BOTANY VOLUME 4 - ECOLOGY - 2007

12. FUNDAMENTALS OF PLANT ECOLOGY

12.2. Stress and Adaptation

Not every deviation from the physiological optimum for growth constitutes stress. Without periodic departures from optimal living conditions, most organisms, including humans, would be utterly incapable of withstanding peak loads. In plants, such stimulating loads that bring the Organism into proper condition include, for example, periodic moisture deficits, Temperature fluctuations, wind stress, and intense variations in solar radiation. All of these have a training and hardening effect, thus acting constructively, even if they temporarily reduce biomass production. Destructive loads, or stress, differ fundamentally from these vital cyclical loads. It is often difficult to prove the NEGATIVE IMPACT OF stress if not all circumstances are taken into account. For instance, one tree under stress may shed all its leaves, whereas the neighbouring tree might pay with its life, which actually benefits the first tree as it gains more space for its own development. One must also consider the difference between the consequences of stress for an isolated individual versus a population within a community (regeneration). Thus, stress is somewhat subjective. The exact same stress factor (e.g., a decrease in available Water) may be destructive for one species while being routine or even beneficial for another. Plant responses to stress vary accordingly, as they depend on both the plant species (and its prior history) and The Nature of the stress factor; therefore, there is no universal scheme for coping with stress. Fig. 12.2 provides an example of a generalized framework for dealing with freezing temperatures and drought.

Class="center">Fig. 12.2. Plant responses to stress. Due to the wide variety of stress factors and plant responses, a comprehensive scheme for coping with stress does not exist. Here, the potential ways of surviving frost or desiccation are schematically presented as an example. If a plant cannot avoid such stressful situations, it must either prevent the consequences within its Tissues or tolerate them in order to survive.

The evolutionary consequences of stress are such that, ultimately, severe loads will be better tolerated by the plant or will cease to be stressful altogether; this phenomenon is referred to as adaptability. Whether this is the actual result of adaptations—through the fixation of traits in the genotype and consequently the Selection process—or merely an "acquired" trait that unexpectedly yields a positive effect, is in most cases impossible to determine. However, these causally important nuances are rarely distinguished in terminology. It is more accurate in this context to speak of adaptive traits rather than adaptation. In other words, this is tolerance achieved through continuous conditioning within the habitat. It involves recurring active processes, such as The formation of a thick leaf cuticle in sun leaves; this is also a form of adaptation. To properly evaluate such functional differences, three categories of adaptive traits are distinguished:

✵ modulational, or acclimative, traits;

✵ modificational traits;

✵ evolutionary, or genetically fixed, traits.

Even though the capacity to develop the first two categories of adaptive traits must also be genetically predetermined, significant differences still exist between them. Modulational or acclimative traits (acclimatization) mean that reversible phenotypic changes occur during the lifespan of a single organ or the entire plant, such as the acquisition of frost hardiness induced by low temperatures. Modificational adaptation primarily involves morphological changes that are irreversible for a given organ. A mature sun leaf can naturally never develop into a shade leaf. Finally, evolutionary (genotypic) adaptation involves heritable properties, such as succulence or Photoperiodism, which can never be modulated or modified, except perhaps within very narrow limits. The diverse adaptive responses and traits of plants are described in chapters 6, 8, 10, and 13.