Human activity is altering the chemistry of the atmosphere, which, in turn, is affecting the physiology and growth of plants. The purpose of this article is to develop four ideas that are currently emerging from the work of a diverse group of plant scientists. (1) Air pollution definitions: The definition of air pollution has been broadened, and research activities are expanding to include analysis of plant responses to a wide range of atmospheric chemicals emitted from anthropogenic sources but not previously considered as air pollutants. Thus experiments with CO2 and other trace gases are being pursued with approaches developed in air pollution research. (2) Air pollution uptake: Efforts are increasing to better quantify air pollution absorption rates through stomata in order to calculate actual dose vs, plant responses. The flux rates of gaseous pollutants into leaves, especially O-3, are largely dependent upon stomatal conductance. Approaches are being developed to calculate stomatal absorption of gaseous pollutants, based on stomatal conductance values for water vapor and ambient air-pollution concentrations. Calculation of air pollution absorption rates will allow responses of plants to pollutants to be assessed in toxicological frameworks and will help characterize the strength of vegetation as sinks for some gaseous pollutants. (3) Compensatory responses: Plant responses to air pollutants can be interpreted as compensatory, i.e., a physiological adjustment to an environmental stress that maximizes productivity above that which would have occurred in the absence of compensation. Examples of compensatory responses to air pollutants are shifts in root-to-shoot ratio and accelerated rates of leaf maturation. Recognition of compensatory responses to air pollutants allows these responses to be placed in a framework that relates to whole-plant processes and ecosystem functions. (4) Air pollution and multiple stresses: Air pollution stress seldom occurs in isolation, and research approaches are being developed around the concept of multiple interacting stresses. Multiple-stress experiments are important because factors such as plant water status, light, and nutrient availability are known to alter plant responses to air pollutants. Multiple-stress studies will involve experiments with model plant species and high degrees of environmental control and monitoring.
