In this article, we will discuss in detail about Plant Water Relation. Independent of the species, plants obtain water from the soil that overcomes the metabolic necessities of plants. Therefore a large amount of water is continuously absorbed from the soil and moved through the plant. We all know that root hairs remain in direct contact with soil water and absorb water and minerals from the soil.

The water is further transported to the stem, leaves, and other parts of the plant either by utilising energy (active transport) or without the expenditure of energy (passive transport). Have you ever thought about what are the internal factors that cause the cell to cell absorption and movement of water inside the plant tissue? Let’s study the major physicochemical phenomena of Plant-Water Relations to know the cell-to-cell movement of water in plants. Continue reading to know more.

Water Potential Definition

All living organisms require energy for growth, to maintain metabolism, and to reproduce. As the water molecules possess kinetic energy, they are in random motion (in liquid and gaseous states), which is rapid and constant.

Thus, water potential is defined as the difference between the energy of water molecules in the pure form and the energy of water in a particular system (in a plant cell) at the same atmospheric pressure and temperature. It is denoted by Psi and measured in Pascal.

Components of Water Potential

The water potential of a living cell has three main components, namely solute potential, pressure potential, and matric potential. Water potential is the sum of all three potentials.

Water Potential\({\rm{ = }}{{\rm{\psi }}_s} + {{\rm{\psi }}_p} + {{\rm{\psi }}_g}\)

Solute potential: It is also known as osmotic potential. It is defined as the amount by which the water potential is reduced as a result of the presence of the solute. The solute potential is proportional to the number of dissolved solute particles. These particles interact with each other and reduce the activity of water molecules, and thereby decrease the potential energy of the water. Thus, adding solute always lowers the water potential. The solute potential is always in negative value.

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Pressure Potential: It is also known as hydrostatic pressure. The pressure potential is physical pressure and may be positive or negative. If hydrostatic pressure is positive, the water potential will be raised, and if the hydrostatic pressure is negative, the water potential will be reduced. The positive hydrostatic pressure is called turgor pressure.  The water potential for pure water is zero.

Matric Potential: It is also known as gravity potential. It indicates the effect of gravity on water potential. It is a component of water potential due to the adhesion of water molecules to non-dissolved structures of the system, i.e. the matrix and plasma membrane. It depends on the factors like height of the water, the density of water, and acceleration due to gravity. If the vertical distance is smaller (less than 5 meters), the matric potential is negligible and therefore ignored. Thus, the equation can be simplified as:

Water Potential\({\rm{ = }}{{\rm{\psi }}_s} + {{\rm{\psi }}_p}\)

Diffusion

Diffusion is defined as the process of random movement of individual molecules from a region of higher concentration to a region of lower concentration.
(i) The rate of diffusion depends on the difference in the concentration of matter. Larger the difference in concentration, the more rapid the process of diffusion. Diffusion is more rapid in gases than liquids. At the state of equilibrium, there is no further movement of molecules.
(ii) In diffusion, the movement of molecules is random and independent of each other. Hence many gases and solutes diffuse simultaneously and independently at different rates.

Examples of Diffusion: Diffusion of sugar and salt molecules in water, diffusion of the fragrance of perfume in the room, etc., are examples of diffusion.

Role of Diffusion in Plant-Life

(i) It helps in the exchange of oxygen and carbon dioxide during photosynthesis and respiration.
(ii) The water vapour diffuses into the atmosphere during transpiration.
(iii) The passive uptake of ions by plant roots occurs through the simple process of diffusion.

Factors Affecting Diffusion

The diffusion rate is affected by the concentration gradient, the permeability of the membrane separating the solute and the solvent, temperature, and pressure. Though diffusion is a slow process, the rate of diffusion is faster if:

(i) The concentration gradient between the two regions is increased.
(ii) The distance between the two regions has increased.
(iii) The area over which diffusion occurs is increased.
(iv) The molecules diffusing are small and fat-soluble.
(v) Numerous and larger size pores in the cell membrane.

Facilitated Diffusion

It is the process of movement of molecules or ions across biological membranes facilitated by membrane proteins without the involvement of metabolic energy.
(i) The rate of facilitated diffusion depends on the size of the substances. The smaller the substance, the faster it will diffuse.
(ii) The facilitated diffusion does not allow the net movement of molecules from lower to higher concentrations. The rate of transport finally reaches the maximum when all transport proteins are utilised.

Diffusion Pressure Deficit (DPD)

The diffusing particles exert a pressure called diffusion pressure that is directly proportional to the concentration of the diffusing particles. The difference in the diffusion pressure of pure water and water in a specific solution is said to be diffusion pressure deficit. The value of the diffusion pressure deficit of a system is equal to its osmotic pressure minus any forces which oppose the osmotic entry of water into the system.

\({\rm{DPD}} = {\rm{OP}} – {\rm{WP}}\left( {{\rm{TP}}} \right)\)

Diffusion pressure deficit determines the direction of the net movement of water. It is always from a cell of lower DPD to a cell of higher DPD.
Diffusion and facilitated diffusion facilitate the short distance transport of water, nutrients, gases, etc., through passive transport.

Permeability

Permeability is defined as the extent to which a membrane either allows or restricts the movement of substances. Based on the permeability, the membrane is of three types:
(i) Permeable membrane: It allows the movement of water and solutes into the interior and exterior of the cell. For example, the cell wall of the plant cell.
(ii) Impermeable membrane: Such membranes prevent the diffusion of water and solutes into the protoplasm of the cell—for example, suberised walls of cork cells, cuticle layer of the leaf.
(iii) Semipermeable membrane: It is almost impermeable to the solute molecules while permeable to the solvent, for example, egg membrane, animal bladder, parchment membrane.
(iv) Selectively permeable membrane:  It allows selective passage of the solute. It is also called a differentially permeable membrane, for example, plasma membrane, tonoplast.

Osmosis

The movement of water or solvent molecules from a region of lower concentration to a region of higher concentration across a semipermeable membrane is called osmosis. The movement of water or solvent molecules continues to occur across the membrane until the equilibrium is reached. It can be said that equilibrium water potential on both sides is balanced. The Discovery of osmosis was made by Pfeffer, and the term was coined by Abbe Nollet.

The Funnel Experiment Demonstrating Osmosis

The phenomenon of osmosis can be easily demonstrated by using an animal membrane such as a fish bladder membrane or egg membrane.
(i) The membrane is tied over the mouth of a thistle funnel with a long stem.
(ii) The funnel is filled with the concentrated sugar solution, with the stem of the funnel partially empty.
(iii) The funnel is then inverted in the beaker filled with distilled water.
(iv) It has been observed that the level of the solution within the stem of the funnel will go up steadily.
(v) The rise in the level is due to the entry of water molecules into the sugar solution in the funnel through the semipermeable membrane by osmosis.
(vi) After some time, the level sugar solution becomes static as there is no more entry of water molecules inside. This is due to the fact that a semipermeable egg or fish bladder membrane allows only the movement of solvent molecules in the thistle funnel.

Types of Osmosis – A Living Cell as an Osmotic System

The cell exhibits two types of osmosis based on the nature of the solution in which the cell is placed. These can be discussed as follows:
(i) Osmotic entry of water into the cell or system when a cell is placed in the pure water or hypotonic solution is called exosmosis. Due to exosmosis, the cell becomes turgid.
(ii) Osmotic withdrawal of water from the cell or system when a cell is placed in the hypertonic solution is called endosmosis. The endosmosis leads to the flaccidity of the cell.

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Plant-Water Relation with Osmosis

(i) Osmosis helps the liquid to move across the biological membrane.
(ii) The phenomenon of endosmosis helps to maintain the turgidity of the cell.
(iii) It plays a vital role in the stomatal movement during transpiration.
(iv) It also affects the absorption of water by roots.

Osmotic pressure

It is the maximum amount of hydrostatic pressure which can be developed in a solution when it is separated from pure water by a semipermeable membrane. The osmotic pressure of the solution depends on its concentration. The more the concentration of solute in the solution, the greater the osmotic pressure of the solution.

Factors Affecting Osmotic Pressure

The osmotic pressure of the solution is affected by the following factors:
(i) The osmotic pressure of the solution depends on its concentration. The higher the concentration, the greater the osmotic pressure, and this allows the entry of a larger amount of solvent by osmosis.
(ii) An increase in temperature and an increase in ionisation of solute molecules also increases osmotic pressure.

Osmotic Potential

Osmotic potential is the potential of the solution that causes the water movement across the semipermeable membrane as a result of dissolved solutes. It is also called solute potential. The osmotic pressure and osmotic potential of pure water are zero. The value of osmotic pressure increases due to the addition of solute particles, whereas the addition of solute particles makes the value of osmotic potential more negative.

\({{\rm{\psi }}_s} = – \pi \)

When the additional pressure is applied during the process of osmosis (more than the osmotic pressure to check the entry of water into the solution), then the water can be made to move out of the solution into the water in the beaker. This process is called reverse osmosis. It can be used to remove salt from the saline water.

Turgor Pressure

When the plant cell is placed in pure water, the water moves into the cell and creates a hydrostatic pressure called turgor pressure that causes the plasmalemma to be pressed against the cell wall. It can be said that the hydrostatic pressure developed inside the cell on the cell wall due to endosmosis is called turgor pressure.

Importance of turgor pressure

Turgor pressure keeps the herbaceous plants upright and supports the fleshy stalk and leaves of shrubs and trees. Changes in turgor pressure also cause movements in plants, such as the opening and closing of stomata.

Wall Pressure

The counter-pressure exerted by the wall over the swelling protoplast is known as wall pressure. The wall pressure is equal to the turgor pressure except in plasmolysed cells. The equilibrium between turgor pressure and wall pressure is required to keep the cell in a complete turgid state. In case the turgor pressure is more than the wall pressure, the cell will burst.

Plasmolysis

Plasmolysis is the condition that occurs when the water tends to move out of the cell, and the cell membrane of the plant cell shrinks away from its cell wall due to the shrinkage of the vacuole and the protoplast. Plasmolysis takes place when the cell is placed in the hypertonic solution. Plasmolysed cell is a state of the cell due to exosmosis.

Types of Plasmolysis

Plasmolysis can be categorised into the following two types:
(i) Incipient plasmolysis: It is a stage of plasmolysis at which the first sign of shrinkage of the cell content from the cell wall is identified. Here, the cell wall exerts no pressure on the cell content. Hence the pressure potential is zero, and the water potential of the cell is equal to the solute potential.
(ii) Evident plasmolysis: It is the stage of plasmolysis at which the protoplasm reaches the limit of concentration, and the cytoplasm, along with the plasma membrane, gets detached from the cell wall and turns spherical in shape due to exosmosis.
In the plasmolysed cell, the pressure potential is zero, and the cell wall is not attached to the protoplasm.

Significance of plasmolysis

(i) Plasmolysis is often being observed in the living cell. Therefore helps to determine whether the cell is living or dead.
(ii) It helps in determining the osmotic pressure of the plant.
(iii) It ensures that the cell wall is elastic and permeable.
(iv) It is dominant in the initial stage of water absorption by roots.

Deplasmolysis

If the plasmolysed cell is placed in the water, the process of endosmosis takes place. Water enters the cell sap, the cell sap becomes turgid, and the protoplasm again assumes its normal shape. This phenomenon is called deplasmolysis. It occurs due to the phenomenon of endosmosis.
Deplasmolysis should be followed immediately after plasmolysis; otherwise, the cell protoplast becomes permanently damaged.

Imbibition

The process of absorption of water by the solid particles of a substance without forming the solution is called imbibition. It is a special type of diffusion in which the water is absorbed by the colloids of the solution along with the concentration gradient. The particles that absorb the water are called imbibants. The liquid which is absorbed is called imbibate. The following  two conditions are necessary for imbibition:
(i) A water potential gradient must exist between the surface of the absorbent and the liquid imbibed.
(ii) A certain degree of affinity between the absorbent and the imbibed liquid.

Interrelationship of DPD, TP, and OP

The difference in the concentration of solutions on two sides of differentially permeable membrane results in the flow of water from the solution of lower concentration to the solution of higher concentration. As the water enters the cell, the turgor pressure of the cell increases simultaneously, and the wall pressure also increases in equal magnitude but in opposite direction. Therefore the actual force responsible for the water into the cell will be, therefore OP – TP
i.e., DPD = OP – WP (As WP = TP)
DPD = OP – TP
In the flaccid cell, TP is zero, therefore,
DPD = OP – 0 = OP
As a result, the water will enter the cell with an equal force to the OP of the cell.
In the turgid cell, OP = TP
i.e., DPD = OP – WP,
Therefore, DPD = zero
As a result, there will be no absorption of water by a fully turgid cell.

When the two aqueous solutions having different osmotic pressure are separated by a semipermeable membrane, their DPD will tend to equate. Cell to cell movement of water depends upon the DPD and not on osmotic pressure and turgor pressure. This can be explained as follows:

Imbibition pressure

The pressure developed by an imbibant when submerged in the pure imbibing liquid is called imbibition pressure. Due to imbibition pressure, the seed absorbs more water that causes the seedling to emerge from the soil and grow.
Dry seeds have high negative water potential. When the dry seeds are placed in water, a steep water potential gradient is established. This allows the absorption of water to the surface of the dry seeds. The imbibition of water into seeds continues until equilibrium. Maximum imbibition is found in Agar.

Summary

Water is the basic necessity of all the living beings on the earth. Like animals, plants also require water for several metabolic activities. Water in plants also maintains the cell shape, regulates the opening and closing of stomata, initiates the germination of seeds, etc. Plants fulfil their water requirement by absorbing the soil water through the root hairs. The water absorbed by the root hairs is transported to different parts of plants. There are certain internal and environmental factors that support the absorption and cell-to-cell movement of water inside the plant tissue. These factors showing plant-water relationships mainly include water potential, diffusion, osmosis, turgor pressure, wall pressure, plasmolysis, deplasmolysis, and imbibition.

Frequently Asked Questions

Q.1. What is osmosis?
Ans: The movement of water or solvent molecules from a region of higher concentration to a region of lower concentration through a semipermeable membrane is called osmosis.

Q.2. What are the two types of diffusion?
Ans: Simple diffusion and facilitated diffusion are the two types of diffusion.

Q.3. What is imbibition?
Ans: Imbibition is the absorption of water by the solid particles of the solution through its general surface.

Q.4. What is water potential in plants?
Ans: The water potential is the measure of the potential energy of water in a particular system or cell.

Q.5. What is plasmolysis in the plant cell?
Ans: Plasmolysis is the shrinkage of protoplasm due to the loss of water from the cell. It happens if the cell is placed in the hypertonic solution.

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We hope this article on Plant Water Relations helps you in your preparation. Do drop in your queries in the comments section if you get stuck and we will get back to you at the earliest.