Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

8. Plant Adaptation and Resistance to Unfavorable Environmental Factors
8.4 Plant Adaptations for Low-Temperature Tolerance

Frost-hardy plants are capable of preventing or neutralizing the damaging effects of low freezing temperatures. These plants employ adaptive mechanisms that reduce cellular dehydration.

1. To prevent intracellular ice formation during freezes, it is paramount that free Water can be rapidly transported Cell/5.html">From Cells to sites of extracellular ice formation, which requires maintaining high membrane permeability under such conditions. This capability is ensured by the specific Lipid Composition of membranes in hardy plants. A universal PLANT RESPONSE TO low temperatures is an increase in the proportion of Unsaturated Fatty acids within the Membrane Lipids. This shifts the lipid phase transition Temperature from a liquid-crystalline to a gel state well below the freezing threshold.

2. Frost tolerance is also facilitated by enhanced synthesis of protective tissue compounds, or cryoprotectants. These primarily include polymers capable of binding significant volumes of water, such as hydrophilic Proteins, mono-, and Oligosaccharides. The water bound within the Hydration shells of these molecules does not freeze or migrate, remaining inside The Cell. In this way, cells are protected from intracellular ice formation and excessive dehydration. In frost-hardy plants, low temperatures stimulate starch Hydrolysis, leading to the accumulation of sugars in the Cytoplasm; many species also show increased synthesis of water-soluble proteins. The higher their concentration, the greater the cell's capacity to survive low-temperature stress. Another Class of cryoprotectant polymers consists of hemicellulose molecules (xylans, arabinoxylans) secreted into The Cell wall. They coat ice crystals and inhibit their growth, resulting in smaller crystals that cause less cellular damage.

3. During winter preparation, frost-hardy plants accumulate reserve substances that can be utilized later when growth resumes. Their resistance to diseases—the risk of which increases significantly when Tissues are damaged by frost—is also crucial.

Plant cold hardiness can be enhanced through hardening.

The Theory of low-temperature hardening was developed by I. I. Tumanov. According to this theory, to acquire frost resistance, plants must undergo three preparatory stages: transition to dormancy, and the First and Second phases of hardening. The transition to dormancy is accompanied by a shift in the phytohormone balance toward decreased auxin and gibberellin levels and increased Abscisic acid content. Treating plants with growth inhibitors during this period (such as chlorocholine chloride or triiodobenzoic acid) increases their resistance to low temperatures. During the first phase of hardening (winter cereals complete this phase in the light at 0.5–2 °C over 6–9 days, while woody plants take about 30 days) at lowered positive temperatures (approaching 0 °C), growth halts (if plants are not already dormant), protective compounds (sugars, soluble proteins, etc.) accumulate in the cells, membrane unsaturation increases, the cytoplasm freezing point drops, and There is a slight decrease in intracellular water, which inhibits intracellular ice formation. During the second phase of hardening (gradual temperature reduction to -10, -20 °C and lower at a rate of 2–3 °C per day), ice forms in the intercellular spaces, and the dehydration-defense mechanisms primed during The first phase become fully operational.

Microelements have a positive effect on both frost and cold hardiness in plants. For instance, zinc increases bound water content and enhances sugar accumulation, whereas molybdenum promotes an increase in total and protein nitrogen. Similar effects are produced by cobalt, copper, vanadium, and other Trace Elements.