We define stress as an environmental factor that reduces the rate of some physiological process (e.g., growth or photosynthesis) below the maximum rate that the plant could otherwise sustain. Stresses can be generated by abiotic and/or biotic processes. Examples of stress include low nitrogen availability, heavy metals, high salinity, and shading by neighboring plants. The immediate response of the plant to stress is a reduction in performance (Fig. 3). Plants compensate for the detrimental effects of stress through many mechanisms that operate over different time scales, depending on the nature of the stress and the physiological processes that are affected. Together, these compensatory responses enable the plant to maintain a relatively constant rate of physiological processes despite occurrence of stresses that periodically reduce performance. If a plant is going to be successful in a stressful environment, then there must be some degree of stress resistance. Mechanisms of stress resistance differ widely among species. They range from avoidance of the stress, e.g., in deep-rooting species growing in a low-rainfall area, to stress tolerance, e.g., in Mediterranean species that can cope with a low leaf water content.
Physiological processes differ in their sensitivity to stress. The most meaningful physiological processes to consider are growth and reproduction, which integrate the stress effects on fine-scale physiological processes as they relate to fitness, i.e., differential survival and reproduction in a competitive environment. To understand the mechanism of plant response, however, we must consider the response of individual processes at a finer scale (e.g., the response of photosynthesis or of light-harvesting pigments to a change in light intensity). We recognize at least three distinct time scales of plant response to stress:
1. The stress response is the immediate detrimental effect of a stress on a plant process. This generally occurs over a time scale of seconds to days, resulting in a decline in performance of the process.
Figure 3. Typical time course of plant response to environmental stress. The immediate response to environmental stress is a reduction in physiological activity. Through acclimation, individual plants compensate for this stress such that activity returns toward the control level. Over evolutionary time, populations adapt to environmental stress, resulting in a further increase in
Figure 3. Typical time course of plant response to environmental stress. The immediate response to environmental stress is a reduction in physiological activity. Through acclimation, individual plants compensate for this stress such that activity returns toward the control level. Over evolutionary time, populations adapt to environmental stress, resulting in a further increase in activity level toward that of the unstressed unadapted plant. The total increase in activity resulting from acclimation and adaptation is the in situ activity observed in natural populations and represents the total homeo-static compensation in response to environmental stress.
2. Acclimation is the morphological and physiological adjustment by individual plants to compensate for the decline in performance following the initial stress response. Acclimation occurs in response to environmental change through changes in the activity or synthesis of new biochemical constituents such as enzymes, often associated with the production of new tissue. These biochemical changes then initiate a cascade of effects that are observed at other levels, such as changes in rate or environmental sensitivity of a specific process (e.g., photosynthesis), growth rate of whole plants, and morphology of organs or the entire plant. Acclimation to stress always occurs within the lifetime of an individual, usually within days to weeks. Acclimation can be demonstrated by comparing genetically similar plants that are growing in different environments.
3. Adaptation is the evolutionary response resulting from genetic changes in populations that compensate for the decline in performance caused by stress. The physiological mechanisms of response are often similar to those of acclimation, because both require changes in the activity or synthesis of biochemical constituents and cause changes in rates of individual physiological processes, growth rate, and morphology. In fact, adaptation may alter the potential of plants to acclimate to short-term environmental variation. Adaptation, as we define it, differs from acclimation in that it requires genetic changes in populations and therefore typically requires many generations to occur. We can study adaptation by comparing genetically distinct plants grown in a common environment.
Not all genetic differences among populations reflect adaptation. Evolutionary biologists have often criticized ecophysiologists for promoting the ''Panglossian paradigm'', i.e., the idea that just because a species exhibits certain traits in a particular environment, these traits must be beneficial and must have resulted from natural selection in that environment (Gould & Lewontin 1979). Plants may differ genetically because their ancestral species or populations were genetically distinct before they arrived in the habitat we are studying or other historical reasons may be responsible for the existence of the present genome. Such differences are not necessarily adaptive.
There are at least two additional processes that can cause particular traits to be associated with a given environment:
1. Through the quirks of history, the ancestral species or population that arrived at the site may have been pre-adapted, i.e., exhibited traits that allowed continued persistence in these conditions. Natural selection for these traits may have occurred under very different environmental circumstances. For example, the tree species that currently occupy the mixed deciduous forests of Europe and North America were associated with very different species and environments during the Pleistocene, 100000 years ago. They co-occur now because they migrated to the same place some time in the past (the historical filter), can grow and reproduce under current environmental conditions (the physiological filter), and out-competed other potential species in these communities and successfully defended themselves against past and present herbivores and pathogens (the biotic filter). 2. Once species arrive in a given geographic region, their distribution is fine-tuned by ecological sorting, in which each species tends to occupy those habitats where it most effectively competes with other plants and defends itself against natural enemies (Vrba & Gould 1986).
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