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Adaptation of plants to atmospheric pollutants.

Man-made air pollutants are a recent phenomenon in the evolutionary experience of plants and animals although natural air pollutants from volcanic eruptions, forest fires and dust storms have accompanied evolution for geological eras. Plants have responded to increasing concentrations of such pollutants as sulphur dioxide, fluorides, photochemical oxidants and acid rain at the community, species, population and individual levels. The lichens and bryophytes have shown particularly dramatic changes in urban and industrial areas. Many species have had their distribution severely limited. Tolerances to sulphur dioxide have evolved in populations of a number of grasses and herbs, and some sulphur dioxide-tolerant lichens have invaded inner city areas. Sensitivity to pollutants is partly a function of substrate chemistry. Synergistic interactions occur between various pollutants and also between pollutants and pathogens. A good deal of genetic variation occurs within crops, and this allows for selection of pollution-tolerant varieties. The nature of specific adaptations is not generally well known although, for sulphur dioxide, recent studies in poplar and spinach strongly suggest that increased production of the enzyme superoxide dismutase may be a key factor. In other adaptations, morphological and anatomical features play a part.