Plants as Accumulators of Atmospheric Emissions

The accumulation of airborne pollutants by plants is a consequence of uptake, usually passive, onto leaf surfaces, through stomata, or indirectly through the soil and root systems. Whether or not a pollutant accumulates depends on whether it can be metabolized, transported from the leaf to the root and excreted, used by the plant’s normal biochemical processes, or stored inside the leaf. All the processes involved depend on the physiological state of the plant to some extent, as well as the plant’s architecture. Consequently, accumulation rates vary greatly across plant species, even when exposed to similar pollutant concentrations. However, the use of carefully designed protocols, which minimize the effects of phenology and differential exposure and which avoid potential artifacts caused by the way samples are processed after sampling and before analysis, has provided reliable indications of the relative exposure of vegetation to different pollutants. If one is interested in potential harmful effects of pollutants on vegetation, then bioaccumulation is perhaps a more meaningful indicator of areas under pollution stress than a simple measure of air concentration or wet deposition, because plant accumulation integrates not just exposure, but the rate of uptake, which may depend on other factors such as climate. Measurements of the accumulation of pollutants in plants have provided spatial patterns that identify local and regional sources in terms of a measurable footprint on the environment, whether or not damage to vegetation is immediately obvious. They have also provided independent evidence of long-term temporal changes where air pollution measurement data did not exist. Their strength is in the relative simplicity of the processing required—and consequent cost saving—compared with instrumental methods of air quality monitoring. Although progress has been made in linking concentrations of nutrients such as N and S in plants to exposure or deposition rates, more data are needed before the relative importance of the different forms of pollutant exposure, and the responses of different species, are understood well enough to provide reliable estimates of deposition rates from foliar measurements. However, for substances that are less biologically active and more difficult to measure directly, such as heavy metals or POPs, the use of accumulation in plants will continue to be a cost-effective method for estimating spatial and temporal patterns of exposure, and for the identification of emission sources.