More Work On Pollution's Impact, For Plants' Sake

Atmospheric emissions from human activity have long been known to be a health hazard. Short chimneys and, later, tall smokestacks have dispersed emissions across our landscapes in ever-widening spheres of influence, to the point where anthropogenic air pollutants today cover broad regions in developing and developed countries across the globe. Scientific study of the impact of these emissions on plants has extended beyond the dramatic local-point source problem to current major programs’ investigating effects of air pollutants on regional ecosystems. Research on air pollutants and their effects on plant life has encompassed many disciplines, thousands of research reports, and many books and symposium volumes. The major issues and current approaches to understanding the role of gaseous air pollutants in modifying plant health involve: (1) description of the mechanism of action at the biochemical, cellular, and tissue levels, (2) characterization of pollutant exposures, (3) interactions of multiple pollutants, (4) whole-plant function, including carbon allocation and stress resistance, and (5) propagation of the stress to population, community, ecosystem, and landscape levels. The policy issues surrounding this topic are quite distinct from the more broadly ranging scientific issues, including the study of physiological mechanisms, population consequences, epidemiology and whole-system function. Resolving the scientific issues at all these levels will require many years and probably a new approach to the manner in which we structure scientific disciplines to tackle interdisciplinary problems. Ozone is probably the most damaging regional-scale air pollutant. It is a secondary pollutant formed by the action of sunlight on various products of fuel combustion, such as nitrogen oxides and hydrocarbons. Ozone currently is the only regionally dispersed air pollutant that has been shown conclusively to damage crops and forests. SO2 is a primary pollutant, in and of itself toxic. SO2 is emitted from stationary-source fuel com bustion (the burning of coal), but various industrial processes also create the pollutant. It is widespread and damages agricultural crops and forests in industrialized countries. Gaseous air pollutants impact virtually all aspects of plant growth and development, either directly or indirectly. Effects occur at various scales in the plant system, beginning at the biochemical level and progressing up to the landscape level. Direct pollutant impacts on leaf structure and function will naturally have indirect effects on most other aspects of plant growth and development. For example, alterations in metabolism and enzyme activities may result in changes in metabolite pools, leading to shifts in the net carbon balance, which cause immediate and long-term consequences in carbon allocation, anatomy and morphology, overall growth, and reproductive processes. While many of the types of impacts have been clearly demonstrated under laboratory or controlled field conditions, or both, current research seeks to understand at what levels and under what conditions these impacts actually occur. Extrapolation from the few studied species to the rest of the plant kingdom (about 350,000 species), or even to other genotypes of the same species, is difficult because the genetic component of sensitivity is apparently high and the pollutant response is strongly controlled by plant conditions and environment. Thus, in addition to the genotype-by-genotype, species-by-species type of empirical dose-response experiments being conducted, a greater understanding of the mechanism of injury and their sequence of occurrence is needed. At the leaf level, while pollutant uptake may be the single most important factor in explaining plant susceptibility to direct effects, other variables mediating these effects will need to be addressed in greater detail in future research, Recent efforts include study of scavenger reactions that consume ozone and repair or compensatory processes. Future efforts to model pollutant impacts on plants will require explicit consideration of the steps in the sequence of injury. Given the difficulties and expense of fully replicated multiple-pollutant experiments for empirical dose-response studies, we can hope to rely on this approach for only a few key species and production systems, leaving out a large part of our agricultural and forestry base and ignoring the bulk of our natural flora. A better description of ambient pollutant conditions is needed: what occurs in the field; which pollutants commonly occur simultaneously; under what climatic conditions do these pollutants usually co-occur what extreme conditions are likely to occur. This type of information would allow researchers to focus on specific scenarios to be used in laboratory and field experiments. Some work of this type has been completed, but the number of fully instrumented nonurban monitoring sites is extremely limited in the United States, and analysis of such data will take some time to complete. We need to expand our knowledge of the multiple abiotic and biotic stress interactions. Partial factorial designs and other statistical approaches are needed to handle the experimental problem of “too many treatment combinations” (multiple pollutants, stresses, and affected plant species). We need pathogen systems that are transportable into the laboratory or field, since at best only a small fraction of the important species/ pathogen systems are “captive.” Disease patterns in space and time are also poorly described. Finally, the direct effect of pollutants on the diseased organisms themselves is not well understood. Current efforts to understand these complex interactions have been limited to demonstrating the types of such interactions that are possible. Quantification of effects on a species-by-species or regional basis is far in the future. To begin to get a grasp on the extent of potential pollutant/stress interac- tions, we need to develop better epidemiological approaches for plants. This could start with more observational data, including better regional coverage of ecosystem health and longer-term records with consistent data and formats. The U.S. Forest Service and the Environmental Protection Agency are currently and jointly pursuing a National Vegetation Survey. This might be expanded to include biosphere reserve sites, long-term ecological research sites, or Forest Service research natural areas, representing a variety of biomes where long-term, intensive research efforts are under way. Fully confronting the complexity of pollutant impacts on both human-dominated and natural ecosystems requires an understanding of population, community, and dynamics of a large number of ecosystem types. Currently, our models at these levels are inadequate. Recent efforts to study forests have focused on the study of branches as more workable units than whole trees in order to extrapolate to forest stands. The success of this approach is uncertain, depending on the degree to which branches are autonomous of the whole tree. At present, policy objectives far exceed research capabilities. Major interdisciplinary efforts, integrated to an extent not yet realized, will be necessary to tackle the complex system-level questions. As a society we must hope that we can control or even reduce emissions; otherwise, our progress in understanding and describing current impacts may be no longer relevant in the changed environment of an increasingly polluted world. Deane Wang and Wieger Schaap are both with the Center for Urban Horticulture at the University of Washington, Seattle. [Ed. note: For further reading, see the authors’ review “Air Pollution Impacts on Plants: Current Research Challenges” in the ISI Atlas of Science: Animal & Plant Sciences 1(1), 33-39,1988, from which the preceding was adapted with permission. Copyright Institute for Scientific Information 1988.]

More Work On Pollution's Impact, For Plants' Sake

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