Damage to plants by cold and frost. In plant ecology, it is customary to distinguish between the effects of cold (low positive temperatures) and frost (negative temperatures). The negative impact of cold depends on the range of temperature decreases and the duration of their exposure. Even non-extreme low temperatures have an adverse effect on plants, since they inhibit basic physiological processes (photosynthesis, transpiration, water exchange, etc.), reduce the energy efficiency of respiration, change the functional activity of membranes, and lead to the predominance of hydrolytic reactions in metabolism. Externally, cold damage is accompanied by a loss of turgor in the leaves and a change in their color due to the destruction of chlorophyll. Growth and development slow down sharply. Thus, cucumber leaves (Cucumis sativus) lose turgor at 3 °C on the 3rd day, the plant withers and dies due to impaired water delivery. But even in an environment saturated with water vapor, low temperatures adversely affect plant metabolism. In a number of species, protein breakdown increases and soluble forms of nitrogen accumulate.
The main reason for the damaging effect of low positive temperatures on heat-loving plants is the disruption of the functional activity of membranes due to the transition of saturated fatty acids from a liquid crystalline state to a gel. As a result, on the one hand, the permeability of membranes for ions increases, and on the other, the activation energy of enzymes associated with the membrane increases. The rate of reactions catalyzed by membrane enzymes decreases more rapidly after a phase transition than the rate of reactions involving soluble enzymes. All this leads to unfavorable changes in metabolism, a sharp increase in the amount of endogenous toxicants, and, with prolonged exposure to low temperatures, to the death of the plant (V.V. Polevoy, 1989). Thus, when the temperature drops to several degrees above O °C, many plants of tropical and subtropical origin die. Their death occurs more slowly than during freezing, and is a consequence of a disorder of biochemical and physiological processes in an organism that finds itself in an unusual environment.
Many factors have been identified that have a detrimental effect on plants at subzero temperatures: heat loss, rupture of blood vessels, dehydration, ice formation, increased acidity and concentration of cell sap, etc. Cell death from frost is usually associated with disorganization of protein metabolism and nucleic acids, as well as with an equally important violation of membrane permeability and cessation of the flow of assimilates. As a result, decay processes begin to prevail over synthesis processes, poisons accumulate, and the structure of the cytoplasm is disrupted.
Many plants, without being damaged at temperatures above 0 °C, are killed by the formation of ice in their tissues. In watered, unhardened organs, ice can form in protoplasts, intercellular spaces and cell walls. G. A. Samygin (1974) identified three types of cell freezing, depending on the physiological state of the organism and its readiness for overwintering. In the first case, cells die after the rapid formation of ice, first in the cytoplasm and then in the vacuole. The second type of freezing is associated with dehydration and deformation of the cell during the formation of intercellular ice (Fig. 7.17). The third type of cell death is observed with a combination of intercellular and intracellular ice formation.
When freezing, as well as as a result of drought, protoplasts give up water, shrink, and the content of salts and organic acids dissolved in them increases to toxic concentrations. This causes inactivation of enzyme systems involved in phosphorylation and ATP synthesis. The movement of water and freezing continue until an equilibrium of suction forces is established between the ice and water of the protoplast. And it depends on temperature: at a temperature of -5 °C, equilibrium occurs at 60 bar, and at -10 °C already at 120 bar (W. Larcher, 1978).
With prolonged exposure to frost, ice crystals grow to significant sizes and can compress cells and damage the plasmalemma. The process of ice formation depends on the rate of temperature decrease. If freezing occurs slowly, the ice will
Rice. 7.17. Scheme of cell damage caused by extracellular ice formation and thawing (after J.P. Palt, P.H. Lee, 1983)
develops outside the cells, and when thawed they remain alive. When the temperature drops quickly, water does not have time to penetrate the cell wall and freezes between it and the protoplast. This causes destruction of the peripheral layers of the cytoplasm, and then irreversible damage to the cell. With a very rapid drop in temperature, water does not have time to leave the protoplast and ice crystals quickly spread throughout the cell. Consequently, the cells freeze quickly if the water has not had time to drain out of them. Therefore, its rapid transport into the intercellular spaces is important, which is facilitated by maintaining high membrane permeability, associated with the high content of unsaturated fatty acids in their composition (V.V. Polevoy, 1989). In hardened plants at subzero temperatures, the membranes “do not freeze”, maintaining functional activity. The frost resistance of the cell also increases if water is firmly bound to the structures of the cytoplasm.
Frost can severely damage the structure of membranes. Membrane proteins are dehydrated and denatured, which inactivates important systems active transport of sugars and ions. The coagulation of proteins under the influence of frost is especially characteristic of southern plants, dying before ice forms. And the frosty breakdown of lipid components of membranes is accompanied by hydrolysis of phospholipids and the formation of phosphoric acid. As a result, damaged membranes lose semi-permeability, water loss from cells increases, turgor drops, intercellular spaces fill with water, and necessary ions are intensively washed out of cells.
Frost also damages the pigment system of plants. Moreover, the effect of temperature stress in winter is often combined with damage to assimilating organs by light. Thus, in the chloroplasts of needles, the electron transport chain is damaged, but this damage is reversible. In wintering plants, the content of carotenoids increases, protecting chlorophyll from damage by light. The preservation of pigments and photosynthesis is important for plant stability in the fall, when protective compounds are synthesized at low positive temperatures, and for overwintering of plants. At negative temperatures, winter cereals partially compensate for the costs of maintaining viability under stressful conditions due to photosynthesis (L. G. Kosulina et al., 1993).
Frost can also cause mechanical damage to plant organisms. In this case, tree trunks and large branches are especially affected. In winter, with strong night cooling, the trunk quickly loses heat. The bark and outer layers of wood cool faster than the inside of the trunk, so significant stress arises in them, which, with rapid temperature changes, leads to vertical cracking of the tree.
In addition, tangential cracks and detachments of the cortex are possible. Frost cracks close when the cambium is active, but if new layers of wood do not have time to form, the cracks spread radially into the trunk. They get infected, which, penetrating into neighboring tissues, disrupts the functioning of the conduction system and can lead to the death of the tree.
Frost damage also occurs during the day. During prolonged frosts, especially in sunny weather, parts of plants rising above the snow can dry out from an imbalance of transpiration and absorption of water from cold soil (compression of cells during dehydration and ice formation, freezing of cell sap is also important). U woody plants in areas with sunny winters ( Eastern Siberia, North Caucasus, Crimea, etc.) there are even winter-spring “burns” on south side branches and young unprotected trunks. On clear winter and spring days, the cells of uncorked plant parts heat up, lose their frost resistance and cannot withstand subsequent frosts. And in the forest-tundra, frost damage can also form in the summer during frosts. Young teenagers are especially susceptible to them. Its cambium cools quickly, since a sufficient heat-insulating bark layer has not yet formed, and therefore the heat capacity of thin trunks is low. These effects are especially dangerous in the middle of summer, when the activity of the cambium is maximum (M.A. Gurskaya, S.G. Shiyatov, 2002).
Compaction and cracking of frozen soil leads to mechanical damage and rupture of roots. Frosty “bulging” of plants, which is caused by uneven freezing and expansion of soil moisture, can also act. In this case, forces arise that push the plant out of the soil. As a result, turfs are turned out, roots are exposed and torn off, and trees fall out. Summarizing the data on winter damage to plants, in addition to cold resistance and frost resistance itself, which reflect the ability to tolerate direct action low temperatures, in ecology they also distinguish winter hardiness - the ability to tolerate all unfavorable winter conditions(freezing, damping off, bulging, etc.). At the same time, plants do not have special morphological adaptations that protect only from cold, and in cold habitats protection is carried out from the entire complex of unfavorable conditions (winds, desiccation, cold, etc.)
Cold affects the plant not only directly (through thermal disturbances), but also indirectly, through the physiological “winter drought.” With intense winter light and warming, the air temperature can exceed the soil temperature. The aboveground parts of plants increase transpiration, and the absorption of water from cold soil is slowed down.
As a result, the osmotic pressure in the plant increases and water deficiency occurs. With prolonged cold and intense insolation, this can even lead to fatal damage. The drying effect of cold is aggravated by winter winds that increase transpiration. And winter drying is reduced by a reduction in the transpiring surface, which occurs during the autumn shedding of leaves. Winter-green plants transpire very heavily in winter. R. Tren (1934) determined that in the vicinity of Heidelberg, leafless shoots of blueberries (Vaccinium myrtillus) transpired three times more intensively than the needles of spruce (Picea) and pine (Pinus). Transpiration of heather (Calluna vulgaris) was 20 times more intense. And shoots of toadflax (Linaria cymbalaria) and Parietaria ramiflora that remained alive until winter on the walls of houses evaporated 30-50 times more intensely tree species. In some habitats, winter drought can be significantly reduced. For example, plants located under snow or in wall crevices spend much less moisture on transpiration and during thaws they can make up for water deficiency.
Completed by: Galimova A.R.
The effect of extreme temperatures on plants
During evolution, plants have adapted quite well to the effects of low and high temperatures. However, these adaptations are not so perfect, so extreme temperatures can cause some damage and even death of the plant. The range of temperatures affecting plants in nature is quite wide: from -77ºС to + 55°С, i.e. is 132°C. The most favorable temperatures for the life of most terrestrial organisms are +15 - +30°C.
High temperatures
Heat-resistant - mainly lower plants, for example, thermophilic bacteria and blue-green algae.
This group of organisms is able to withstand temperature increases up to 75-90°C;
Plant resistance to low temperatures is divided into:
Cold resistance;
Frost resistance.
Cold resistance of plants
the ability of heat-loving plants to tolerate low positive temperatures. Heat-loving plants suffer greatly under positive conditions. low temperatures. External symptoms of plant suffering include wilting of leaves and the appearance of necrotic spots.
Frost resistance
the ability of plants to tolerate negative temperatures. Biennials and perennials, growing in temperate zones, are periodically exposed to low negative temperatures. Various plants have different resistance to this effect.
Frost-resistant plants
Effect of low temperatures on plants
With a rapid decrease in temperature, ice formation occurs inside the cell. With a gradual decrease in temperature, ice crystals form primarily in the intercellular spaces. The death of a cell and the organism as a whole can occur as a result of the fact that ice crystals formed in the intercellular spaces, drawing water from the cell, cause its dehydration and at the same time exert mechanical pressure on the cytoplasm, damaging cellular structures. This causes a number of consequences - loss of turgor, increased concentration of cell sap, a sharp decrease in cell volume, and a shift in pH values in an unfavorable direction.
Effect of low temperatures on plants
The plasmalemma loses semipermeability. The work of enzymes localized on the membranes of chloroplasts and mitochondria, and the associated processes of oxidative and photosynthetic phosphorylation are disrupted. The intensity of photosynthesis decreases, and the outflow of assimilates decreases. It is the change in membrane properties that is the first cause of cell damage. In some cases, membrane damage occurs during thawing. Thus, if the cell has not undergone the hardening process, the cytoplasm coagulates due to the combined influence of dehydration and mechanical pressure of ice crystals formed in the intercellular spaces.
Plant adaptation to negative temperatures
There are two types of adaptations to negative temperatures:
avoiding the damaging effect of a factor (passive adaptation)
increased survival (active adaptation).
Temperature is the most important factor determining the possibilities and timing of crop cultivation.
The biological and chemical processes of transformation of nutrients occurring in the soil are directly dependent on temperature regime. Heat supply to crops is characterized by the sum of average daily air temperatures above 10°C during the growing season. Both high and low temperatures disrupt the course of biochemical processes in cells, and thus can cause irreversible changes in them, leading to the cessation of growth and death of plants. An increase in temperature to 25-28°C increases the activity of photosynthesis, and with its further growth, respiration begins to noticeably predominate over photosynthesis, which leads to a decrease in plant weight. Therefore, most agricultural crops at temperatures above 30°C, wasting carbohydrates on respiration, as a rule, do not produce an increase in yield. Temperature reduction environment from 25 to 10°C reduces the intensity of photosynthesis and plant growth by 4-5 times. The temperature at which the formation of photosynthetic products is equal to their consumption for respiration is called the compensation point.
Most high intensity Photosynthesis in plants of temperate climates is observed in the range of 24-26°C. For most field crops, the optimal temperature during the day is 25°C, at night - 16-18°C. When the temperature rises to 35-40°C, photosynthesis stops as a result of disruption of biochemical processes and excessive transpiration (Kuznetsov, Dmitrieva, 2006). A significant deviation of temperature from the optimal one, either upward or downward, noticeably reduces enzymatic activity in plant cells, the intensity of photosynthesis and the supply of nutrients to plants.
Temperature has a big impact on root growth. Low (< 5°С) и высокие (>30°C) soil temperatures contribute to the superficial location of roots, significantly reducing their growth and activity. In most plants, the most powerful branched root system is formed at a soil temperature of 20-25°C.
When determining the timing of fertilizer application, it is important to take into account the significant influence of soil temperature on the supply of nutrients to plants. It has been established that at temperatures below 12°C, the use of phosphorus, potassium and microelements from soil and fertilizers by plants is significantly impaired, and at temperatures below 8°C, the consumption of mineral nitrogen is also noticeably reduced. For most agricultural crops, a temperature of 5-6°C is critical for the supply of basic nutrients to the plants.
The heat supply of the growing season is largely determined by the structure of sown areas and the possibility of growing more productive late-ripening crops that can be used for a long time solar energy to form a crop or carry out repeated sowings after early harvested crops.
In the conditions of the Non-Chernozem Zone of Russia, there is a direct dependence of the productivity of agricultural crops on the sum of temperatures. In the forest-steppe and steppe zones, under irrigated conditions, no reliable connection has been established between the number of positive temperatures and agricultural yields. In the central and southern regions of the country, an increase or decrease in temperature by 2-3 °C does not have a significant effect on plant productivity.
Temperature also has a great influence on the vital activity of soil microflora, which determines the mineral nutrition of plants. It has been established that the greatest intensity of ammonification of organic residues in the soil under the influence of microorganisms occurs at a temperature of 26-30°C and soil moisture of 70-80% of HB. Deviation of temperature or humidity from optimal values significantly reduces the intensity of microbiological processes in the soil.
The moisture supply of plants has a great influence on the intensity of photosynthesis and the efficiency of fertilizers. The degree of opening of stomata, the rate of entry of CO 2 into the leaves and the release of O 2 depend on the turgor state of plants. In conditions of drought and excessive humidity, the stomata usually close and the assimilation of carbon dioxide (photosynthesis) stops. The highest intensity of photosynthesis is observed with a slight water deficit in the leaf (10-15% of full saturation), when the stomata are maximally open. Only under conditions of optimal water regime does the root system of plants exhibit the most high activity consumption of nutrients from soil solution. A lack of moisture in the soil leads to a decrease in the rate of movement of water and nutrients through the xylem to the leaves, the intensity of photosynthesis and a decrease in plant biomass.
Not only the amount of precipitation is important, but also the dynamics of its distribution during the growing season in relation to individual crops. The productivity of agricultural crops is largely determined by the availability of moisture during the most critical phases of plant growth and development.
For the Non-Chernozem Zone, a dark correlation has been established between yield and precipitation in late May - early June for grain crops, in July - August for potatoes, corn, root crops and vegetable crops. Lack of moisture during these periods significantly reduces plant yield and the effectiveness of fertilizers.
Application of nitrogen and phosphorus-potassium fertilizers significantly increases the moisture deficit, since in proportion to the increase in the yield of the above-ground mass, water consumption also increases. It has been established that in fertilized fields the drying effect of plants on the soil begins to manifest itself earlier and to a greater depth than in unfertilized fields. Therefore, when there is a moisture deficit, fertilized fields are sown as early as possible, so that by the time drought sets in and the top layer of soil dries out, the roots reach the lower, more moist horizons. The most important measures for moisture accumulation in steppe regions are snow retention, early harrowing to seal up moisture, and early sowing.
In the forest-steppe and dry-steppe zones, moisture availability is one of the the most important factors productivity of agricultural crops.
In zones of sufficient and excessive moisture, the leaching water regime has a great influence on the supply of nutrients to plants, since a significant amount of nitrogen, calcium, magnesium and soluble humic substances are removed from the root layer of soil with the downward flow of water. This regime is created, as a rule, in autumn and early spring.
Great influence on crop yields, fertilizer efficiency, lines and agricultural practices field work exposure and relief of the fields have an impact, since slopes of different exposure and steepness differ significantly in the content of humus and nutrients in the soil, thermal and water regimes, and the responsiveness of agricultural plants to fertilizers. The soils of the northern and northeastern slopes, as a rule, are more humified, better provided with moisture, higher snow cover, thaw later compared to the southern slopes and, as a rule, have a heavier granulometric composition. The soils of the southern and southwestern slopes are warmer than the northern ones, thaw earlier, are characterized by intense flood runoff of melt and storm water, hence, as a rule, they are more eroded and contain fewer silt particles. In the soils of the southern slopes, the mineralization of stubble-root residues and organic fertilizers flows more intensely, so they are less humified. The higher the snow cover, the shallower the soil freezing depth, the better it absorbs spring melt water and floods destroy the soil less.
It is important to take into account the characteristics of soils of different exposures when planning the timing of field work and the need for equipment for applying fertilizers, since after completion of field work on the southern slopes it is used in fields with a northern exposure.
Despite the great dependence of the growth and development of plants on their supply of moisture and heat, the decisive role in the formation of agricultural yields in the Non-Black Earth Zone and many other regions belongs to soil fertility and the use of fertilizers.
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Effect of air temperature
The life processes of each plant species are carried out at a certain thermal mode, which depends on the quality of heat and the duration of its exposure.
Different plants need different quantities heat and have different abilities to tolerate deviations (both downward and upward) of temperature from the optimal.
Optimal temperature- the most favorable temperature for a certain type of plant at a certain stage of development.
The maximum and minimum temperatures that do not disrupt the normal development of plants determine the temperature limits permissible for their cultivation in appropriate conditions. A decrease in temperature leads to a slowdown in all processes, accompanied by a weakening of photosynthesis and inhibition of the formation organic matter, respiration, transpiration. An increase in temperature activates these processes.
It is noted that the intensity of photosynthesis increases with increasing temperature and reaches a maximum in the region of 15-20℃ for plants of temperate latitudes and 25-30℃ for tropical and subtropical plants. The daily temperature in autumn interiors almost never drops below 13℃. In winter it is between 15-21℃. In spring, temperature fluctuations increase. It reaches 18-25℃. IN summer time the temperature remains relatively high throughout the day and is 22-28℃. As you can see, the indoor air temperature is almost within the temperature range required for the photosynthesis process to occur throughout the year. Temperature is therefore not such a limiting factor in room conditions, as the lighting intensity.
IN winter period indoor pets feel normal at lower temperatures, because... many of them are at rest, while in others the growth processes slow down or temporarily stop. Therefore, the need for heat is reduced compared to summer.
The influence of light on plant growth – photomorphogenesis. Effect of red and far-red light on plant growth
Photomorphogenesis- these are processes occurring in a plant under the influence of light of different spectral composition and intensity. In them, light acts not as a primary source of energy, but as signal means, regulating processes of plant growth and development. You can draw some analogy with street traffic light, automatically regulating traffic. Only for control, nature chose not “red - yellow - green”, but a different set of colors: “blue - red - far red”.
And the first manifestation of photomorphogenesis occurs at the moment of seed germination.
I already talked about the structure of the seed and the characteristics of germination in the article about seedlings. But details related to signal by the action of light. Let us fill this gap.
So, the seed woke up from hibernation and began to germinate, while being under a layer of soil, i.e. in darkness. Let me note right away that small seeds, sown superficially and not sprinkled with anything, also germinate in darkness at night.
By the way, according to my observations, in general, all raasada standing in a bright place germinates at night and you can see mass shoots in the morning.
But let's return to our unfortunate hatched seed. The problem is that even having appeared on the surface of the soil, the sprout does not know about it and continues to grow actively, reaching for the light, for life, until it receives a special signal: stop, you don’t have to rush any further, you are already free and will live. (It seems to me that people themselves did not invent a red brake light for drivers, but stole it from nature...:-).
And it receives such signal not from air, not from moisture, not from mechanical impact, but from short-term light radiation containing red part of the spectrum.
And before receiving such a signal, the seedling is in the so-called etiolated condition. In which it has a pale appearance and a hooked, bent shape. The hook is an exposed epicotyl or hypocotyl, needed to protect the bud (growth point) when pushing through thorns to the stars, and it will remain if growth continues in the dark and the plant remains in this etiolated state.
Germination
Light plays an extremely important role in plant development. Changes in plant morphology under the influence of light radiation are called photomorphogenesis. After the seed germinates through the soil, the first rays of the sun cause radical changes in the new plant.
It is known that under the influence of red light the process of seed germination is activated, and under the influence of far-red light it is suppressed. Blue light also inhibits germination. This reaction is typical for species with small seeds, since small seeds do not have a sufficient supply of nutrients to ensure growth in the dark while passing through the thickness of the earth. Small seeds germinate only when exposed to red light transmitted thin layer earth, while only short-term irradiation is enough - 5-10 minutes per day. An increase in the thickness of the soil layer leads to an enrichment of the spectrum with far-red light, which suppresses seed germination. In plant species with large seeds containing a sufficient supply of nutrients, light is not required to induce germination.
Normally, a root first sprouts from a seed, and then a shoot appears. After this, as the shoot grows (usually under the influence of light), secondary roots and shoots develop. This coordinated progression is an early manifestation of the phenomenon of coupled growth, where root development influences shoot growth and vice versa. To a greater extent, these processes are controlled by hormones.
In the absence of light, the sprout remains in the so-called etiolated state, and has a pale appearance and a hooked shape. The hook is an exposed epicotyl or hypocotyl that is needed to protect the growing point during germination through the soil, and it will remain if growth continues in the dark.
Red light
Why this happens - a little more theory. It turns out that, in addition to chlorophyll, in any plant there is another wonderful pigment, which has a name - phytochrome. (A pigment is a protein that has selective sensitivity to a certain part of the white light spectrum.)
Peculiarity phytochrome is that it can take two forms With different properties under influence red light (660 nm) and distant red light (730 nm), i.e. he has the ability to phototransformation. Moreover, alternating short-term illumination with one or another red light is similar to manipulating any switch that has the “ON-OFF” position, i.e. The result of the last impact is always preserved.
This property of phytochrome ensures monitoring of the time of day (morning-evening), controlling frequency life activity of the plant. Moreover, love of light or shade tolerance of a particular plant also depends on the characteristics of the phytochromes it contains. And finally, the most important thing - flowering plants are also controlled... phytochrome! But more on that next time.
In the meantime, let's return to our seedling (why is it so unlucky...) Phytochrome, unlike chlorophyll, is found not only in leaves, but also in seed. Participation of phytochrome in the process of seed germination for some plant species are as follows: simply red light stimulates seed germination processes, and far red - suppresses seed germination. (It is possible that this is why the seeds germinate at night). Although this is not a pattern for everyone plants. But in any case, the red spectrum is more useful (it stimulates) than the far red spectrum, which suppresses the activity of life processes.
But let’s assume that our seed was lucky and it sprouted, appearing on the surface in an etiolated form. Now that's enough short-term lighting the seedling to start the process deetiolation: the growth rate of the stem decreases, the hook straightens, chlorophyll synthesis begins, the cotyledons begin to turn green.
And all this, thanks red to the world In solar daylight there are more ordinary red rays than far red rays, so the plant is highly active during the day, and at night it becomes inactive.
How can one distinguish between these two close parts of the spectrum “by eye” for a source of artificial lighting? If we remember that the red area borders on the infrared, i.e. thermal radiation, then we can assume that the warmer the radiation “feels to the touch”, the more infrared rays it contains, and therefore far red Sveta. Place your hand under a regular incandescent light bulb or a fluorescent fluorescent lamp - and you will feel the difference.
Determination of cold resistance of plants
The concept of low-temperature stress (cold shook) includes the entire set of plant responses to the effects of cold or frost, and reactions corresponding to the plant genotype and manifested at different levels of organization of the plant organism from molecular to organismal.
Cold tolerance is the ability of heat-loving plants to tolerate the effects of low positive temperatures.
For most crops, low positive temperatures are almost harmless. Individual organs of heat-loving plants have different resistance to cold. In corn and buckwheat, the stems die off most quickly, in rice the leaves are less resistant, in soybeans the petioles are damaged first and then the leaf blades, and in peanuts the root system is most sensitive to cold.
When exposed to cold, leaves lose turgor due to disruption of water delivery to transport organs, which leads to a decrease in intracellular water content. Hydrolytic processes intensify, resulting in the accumulation of non-protein nitrogen (proline and other nitrogenous compounds) and monosaccharides. The heterogeneity and amount of protein, especially low molecular weight (26, 32 kDa), increase.
The permeability of membranes increases. This reaction is one of the primary mechanisms of cold exposure. The change in the state of membranes at low temperatures is largely associated with the loss of calcium ions. In winter wheat, if the impact is not too strong, cell membranes lose calcium ions, permeability increases; various ions, primarily potassium, as well as organic acids and sugars from the cytoplasm enter the cell wall or intercellular spaces. Calcium ions also enter the cell wall, but their concentration also increases in the cytoplasm, and H+-ATPase is activated. Active proton transport triggers secondary active transport, and potassium ions return to the cell. As a result, the absorption of water and those substances that leave the cell increases, i.e. cell sap from the extracellular space enters it, which leads to restoration of its condition after damage (Fig. 24a).
When exposed to lower temperatures, the loss of calcium ions by the membranes is very high. As a result of strong exposure, the amount of calcium ions in the cytoplasm increases, and membrane structures are disrupted, as well as the functions of membrane-bound enzymes. H+-ATPase is inactivated, and phospholipids, on the contrary, are activated, which causes ion leakage and stimulates the degradation of membrane lipids. In this case, the damage becomes irreversible.
The change in membrane permeability is also associated with shifts in the fatty acid components: saturated fatty acids move from the liquid crystalline state to the gel state earlier than unsaturated fatty acids. Therefore, the more saturated fatty acids in the membrane, the stiffer it is, i.e. less labile. By increasing the level of unsaturated fatty acids, it was possible to reduce sensitivity to low temperatures.
Membrane disintegration is also facilitated by an increase in the content of free radicals, indicating increased lipid peroxidation (LPO). For example, in rice at 2ºC, the activity of the antioxidant enzyme SOD in the tissues decreased and the content of malondialdehyde (MDA), the final product of LPO, increased. When treated with tocopherol, the amount of MDA decreased.
Violation of the integrity of the membranes leads to the disintegration of cellular structures: mitochondria and chloroplasts swell, the number of cristae and thylakoids in them decreases, vacuoles appear, the ER forms concentric circles, including the tonoplast inside the vacuole. These are non-specific changes.
Due to the disintegration of thylakoid membranes of chloroplasts, photosynthesis is disrupted, which applies to both the ETC and the enzymes of the Calvin cycle.
Damage to the respiratory process is also observed with cold exposure, decreased energy efficiency associated with additional costs to maintain metabolism. The activity of the alternative breathing pathway increases. In some cases, for example in aroids, the intensification of this pathway contributes to an increase in the temperature of flowers in cold weather, which is necessary for evaporation essential oils that attract insects. The ratio of respiratory pathways also changes in favor of the pentose phosphate pathway.
In heat-loving plants, complete inhibition of photosynthesis occurs at 0°C, because chloroplast membranes are disrupted and electron transport and photosynthetic phosphorylation are uncoupled. In non-cold-resistant corn varieties, chloroplasts disintegrate and pigments are destroyed 20 hours after exposure to a temperature of +30C. In cold-resistant hybrids, such as corn, the temperature of +3°C does not affect the composition of pigments and the structure of chloroplasts.
The effect of temperature on photosynthesis depends on light exposure. The formation of chlorophyll in cucumber leaves at a hardening temperature (+15°C) is inhibited less at lower light levels. Growth is inhibited, the balance of phytohormones changes - the ABA content increases (mainly in resistant varieties and species), and auxin decreases. A decrease in temperature also leads to changes in transport processes: the absorption of NO3 weakens, and NH4 increases, especially in adapted plants. The transport of NO3 from roots to leaves is most vulnerable to low temperatures.
Prolonged exposure to low temperatures leads to the death of the plant. The main reasons for plant death are an irreversible increase in membrane permeability, damage to cell metabolism, and accumulation of toxic substances.
