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Plant water relations are critical as water is crucial for both plants and animals. It acts as a dissolving medium for many compounds. Plants take in a lot of water on a daily basis, and a lot of it is lost through transpiration. Varying types of plants have different water requirements. Plant water relation refers to the amount of water that the plant restores within its cells. It absorbs water from the soil and the complete process starts from the absorption of water and continues till evaporation of the water from the leaves.
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Key Takeaways: Plants, water potential, diffusion, osmosis, solute potential, pure water, pressure, semipermeable membrane, solution
Water Potential
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The kinetic energy of water molecules is a known quantity. The kinetic energy or water potential of a system increases as the amount of water in the system increases. Water molecules flow from the higher-energy system to the lower-energy system when two systems containing water come into contact. Pascals are a unit of measurement for water potential. At ordinary temperature, pure water has no water potential.
When a specific amount of solute is introduced to pure water, the concentration of the water falls, and the water potential falls as well. Solute potential is the amount by which the water potential decreases. This is always negative, and as the amount of dissolved solutes increases, the value of solute potential drops. When pure water is subjected to a pressure greater than atmospheric pressure, the value of water potential rises. Turgid cells are formed when water enters a plant cell via diffusion and exerts pressure on the cell walls. The pressure potential rises as a result. This is usually a positive number. The sum of the solute and pressure potentials is the water potential.
The video below explains this:
Plant Water Relations Detailed Video Explanation:
Diffusion
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Individual molecules migrate in all directions from a location of higher concentration to a region of lower concentration by diffusion. The flow of molecules is proportional to the concentration difference. In gases, diffusion is faster than in liquids. When it comes to diffusion, the movement is random and unrelated to one another. It transfers solvent to solution along a concentration gradient and kinetic energy.
Diffusion pressure is the imaginary potential of solute particles to diffuse from a higher concentration to a lower concentration (DP). A pure solvent's value is always larger than its solution's value. The diffusion pressure of a sugar solution formed in water is lower than that of water.Temperature, kinetic energy, and diffusion pressure gradient are all directly related to diffusion rate. Diffusion rate is inversely related to medium density, humidity, solute size, molecular weight, and distance between diffusing particles.
Diffusion pressure is constant in all liquids. The diffusion pressure of a pure solvent is the highest. When a certain amount of solute is added, the diffusion pressure drops. Diffusion pressure deficit refers to a decrease in the diffusion pressure of a solution as a result of the addition of a solute (DPD).
Osmosis
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The passage of water across a semipermeable membrane is known as osmosis. Water travels from a higher-concentration zone to a lower-concentration region until equilibrium is attained.
Again, two processes are involved: endosmosis and exosmosis. Endosmosis occurs when water diffuses inward through a semipermeable membrane when the surrounding medium is less concentrated, whereas exosmosis happens when water diffuses outward through a semipermeable membrane when the surrounding medium is more concentrated.
Plasmolysis
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Plasmolysis is a process in which the cell membrane shrinks away from the cell wall as water travels out of the cell. When a cell is placed in a hypertonic solution, this happens (which has more solutes). The cytoplasm and then the vacuole both lose water. When the cell is submerged in an isotonic solution, there is no net movement of water; however, when the cell is submerged in a hypotonic solution, water moves into the cell and puts turgor pressure on its walls.
Imbibition
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It is a process in which solids absorb water and their volume grows. Absorption of water by seeds and dry wood is an example of this. The water is moving along a concentration gradient in this case.
Things to Remember
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- The osmotic potential of water is its permeability in comparison to pure water. As a result, the purest form of water is said to have no osmotic potential.
- Water also aids in the absorption of minerals from the soil and the transfer of mobile nutrients such as nitrogen, potassium, phosphorus, phosphate, chlorine, magnesium, zinc, calcium, molybdenum, iron, sulphur, copper, and boron.
- Water potential refers to the tendency of water to flow or move from one place to another as a result of various factors such as gravity, osmosis, mechanical pressure, capillary action, and so on.
- The seed, as well as semi-permeable or porous material like wood or colloid, absorb water from the soil during the imbibition process.
- Plants can be harmed by excessive watering. When there is too much water in the soil, it evaporates, leaving all of the minerals, including salts, at the surface. It has the potential to deplete the soil.
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Sample Questions
Ques: Give a brief description of plant water relations. (5 marks)
Ans: One of the most important requirements for plants is water. Water makes up around 90% of a plant's body. In the presence of water, green plants carry out activities such as photosynthesis, in which they absorb carbon dioxide from the atmosphere and release oxygen during the process.
Water also aids in the absorption of minerals from the soil and the transfer of mobile nutrients such as nitrogen, potassium, phosphorus, phosphate, chlorine, magnesium, zinc, calcium, molybdenum, iron, sulphur, copper, and boron. Water also aids in maintaining cell wall turgidity. Water is also required for cell enlargement via turgor pressure and cell division, as well as plant growth. Water evaporates from the leaves' stomata, which serves to create low pressure in the vacated area, allowing more water to occupy the space and ensuring a steady supply of mobile nutrients. Another significant function of water in plants is that it aids the plant's ability to avoid drying out. Furthermore, water is essential in processes such as nitrogen fixation.
Ques: What are some of the forms of water in soil? (3 marks)
Ans: In the soil, there are three main forms of water.
- Free water: After the rains, some water, along with some minerals, falls into the earth's lower strata due to gravity. This is referred to as gravitational or free water, and it is inaccessible to plants.
- Hygroscopic water: In addition to free water, each particle of soil absorbs some ingested water. Because the imbibition force is stronger, the plants are unable to absorb from the soil. This is referred to as hygroscopic water, which cannot be absorbed by plants from the soil.
- Capillary water: Each soil particle is surrounded by a thin film of water, which is attached to the soil by the capillary force, and is so referred to as capillary water.
Ques: Explain water potential. (4 marks)
Ans: Water potential is the amount of potential energy that water possesses.
Water potential refers to the tendency of water to escape the system.
The phrase is frequently used to describe the direction in which water will flow from one cell to the next, or from one area of the plant to the next. Water always flows from a higher-water-potential zone to a lower-water-potential region. Water's osmotic flow necessitates the completion of specific tasks. The differential in water free energy on both sides of the selectively permeable membrane is the main driving force behind this movement.
Water potential (w) is the free energy molecule for water. The pressure of water is used to determine its potential. Pascal, Pa. is the unit.
At atmospheric pressure, pure water has no water potential. As a result, under atmospheric pressure, all solutions have a lower water potential than water, implying that they have a negative value. The difference between the potential of a solution in a given state and the potential of the same solution in a standard state is stated as the water potential w.
The addition of solutes lowers the water potential. Because pure water has no water potential, all other water potential values will be negative. Water moves from a zone of higher water potential (i.e. less negative) to a region of lower water potential in osmotic or other systems (i.e. more negative).
Ques: What are the factors affecting water potential? (5 marks)
Ans: Three factors influence the water potential of any solution:
- Concentration
- Pressure
- Gravity
The following equation can be used to represent this:
ψw = ψs + ψp + ψg
ψs = effect of solutes (i.e. solute potential or osmotic potential). Solutes in a cell reduce the water potential, or free energy of water.
ψp = effect of pressure (i.e. pressure potential or hydrostatic pressure). The positive hydrostatic pressure is termed as turgor pressure.
ψg = effect of gravity (i.e. gravity potential). The influence of gravity on water potential is referred to as this term. It is determined by the water's height. The g is insignificant if the vertical height is less than five metres.
Only s and p are necessary and regarded in plant cells, i.e., ψw = ψs + ψp
According to the equation, as water comes into the cell from outside, the hydrostatic pressure, i.e. pressure potential (ψp) increases, that results in an increased water potential (ψw) of the cell, and the difference between the inside and outside ψw is lowered.
When the concentration of a solute increases in a cell, the solute potential (ψs) decreases, and consequently the water potential (ψw) decreases.
As a result of the water potential gradient, water from the outside enters the cell. If you apply pressure to the cell, the water will move out. External pressure elevates the cell's water potential (ψw), causing the difference in water potential within and outside to be large enough for water to escape. As a result, the amount of solute and the external pressure are the two main elements that influence the water potential.
Ques: Explain osmosis. (3 marks)
Ans: It's a different kind of diffusion.
It involves moving water from low concentrations to high concentrations over a semipermeable membrane. The solvent travels from a concentrated solution to a dilute solution when concentrated and dilute solutions are separated by a semipermeable membrane. Osmosis is the term for this process.
Osmosis is the process of moving a solvent from a higher water potential to a lower water potential, or from a high free energy to a lower free energy state. In isotonic solutions, osmosis does not occur. Only live cells are capable of osmosis. Osmosis continues until the hydrostatic and osmotic pressures reach a point of equilibrium. Endo-osmosis and exo-osmosis are two types of osmosis. The transfer of solvent into the cells from the surrounding environment occurs during endosmosis, and the cell becomes turgid. Exo-osmosis occurs when a solvent moves from inside the cell to outside the cell, causing the cell to become flaccid.
Ques: What is turgor pressure? (3 marks)
Ans: Turgor pressure is the hydrostatic pressure created when solvent particles enter the cell and the cell membrane pushes against the cell wall (TP).
Turgor pressure applies only to osmotic solutions.
The turgor pressure in a flaccid cell is zero, while in a turgid cell, the OP is maximal and the TP is near the minimum. Negative TP is thought to be present in plasmolyzed cells. Turgor pressure keeps the cell turgid and growing. It gives plumule the energy it needs to emerge and facilitates radicle penetration into soil during seed germination.
Ques: What is diffusion pressure deficit? (3 marks)
Ans: Diffusion pressure deficit is the difference between the pure solvent's diffusion pressure and its solution's diffusion pressure.
The osmotic parameter DPD is the reason for water entering the plant cell.
DPD determines the direction of osmosis.
Suction pressure is another term for it.
DPD stands for the difference between OP and TP.
OP-TP = DPD
OP is usually higher than turgor pressure.
Ques: What is imbibition? (3 marks)
Ans: It's a sort of diffusion in which water moves up and down a diffusion gradient.
Specific dry and half-dried materials absorb water as a result of this process.
Adsorbents are substances such as fibres, wood fragments, proteins, and sponges.
For imbibition to occur, an adsorbent is required.
Imbibition is a mechanism that allows the cell wall and protoplasm to absorb water.
The following are the two prerequisites for imbibition:
- Between the adsorbent's surface and the liquid ingested, there is a water potential gradient.
- Compatibility of the adsorbent with the ingested liquid
Imbibition is an exothermic process because heat is emitted during the process.
When it comes to dry seeds, imbibition is crucial before germination.
Water penetrates the seed initially during germination due to imbibition pressure or matrix potential (DPD=IP).
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