Cold stress in plants
Cold stress in plants refers to stress , an exposure to external factors, of plants at low temperatures . In tropical plants, deadly cold stress can already exist at +10 ° C. Cold stress also includes the effects of frost , temperatures below the freezing point of water . The ability to withstand frost is known as frost or winter hardiness .
Geographical distribution
A third of the earth's land area is never affected by frost. These are the tropical areas with the exception of the high mountains ; near the coast, the frost-free areas also extend over the tropics . On around 43% of the land area there is severe frost with an average annual minimum of below −20 ° C. Plants can prepare for periodically recurring frosts; they only suffer damage in extremely cold winters. Episodic frost events such as late frosts usually only reach −5 to −8 ° C, but can be dangerous for plants because the frost hits them during sensitive phases of life. In tropical high mountains, frost occurs every night, these frosts reach −10 to −12 ° C, but last only a few hours (frost change climate).
Primary cold effects
At lower temperatures, chemical processes run more slowly, and equilibrium reactions shift in the direction of energy release ( Le Chatelier's principle ). For plants, this means less energy from the operating metabolism, less nutrient and water absorption from the soil, less productive biosyntheses and, as a result, a cessation of growth. The individual life processes are differently sensitive to cold. First, the protoplasmic flow stops , and photosynthesis is also stopped very quickly. Plasmolysis and vital staining are retained the longest.
Plants sensitive to cold
Plants or plant organs sensitive to cold die at temperatures between +10 and 0 ° C. This includes many tropical plants and often also the flower systems and fruits of plants whose other organs are quite insensitive to cold. The degree of cold damage in a plant species depends on the cooling depth, the duration and the speed of cooling or reheating. The first damage is usually still reversible. First, the lipids of the biomembrane change from a liquid-crystalline to a gel-like state. This reduces the selectivity of the membrane, the exchange of substances between the cell compartments is no longer adequately controlled, and cell constituents can diffuse outwards . The photosynthesis is inhibited, the respiration increased, the metabolism gets out of balance. Stress metabolites and toxic metabolic products can accumulate, which ultimately leads to cell death and further to the death of organs or the whole plant.
Freeze
When freezing, the location of the ice formation is essential. In plants, ice is created first in the places that cool the fastest and freeze out the easiest. So in the most exposed plant organs and then in the intercellular areas of the leaves, mostly needles, and peripheral vascular bundles . From these locations, ice formation progresses rapidly along the vascular bundles and within homogeneous tissue. Lignified or cutinized cell walls hinder the spread of ice formation.
Water-rich, uncured cells freeze intracellularly. The ice crystals that develop inside the cell usually destroy vital structures of the plasma. Often, however, the ice is formed outside the protoplast in the intercellular areas or between the cell wall and protoplast. This extracellular ice formation then acts like a dehydration, water is removed from the protoplast and the dissolved substances are concentrated. The cell membranes are osmotically stressed and the cells shrink. From a certain degree of dehydration , the cells are irreversibly damaged.
Survival of frost exposure
Plants that grow in frost-prone areas have developed different strategies to survive the frost events.
Frost protection
The protection against frost consists in the thermal insulation and the reduction of the heat radiation . Examples of this are the retreat of the wintering organs under a cover of leaves or underground ( geophytes ) or the shedding of frost-sensitive organs before the onset of frost periods - such as the fall of leaves from wood plants. In the tropical high mountains of the giant rosette plants for the short nocturnal frosts, it is enough to close the leaves together over the sensitive shoot tips to reduce the cooling .
Freezing point depression and hypothermia
A freezing point depression is a strategy, a Fully freezing of water in the protoplasm to prevent at temperatures below 0 ° C. Dissolved substances that are actively enriched in the cell sap lower the freezing point to an average of −1 to −5 ° C. It represents a moderate but safe frost protection.
Hypothermia is unstable in water-rich, large-cell parenchyma and in the xylem (transient hypothermia) and can only be maintained here for a few hours. For frost heave occurs when the following mechanism does not intervene quickly enough.
A third form of protection is translocated ice formation. It always occurs in the xylem and in some seeds, buds and bark tissue and consists in the fact that water from the tissues into the intercellular or other cavities, e.g. B. inactive xylem elements, is transferred and freezes here to ice. The cell sap is thereby concentrated and thus intracellular freezing is delayed.
In some particularly hard frost tree species occurs in the protoplasm into a glass ( vitrification ). This is achieved through high concentrations of sucrose and other sugars. In this state, the plants could theoretically withstand temperatures in the vicinity of absolute zero .
Freeze Resistance
Freeze-resistant (freeze-tolerant) plants can survive the freezing of their protoplasm. This form of frost resistance is necessary in areas with severe frost. In order to achieve resistance to freezing, phospholipids that are stable to the cold are built into the biomembrane and soluble carbohydrates , polyols , low molecular weight nitrogen compounds ( amino acids , polyamines ) and water-soluble proteins are accumulated in the cytoplasm . Frost protection proteins (AFPs), hydrophilic proteins that bind irreversibly to ice crystals and prevent their further growth, play a role in preventing freezing. AFPs are mainly known from hardy crops ( rye , wheat , barley, etc.).
Hardening
Plants are not always freeze tolerant. Practically all plants are sensitive to cold during growth phases. Land plants in seasonal climates acquire the ability to survive ice formation in autumn through hardening processes. The prerequisite for this is the cessation of growth. In the case of many wood plants, hardening is achieved through prolonged exposure to low temperatures close to freezing point.
The hardening takes place in three steps:
- During the pre-hardening process, sugar and other substances are accumulated, the cells lose water, and the vacuole fissures into many small vacuoles. All of this leads to an increase in the stability of the biomembranes (e.g. through the incorporation of disulfide bridges )
- In the next step, the enzymes are converted so that the viscosity of the cytoplasm is increased
- Finally, there is a sharp increase in the sugar concentration in the cytoplasm
The more stable biomembranes and the more viscous, highly sugar-containing plasma now protect the cells from dehydration through extracellular ice formation.
"Softening" takes place in spring, which takes place within a few days.
Indirect frost effects
Winter frosts often occur together with other environmental influences. These include the freezing of water in the ground, snowfall and the formation of snow cover. Long snow cover reduces the growing season due to lack of light . On ski slopes this leads to 20 to 30%, in extreme cases up to 70% yield losses in grassland use . Layers of ice hinder the gas exchange between the plants. The freezing of the ground together with a little snow cover causes frost-dryness .
supporting documents
- Walter Larcher: Ecophysiology of plants . 5th edition, Ulmer, Stuttgart 1994, pp. 280-296, ISBN 3-8252-8074-8
- Peter Schopfer, Axel Brennicke: Plant Physiology . Elsevier, Munich 2006, pp. 596-601, ISBN 978-3-8274-1561-5