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Heat, as well as light, moisture, air, nutrients, is one of the basic conditions for plant life. All living plant cells are separated from the environment by cell membranes.
Membranes are composed of lipid and protein molecules, and it is the lipid composition of the membranes that significantly changes depending on the light, temperature and acidity. In particular, under the influence of high temperature in the membranes, the amount of unsaturated phospholipids increases, which increases the permeability of the membranes, increases the viscosity of the cytoplasm, and begins to produce amino acids (proline) capable of osmoprotection (prevent dehydration by retaining water and maintains electrolytic balance), organic acids that bind ammonia.
These adaptive changes in metabolism are genetically determined and are expressed in different plant families to varying degrees. More precisely, due to the ability of plants to adapt to high temperatures, they are classified as:
- non-heat resistant plants - they are damaged already at + 30-40 ° С (aquatic flowering plants, most of mesophytes and hygrophytes).
- heat-tolerant plants - which tolerate short-term (about half an hour) heating up to + 50-60 ° С (plants of dry habitats with strong insolation - steppes, deserts, savannas, dry subtropics).
Mesophytes (first group) include the majority of indoor plants that live in nature in conditions of sufficient, but not excessive moisture. Mesophytes are examples: dracaena, citrus, ficus, palms, roses, petunias, etc. Plants hygrophytes are moisture-loving plants, in room culture: fittonias, begonias, philodendrons, etc. These non-heat-resistant plants are the first to wither at high temperatures.
So, when the ambient temperature rises, in order not to overheat, the plants reduce their temperature due to transpiration (evaporation of water by leaves). If the temperature continues to rise, the intensity of respiration begins to prevail over the processes of photosynthesis, the formation and accumulation of organic substances in the plant stops, and a water deficit occurs. Subsequently, cell membranes are damaged and proteins are denatured. The plant first withers, stops growing, then dies. And if some higher plants can tolerate short-term heating up to + 50-60 ° C, then active photosynthesis, even in desert plants, is inhibited at temperatures exceeding + 40 ° C.
The temperature limits within which the processes of photosynthesis are possible are different for different plants. In addition, temperature affects photosynthesis depending on the light. Thus, at low illumination, photosynthesis does not depend on temperature in any way; in poor lighting conditions, photosynthesis proceeds at the same rate both in warmth (+ 18-20 ° С) and cool (+ 8-12 ° С). With good lighting, lowering the temperature inhibits the process of photosynthesis, decreasing the activity of enzymes involved in dark reactions. In most indoor plants, the intensity of photosynthesis reaches a maximum in the temperature range of 20-25 ° C, with a further increase in temperature, it decreases, and at a temperature of 40 ° C, photosynthesis almost completely stops.Therefore, the maximum limit of optimal temperatures for plant growth points to 28 ° C on our graph.
Why do you need to know all this? This is important, first of all, for those who take out their plants for the summer on a glazed balcony or loggia. The fact is that in summer the temperature in the shade on hot days rises to + 26-28 ° C (in central Russia). If the balcony faces the shady side, with the windows closed, the temperature there can reach + 35 ° C. And if the balcony is on the sunny side - everything is + 50 ° С. Those. if you accidentally forget to open the windows, your plants will at least slow down in growth, and will be severely dehydrated, at worst, they will simply burn out and die.
According to the degree of adaptation of plants to conditions of low temperatures, there is the following classification:
- non-cold-resistant plants - these are the majority of rainforest plants, they are not able to withstand a decrease in temperature to + 5-8 ° C. Those. plant cells are damaged, even before the temperature drops to the freezing point of water. In the encyclopedia of indoor plants for such plants, a temperature minimum of 12 ° C is indicated.
- non-frost-resistant plants are plants that tolerate a strong decrease in positive temperatures, but the limit lies at a point slightly above 0 ° C. Such plants include the so-called greenhouse plants, for example, acacia, cypress, laurel, citrus fruits, etc.; as well as some cacti and leafy succulents.
- frost-resistant plants - plants that remain viable even at subzero temperatures, because even when frozen, crystalline ice does not form in their cells (many plants of temperate latitudes). I must say that in a potted culture, even initially frost-resistant plants, for example, a birch planted in a pot as a potensai (for the formation of a bonsai), risk freezing out in winter. Therefore, even frost-resistant plants, when grown as indoor and greenhouse plants, are kept in winter at temperatures in the range of + 1-5 ° C, and only a short-term decrease to -3-4 ° C is allowed.
Warm for plants
Simplifying all classifications, for an ordinary grower we divide (conditionally) all plants by temperature into: indoor plants in warm rooms and plants in cool rooms. In warm rooms, the temperature fluctuates between 16-20 ° C in winter (normal room temperature in summer), in cool rooms - on average, within 12-14 ° C (the lower limit depends on the specific species, for example, for myrtle, some palms may be 6- 8 ° C). In warmer rooms, plants from tropical countries are kept. Plants from subtropical countries and hardy plants from tropical countries thrive and thrive in cooler environments.
There are plants that are not demanding on the room temperature (aloe, aspidistra, dracaena, monstera, ficus), i.e. they can grow well in both warm and cool rooms, if the balance of light and watering is maintained. Those. lighting is always equally good, and watering decreases with decreasing temperature, and the colder, the drier, until the soil is completely dry in the pot. This is because the absorption of water by the roots of plants is difficult at low temperatures (as well as with great dryness of the soil, salinity or strong acidity). And if a plant is watered in excess of what it can absorb, absorb and use up, the roots simply rot.
Some plants are very sensitive to very sharp daily temperature changes, when the difference is 10-15 ° C. But this is only if the plant is in moist soil. It should be remembered that in winter the temperature near the window is 1-2 ° C lower than in the room, and on the windowsill it is 3-4 ° C lower (the difference can reach 5-8 ° C). In addition, the soil in the pot (around the roots) is also lower than the ambient temperature by about 1-2 ° C. If the plant stands on a windowsill with 16 ° C, then the temperature in the roots after watering can be only 13 ° C. And this is already on the verge, for some plants. Therefore, in order not to guess on the coffee grounds, be guided by the thermometer located on the windowsill. However, if the roots of the plant are in only slightly moistened soil (i.e. moisture is not even felt by the hands, it is somewhere in the very depths of the pot,the ground is not damp) or practically in dry ground, then even significant temperature fluctuations are not afraid of him. Even a small draft is not terrible if it does not cause cooling of the earth in the pot.
Effect of temperature on plants
There are plants that need daily temperature fluctuations when the daytime temperature is much higher than the nighttime. These are many orchids, succulents, cacti, in their historical homeland these fluctuations reach a corridor of 20-40 ° C! And if you do not provide them with these differences, the plants will not bloom. But that's not all. Many plants, in addition to daily temperature fluctuations, require special light conditions to set flower buds. For example, the well-known azalea and the Decembrist (Schlumberger) need a temperature drop to 10-15 ° C for flowering in conditions of short daylight hours.
If specific temperatures are not indicated for a plant, but it is said as follows: moderate, not lower than 12 ° C, this does not mean that the plant should grow at 13 ° C constantly. The fact is that the lower limit is indicated, which is possible for a short time, a day or two. But if the plant is constantly grown at its temperature limit, then it will hurt (unless the plant is dormant). The temperature for good growth and development must be optimal. Those. for plants that need a moderate temperature - 15-18 ° С, thermophilic - 18-24 ° С. Special mention should be made of the harmful effect of too high temperatures in combination with dry air and lack of light. The high temperature encourages the plant to grow, and the lack of light affects the processes of photosynthesis. As a result, the shoots are stretched, internodes increase,and the leaves turn pale and shrink. A vivid example of such a harmful effect can be seen on indoor roses, they need a temperature of about 10 ° C in winter.