ABSTRACT
Air pollutants generally initially affect shoot growth, because the pollutants are delivered directly to photosynthetic cells. In addition, the apoplast solution is an important site of interaction between plants and pathogens, which helps to explain the effects of some air pollutants. A critical level is an atmospheric concentration of a pollutant which, if exceeded, can have an adverse effect on plants. Thus, globally, there is a range of air pollutants that have established adverse effects on plants at thresholds that are in some cases being exceeded with increasing frequency. Pollutants can be scavenged when they come into contact with water because of Brownian motion, turbulent airflows, coagulation of droplets, and movement under gravity. Atmospheric particulates are probably the most widespread air pollutant. Many trees also accumulate particular pollutants in particular tissues, especially the cuticle. Plants have been naturally exposed to similar, and sometimes the same, compounds as those that human activity produces as air pollutants.
