Plant Physiology - Musienko M.M. 2001

Adaptation and Plant Stress Resistance Mechanisms
Drought and Heat Resistance

Water and high-Temperature stresses cause numerous Functional and Structural Changes in the vital activity of the plant Organism, reduce productivity, and their deep, prolonged action leads to irreversible reactions and plant death.

According to the nature of adaptation to water stress, plants are divided into three groups:

· ephemerals, which avoid the NEGATIVE IMPACT OF water deficit, vegetate only for a short period, and survive drought in a dormant state;

· drought-resistant plants, which are able to use water reserves from deep soil horizons or store water within the plants themselves, such as xerophytes;

· drought-tolerant plants, which do not lose their viability under significant water loss.

Water stress occurs in plants when Transpiration exceeds water absorption by the ROOT.

Two Major Groups of species differing in drought tolerance have been identified. These are poikilohydric plants, which are unable to regulate their water regime, and homeohydric plants, which are able to maintain Hydration to a certain extent under water deficit.

To elucidate the physiological mechanisms of resistance, The sequence of damage to individual vital Functions as water deficit and overheating develop is of essential significance. Cell growth is the most sensitive to water scarcity, while synthetic processes, in particular Protein Synthesis, are slightly less sensitive.

The response of mesophytes to drought has a biphasic character. Short-term drought causes a decrease in Cytoplasm viscosity, whereas most physiological functions intensify against the Background of slowed growth processes. These changes are likely associated with adaptive rearrangements of METABOLISM and protoplast Structure, which promotes survival under stress conditions (Protsenko, Musienko, 1975). Prolonged and severe drought causes degradative changes, and the entire orientation of metabolism shifts towards Breakdown and Oxidation (Shmatko, 1989; Zholkevich, 1992). According to T.M. Pustovoitova and V.M. Zholkevich, drought at its various phases causes the following changes in the plant organism:

As can be seen, growth proved to be the most sensitive both to The impact of drought and to other stress factors. SHOOT growth processes are more sensitive to drought compared to The Root System. The question of which phase of cell growth is most sensitive to moisture deficit remains debatable: some scientists consider the division phase, while others consider the extension growth phase.

It is known that growth intensity is regulated by phytohormones. As drought intensifies, stress Hormones—Ethylene and ABA—accumulate in Tissues, and the levels of Auxins, Gibberellins, and Cytokinins decrease. A special role under moisture deficit, as well as under high-temperature conditions, is played by ABA, which accumulates in all plant Organs. The signal for its formation is the loss of turgor. Possibly, the decrease in turgor causes Conformational Changes in membranes, which stimulates ABA formation. The site of synthesis may be the cytoplasm, but it is found in significant amounts in the Chloroplasts of the leaf mesophyll—up to 10 ^-5 M.

Under drought conditions, ABA moves from the Chloroplasts of mesophyll cells to the epidermis into the guard cells of Stomata, causing them to close. The question regarding the synthesis of ABA by the guard cells themselves also remains open. Abscisic acid also accumulates in roots and can even be released into the environment, exerting an allelopathic effect on other plants.

The increase in ABA content in various plant organs under such conditions is associated with its Participation in the regulation of rapid and prolonged defense reactions. Under osmotic stress, ABA protects the limiting cell membranes from dehydration, increases the content of osmotically active substances (Proline, potassium), and causes the accumulation of stress proteins, including osmotin.

The combination of defense properties manifested under the Influence of the phytohormone also increases the heat resistance of plants. A negative effect of ABA has also been noted, in particular, an increase in its content during Critical Periods of ontogenesis—flowering—stimulated the appearance of sterile pollen.

Another stress hormone that signals the onset of drought is ethylene. Research into The Role of other phytohormones in the adaptation of plants to drought and overheating continues.

Recently, a new type of phytohormones has been discovered—brassinosteroids, which stimulate The formation of auxins, ethylene, and Key Enzymes of NUCLEIC ACID METABOLISM. It is anticipated that they are promising as anti-stress agents. Research into The Physiological Role of phytohormones in adaptation to drought has become the basis for the targeted application and creation of new synthetic growth regulators used to increase plant resistance.

A protective role is also played by an increase in the content of substances such as proline, betaines, and Polyamines. During the drought period, proline is formed in all plant organs up to 200 mmol/g of dry matter mass. The stimulus for its accumulation is A change in osmotic potential. The role of proline in adaptation is associated with its osmotic properties. Substances that maintain cell turgor and protect them from damage also include betaines—Glycine betaine, proline betaine, Alanine betaine, and others, as well as polyamines.

It should be noted that there is no universal, uniform physiological response to water deficit or overheating. Responses to stress conditions cause changes not only in growth, but also in photosynthesis, respiration, and at THE MOLECULAR LEVEL, including Membrane regulation and Gene Expression.

Water stress disrupts the liquid-crystalline structure of polar Lipids in the thylakoid membranes of chloroplasts, reducing its viscosity. During the action of the stress factor, the fatty acid content increases, the phospholipid content decreases, and their transition into a gel state is observed.

One of the factors of the adverse impact of drought on the plant organism is high temperature. The thermal regime of the environment significantly affects plant metabolism, growth, development, and productivity (Musienko et al., 1985). The following zones of PLANT RESPONSE TO temperature are distinguished: the adaptation zone, the damage zone, and the death zone due to the impact of high temperature. High-temperature stress can be both prolonged and sharp short-term.

High temperatures inhibit cytoplasm movement, reduce the mitotic index in the first 3–4 hours of heat stress, and cause changes in Protein Structure and characteristic interactions between the enzyme and substrate. Extreme temperatures cause nuclear destruction, Denaturation and breakdown of Nucleic Acids, and changes in Transcription and Translation Processes.

Gene activity under overheating conditions is inferred from marker proteins, which are stress proteins. Under sudden stress, heat Shock proteins are formed under The Influence of pre-lethal temperatures, which probably confirms their predominant formation During the first adaptive phase. Such proteins have been found in The Nucleus, nucleoli, chloroplasts, Mitochondria, and Cytoskeleton.

High temperatures damage thylakoid membranes and disrupt The activity of the donor part of PS II and phosphorylation. Therefore, to clarify the specifics of adaptation to overheating and drought, The Study of the photosynthetic apparatus and the anatomical-morphological and physiological-Biological features of C₃, C₄, and CAM types of photosynthesis in plants is of considerable interest. During adaptation to overheating and drought, respiration intensity also increases, and its energy is expended on repair processes.

In recent years, interest has grown in studying the role of the cytoskeleton in information transmission and substance transport throughout the plant. It is hypothesized that the translocation of the 45 kDa cytoskeletal protein into the nuclear fraction under stress may serve as a signal of the onset of adverse conditions.

Thus, Drought and Heat resistance, which develop under conditions of water deficit and overheating, are genetically determined traits that manifest differently in various plant species.

Assessing functional and structural changes under conditions of drought and overheating is crucial for determining the level of adaptive resilience and predicting plant endurance.