Theoretical aspects of pollutant stress.

Symptoms of tree damage observed in Europe and overseas are often described as premature senescence. However, our own experiments with fumigated poplars have shown that the term premature senescence is misleading if undue emphasis is placed on features common to both normal and premature senescence. Consequently, the term should be replaced by a concrete description of the stress symptom noted, e.g. premature leaf loss or premature starch degradation. Such stress symptoms are usually unspecific - just like the mechanisms of stress avoidance or stress tolerance themselves. A stress model taking account of ecological, physiological and evolutionary aspects is presented; it is shown that the different stress phases are influenced by various factors, e.g. the photosynthetic capacity of the leaves. The stress reactions displayed by fumigated poplars serve as illustrations for this model. Different species of the genus Populus can be classified as more or less pure C-strategists. It is postulated that this group of plants, in particular, is disturbed by various stress parameters, such as air pollutants, if shoot growth or metabolite transport is impaired.