Therefore, a small difference in relative humidity leads to a significant depression of its water potential. So, at a relative humidity of 100%, the water potential is zero, at a humidity of 99.6 - 0.5 MPa, at 99 and 97%, it is -1.36 and -4.0 MPa, respectively. The relative humidity of air in the summer most often does not exceed 60%, the water potential at the same time decreases to -68 MPa, and during a dry wind (30% humidity) the depression of water potential reaches 200 MPa.
Cuticular transpiration. From the very beginning of the existence of land plants, a dilemma arose: CO assimilation2 from the atmosphere requires intensive gas exchange, the prevention of significant water loss is possible only if there is good insulation from the ambient air, which has extremely low water potential values. The main problem of gas exchange, as O. Shtokker (1923) put it, is “maneuvering between thirst and hunger”.
The maintenance of leaf water homeostasis is achieved by the presence of epithelial epidermal tissue, which is a layer of rather adjacent cells, the outer walls of which are thickened. Outside the epidermis is covered with a cuticle, which includes cutin - polymeric esters of oximonocarbonic acids containing 16-28 carbon atoms and 2-3 hydroxyl groups. The cuticle also includes plates wax. The tubules, filled with a loose network of cellulose fibrils, depart from the plasmalemma of the epidermal cells to the cuticle. They serve as polar pathways for absorption and excretion of substances by the leaf.
Cuticular diffusion resistance In most cases very large. It depends on the thickness of the cuticle, the location, density and number of layers of kutan and wax. The formation of insulating structures on the surface of leaves is strongly affected by the species characteristics and age of plants, as well as the conditions for their growth. With atmospheric and soil drought, adult leaves usually have a thicker cuticle and a thicker wax coating.. Cuticular protection from transpiration is very effective. In adult leaves, cuticular transpiration accounts for 10–20% of total water evaporation. For example, in pome fruit trees, it measures 120–160 mg / dm 2 h.
The cuticle has a unique property due to the peculiarities of its composition - to change the hydraulic conductivity depending on the water content. When the outer layers of the epidermis are drying, the hydrophobic layers of the cuticle move closer together.therefore, the cuticular resistance between the interet is doubled. At low temperatures, it also increases. And vice versa, with increasing hydration of the epidermis, the cuticle swells due to the hydration of carboxyl and hydroxy groups, loosened, cuticular resistance to diffusion is significantly reduced and transpiration increases. Thus, the loss of water through the cuticle regulated by watering sheet. At night, for example, with a stronger swelling of the cuticle, cuticular transpiration is more intense than during the day. Moistened leaves can absorb water through the cuticle.
Normal transpiration. The main route of mesophyll leaf communication with the atmosphere is stomata. The stomatal transpiration process can be divided into a number of stages (See figure!).
Per es and e t a p - evaporation of water from the cell surface into the intercellular spaces. Each cell of the mesophyll, at least with one of its sides, borders on the extracellular space. The surface of all cell walls in contact with the intercellular spaces exceeds the surface of the sheet approximately in 10-30 time. It should be noted that already at this stage the plant is able to regulate transpiration. Reduction of water evaporation in the intercellular space is achieved by at least two mechanisms. The first due to changes in the water holding capacity of the cytoplasm by increase osmotic and colloidal water binding, its compartmentation in individual cell organelles and reduce membrane permeability. Second the mechanism is associated with a decrease in cell wall hydration. He got the name the mechanism of beginning drying, or root regulation of transpiration.
With a decrease in water supply by the root and an increase in water-holding capacity Therefore, with open stomata, transpiration decreases due to a decrease in the amount of water vapor in the intercellular spaces. it extraordinary method of regulating transpiration, which represents an undoubted benefit for the plants in the leaf as it allows to reduce the consumption of water without harming the assimilation of carbon dioxide. Cotton this method of regulation of transpiration prevails. In plants of moderate extraordinary regulation of the zone is expressed less: transpiration can be reduced by 30% without closing the stomata. The possibilities of extra-regulatory regulation largely depend on the growing conditions and the age of the plants. For example, the optimization of mineral nutrition increases the water-holding capacity of tissues, and decreases with aging of the leaves.
Second phase - the release of water vapor from the intercellular spaces through the stomatal crevices. The number of stomata and their location vary greatly among different plant species. Xerophytes - plants of dry habitats - per unit area usually have fewer stomata than mesophytes. In addition, their stomata are often recessed into the highly cutinized leaf surface, which reduces water loss. In most crops, stomata are mainly located from the bottom of the sheet (tab.). This is one of the devices for less water.
The number of stomata at crops, pieces / cm 2
What is transpiration
Transpiration - is a controlled physiological process of water movement through the organs of the plant organism, resulting in its loss through evaporation.
Thus, under the influence of atmospheric factors, the water reserves in the aboveground organs of the plant are constantly consumed and, therefore, must be replenished all the time due to new inputs. As the water evaporates in the cells of the plant, a certain sucking force arises, which “pulls” water from the neighboring cells and so along the chain - up to the roots. Thus, the main “engine” of water flow from the roots to the leaves is located in the upper parts of the plants, which, to put it simply, work like small pumps. If you delve into the process a little deeper, the water exchange in plant life is the following chain: drawing water out of the soil by the roots, lifting it to the aboveground organs, evaporating. These three processes are in constant interaction. In the cells of the plant's root system, the so-called osmotic pressure is formed, under the influence of which the water in the soil is actively absorbed by the roots.
When, as a result of the emergence of a large number of leaves and an increase in the ambient temperature, the water begins to be sucked out of the plant by the atmosphere itself, there is a pressure deficit in the vessels of the plants, which is transmitted down to the roots and pushes them towards the new “work”. As you can see, the root system of the plant pulls water from the soil under the influence of two forces - its own, active and passive, transmitted from above, which is caused by transpiration.
What role does transpiration play in plant physiology?
The process of transpiration plays a huge role in plant life.First of all, it should be understood that It is transpiration that provides plants with overheating protection. If on a bright sunny day we measure the temperature of a healthy and faded leaf in the same plant, the difference can be up to seven degrees, and if a faded leaf in the sun can be hotter than the surrounding air, then the temperature of the transpiring leaf is usually several degrees lower ! This suggests that the transpiration processes that take place in a healthy leaf allow it to self-cool itself, otherwise the leaf overheats and dies.
Finally, transpiration is an amazing force that can cause water to rise inside the plant throughout its height, which is of great importance, for example, for tall trees, the upper leaves of which, due to the process under consideration, can receive the required amount of moisture and nutrients.
Types of transpiration
There are two types of transpiration - stomatal and cuticular. In order to understand what is the one and the other species, we recall from the lessons of botany the structure of the leaf, since this particular organ of the plant is the main one in the process of transpiration.
So, The sheet consists of the following fabrics:
- skin (epidermis) is the outer covering of the leaf, which is a single row of cells, tightly interconnected to ensure the protection of internal tissues from bacteria, mechanical damage and drying. On top of this layer is often an additional protective wax, called the cuticle,
- the main tissue (mesophyll), which is located inside two layers of the epidermis (upper and lower),
- veins along which water moves and nutrients dissolved in it,
- The stomata are special locking cells and the opening between them, under which there is an air cavity. The stomatal cells are able to close and open depending on whether there is enough water in them. It is through these cells that the process of water evaporation and gas exchange is mainly carried out.
First, the water begins to evaporate from the surface of the main tissue of the cells. As a result, these cells lose moisture, water menisci in the capillaries are bent inward, the surface tension increases, and the further process of water evaporation becomes difficult, which allows the plant to significantly save water. Then the evaporated water goes out through the stomatal crevices. As long as the stomata are open, water evaporates from the leaf at the same rate as from the water surface, that is, the diffusion through the stomata is very high.The fact is that with the same area, the water evaporates more quickly through several small holes located at some distance than through one large one. Even after the stomata are closed in half, the intensity of transpiration remains almost as high. But when the stomata close, transpiration decreases several times.
The number of stomata and their location in different plants is not the same, in some species they are only on the inner side of the leaf, in others - both from above and below, however, as can be seen from the above, not so much the number of stomata affects the evaporation rate, but the degree of their openness: if there is a lot of water in the cell, the stomata open, when a deficiency occurs - the closing cells are straightened, the stomatal gut width decreases - and the stomata close.
The cuticle, as well as the stomata, has the ability to respond to the degree of saturation of the leaf with water. The hairs on the leaf surface protect the leaf from air and sunlight movements, which reduces water loss. When the stomata are closed, cuticular transpiration is particularly important. The intensity of this type of transpiration depends on the thickness of the cuticle (the thicker the layer, the less evaporation). The age of the plant is also of great importance - water leaves on mature leaves make up only 10% of the entire transpiration process, while on young ones they can reach up to half. However, an increase in cuticular transpiration is observed on too old leaves, if their protective layer is damaged by age, cracks or cracks.
Factors affecting the process of transpirationAs mentioned above, the intensity of transpiration is determined primarily by the degree of saturation of the plant leaf cells with water. In turn, this condition is mainly affected by external conditions - humidity, temperature, and the amount of light.
It is clear that with dry air the evaporation processes occur more intensively. But soil moisture affects transpiration in the opposite way: the drier the land, the less water gets into the plant, the greater its deficit and, accordingly, less transpiration.
With increasing temperature, transpiration also increases. However, perhaps the main factor affecting transpiration is still light. When the leaf absorbs sunlight, the leaf temperature increases and, accordingly, the stomata open and the transpiration rate increases.
Based on the influence of light on the movements of stomata, there are even three main groups of plants according to the daily course of transpiration. In the first group, the stomata are closed at night, in the morning they open and move during daylight hours, depending on the presence or absence of water deficit. In the second group, the night state of the stomata is a “changeling” of daytime (if they were open during the day, closed at night, and vice versa). In the third group, during the daytime the state of the stomata depends on the saturation of the leaf with water, but at night they are always open. As examples of the representatives of the first group, some cereal plants can be cited; to the second group include fine-leaved plants, for example, peas, beets, and clover; to the third group, cabbage and other representatives of the plant world with thick leaves.But in general it should be said that at night, transpiration is always less intense than during the day, because at this time of day the temperature is lower, there is no light, and humidity, on the contrary, is increased. During daylight hours, transpiration is usually most productive at noontime, and with a decrease in solar activity, this process slows down.
The ratio of the intensity of transpiration from a unit of surface area of a sheet per unit of time to the evaporation of a similar area of free water surface is called relative transpiration.
How is the water balance adjustment
The plant absorbs most of the water from the soil through the root system.
In addition to the roots, some plants have the ability to absorb water and ground organs (for example, mosses and lichens absorb moisture throughout its surface).
The water entering the plant is distributed throughout all its organs, moving from cell to cell, and is used for the processes necessary for the life of the plant. A small amount of moisture is spent on photosynthesis, but most of it is necessary to maintain the fullness of tissues (the so-called turgor), as well as to compensate for losses from transpiration (evaporation), without which the vital activity of the plant is impossible. Moisture evaporates on any contact with air, so this process occurs in all parts of the plant.
If the amount of water that is absorbed by the plant is harmoniously coordinated with its spending on all of these goals, the water balance of the plant is settled correctly, and the body develops normally. Violations of this balance can be situational or prolonged. In the process of evolution, many terrestrial plants have learned to cope with short-term fluctuations in the water balance, but long-term disruptions in the water supply and evaporation processes, as a rule, lead to the death of any plant.
Transpiration - what is it
If we talk about this concept in more detail, then transpiration is nothing but evaporation into the atmosphere of moisture from the leaves and stems of living plants. This phenomenon helps the water that the root system sucks, sometimes from fairly deep layers of soil (in deserts, roots can go even twenty meters deep), climb the stems or trunks to the leaves, flowers, fruits, delivering the necessary minerals and minerals to all parts of the plant organism. items. And a new portion of water with nutrients is “sucked in” due to transpiration in plants: the place is freed by evaporation of the used moisture through small pores on the leaves located on the back side. The intensity of the movement of water depends on external factors - time of day, temperature and humidity. In other words, the plant transpires when the humidity of the air inside it is higher than the humidity of the surrounding atmosphere. It is proved that ten percent of all moisture that evaporates on the surface of the Earth is attributed precisely to the plant world of our planet.
The biological significance of transpiration
Paraphrasing a well-known expression, one can say: if a phenomenon exists, it means that it is necessary for something. This is true with respect to transpiration. For plants, it is vital, and it is wrong to consider it fatal to the world of flora.
- The process of transpiration provides a constant movement of water "from toe to the top" - through the roots, stems, leaves.
- Thus it is possible to regulate the temperature and water regimes. At the hottest time of a summer day, the leaves are usually cooler than the surrounding atmosphere by three to eight degrees.
- Испарение помогает разгрузить растение от излишка влаги внутри и дать место новой партии воды, полной питательными микроэлементами.
- Transpiration prevents overheating and burns of leaves at high temperatures or direct sunlight.
But if the water leaves more than the plant manages to “drink” from the ground by its roots, it is in danger:
- moisture deficit
- decrease in the intensity of photosynthesis,
- metabolic disorders within the plant organism.
The result can be not just wilting, but even death. And yet, if the conditions are not extreme, the plant is able to independently regulate the level of evaporation. If there is not enough water to the surface from where it evaporates, transpiration slows down.
Water movement processes
As we have already found out, transpiration is a natural physiological process in the plant kingdom. Its main body is the leaf. Since the leaves of the plants are many, they form a large enough area for evaporation. As a result, the water potential decreases, and this is a signal for leaf cells to absorb water from the xylem veins. According to the principle of a falling domino, the movement of water from the roots along the xylem to the leaves is provoked. Formed something akin to the top end engine. And the more active the transpiration, the more powerful the upper “engine”, and the stronger the suction force of the “engine” of the lower - the root system.
From the stem, the water moves into the leaf, passing through the veins through the petiole. On the way, the veins "run up", the number of conductive elements becomes smaller. The veins themselves are transformed into individual tracheids, which form a very dense network. Delay moisture in a sheet of single-layer epidermis with a cuticle on its surface. The water turned into steam escapes through the stomata - special numerous micron-sized openings that the plant is able to expand or contract depending on external conditions.
Mechanism and intensity of transpiration
Plants absorb only a small part of the total volume of water that is extracted from the soil - 0.2 percent, sometimes a little more. Everything else evaporates into the air. The mechanism of the upper end of the engine is quite simple. It is based on the fact that there is usually not enough water vapor in the atmosphere, which means that its water potential can be characterized as negative. For example, at a relative humidity of 90 percent, the atmospheric pressure is 140 bar. And in the overwhelming majority of representatives of the kingdom of flora, the pressure inside the leaf varies between 1 and 30 bars. Such a large gap and provides transpiration. Water deficiency, going down the cells from the leaves along the stems, inevitably reaches the roots. This forces the lower engine to "run", sucking water from the ground. And evaporation from the surface of the leaves raises it, along with all the mineral salts, back to the top.
There are several factors that influence the intensity of transpiration.
- "Filled" plants with water. When it reaches a critical level, the stomata narrow.
- Air saturation with carbon dioxide. Most plants respond to excessive concentration by stomatal closure.
- Lighting. Usually when light, the stomata are open. It's getting dark - they are closing.
- Air temperature. Crossing over 35-40 ° С, it provokes stoma closure.
- The surface temperature of the sheet itself. Heated for every 10 ° C, the sheet gives up twice the moisture.
- Air humidity and wind speed. The drier the atmosphere, the higher the transpiration.
Water evaporation by plants takes place in three phases:
- Promotion from the veins to the upper layers of the mesophyll.
- Evaporation from the cell walls into the intercellular spaces and voids around the stomata, followed by exit to the outside.
- The last stage is divided into:
- transpiration through the stomata - mouth,
- evaporation into the atmosphere directly through the cells of the epidermis - cuticular transpiration.
It can be divided into two stages.
- The transition of water from the drip state (in this form it resides in the cell membranes) to gaseous in the intercellular spaces. At this time, the plant is able to regulate the power of transpiration. If it lacks water, there is a strong tension in the root and stem vessels that delays the movement of water to the leaf cells. And evaporation is slowing.
- The release of steam to the surface through the stomata. As soon as the water vapor leaves the intercellular voids, they are filled again by moving cells out of the sheath. The main lever for coordinating transpiration is the degree of stoma openness.
During the day, the plants "breathe" with different strength.
- If it is clear and not very dry outside, they make the first deep "breath" at dawn, when the stomata open to the maximum width. In the afternoon, they gradually narrow and close when the sun sets.
- In dry weather, this happens much earlier - by ten or eleven o'clock. As soon as in the evening the heat subsides, they open again until sunset.
- When the sky is covered with clouds, the stomata are usually open until the evening, but not very wide.
Daily variations in water loss are comparable to stomatal movement. Transpiration is slightly ahead of the ingress of moisture, which cannot pass through plant cells at the same rate. Therefore, in the daytime there is a certain deficit. But at night, when the stomata are closed and “sleeping”, it is filled. But in many ways the situation depends on the region where the plant lives, and its type. So, in cactus and euphorbia stomata open exclusively at night.
In a temperate climate, about 300 grams of water is used to accumulate one gram of dry matter. In general, this figure can vary from 125 to 1000 grams.