Plant Physiology - Musienko M.M. 2001

Plant Adaptation and Resistance Mechanisms
Cold and Frost Resistance

The Geographical Distribution of various plant species is closely linked to Temperature factors. Among the many exogenous factors to which plants can develop resistance, processes that enable them to maintain viability under low-temperature conditions occupy a special place.

It is well known that tropical plants cannot withstand even slight temperature drops to +10°.. +12 °С. The temperature limits for the survival of various organisms are quite broad. The highest temperatures have been recorded for Bacteria (+88 °С) and blue-green Algae from hot springs (+85 °С). Alongside thermophilic organisms, the flora of the Yakutia region, where temperatures drop to -68 °С, numbers about 200 species.

Research into the mechanisms of temperature adaptation in plant organisms largely focuses on studying how the rates of various PHYSIOLOGICAL AND BIOCHEMICAL processes depend on temperature. This dependency is represented by curves featuring three cardinal points: minimum, optimum, and maximum. Based on THE POSITION OF these temperature cardinal points, plants are classified as thermophilic or frigophilic. The former are heat-loving with high cardinal points, while the latter are cold-tolerant and grow at lower temperatures.

The ability of heat-loving plants to withstand temperatures slightly above 0 °С is referred to as chilling resistance, whereas the ability of plant organisms to withstand temperatures below 0 °С is known as frost resistance.

The primary cause of injury in heat-loving plants at low positive temperatures is the disruption of membrane functional activity resulting from the transition of their saturated Fatty acids from a liquid-crystalline to a gel state, which triggers metabolic changes. Under these conditions, the optimal Structure OF THE pigment-lipid-protein complex is disrupted, energy-storing Functions are suppressed, and alterations in the Water balance are observed.

Presowing seed hardening with low positive temperatures enhances chilling resistance. Fundamental studies dedicated to developing the Physiological foundations of plant frost resistance (Maksimov, 1952; Protsenko, 1958, 1969; Solovyova, 1967, 1982) have demonstrated that adaptation to low temperatures is accompanied by profound Changes in the intensity and direction of METABOLISM, which help create conditions for the synthesis and accumulation of certain compounds, most notably CARBOHYDRATES, Proteins, and Nucleic Acids.

It has been established that injury and death in overwintering plants are also caused by the freezing of water within intercellular spaces and Cells, which is accompanied by dehydration, osmotic Shock, and mechanical damage to membranes. The Development of frost resistance in overwintering plants during ontogeny is viewed as a chain of adaptive readjustments in carbohydrate, amino acid, protein, and Lipid Metabolism, as well as shifts in oxidation-reduction, energy, and other functions.

The reduction in frost sensitivity is primarily explained by complex changes occurring in The chemical composition of intracellular substances and the appearance of so-called cryoprotectants. These primarily include polymers capable of binding water—hydrophilic proteins, soluble Polysaccharides, hemicellulose molecules, and the like. Some plants also form other substances, such as Tannins and hexahydric alcohols (mannitol, sorbitol), alongside shifts in the physicochemical composition of biocolloids.

Establishing a high energy potential is the foundation of the low-temperature adaptation process. Under overwintering conditions, plants shift to an Anaerobic Respiration pathway to generate additional ATP reserves. High frost resistance is associated with energy generation at low temperatures on mitochondrial membranes. Damaging low temperatures inhibit the oxidative and phosphorylating activity of Cell/35.html">Mitochondria, causing a decrease in Oxygen Uptake Rate and the ATP/oxygen ratio. In studies by I.I. Tumanov on the temperature and light conditions of plant hardening, it was established that increased frost resistance is achieved through a gradual decrease in temperature against the Background of a shortening daylight period.

Plant Hardening

Plant frost resistance depends on the physiological state in which they enter the winter period. According to I.I. Tumanov's theory, plants undergo three stages of preparation for winter: transition to dormancy and two stages of hardening. At The First stage, during the transition to dormancy, a decrease in auxin and gibberellin content and an increase in Abscisic acid are observed.

Winter cereals undergo the first stage of hardening in the light at temperatures ranging from +5 to -5 °C over several days, whereas woody plants require several weeks.

Against the background of gradually ceasing growth processes, changes occur in the metabolism and ULTRASTRUCTURE OF THE Cytoplasm, primarily driven by the accumulation of soluble carbohydrates, the transformation of Enzymes, and Membrane Proteins. The Second Stage of hardening takes place as the temperature gradually drops to -10... -15 °С and lower, resulting in the stabilization of the cytoplasmic structure and the efflux of water to extracellular ice, which enables the cells to withstand the dehydration of macromolecules and membrane structures.

Consequently, Introduction/36.html">Biological Membranes are the structures most sensitive to low temperatures; among their molecular components, Protein Complexes are the most stable, Glycoproteins and Glycolipids occupy an intermediate position, and Lipids are the most sensitive to temperature drops. Therefore, adaptation processes and resistance to low temperatures are largely driven by modifications in Membrane Lipids. Under these conditions, Changes in membrane structures begin with the physicochemical reorganization of lipid-lipid and lipid-protein complexes, culminating in cellular biochemical pathology. This encompasses elements such as Lipid Peroxidation of membranes, the activation of lysosomal Hydrolases and certain membrane-bound enzymes, and the disruption of active and passive transport processes.

Lipid peroxidation is regulated by antioxidants, a role fulfilled within the Organism by tocopherol, ubiquinone, carotenoids, Phospholipids, Glutathione, and others. Plants with varying degrees of frost resistance differ in their content of lipid antioxidants, and their natural hardening is accompanied by an increase in the antioxidant activity of lipids.

Thus, high frost resistance in plants is observed only As a result of prolonged and complex preparation for winter during the hardening process. Consequently, hardening is essentially the realization of a previously latent genotypic trait representing the potential capacity to increase frost resistance.

The adaptation of a plant organism to temperature factors during ontogeny is referred to as ontogenetic adaptation to low (or high) temperatures, whereas genetically determined frost (heat) resistance is termed genetic adaptation.