Water Uptake in Plants
In plants, both the structure and the composition are in a state of continuous change. This constant change is typical of vitality. Therefore, examining only the structure and composition of a plant does not give us a complete explanation of life events, i.e. physiological events. It is also necessary to examine the changes in the substances that make up the plant and the special shape changes in each plant. All the processes of change of substances in plants are called metabolism in general terms.
In order for the metabolic events occurring in plant cells to occur, there must be water activity in the plant.
Composition of Plant Body
Water constitutes a large part of plants and plant organs. Water is an essential substance for all vital activities of plants. Water makes up about 82% of the cell cytoplasm. It is also important in terms of being a solvent of various nutritive mineral substances that have very important roles in the life of plants. Moreover, water is required for all metabolic activities to occur.
Dry matter forming the plant; consists of organic and inorganic compounds. Organic compounds are carbohydrates, proteins and fats. Inorganic compounds are various minerals.
Studies have shown that 92 elements found on earth can also be found in the structure of plants. However, it has been understood that especially 10 elements are essential. These elements are called essential elements or macro elements. These elements are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), sulfur (S) and phosphorus (P).
In the researches carried out by plant physiologists, it has been understood that boron (B), zinc (Zn), copper (Cu), manganese (Mn), molyptene (Mo) and chlorine (Cl) are also necessary for plants. However, since it was understood that less amounts of these elements were sufficient for plants, they were called microelements.
It has been understood that the elements defined as microelements above and many other elements besides these are found in very low concentrations in plants and they affect plant metabolism with certain effects. These elements are also called trace elements.
The most important functions of macro elements in plant metabolism are:
– Carbon: It is the basic element of carbohydrates.
– Nitrogen: It takes place in the protein structure.
– Phosphorus: It takes place in the structure of some proteins. It also participates in the structure of ATP, DNA, RNA in the cell. It is essential for the building blocks that make up the cell nucleus. It also takes part in carbohydrate metabolism.
– Sulfur: It participates in the structure of some proteins.
– Magnesium: It is used to make chlorophyll.
– Iron: It acts as an enzymatic factor in the production of chlorophyll.
– Calcium: It is an essential substance for cell wall construction.
– Potassium: It takes part in growth and division events. It also has an enzymatic effect on chlorophyll production and carbohydrate metabolism.
In order for a plant to maintain its vital events normally, it must find the necessary elements. However, if even one of the necessary elements is not sufficient, the development of the plant is limited depending on this element. If only one slice of the barrel made of many slices is half, it takes as much water as the half part.
Removal of Water and Dissolved Substances
Except for aquatic plants and a few plants that are in the air and are capable of making use of water, all other plants have to take water from the soil. This; It is found in the soil, stuck to the soil particles and filling the spaces between the particles. For this reason, the sizes and compositions of soil particles differentiate the water holding capacity of the soil.
In order for the plants to absorb the water between the soil particles, the plants must have suction power. Plants provide this suction force in two ways. These;
– Swelling abilities of the substances that make up the structure of plants,
– Density of sap of plant cells.
Swelling
If the mouth of a water-filled jug is closed with a stopper made of dry wood, after a while, it is seen that the stopper is stuck in the mouth of the jug. We call this situation a plug. When swelling is mentioned, it is understood that colloid (particles of various substances suspended in a liquid) absorb water and increase in volume without losing their solid structures, which are the characteristics of objects. Colloid bodies are structures formed by the combination of many molecules. The structures formed by the combination of many molecules that make up the colloids are also called micelles. The spaces between the micelles are also called intermicelle spaces. When such a colloidal substance absorbs water, the water fills between the micelles and the micelles move away from each other. As a result, the initial volume of the swelling substance increases. Sometimes it is seen that the vegetative structures swell up to 15-100 times the initial volume. The swelling capacity of each plant organ is different. Most swelling is seen in seeds.
Diffusion
Matter molecules move in their environment with their own kinetic energy. This movement changes according to the gas, liquid and solid form of the substance. Gas molecules are more mobile than liquid and solid molecules.
The diffusion of molecules from where the densities are high to where they are less is called diffusion. The cell performs most of the substance exchange by diffusion. Diffusion takes place under the control of the cell membrane. In other words, substances large enough to pass through the cell membrane are taken or given by diffusion. For example; Small molecules such as oxygen, carbon dioxide, amino acids and fatty acids are exchanged by diffusion, while large substances such as starch and glycogen cannot be exchanged by diffusion.
The factors affecting the diffusion rate are:
– If there is no density difference between the substance molecules inside and outside the cell, there will be no diffusion. As the density difference increases, the diffusion rate increases.
– Fat-soluble substances also pass more easily than insoluble ones (vitamins A, D, E, K).
– It passes easily in oil-soluble substances (such as alcohol and ether).
– As the temperature increase increases the kinetic energy of the molecules, it accelerates the diffusion.
– The smaller the molecules, the faster the diffusion. They slow down as they grow.
– As the surface (number of pores) increases, diffusion accelerates.
– Neutral molecules pass through more easily than ions.
In cases where the diffusion rate in cells is not sufficient, it may be necessary to increase the diffusion rate. Cells use carrier proteins in the cell membrane to accelerate the passage of certain substances. The passage of substances such as glucose through the cell membrane with the help of carrier proteins is called facilitated diffusion.
Osmosis
The exchange of certain substances in the cell is one of the basic physiological events of its vitality. Plant cell membranes do not allow every substance to pass equally. These properties of membranes, which determine the passage of certain substances through themselves, are called permeability. Some membranes pass through any substance. These are called permeable membranes. Some substances pass through some membranes, some do not. These membranes are also called semi-permeable membranes.
Diffusion of water molecules under the control of a semipermeable membrane is called osmosis. Osmosis is the passage of solute through a semipermeable membrane. In order to understand the osmosis phenomenon, we need to know the solutions. Solutions consist of solvent and solute. There are three types of solutions according to the solution inside the cell, that is, the cell sap.
– Isotonic solution: It is the solution whose solute concentration is equal to the cell sap.
– Hypotonic solution: It is the solution in which the solute concentration is low compared to the cell. Example: Pure water.
– Hypertonic solution: It is the solution in which the solute concentration is higher than the cell.
In the event of osmosis, which is a special form of diffusion, water molecules pass from the environment where they are very concentrated to the environment where they are less dense. From two solutions with a semi-permeable membrane between them, the side with more solute absorbs water from the side with less solute. This suction force is called osmotic pressure. Osmotic pressure decreases as the amount of water in the cell increases, and increases as the amount of water decreases. Osmotic pressure is made from the environment towards the cell membrane. On the other hand, there is also the pressure applied from inside the cell to the wall. This pressure is called turgor pressure. Turgor pressure is caused by water entering the cell. As water enters the cell, the turgor pressure increases and the osmotic pressure decreases. If the osmotic pressure is greater than the turgor pressure, the cell takes in water. The difference between the osmotic pressure and the turgor pressure is called the suction force. If the osmotic pressure and the turgor pressure are equal, the cell is in osmotic equilibrium.
Plant cells take water and dissolved substances from their environment. The living cell has a semipermeable membrane, cellulose, cell wall, vacuole filled with a more or less dense solution. Living things exchange substances according to osmotic rules. The osmosis event ends in the case of density equality. In order for the cell to survive, osmosis must continue. This is achieved through metabolic activities.
Turgor
The stretching of the cell membrane by taking in water is called turgor. This pressure inside the cell is called turgor pressure. Turgor pressure gives resistance and uprightness to herbaceous plants. This is why the leaves of the potted flowers in our house wither when they are dehydrated and return to their original state when water is given. The ability of each tissue to absorb water in plants is different. As a result of different water uptake in the internal and external tissues of an organ, different turgor pressures occur. This situation creates twisting, orientation, and startle movements.
Plasmolysis
If the plant cell is placed in a solution that is denser than the density of its own vacuole sap, then the cell gradually loses water and releases water to the external environment. As a result, the cell shrinks. This event continues until the density of the cell sap is equal to the density of the external environment. This condition is called plasmolysis.
If the cell, which has been plasmolyzed in this way, is placed in pure water, it takes water and returns to its original state. This condition is called deplasmolysis.
The permeability of protoplasmic membranes is related to the vitality of the cell. Damage to cell vitality for various reasons causes the selective permeability of the cell membrane to change. In general, permeability increases with cell death. Substances in cell vacuoles come out. This condition is called exosmosis. For example, if a flower or fruit is thrown into the boiling water, their color immediately changes to the water. However, this is not the case in cold water. Hot water causes the plant to die and permeability increases.
Ion Accumulation and Antagonism (Contrast)
Mineral salts are of great importance for plants. While some mineral salts are used extensively in the plant cell, a very small amount of some mineral salts is sufficient for the plant to survive. Excessive accumulation of mineral salts taken with water in the cell may be objectionable. According to the osmotic rules, when the concentration of a certain ion in the cell is equal to that in the external environment, its uptake stops. However, some ions accumulate in the vacuole to a much higher density than the environment they are in. This increases the osmotic pressure of the cell. This situation is called ion accumulation.
In non-living cells, ion accumulation stops. This ion accumulation occurs by active transport.
Although the ions of many salts have toxic effects for the plant when they are alone in the environment where the plant lives, when the same ion is given to the plant with another ion, no toxic effect is observed. In this case, the second ion eliminates the poison effect of the first. This is called ion antagonism. For example, when sodium chloride and calcium chloride are given separately to wheat seedlings, it is seen that the seedlings do not develop. When the two salts are mixed and given in certain proportions, it is seen that the seedlings develop normally.
Factors Affecting Plants’ Water Uptake
The factors affecting the water intake of plants can be divided into two main groups as environmental factors and plant factors.
– Environmental factors:
Environmental factors have a great effect on the water intake of plants. First of all, there should be enough water in the soil that the plants can take. Plants need a large amount of smooth and rapid water flow in the soil. By moving through mass movement or diffusion, water in the soil can reach the root point more quickly and in the desired amount. However, plant roots always grow themselves and move towards the direction of the water, without waiting for the water in the soil to reach them. Plant movement in the soil towards the location of water is also defined as hydrotropism. This situation gains importance with the distribution of plant roots in the soil, the amount and depth.
Plants can take up to 15 atmospheres of water held in the soil. Plants cannot benefit from water held with more force than this value.
The amount and types of salts in the soil also affect water intake. An increase in salt concentration in the soil causes an increase in osmotic pressure. If a pressure higher than the osmotic pressure in the roots occurs, the roots cannot take the water from the soil. They even start to give out the water in their own body. However, here, active water intake is activated and some plants can continue to receive water despite a certain amount of soil salinity (high pressure). These plants are shown as partially drought and salinity tolerant plants.
Soil temperature also has an effect on the water uptake power of plants. In this respect, it is desirable that the soil temperature be between 8-25 degrees on average. A decrease in temperature reduces water intake, especially in plants that enjoy heat. For example, when the temperature drops from 20 degrees to 10 degrees in greenhouse tomatoes, the water intake decreases by 20-30%. On the other hand, water intake increases by 30-40% in cabbages, which are cool climate plants. However, when the soil temperature drops below 5 degrees in cabbage, the water intake decreases. Each plant has the best possibility of getting water at a certain temperature.
We can list the factors that limit the water intake of plants at low degrees as follows:
– Root growth is restricted and root activity is reduced.
– Membrane permeability of stem cells decreases.
– The activity and permeability of the cell protoplasm decreases.
– The stickiness (viscosity) of the water increases.
– The vapor pressure of the water decreases.
– Water movement from the soil to the root is reduced.
As the water intake decreases at low temperatures, the same effect occurs when it is increased to very high temperatures.
Another factor affecting the plant’s water intake is soil air. As the amount of oxygen in the soil decreases and the amount of carbon dioxide increases, the roots slow down to take up water. The airless soil environment increases the viscosity of the root cells and reduces the ingress of water to the root. Therefore, the plants cannot take the water. Heavy and long-term rains, elevation of the ground water level and unconsciously excessive and frequent irrigations cause an increase in the amount of water in the soil, filling all the voids in the soil with water, and as a result, the soil becomes airless. As it is known, plants need a certain amount of air as well as water in the soil in order to maintain their normal development.
As a result of the increase in the amount of water in the soil and the corresponding decrease in the amount of oxygen;
– Proliferation of stem cells by dividing slows down and root development cannot be achieved at the desired level.
– The activity of soil microorganisms, which decomposes the organic matter in the soil and transforms it into the nutrient form that the plants will take, slows down.
– Harmful compounds are formed in the soil that prevent the uptake of plant nutrients.
All these factors affect plant development, so there is a decrease in yield.
The water needs of the plant; irrigation method, amount and number of irrigation water, temperature, relative humidity of the air, wind speed, condition of the day and season, etc. many factors come into play.
Plant water consumption; It is determined for different time intervals as daily, monthly and seasonal. Each of the specified periods is highly influential on some decisions to be made in the design and operation of irrigation systems.
Plant factors:
The uptake of water in the soil by plants can also vary considerably by some plant-related characteristics. Since the distribution and structural features of a plant can change under different environmental factors, there may be differences in its functions. Accordingly, the plant water consumption values also change during the development period. This situation will vary according to plant genus and species, as well as environmental factors and cultural processes applied in a plant’s life process. The differences in question are the development time of the plant, its anatomical structure, the amount of root development, root depth, the water absorption power of the roots, the growth balance and connection between the above-ground organs and the soil organs, etc. many factors such as
Among the factors mentioned above, especially plant root development plays the most effective and major role in terms of plant-water relationship. In addition, many side factors such as root growth rate, root depth, amount of root sucking hairs, botanical structure of the root, and suction power can be counted.
Determining the irrigation time by looking at the plants is a method generally applied in practice. In this method, the wilting state of the leaves is generally taken into account. There are two types of wilting (wilt) in plants, temporary and permanent. In temporary wilt, there is sufficient water in the soil. The plant is in the morning turgor state. There is no sign of thirst during these hours. However, the increase in air temperature and light intensity towards noon causes the transpiration in the plant to rise above its peak. Meanwhile, when the water taken by the roots is less than the water lost by the plant’s leaves, that is, in cases where the water taken cannot meet the lost water, plasmolysis occurs in the plant cells and the plant starts to wilt. The leaves are bent, twisted, drooping, the shoot tip is curled. Towards evening, when bad weather conditions such as excessive heat and light intensity disappear, the water balance in the plant is re-established. The water taken becomes more state than the water lost. Plant cells are filled with water and turgor occurs. As a result, the leaves and shoot of the plant become stretched and erect. This wilting in the plant is called “temporary wilt” and this event is called “physiological drought”.
If the plant shows a wilt even in normal living conditions in the morning and evening hours, this wilt is called “continuous wilt”. Persistent wilting occurs as a result of decreasing water in the soil. Therefore, plant growth slows down or even stops in continuous wilting. The leaves of the plant first become shriveled, curled, and then their color darkens. As a result of this, the leaves start to turn yellow first and the leaf fall accelerates from the bottom. The shoot tip dries up. In case of continued thirst, the plant will finally dry out completely.
Sometimes, although there is water in the soil and the weather conditions are suitable for growing plants, permanent wilting can be seen in the plants. This pallor is not due to lack of water. In this wilt, one should first look for discomfort on one side of the plant. Situations such as root diseases and pests infecting or killing the roots, destruction of plant transmission pipes by any disease or mechanical damage may have resulted in this result. In addition, situations such as increased soil salinity, lack of air in the soil, and insufficient soil temperature may have a similar effect.
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