Heat, along with light, water, air, and nutrients, is one of the basic conditions for plant life. All living plant cells are separated from their environment by the cell membrane.
The membrane is composed of lipid and protein molecules, and it is the lipid composition of the membrane that changes considerably with light, temperature, and acidity. In particular, under the influence of high temperatures, the number of unsaturated phospholipids in the membrane increases. The membrane's permeability, the cytoplasm's viscosity increases, amino acids (proline) capable of osmotic protection (preventing dehydration by retaining water and maintaining electrolytic balance), and organic acids that bind ammonia to begin to be produced. You will learn more about Plant Temperature System by Bubgo article.
These adaptive metabolic changes are genetically determined and expressed in different plant families to varying degrees. Due to their adaptability to high temperatures, plants are categorized as.
1. Heat-tolerant plants, already impaired at 86-104 °F (30-40°C) (aquatic flowering plants, most mesophytes, and wet plants)
2. Heat-tolerant plants that tolerate short-term (about half an hour) heating to 122-140 °F (50-60 °C) (plants in dry habitats with strong sunlight - grasslands, deserts, savannas, dry subtropics).
Intermediate plants (Group I) include most houseplants that live in inadequate but excessively humid conditions. Examples of mesophytes are lobelia, citrus trees, Ficus, palm trees, roses, petunias, etc. Water-loving plants are moisture-loving plants cultivated indoors: delphiniums, begonias, azaleas, etc. These heat-tolerant plants will wilt first in high temperatures.
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Therefore, if the ambient temperature rises, the plant reduces its temperature by transpiration (water evaporation from the leaves) to avoid overheating. If the temperature continues to rise, the intensity of respiration begins to dominate the process of photosynthesis, the formation and accumulation of organic matter in the plant stop, and water deficit occurs. Subsequently, cell membrane damage and protein denaturation occur. Plants first wilt, stop growing, and then die. Although some higher plants can tolerate short-term heating of 122-140 °F (50-60°C), active photosynthesis in even desert plants shuts down at temperatures above 104 °F (40°C).
The temperature limitation of photosynthesis varies from plant to plant. In addition, the effect of temperature on photosynthesis depends on the light level. Thus, in low light, photosynthesis is not affected by temperature, i.e., it proceeds at the same rate in low light under warm 64-68 °F (18-20 °C) and cool 46-53 °F (8-12 °C) conditions. In good light, lower temperatures inhibit photosynthesis by reducing the activity of enzymes involved in dark reactions.
For most houseplants, photosynthesis reaches its maximum intensity in the temperature range of 68-77 °F (20-25 °C), decreases with further temperature increases, and is almost completely terminated at 104 °F (40 °C). Thus, on our graph, the maximum limit of the optimal temperature for plant growth points to 82 °F (28 °C).
Why do you need to know this? This is especially important if you bring your plants on a glassed-in balcony or log cabin in summer. In summer, the temperature can rise to 78-82 °F (26-28 °C) in the shade on hot days. If your balcony is in the shade, the temperature can reach 95 °F (35 °C) when the windows are closed. And if your patio is on the sunny side, that's all 122 °F (50 °C). If you accidentally forget to open a window, your plants will at least retard growth, become severely dehydrated, or in the worst-case scenario, freeze to death.
These plants can be classified as follows, based on their adaptability to cold temperatures.
1. Cold intolerant plants are most rainforest plants that cannot tolerate temperature drops to 41-46 °F (5-8°C). This means that the plant cells are destroyed before the temperature has dropped to the freezing point of water. The Encyclopedia of House Plants lists the minimum temperature for such plants as 53 °F (12 °C).
2. Frost-resistant plants are those that can tolerate a strong drop in positive temperature, but the limit is above 32 °F (0 °C). They include so-called greenhouse plants such as acacias, cypresses, laurels, citrus, etc., as well as some cacti and leafy succulents.
3. frost-resistant plants are those that remain viable even at subzero temperatures since even freezing does not form crystalline ice in their cells (many plants come from temperate regions). It must be noted that in pot culture, even initially frost-resistant plants, such as birch trees grown in pots (forming bonsai), are at risk of freezing in winter. Therefore, even frost-tolerant plants, when grown as indoor and greenhouse plants, should be kept at a winter temperature of 33-41 °F (1-5 °C), allowing only short periods to drop to 26-24 °F (-3 to -4 °C).
Simplifying all classifications, we (by convention) divide all plants into warm and cool room plants for the average florist. Warm room temperatures in winter vary between 60-68 °F (16-20 °C) (the usual room temperature in summer), while cool room temperatures average between 53-57 °F (12-14 °C) (the lower limit depends on the species, e.g., for myrtle, some palms maybe 42-46 °F (6-8 °C). Tropical plants are kept in warmer areas. In subtropical countries, hardy plants from tropical countries grow and develop well in cooler rooms.
There are plants that are not affected by temperature (aloe, celestial, lobelia, water monster, ficus) that grow well in both warm and cool rooms as long as there is a balance between light and watering. That said, light is always just as good. Watering should be reduced when the temperature drops, and the colder the weather, the drier it gets until the soil in the pot is completely dry. This is because the plant's roots have difficulty absorbing water at low temperatures (and when the soil is very dry, salty, or highly acidic). And if the plant is watered more than it can absorb, digest, and utilize, the roots will rot.
Some plants are susceptible to very sudden temperature changes of 50-59 °F (10-15°C) per day. But this is only when the plant is in moist soil. Keep in mind that in winter, temperatures near windows are 33-35 °F (1-2°C) cooler than indoors, while temperatures on windowsills are 37-39 °F (3-4°C) cooler) the difference can be as high as 41-46 °F (5-8°C).
In addition, the soil in the pot (around the roots) is also about 33-35 °F (1-2 °C) cooler than the ambient temperature. If the plant is standing on a windowsill at 60 °F (16 °C), the temperature at the roots after watering may only be 55 °F (13 °C). And this is already at the edge for some plants. So, to avoid guessing by coffee grounds, refer to the thermometer on the windowsill. However, if the plant's roots are in slightly moist soil (i.e., you can't even feel the moisture with your hands, it's somewhere in the deepest part of the pot and the soil is not wet) or almost dry soil, then even large temperature changes are not a problem. Even a slight breeze will not harm the soil in the pot as long as it does not cause it to become cold.
Some plants require daily temperature changes if the daytime temperature is significantly higher than the nighttime temperature. These plants include many orchids, succulents, and cacti, which have experienced temperature changes of up to 68-104 °F (20-40°C) in their historic home! All of these plants have one thing in common: their growing environment. And if you don't provide them with these fluctuations, the plants won't bloom. But that's not all. Many plants need special light conditions in addition to diurnal temperature variations to grow flower buds. For example, the famous Rhododendron and Schlumberger require lower temperatures of 50-59 °F (10-15°C) to bloom under short daylight conditions.
If you do not specify a specific temperature for the plant, but instead say a moderate temperature of at least 53 °F (12 °C), this does not mean that the plant should always grow at 55 °F (13 °C). The problem is that a lower limit is specified that is possible for a short period of time, i.e., one or two days. But if a plant is grown at its temperature limit all the time, it will get sick (unless the plant is dormant). The temperature must be optimal for good growth and development. That is, 59-64 °F (15-18 °C) for plants requiring moderate temperatures and 64-75 °F (18-24 °C) for thermophilic plants. Special mention should be made of the detrimental effects of too high temperatures combined with dry air and lack of light. High temperatures encourage plant growth, while the lack of light affects photosynthesis. As a result, shoots stretch out, internodes become longer and leaves become paler and smaller. A typical example of this detrimental effect is indoor roses, which require winter temperatures of about 50 °F (10 °C).
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