Leaf Wettability as a Measure of Air Pollution Effects

Abstract

Droplet contact angle (DCA) is a technique that can be used to measure wettability and, in turn, provide an assessment of the physical and chemical characteristics of a surface. As adapted to plant biology, DCA measurements have been useful in characterizing changes in the type or condition of leaf epicuticular waxes. Environmental as well as temporal factors can modify the biophysical features of epicuticular wax surfaces and thereby affect DCA measurements. An understanding of the role of these non–pollutant factors is necessary before pollution damage can be accurately assessed. Controlled chamber experiments and field pollutant gradient studies have shown that DCA is generally reduced when plants are exposed to air pollutants such as ozone, S02, and acidic fog. In some cases, environmental influences, such as temperature, have been separated from the pollutant effect. However, mixtures of anthropogenic pollutants or anthropogenic and natural compounds (sea salts, dust particles) which are often present in field studies can confound the interpretation of DCA measurements. A few studies that attempt to separate these factors have been conducted, but more are needed before the potential for using DCA measurements in long–term bioindicator studies can be fully realized. Some studies have demonstrated that pollutants do not necessarily affect leaf surfaces in a uniform patter, but rather are specific for certain structures such as stomates or trichomes; deposition levels can also be different on ad– and abaxial surfaces. The degree to which these inhomogeneities of action can affect DCA measurements needs further study.