What is the effect of temperature on plants? The influence of temperature on plant development. Is soil temperature important?

Affects plants, changes the rate of growth and development, absorption, assimilation and movement of water and elements of mineral nutrition and synthesis organic compounds. Soil temperature determines the rate of seed germination, as well as the degree of activation of beneficial and phytopathogenic microorganisms that damage seeds and reduce field germination. Crops vary greatly in the temperature range at which seeds germinate.
Lettuce, spinach, parsnip and onion seeds tend to germinate coldly. They begin to germinate at the temperature of melting ice (0°C). The process of germination, as well as the formation of a seedling, takes a very long time - 21...65 and 49...136 days, respectively. Different crops also differ greatly in the upper temperature limit of seed germination. Thus, at temperatures above 25 °C, lettuce seeds do not germinate, above 30 °C - spinach and parsnips, above 35 °C - carrots, corn, tomatoes, peppers, beans.
With increasing temperature, the rate of seed germination and emergence of seedlings increases to a certain limit. At the upper temperature limit of seed germination and seedling formation in onions, carrots, tomatoes and asparagus, it decreases.
The germination of a seed, that is, the formation of a root, has a lower temperature minimum than the growth of the subcotyledon, which is associated with the emergence of the seedling to the soil surface. Thus, asparagus seeds begin to germinate at 5 °C, and seedlings appear at 10 °C and above, but better at 20...25 °C. In beans, peppers and okra, seeds germinate at 10 °C, and seedlings form at 15 °C. In the zone of extreme temperatures, the roots of not all sprouted seeds form root hairs, which affects their absorption capacity, and not all sprouted seeds sprout, that is, field germination is reduced.
Field germination is especially strongly reduced when sown in cold soil in heat-demanding crops, which is largely due to the activation of soil pathogens. Field germination can be increased by treating and hardening the seeds and disinfecting the soil.
Root systems vegetable crops have lower temperature optimums than the above-ground parts of plants, but their range of tolerance is much narrower, that is, they are less cold- and heat-resistant. Root systems react more painfully than above-ground ones to sudden temperature fluctuations, which often happens in hydroponic culture and when growing container seedlings.
A decrease in soil temperature reduces the supply of water to heat-demanding crops (physiological drought), which occurs when watering cucumber and melons cold water. IN hot weather moisture deficiency often leads to the death of crops. At the northern borders of the cucumber crop, there are frequent cases of crop death on hot days following rains, accompanied by a significant decrease in air and soil temperatures.
The effect of low soil temperature is manifested in the degree of absorption of mineral nutrition elements, especially phosphorus, and often nitrogen due to the weakening of the activity of nitrifying bacteria. Phosphorus deficiency is especially severe in cold soils in tomatoes when the temperature drops below 15 °C.
The temperature of the substrate affects not so much the absorption of mineral nutrition elements, but rather their movement into the above-ground system.
Soil temperature determines the degree of activation of soil pathogens and the resistance of plants to them. At low soil temperatures (0...10 °C), fungi from the genera Pythium and Rhizoctonia are activated, affecting seeds, seedlings and plants, especially heat-loving crops. At high soil temperatures (20...30 °C) there is danger from fungi from Fusarium genera and Verticillium. At a temperature of about 20 ° C, cabbage clubroot is very harmful.
The influence of soil temperature is realized in the accumulation of plant biomass, the size of the root and above-ground systems, the rate of growth and the passage of phenophases. Soil temperatures below optimal retard the growth of roots and above-ground systems, lead to a reduction in the size of leaves and the entire plant, and delay the rate of onset of phenophases. Cucumber and tomato plants branch weaker and bear fruit. In experiments with cucumber varieties Vyaznikovsky and Muromsky, a complete absence of fruiting was observed at soil temperatures of 12...14 °C. The plants bloomed, but did not form ovaries. At temperatures of 15...20 °C the plants bore fruit normally.
The optimal temperature for the formation of potato tubers is 17...19 °C. When exposed to low temperatures for a long time (below 5 °C), the planted tubers fail to sprout; they form stolons with small nodules (babies). At a temperature of 28 °C, tuberization stops.
Extremely high soil temperatures suppress the growth of root and above-ground systems, delay the formation of cabbage heads, and fruit formation in tomatoes, cucumbers, and peppers. At the soil surface level, where the temperature is especially high, the phloem of the stem often dies, which leads to the death of plants.

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Ministry of Education of the Russian Federation

State educational institution

higher professional education

IRKUTSK STATE UNIVERSITY

(GOU VPO ISU)

Department of Hydrology

Effect of temperature on plants

Supervisor

Associate Professor, Ph.D. Mashanova O.Ya.

Voloshina V.V.

study group 6141

Irkutsk, 2010

Introduction

The adaptation of plant ontogeny to environmental conditions is the result of their evolutionary development(variability, heredity, selection). Throughout the phylogenesis of each plant species, in the process of evolution, certain individual needs for living conditions and adaptability to the ecological niche it occupies have developed. Moisture and shade tolerance, heat resistance, cold resistance and other ecological characteristics of specific plant species were formed during evolution as a result of long-term action of appropriate conditions. So, heat-loving plants and plants short day characteristic of southern latitudes, less demanding of heat and plants have a long day- for the northern ones.

In nature, in one geographical region, each plant species occupies an ecological niche corresponding to its biological features: moisture-loving - closer to water bodies, shade-tolerant - under the forest canopy, etc. The heredity of plants is formed under the influence of certain conditions external environment. The external conditions of plant ontogenesis are also of great importance.

In most cases, plants and crops (plantings) of agricultural crops, experiencing the effect of certain unfavorable factors, show resistance to them as a result of adaptation to the conditions of existence that have developed historically.

1. Temperature as a biological factor

Plants are poikilothermic organisms, i.e. their own temperature is equalized with the temperature of their environment. However, this correspondence is incomplete. Of course, the heat released during respiration and used in synthesis is unlikely to play any ecological role, but still the temperature of the above-ground parts of the plant can differ significantly from the air temperature as a result of energy exchange with the environment. Thanks to this, for example, plants of the Arctic and high mountains, which inhabit places protected from the wind or grow close to the soil, have a more favorable thermal regime and can quite actively support metabolism and growth, despite constantly low air temperatures. Not only individual plants and their parts, but also entire phytocenoses sometimes exhibit characteristic deviations from air temperature. On one hot summer day in Central Europe, the temperature on the surface of the crowns in forests was 4 °C, and in meadows - 6 °C higher than the air temperature and 8 °C (forest) or 6 °C (meadow) lower than the surface temperature soil devoid of vegetation.

To characterize thermal conditions plant habitats, it is necessary to know the patterns of heat distribution in space and its dynamics over time, both in relation to general climatic characteristics and specific plant growth conditions.

A general idea of ​​the supply of heat to a particular area is given by such general climatic indicators as the average annual temperature for a given area, the absolute maximum and absolute minimum (i.e., the highest and lowest temperatures recorded in this area), the average temperature of the warmest month ( in most of the northern hemisphere it is July, in the southern hemisphere it is January, on the islands and coastal areas it is August and February); the average temperature of the coldest month (in the continental regions of the northern hemisphere - January, in the southern hemisphere - July, in coastal regions - February and August).

To characterize the thermal living conditions of plants, it is important to know not only the total amount of heat, but also its distribution over time, on which the capabilities depend. growing season. The annual dynamics of heat is well reflected by the course of average monthly (or average daily) temperatures, which are not the same at different latitudes and at different types climate, as well as the dynamics of maximum and minimum temperatures. The boundaries of the growing season are determined by the duration of the frost-free period, the frequency and degree of probability of spring and autumn frosts. Naturally, the vegetation threshold cannot be the same for plants with different attitudes to heat; for cold-resistant cultural species Conventionally, 5°C is accepted, for most temperate zone crops 10°C, for heat-loving crops 15°C. It is believed that for natural vegetation of temperate latitudes the threshold temperature for the onset of spring phenomena is 5°C.

IN general outline the speed of seasonal development is proportional to the accumulated sum of temperatures (it is worth comparing, for example, the slow development of plants in a cold and long spring or the “explosive” beginning of spring during a strong heat wave). From this general pattern There are a number of deviations: for example, too high temperatures no longer accelerate, but retard development.

2. Plant temperature

Along with thermal performance environment it is necessary to know the temperature of the plants themselves and its changes, since it is this that represents the true temperature background for physiological processes. Plant temperature is measured using electric thermometers with miniature semiconductor sensors. In order for the sensor not to affect the temperature of the organ being measured, its mass must be many times less than the mass of the organ. The sensor must also be low-inertia and quickly respond to temperature changes. Sometimes thermocouples are used for this purpose. Sensors are either applied to the surface of a plant, or “implanted” into stems, leaves, or under the bark (for example, to measure the temperature of the cambium). At the same time, be sure to measure the ambient air temperature (by shading the sensor).

Plant temperatures are highly variable. Because of turbulent flows and continuous changes in the temperature of the air immediately surrounding the leaf, the action of the wind, etc. the temperature of the plant varies with a range of several tenths or even whole degrees and with a frequency of several seconds. Therefore, “plant temperature” should be understood as a more or less generalized and fairly conventional value characterizing the general level of heating. Plants, as poikilothermic organisms, do not have their own stable body temperature. Their temperature is determined by the thermal balance, i.e., the ratio of energy absorption and release. These values ​​depend on many properties of both the environment (size of radiation arrival, ambient air temperature and its movement) and the plants themselves (color and other optical properties plants, size and arrangement of leaves, etc.). The primary role is played by the cooling effect of transpiration, which prevents very strong overheating in hot habitats. This is easy to demonstrate in experiments with desert plants: you just need to smear Vaseline on the surface of the leaf on which the stomata are located, and the leaf dies before your eyes from overheating and burns.

As a result of all these reasons, the temperature of plants usually differs (sometimes quite significantly) from the ambient temperature. In this case, three situations are possible:

· the plant temperature is higher than the ambient air temperature (“supratemperature” plants, according to O. Lange’s terminology),

below it (“sub-temperature”),

· equal or very close to it.

The first situation occurs quite often in a wide variety of conditions. A significant excess of plant temperature over air temperature is usually observed in massive plant organs, especially in hot habitats and with low transpiration. Large fleshy stems of cacti, thickened leaves of euphorbias, sedums, and young plants, in which the evaporation of water is very insignificant, become very hot. Thus, at an air temperature of 40-45°C, desert cacti heat up to 55-60°C; in temperate latitudes summer days succulent leaves of plants from the genera Sempervivum and Sedum often have a temperature of 45°C, and inside the rosettes of the young - up to 50°C. Thus, the temperature rise of the plant above the air temperature can reach 20°C.

Various fleshy fruits are strongly heated by the sun: for example, ripe tomatoes and watermelons are 10-15 ° C warmer than the air; the temperature of red fruits in mature cobs of arum - Arum maculatum reaches 50°C. There is quite a noticeable increase in temperature inside a flower with a more or less closed perianth, which retains the heat that is released during respiration from dissipation. Sometimes this phenomenon can have significant adaptive significance, for example, for flowers of forest ephemeroids (scilla, corydalis, etc.), in early spring when the air temperature barely exceeds 0°C.

Peculiar and temperature regime such massive formations as tree trunks. U single standing trees, as well as in deciduous forests in the “leafless” phase (spring and autumn), the surface of the trunks heats up greatly during the daytime, and to the greatest extent on the south side; the cambium temperature here can be 10-20°C higher than at north side, where it is at ambient temperature. On hot days, the temperature of dark spruce trunks rises to 50-55°C, which can lead to cambium burns. The readings of thin thermocouples implanted under the bark made it possible to establish that the trunks tree species are protected differently: in birch, the cambium temperature changes faster in accordance with fluctuations in the outside air temperature, while in pine it is more constant due to the better heat-protective properties of the bark. Warming tree trunks and leafless spring forest significantly affects the microclimate forest community, since trunks are good heat accumulators.

The excess of plant temperature over air temperature occurs not only in highly heated, but also in colder habitats. This is facilitated by the dark color or other optical properties of plants that increase absorption solar radiation, as well as anatomical and morphological features that contribute to a decrease in transpiration. Arctic plants can warm up quite noticeably: one example is the dwarf willow - Salix arctica in Alaska, whose leaves are 2-11°C warmer than the air during the day and even at night during the polar “24-hour day” - by 1-3°C. Another interesting example heating under snow: in the summer in Antarctica, the temperature of lichens can be above 0°C even under a layer of snow of more than 30 cm. Obviously, in such harsh conditions natural selection retained forms with the darkest color, in which, thanks to such heating, a positive balance of carbon dioxide gas exchange is possible.

The needles of coniferous trees can be heated quite significantly by the sun's rays in winter: even at negative temperatures, it is possible to exceed the air temperature by 9-12°C, which creates favorable opportunities for winter photosynthesis. It was experimentally shown that if a strong flow of radiation is created for plants, then even at a low temperature of the order of - 5, - 6 ° C, the leaves can heat up to 17-19 ° C, i.e., photosynthesize at quite “summer” temperatures.

A decrease in plant temperature compared to the surrounding air is most often observed in highly illuminated and heated habitats (steppes, deserts), where the leaf surface of plants is greatly reduced, and increased transpiration helps remove excess heat and prevents overheating. In intensively transpiring species, leaf cooling (the difference with air temperature) reaches 15°C. This is an extreme example, but a decrease of 3-4°C can protect against harmful overheating.

In the most general terms, we can say that in hot habitats the temperature of the above-ground parts of plants is lower, and in cold habitats it is higher than the air temperature. This pattern can be traced in the same species: for example, in the cold mountain belt North America, at altitudes of 3000-3500 m, the plants are warmer, and in the low mountains the air is colder.

The coincidence of plant temperature with the ambient air temperature is much less common in conditions that exclude a strong influx of radiation and intense transpiration, for example, in herbaceous plants under the canopy of shady forests (but not in the sun's glare), and in open habitats - in cloudy weather or in the rain.

There are different biological types of plants in relation to temperature. In thermophilic, or megathermic (heat-loving) plants, the optimum lies in the region elevated temperatures. They live in areas of tropical and subtropical climates, and in temperate zones - in highly heated habitats. Low temperatures are optimal for cryophilic or microthermal (cold-loving) plants. These include species that live in polar and high-mountain regions or occupy cold ecological niches. Sometimes an intermediate group of mesothermic plants is distinguished.

3. Effect of temperature stress

Heat and frost harm vital functions and limit the spread of the species depending on their intensity, duration and frequency, but above all on the state of activity and the degree of hardening of the plants. Stress is always an unusual load, which does not necessarily have to be life-threatening, but which certainly causes an “alarm reaction” in the body, unless it is in a pronounced state of numbness. Dormant stages, such as dry spores, as well as poikilohydric plants in a dried state, are insensitive, so that they can survive without damage any temperature recorded on Earth.

Protoplasm initially responds to stress with a sharp increase in metabolism. An increase in breathing intensity, which is observed as a stress reaction, reflects an attempt to correct existing defects and create ultrastructural prerequisites for adaptation to a new situation. A stress reaction is a struggle between adaptation mechanisms and destructive processes in protoplasm leading to its death.

Cell death from overheating and cold

If the temperature passes a critical point, cellular structures and functions can be damaged so suddenly that the protoplasm immediately dies. In nature, such sudden destruction often occurs during episodic frosts, such as late frosts in the spring. But damage can also occur gradually; individual vital functions are thrown out of balance and inhibited until, finally, the cell dies as a result of the cessation of vital processes.

3.1 Damage pattern

Different life processes are not equally sensitive to temperature. First, the movement of protoplasm stops, the intensity of which directly depends on the energy supply due to respiration processes and on the presence of high-energy phosphates. Then photosynthesis and respiration decrease. Heat is especially dangerous for photosynthesis, while respiration is most sensitive to cold. In plants damaged by cold or heat, respiration levels fluctuate greatly after returning to temperate conditions and are often abnormally elevated. Damage to chloroplasts leads to long-term or irreversible inhibition of photosynthesis. In the final stage, the semi-permeability of biomembranes is lost, cellular compartments are destroyed, especially plastid thylakoids, and cell sap is released into the intercellular spaces.

3.2 Causes of death due to overheating

High temperature quickly leads to death due to membrane damage and primarily as a result of inactivation and denaturation of proteins. Even if only a few, particularly heat-labile enzymes fail, this leads to a disorder in the metabolism of nucleic acids and proteins and, ultimately, to cell death. Soluble nitrogen compounds accumulate in such high concentrations that they diffuse out of the cells and are lost; In addition, toxic decomposition products are formed, which can no longer be neutralized during metabolism.

3.3 Death from cooling and frost

plant temperature overheating frost

When protoplasm is damaged by cold, one must distinguish whether it is caused by the low temperature itself or by freezing. Some plants of tropical origin are damaged even when the temperature drops to a few degrees above zero. Like death from overheating, death from cooling is also primarily associated with disorganization of the metabolism of nucleic acids and proteins, but disturbances in permeability and cessation of the flow of assimilates also play a role here.

Plants that are not harmed by cooling to temperatures above zero are damaged only at temperatures below zero, that is, as a result of the formation of ice in the tissues. Water-rich, unhardened protoplasts can freeze easily; In this case, ice crystals instantly form inside the cell, and the cell dies. Most often, ice is formed not in protoplasts, but in intercellular spaces and cell walls. This ice formation is called extracellular. Crystallized ice acts like dry air, since the vapor pressure above the ice is lower than above the supercooled solution. As a result, water is removed from the protoplasts, they are greatly compressed (by 2/3 of their volume) and the concentration of dissolved substances in them increases. The movement of water and freezing continue until an equilibrium of suction forces between ice and water is established in the protoplasm. The equilibrium position depends on temperature; at a temperature of -5°C, equilibrium occurs at approximately; 60 bar, and at - 10 ° C - already at 120 bar. Thus, low temperatures act on protoplasm in the same way as desiccation. The frost resistance of the cell is higher if the water is firmly bound to the structures of the protoplasm and is osmotically bound. When the cytoplasm is dehydrated (it makes no difference whether as a result of drought or freezing), membrane-associated enzyme systems are inactivated - systems primarily involved in ATP synthesis and phosphorylation processes (Heber and Santarius, 1979). Inactivation is caused by excessive and therefore toxic concentrations of ions. salts and organic acids in unfrozen residual solution. On the contrary, sugars, sugar derivatives, certain amino acids and proteins protect biomembranes and enzymes from harmful substances(Maksimov, Tumanov, Krasavtsev, 1952). Along with this, there are indications that proteins become denatured when frozen, which also leads to membrane damage (Levitt 1980).

3.4 Thermal stability

Thermal tolerance is the body's ability to tolerate extreme heat or cold without permanent damage. Thermal resistance of a plant consists of the ability of protoplasm to tolerate extreme temperatures (tolerance according to J. Levitt) and the effectiveness of measures that slow down or prevent the development of damage (avoidance).

Measures to avoid damage

Possible ways to protect cells from temperature damage are few and not very effective. Insulation against overheating and cooling can only provide short-term protection. Thus, for example, in the dense crowns of trees or in cushion plants, the buds of leaves and flowers located deep and closer to the ground are less in danger of freezing as a result of the loss of heat by radiation than the outer parts of the plant. Conifer species with particularly thick bark are better able to withstand fires in the undergrowth. Two protective measures are of general importance: slowing down the formation of ice in tissues and (in hot weather) cooling by reflecting incident rays and using transpiration.

3.5 Stability of protoplasm

Plants can withstand prolonged and regularly repeated exposure to extreme temperatures only if the protoplasm itself is heat- or frost-resistant. This feature is genetically determined and therefore different types and even varieties are expressed to varying degrees. However, this is not a property that is inherent in the plant constantly and always to the same extent. Sprouts, spring shoots woody plants During the period of their intense stretching, cultures of microorganisms in the exponential growth phase are unlikely to be able to harden and are therefore extremely sensitive to temperature.

Ice resistance and frost hardening

In areas with seasonal climate land plants acquire “ice resistance” in the fall, i.e., the ability to tolerate the formation of ice in the tissues. In the spring, with the buds opening, they again lose this ability, and now freezing leads to their freezing. Thus, cold resistance perennial plants outside the tropics fluctuates regularly throughout the year between a minimum value during the growing season and a maximum in winter time. Ice resistance develops gradually in autumn. The first prerequisite for this is the transition of the plant to a state of readiness for hardening, which occurs only when growth ends. If readiness for hardening has been achieved, then the hardening process can begin. This process consists of several phases, each of which prepares the transition to the next. Hardening to frost, in winter cereals and fruits; trees (these plants have been studied most thoroughly) begins with multi-day (up to several weeks) exposure to temperatures just above zero. At this phase, preceding hardening, sugars and other protective substances accumulate in the protoplasm, the cells become poorer in water, and the central vacuole breaks up into many small vacuoles. Thanks to this, the protoplasm is prepared for the next phase, which takes place during regular mild frosts from - 3 to - 5 ° C. In this case, the ultrastructures and enzymes of the protoplasm are rearranged in such a way that the cells tolerate dehydration associated with the formation of ice. Only after this can plants, without being exposed to danger, enter the final phase of the process; hardening, which, with continuous frost of at least -10 to -15 ° C, makes protoplasm extremely frost-resistant.

Effective temperature zones are different for different species. Birch seedlings ready for hardening, which before the start of the hardening process would have frozen out at a temperature of - 15 to - 20 ° C, are transferred after the end of the first hardening phase; already - 35 °C, and when fully hardened, they can even withstand cooling to - 195 °C. Thus, the cold itself stimulates the hardening process. If the frost weakens, then the protoplasm again goes into the first phase of hardening, however, resistance can again be raised by cold periods to highest level while the plants remain dormant.

IN winter period The seasonal course of frost resistance is superimposed on short-term (induced) adaptations, thanks to which the level of resistance quickly adapts to weather changes. Cold contributes most to hardening at the beginning of winter. At this time, resistance can rise to its highest level in a few days. A thaw, especially at the end of winter, causes a rapid decrease in the resistance of plants, but in the middle of winter, after being kept for several days at a temperature of +10 to +20 ° C, the plants lose their hardening to a significant extent. The ability to change frost resistance under the influence of cold and heat, i.e., the range of inducible resistance adaptations, is a constitutional trait of individual plant species.

After the end of winter dormancy, the ability to harden and at the same time high degree hardening is quickly lost. In spring there is a close connection between the activation of bud break and the progress of resistance changes

Conclusion

The forms of adaptations in plants are infinitely diverse. All vegetable world Since its appearance, it has been improving along the path of expedient adaptations to living conditions.

Plants are poikilothermic organisms. Damage begins at the molecular level with dysfunction of proteins and nucleic acids. Temperature is a factor that seriously affects the morphology and physiology of plants, requiring changes in the plant itself that could adapt it. Adaptations of plants to different temperature conditions even within the same species are different.

At high temperatures, adaptations such as dense leaf pubescence, a shiny surface, a decrease in the surface that absorbs radiation, a change in position relative to the heat source, increased transpiration, a high content of protective substances, a shift in the temperature optimum of the activity of the most important enzymes, a transition to a state of suspended animation, occupation microniches protected from insolation and overheating, shifting the growing season to a season with more favorable thermal conditions.

Adaptations to cold are as follows: pubescence of bud scales, thick cuticle, thickening of the cork layer, pubescence of leaves, closing of rosette leaves at night, development of dwarfism, development of creeping forms, cushion growth form, development of contractile roots, increased concentration of cell sap, increased proportion of colloid-bound water , suspended animation

According to different heat resistance, species are distinguished: non-cold-resistant, non-frost-resistant, ice-resistant, non-heat-resistant, heat-tolerant zukaryotes, heat-tolerant prokaryotes.

List of used literature

1. Alexandrov V.Ya. Cells, macromolecules and temperature. L.: Nauka, 1975. 328 s

2. Voznesensky V. L., Reinus R. M. Temperature of assimilating organs desert plants// Bot. zhurn., 1977; t. 62. N 6

3. Goryshina T.K. Early spring ephemeroids forest-steppe oak forests. L., Publishing house Leningrad. un-ta. 1969

4. Goryshina T.N. Ecology of plants uch. A manual for universities, Moscow, V.

5. Kultiasov I.M. Plant ecology M.: Moscow University Publishing House, 1982 33-89 p.

6. Larcher V. Plant ecology M.: Mir 1978, 283-324c.

7. Maksimov N. A. Selected works on drought resistance and winter hardiness of plants M.: Publishing House AN-USSR.-1952 vol. 1-2

8. Polevoy V.V. Plant Physiology 1978 414-424s.

9. Selyaninov G. T. On the methodology of agricultural climatology. Works on agriculture meteorology, 1930, v. 22

10. Tikhomirov B. A. Essays on the biology of plants in the Arctic. L., Publishing House of the USSR Academy of Sciences, 1963

11. Tumanov I.I. Causes of plant death in the cold season and measures to prevent it. M., Knowledge, 1955

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The negative impact of cold depends on the range of temperature decreases and the duration of their exposure. Even non-extreme low temperatures adversely affect plants because:

• inhibit basic physiological processes (photosynthesis, transpiration, water exchange, etc.),
• reduce energy efficiency breathing,
• change the functional activity of membranes,
• 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. The main reason damaging effect low positive temperature on heat-loving plants - 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 when long-term action low temperature - to the death of the plant.

It has been established that the action low negative temperatures depends on the state of the plants and, in particular, on the hydration of the body tissues. Thus, dry seeds can tolerate temperatures down to -196°C (liquid nitrogen temperature). This shows that the detrimental effect of low temperature is fundamentally different from the effect high temperature, causing direct protein coagulation.

Main damaging effect ice formation affects the plant organism. In this case, ice can form as inside the cell and outside the cell. With a rapid decrease in temperature, ice formation occurs inside the cell (in the cytoplasm, vacuoles). With a gradual decrease in temperature, ice crystals form primarily in the intercellular spaces. The plasma membrane prevents ice crystals from penetrating into the cell. The contents of the cell are in a supercooled state. As a result of the initial formation of ice outside the cells, the water potential in the intercellular space becomes more negative compared to the water potential in the cell. There is a redistribution of water. The balance between the water content in the intercellular spaces and in the cell is achieved thanks to:

• or the outflow of water from the cell,
• or the formation of intracellular ice.

If the rate of water outflow from the cell corresponds to the rate of temperature decrease, then intracellular ice does not form. However, the death of the 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,
• increasing the concentration of cell sap,
• a sharp decrease in cell volume,
• a shift in pH values ​​in an unfavorable direction.

Plant resistance to low temperatures is divided into cold resistance and frost resistance.

Cold resistance of plants– the ability of heat-loving plants to tolerate low positive temperatures. A number of adaptations have a protective value under the influence of low positive temperatures on heat-loving plants. First of all, it is maintaining membrane stability and prevention of ion leakage. Resistant plants are characterized by a higher proportion of unsaturated fatty acids in the composition of membrane phospholipids. This allows you to maintain the mobility of the membranes and protects them from destruction. In this regard, the enzymes acetyltransferase and desaturase play an important role. The latter lead to the formation of double bonds in saturated fatty acids.

Adaptive reactions to low positive temperatures are manifested in the ability to maintain metabolism when it decreases. This is achieved by a wider temperature range of enzyme operation and the synthesis of protective compounds. In resistant plants, the role of the pentose phosphate respiration pathway increases, the efficiency of the antioxidant system increases, and stress proteins are synthesized. It has been shown that under the influence of low positive temperatures the synthesis of low molecular weight proteins is induced.

To increase cold resistance, pre-sowing seed soaking is used. The use of microelements (Zn, Mn, Cu, B, Mo) is also effective. So, soaking seeds in solutions boric acid, zinc sulfate or copper sulfate increases the cold resistance of plants.

Frost resistance of plants– the ability of plants to tolerate negative temperatures.

Plant adaptations to negative temperatures . There are two types of adaptations to negative temperatures:

• avoidance of the damaging effect of the factor (passive adaptation),
• increased survival (active adaptation).

Avoidance of damaging effects low temperatures is achieved primarily due to short ontogenesis - this leaving in time. U annual plants life cycle ends before the onset of negative temperatures. These plants have time to produce seeds before the onset of autumn cold weather.

Most perennials lose their above-ground organs and overwinter in the form of bulbs, tubers or rhizomes, well protected from frost by a layer of soil and snow - this is care in space from the damaging effects of low temperatures.

Hardening is a reversible physiological adaptation to adverse effects that occurs under the influence of certain external conditions, refers to active adaptation. The physiological nature of the process of hardening to negative temperatures was revealed thanks to the work of I.I. Tumanov and his school.

As a result of the hardening process, the body's frost resistance increases sharply. Not all plant organisms have the ability to harden; it depends on the type of plant and its origin. Plants of southern origin are not capable of hardening. In plants of northern latitudes, the hardening process is confined only to certain stages of development.

Plant hardening occurs in two phases:

First phase hardening takes place in the light at slightly lower temperatures above zero (about 10°C during the day, about 2°C at night) and moderate humidity. During this phase, a further slowdown and even a complete stop of growth processes continues.

Of particular importance in the development of plant resistance to frost during this phase is the accumulation of cryoprotective substances that perform protective function: sucrose, monosaccharides, soluble proteins, etc. Accumulating in cells, sugars increase the concentration of cell sap and reduce water potential. The higher the concentration of the solution, the lower its freezing point, so the accumulation of sugars stabilizes cellular structures, in particular chloroplasts, so that they continue to function.

Second phase hardening occurs with a further decrease in temperature (about 0°C) and does not require light. In this regard, for herbaceous plants it can also occur under snow. During this phase, water outflows from the cells, as well as a restructuring of the protoplast structure. New formation of specific, dehydration-resistant proteins continues. Important has a change in the intermolecular bonds of cytoplasmic proteins. During dehydration, which occurs under the influence of ice formation, protein molecules come closer together. The connections between them are broken and are not restored in their previous form due to too close proximity and deformation of the protein molecules. Due to this great importance has the presence of sulfhydryl and other hydrophilic groups, which promote water retention and prevent the proximity of protein molecules. Restructuring of the cytoplasm helps to increase its permeability to water. Thanks to faster water outflow, the risk of intracellular ice formation is reduced.

In relation to temperature there are following types plants:

• 1. Thermophiles, megathermic, heat-loving plants, the temperature optimum of which lies in the region of elevated temperatures.
• 2. Cryophiles, microthermic, cold-loving plants, the temperature optimum of which lies in the low temperature region.
• 3. Mesothermic plants are an intermediate group.

The tolerance of plants to extreme temperatures characterizes their heat resistance and frost resistance. To the effect of temperature as a factor, land plants have developed a number of adaptations.

So, the plant protects from overheating:

• 1. Transpiration (evaporation of 1 g of water at 20° requires 500 kcal)
• 2. Shiny surface, dense pubescence, vertical arrangement narrow leaf blades (fescue, feather grass), general reduction of the leaf surface - that is, all those devices that serve to weaken the influence of solar radiation.
• 3. Cork on the bark, air cavities on the root collar - adaptations characteristic of desert plants.
• 4. A peculiar adaptation is the occupation of certain ecological niches by plants, protected from overheating.
• 5. Surviving the hottest months in a state of suspended animation or in the form of seeds and underground organs.

Special adaptations to the effects of cold plants do not, but from the whole complex of unfavorable factors associated with it ( strong winds, the possibility of drying out) the plant is protected by such morphological features as pubescent bud scales, tarred buds, a thickened cork layer, and a thick cuticle. A peculiar adaptation to cold is observed in the highlands of Africa in rosette lobelia trees; during the night cold, the rosettes of leaves close.

Protection from the cold also contributes to:

• 1. Small size, dwarfism, or nanism. For example, at dwarf birch and willows - Betula nana, Salix polaris.
• 2. Creeping forms - stlantsy.
• 3. Surviving the hottest months in a state of suspended animation or in the form of seeds or underground organs.
• 4. Special life form cushion plants (in heather) are able to maintain a temperature in the thicket of branches 13°C higher than the ambient temperature.
• 5. Development contractile- contractile roots. In autumn, such roots dry out, shrink and press the wintering buds deep into the soil, which interferes with the buoyant force of permafrost).

Physiological methods of protection from cold are more typical for plants in temperate regions.

• 1. Reducing the freezing point of cell sap (more soluble sugars, increasing the proportion of colloid-bound water). In general, plants are worse adapted in this regard than insects.
• 2. Decrease in temperature optimum of physiological processes. In arctic lichens, for example, photosynthesis is optimal at 5° and possible at -10°
• 3. Snowy growth in the pre-spring period in scillas, tulips and other ephemeroids.
• 4. Anabiosis- an extreme measure of plant protection - a state of dormancy, during which the plant can tolerate down to -200°C. In the state of winter dormancy, a distinction is made between a phase of deep or organic dormancy, when cut branches do not bloom in the warmth, and a phase of forced dormancy at the end of winter. The signal for the onset of rest is the shortening of the day.