 2000. A conceptual ozone dose-response model to develop a standard to protect vegetation. Atmospheric Environment. 34: 745-759.View Document

Musselman, Robert C.; Massman, William J. 1999. Ozone flux to vegetation and its relationship to plant response and ambient air quality standards. Atmospheric Environment. 33: 65-73.View Document

Musselman, Robert C.; Minnick, Tamera J. 2000. Nocturnal stomatal conductance and ambient air quality standards for ozone. Atmospheric Environment. 34: 719-733.View Document