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Transpiration.
Its types and meaning
The basis of water consumption by the plant organism is the process of evaporation — the transition of water from a liquid to a vaporous state, which occurs when the plant organs come into contact with an unsaturated water atmosphere.
However, this process is complicated by the physiological and anatomical features of the plant, and it is called transpiration.
The amount of water evaporated by the plant is many times greater than the volume of water contained in it.
Economical water consumption is one of the most important problems of agricultural practice.
K. A. Timiryazev called transpiration, in the volume in which it goes, a necessary physiological evil.
Indeed, in normally flowing sizes, transpiration is not necessary.
So, if you grow plants in conditions of high and low humidity, then, naturally, in the first case, transpiration will go with a much lower intensity.
However, the growth of plants will be the same or even better where the humidity is higher and the transpiration is less.
At the same time, transpiration to a certain extent is useful to the plant organism.
Fig.
1. Transpiration
Transpiration saves the plant from overheating, which threatens it in direct sunlight.
The temperature of a strongly transpiring leaf can be about 7 °C lower than the temperature of a wilting, non transpiring leaf.
This is especially important due to the fact that overheating, destroying chloroplasts, dramatically reduces the photosynthesis process (the optimal temperature for the photosynthesis process is about 30-33 °C).
It is due to the high transpiring ability that many plants tolerate elevated temperatures well.
Transpiration creates a continuous flow of water from the root system to the leaves, which connects all the organs of the plant into a single whole.
Soluble mineral and partially organic nutrients move with the transpiration current At the same time, the more intense the transpiration, the faster this process goes.
As already mentioned, the mechanism of intake of nutrients and water into the cell is different.
However, a certain amount of nutrients can be supplied passively, and this process can accelerate with an increase in transpiration.
There are two types of transpiration: stomatal evaporation of water through the stomata and cuticular evaporation of water through the entire surface of the leaf blade.
For the first time, the distinction between cuticular and stomatal transpiration was introduced in 1877.
It is easy to see that evaporation really goes not only through the stomata, but also through the cuticle.
So, if you take leaves whose stomata are located only on the lower side (for example, apple leaves), and cover this side with vaseline, then the evaporation of water will continue, although in a significantly reduced size.
Therefore, a certain amount of water evaporates through the cuticle.
Cuticular transpiration
Outside, the leaves have a single layer epidermis, the outer walls of the cells of which are covered with cuticle and wax, forming an effective barrier to the movement of water.
Hairs are often developed on the surface of the leaves, which also affect the water regime of the leaf, since they reduce the speed of air movement over its surface and scatter light, thereby reducing water loss due to transpiration.
The intensity of cuticular transpiration varies in different plant species.
In young leaves with a thin cuticle, it can account for about half of the entire transpiration.
In mature leaves with a more powerful cuticle, cuticular transpiration is equal to 1/10 of the total transpiration.
In aging leaves, due to damage to the cuticle, it can increase.
Thus, cuticular transpiration is mainly regulated by the thickness and integrity of the cuticle and other protective integumentary layers on the surface of the leaves.
Cuticular transpiration usually accounts for about 10% of the total water loss of the leaf.
However, in some cases, in plants whose leaves are characterized by weak cuticle development, the proportion of this type of transpiration can increase to 30%.
The age of the sheet also matters.
Young leaves, as a rule, have a poorly developed cuticle and, consequently, more intense cuticular transpiration.
In old leaves, the proportion of cuticular transpiration increases again, since, although the cuticle retains a sufficient thickness, cracks appear in it, through which water vapor easily passes.
Cracks in the cuticle can also appear after the temporary wilting of the leaves, due to which transpiration increases.
There is evidence that cuticular transpiration is less dependent on environmental conditions compared to stomatal transpiration.
Stomatal transpiration
The main part of the water evaporates through the stomata.
Stomata play an important role in the gas exchange between the leaf and the atmosphere, as they are the main route for water vapor, carbon dioxide and oxygen.
Stomata are located on both sides of the leaf.
There are plant species in which the stomata are located only on the underside of the leaf.
On average, the number of stomata ranges from 50 to 500 per 1 mm2.
Transpiration through the stomata proceeds almost at the same speed as from the surface of pure water.
This is explained by I. Stefan's law: through small holes, the rate of gas diffusion is proportional not to the area of the hole, but to the diameter or length of the circle.
Therefore, although the area of the stomatal openings is small in relation to the area of the entire leaf (0.5-2 %), the evaporation of water through the stomata is very intense.
Transpiration consists of two processes:
1.
the movement of water in the sheet from the xylem vessels along the symplast and, mainly, along the cell walls, since water transport meets less resistance in the walls
2.
evaporation of water from the cell walls into the intercellular and subcellular cavities, followed by diffusion into the surrounding atmosphere through the stomatal slits.
The lower the relative humidity of atmospheric air, the lower its water potential.
If the water potential of the air is less than the water potential of the subcutaneous cavities, then the water molecules evaporate outwards.
The main factor affecting the opening and closing of stomata is the water content in the leaf, including in the closing cells of the stomata.
The cell walls of the closing cells have unequal thickness.
The inner part of the wall adjacent to the stomatal fissure is thicker, and the outer part is thinner.
As the closing cell osmotically absorbs water, the thinner and more elastic part of its cell wall stretches and pulls the inner part of the wall.
The closing cells take a semicircular shape and the stomata open.
When there is a lack of water, the closing cells straighten and the stomatal gap closes.
In addition, as the water deficit increases, the concentration of the growth inhibitor abscisic acid increases in the plant tissues.
It suppresses the activity of H+ pumps in the plasmalemma of the closing cells, as a result of which their turgor decreases and the stomata close.
Abscisic acid also inhibits the synthesis of the enzyme α amylase, which leads to a decrease in starch hydrolysis.
Compared with low molecular weight carbohydrates, starch is not an osmotically active substance, so the sucking force of the closing cells decreases, and the stomata close.
Unlike other cells of the epidermis, the closing cells of the stomata contain chloroplasts.
The synthesis of carbohydrates during photosynthesis in the closing cells increases their sucking force and causes the absorption of water, thereby contributing to the opening of the stomata.
The condition of the stomata depends on carbon dioxide.
If the concentration of CO2 in the subcostal cavity falls below 0.03%, the turgor of the closing cells increases and the stomata open.
An increase in the concentration of CO2 in the air causes the closure of the stomata.
This occurs in the intercellular cells of the leaf at night, when, as a result of the lack of photosynthesis and continued respiration, the level of carbon dioxide in the tissues increases.
This effect of carbon dioxide explains why the stomata are closed at night and open with the sunrise.
The pH shift to the alkaline side due to a decrease in the concentration of CO2 increases the activity of enzymes involved in the breakdown of starch, whereas at an acidic pH, with an increase in the content of CO2 in the intercellular cells, the activity of enzymes that catalyze the synthesis of starch increases.
In the light, the closing cells of the stomata contain significantly more potassium than in the dark.
When the stomata are opened, the potassium content in the closing cells increases by 4 times, while its content in the accompanying cells decreases.
An increase in the content of ATP in the closing cells of the stomata during their opening was found.
ATP formed during photosynthetic phosphorylation in the closing cells is used to enhance the intake of potassium.
The increased intake of potassium ions increases the sucking force of the closing cells.
In the dark, potassium ions are released from the closing cells and the stomata are closed.
The frequency of the daily course of transpiration is observed in many plants, but the stomata function differently in different plant species.
In trees, shade tolerant plants, many cereals and other hydrostable species with perfect regulation of stomatal transpiration, water evaporation begins at dawn, reaches a maximum in the morning.
At noon, transpiration decreases and increases again in the afternoon hours when the air temperature decreases.
Such a course of transpiration leads to insignificant daily changes in osmotic pressure and water content in the leaves.
In plant species that are able to tolerate sharp changes in the water content in the cells during the day, that is, in hydrolabile species, a single vertex diurnal course of transpiration is observed with a maximum at noon.
In both cases, transpiration is minimal or completely stops at night.
According to the ability to regulate their water metabolism, plants are divided into poikilohydric and homoyohydric.
Poikilohydric (from the Greek poikilos — various, diverse and hydor — water) are plants that cannot regulate their own water exchange.
This group includes soil algae, lichens, mosses, ferns and some angiosperms.
Homoyohydric (from Greek. homoios similar, identical and hydor water) are called plants that regulate their water exchange.
Angiosperms are homohydric plants.
There are two types of transpiration regulation: stomatal and non stomatal.
Stomatal regulation is carried out by opening and closing the stomata.
Closing the stomata by half has little effect on the intensity of transpiration, which follows from Stefan's law.
Their complete closure reduces transpiration by about 90 %.
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