Info

Stress Concepts

Based on physical principles, Levitt (1980) published a theoretical understanding of stress reactions that is applicable to all groups of organisms, as illustrated by an abstract experiment. It is known as the physical stress concept (Fig. 1.1.5). A body is deformed if it is stretched by a force (stress); this deformation is at first reversible ("elastic"), but upon intensifying the force it becomes irreversibly ("plastic") deformed and finally breaks. The change in the body caused by the force is called strain. The force required to produce a unit of change is the elastic modulus, M. In this sense, elasticity does not mean expansion in the sense of maximum elastic deformation. The modulus of elasticity M corresponds in principle to s, the elastic modulus of a cell wall, which is a measure of the cell wall's flexibility (see, e.g., Fig. 1.5.2)

Stress, strain and damage

force stress deformation strain

Stress, strain and damage

Stress avoidance

Strain avoidance

Stress tolerance

Fig. 1.1.5. The physical stress concept of Levitt (1980)

Stress avoidance

Strain avoidance

Stress tolerance

According to this relation, M is also a measure of the resistance of the system to an externally applied force on the system.

In biological systems, stress is not commonly a single physical force affecting the organism, but a load from many individual environmental factors. Primarily, metabolic processes are changed or deformed. The concept by Levitt convincingly explains the relation of stress and strain, but it can be applied to biological systems only to a limited extent, as the following, biologically important parameters are lacking:

• Time factor: In a physical system, the amount of stress equals the strength of stress; in a biological system, the amount of stress is the product of the intensity of stress and duration of stress. For example, if one cools the tropical ornamental Saintpaulia ionantha (African violet) for a short time (6 h) to 5 °C

Fig. 1.1.5. The physical stress concept of Levitt (1980)

and then returns it to the original temperature, some of the metabolic reactions may change their rates in accordance with their activation energy (Qi0), but the increase or decrease in metabolite pools is not changed so dramatically that the plant is damaged. However, if the plant is left for a longer period (48 h) at 5 °C, metabolic chaos results, as individual metabolite pools empty whilst others grow disproportionally. The plant is damaged, in other words: Elastic strain has passed into plastic strain (Fig. 1.1.6). Repair: Plastic change or deformation is not completely irreversible. In most cases, the organism is able to repair the damage, if it is not too severe. One example is DNA repair after damage by UV irradiation. Plastic strain can change to elastic strain (see Box 1.2.4). Because of the open life form of plants, "repairs" can also be accomplished through premature senescence or shedding of damaged

Duration of chilling (h)

Fig. 1.1.6. Chilling damage to the African violet (Saintpaulia ionantha). Below the threshold temperature of +8°C leaves suffer chilling damage, recognisable by the incidence of necroses. Strength of stress can be estimated as the product of cold stress (above the freezing point) multiplied by the duration of exposure; it is proportional to the extent of damage. (After Larcher and Bodner 1980)

Duration of chilling (h)

Fig. 1.1.6. Chilling damage to the African violet (Saintpaulia ionantha). Below the threshold temperature of +8°C leaves suffer chilling damage, recognisable by the incidence of necroses. Strength of stress can be estimated as the product of cold stress (above the freezing point) multiplied by the duration of exposure; it is proportional to the extent of damage. (After Larcher and Bodner 1980)

organs, for example, of leaves damaged by radiation or drought.

• Adaptation: Under stress conditions organisms are able to adapt to more stressful living conditions by changing their elastic modulus and thus their resistance to stress. Such adaptations occur in stress reactions which are not part of the "normal" metabolic changes, for example, the induction of CAM by salt stress or drought (see Chap. 1.5.3.1). Adaptation may be achieved in two different ways, by avoidance of the stress or strain, as in CAM plants, which can close their stomata during the hot and dry daytime because of their ability to fix C02 during the night, or by developing an intrinsic tolerance, for example, by water storage and by increasing (numerically decreasing) the water potential of the plant.

• Seasonally recurring stress requires timely hardening of the plants, for example, development of frost resistance in the case of perennials (see Chap, 1.3.6.7). It might be assumed that such plants, once they have become frost-resistant, would maintain this capacity. However, frost hardening involves not only metabolic changes, but also alterations of the cellular ultrastructure. Frost-hardened plants

Was this article helpful?

0 0

Post a comment