Water Relations in Plants

Figure 1 A close-up of watering plants

Role of water in plants

The water has following important roles in the life of plants:

1. Water acts as a universal solvent to dissolve different types of nutrients.
2. Water is a reaction medium to many bio-chemical processes occurring in plants. 
3. Water is a reactant that participates directly in many physiological processes like photosynthesis, respiration, and growth, etc. 
4. Water transports nutrients throughout the plant body. 
5. Water maintains turgidity of the cells. 
6. It helps in the mobility of gametes and dispersal of spores, fruits, and seeds. Water serves to regulate heat in plants.

Diffusion

Diffusion is the process of random movement of molecules from a region of their higher concentration to a region of lower concentration. Diffusion slowly occurs but it can be speeded up by stirring or heating the molecules. That movement occurs spontaneously and passively in all directions (see Figure 2) till it reaches equilibrium.

The pressure exerted by the diffusing ions or molecules is called diffusion pressure (DP). The diffusion pressure is proportional to the concentration of diffusing particles. Pure water has the maximum diffusion pressure. When solute particles are added the diffusion pressure lowers. Hence, the diffusion pressure a water in a solution is lower than that of pure water.

Figure 2 A experiment showing the process of diffusion in liquid medium

The maximum pressure exerted in a solution separated from pure water by a semi-permeable membrane is called an osmotic pressure (OP).

The hydrostatic pressure developed inside the turgid cell on the cell wall due to endosmosis is called turgor pressure (TP). The cell becomes turgor due to the entry of water when it is placed in hypotonic solution (pure water or in a solution of lower concentration).

The difference between diffusion pressure of pure solvent and solution is called diffusion pressure deficit (DPD).

Significance of diffusion

1. The diffusion helps in the gas exchanges during photosynthesis and respiration. 
2. The diffusion is involved in the transpiration, in which water is lost as the water vapour. 
3. As diffusion diffuses aroma of flowers, it helps in pollination by attracting insect pollinators. 
4. During passive uptake of salts, the ions are absorbed by the diffusion. 
5. Diffusion to translocate the food materials in the different parts of the plant. 
6. Diffusion spreads the ions and other substances throughout the protoplasm.   

Osmosis

The movement of water (solvent) from the region of its higher concentration to the region of its lower concentration through a semipermeable membrane is called osmosis. The semipermeable membrane is a membrane that allows the passage of water or solvent but not the passage of solutes (see Figure 3).

Figure 3 Schematic representation of the process of osmosis

The osmosis is of two types – exosmosis and endosmosis.

Exosmosis

The outward movement of water or solvent from a cell, when it is placed in a hypertonic solution is called exosmosis. A solution is hypertonic when it has higher concentration of solutes than that of a cell sap or any other solution. During exosmosis, water diffuses out from a cell and a cell becomes flaccid.

For example, when a bunch of fresh grapes is placed in a concentrated sugar solution (hypertonic), grapes shrink after a few hours due to the loss of water content. Here, the skin of grapes acts as differentially permeable membrane (see Figure 4).

Endosmosis

The entry of water or solvent into a cell when it is placed in hypotonic solution is celled endosmosis. A solution is hypotonic when it has lower concentration of solutes than that of a cell sap or any other solution. Endosmosis makes cell turgid.

For example, when some dry raisins are immersed in a dish of water, raisins become swollen after a few hours due to passage of water into them (see Figure 4).

Figure 4 An experiment showing the process of osmosis: A. endosmosis; B. exosmosis

Significance of osmosis

1. Roots absorb the water by osmosis. 
2. Osmosis makes movement of water from one cell to another. 
3. Normal growth and development in plants is possible due to the turgidity of cells by osmosis. 
4. Plant organs are rigid due to turgidity caused by osmosis. 
5. Osmosis regulates the opening and closing of stomata due to turgor changes in guard cells and assists in photosynthesis, gas exchange, transpiration, etc. 
6. Osmosis helps to control dehiscence of anthers, sporangia, and fruits, etc.

Plasmolysis

When a plant cell is placed in hypertonic solution, the protoplasm shrinks from the cell wall due to loss of water from such cell (see Figure 5). This shrinkage of protoplasm from the cell wall under the action of some strong solution is called plasmolysis.

Figure 5 Different stages of plant cells during plasmolysis

Significance of plasmolysis

1. Plasmolysis is a vital phenomenon that explains the process of osmosis. 
2. It also proves the permeability of the cell wall and semi-permeability of the cell membrane. 
3. As the plasmolysis does not occur in dead cells, it helps in determining the living and dead cell. 
4. It is also used to determine the osmotic pressure of a cell.

De-plasmolysis

The phenomenon of absorption of water by a plasmolysed cell through endosmosis to regain the original form of its protoplasm is called de-plasmolysis.

Wilting

During hot and dry weather, the individual cells of leaves and other softer parts become flaccid due to excessive loss of water from the plant organs (see Figure 6). As the turgidity is lost, shoots drop down and leaves lose rigidity is known as wilting. When the plants regain back their turgidity due to continuous absorption of water from the soil, it is known as temporary wilting. However, if the plants are unable to regain back their turgidity, it is called permanent wilting.

Figure 6 Wilting of Cucumber plants in greenhouse
Image source: Shenglian Lu, Chunjiang Zhao, Xinyu Guo, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Imbibition

The increase in the volume of solids due to the absorption of water or a liquid is called imbibition. The imbibition results in a subsequent increase in weight. The examples of imbibition are swelling of dry fruits or seeds and the tightening of window or door pane during rainy season.

Ascent of sap

Ascent of sap is defined as the upward movement of water and dissolved mineral salts from roots to the leaves and other aerial parts against the force of gravity. The ascent of sap occurs through the xylem.

Figure 7 A diagram showing ascent of sap

Mechanism of ascent of sap

Among the numbers of theories to explain ascent of sap, some of the important theories are vital force theory and physical force theory.

A. Vital force theory

According to vital force theory, the metabolic activity of the living cells is responsible for ascent of sap. Westermaier (1883-84) suggested that the force for the upward conduction of water is provided by xylem parenchyma cells and that tracheids and vessels simply act as water reservoirs.

Root pressure theory

The root pressure theory explains that the sap in the xylem is forced upwards under hydrostatic pressure i.e. root pressure developed in roots. This theory is applicable to plants like grapevines which generate considerable amount of root pressure. However, in many plants the root pressure is not observable and, in some plants, it is so low that cannot do upward translocation of sap even up to a short distance.

B. Physical force theory

According to physical force theories, the ascent of sap is purely involving physical forces. Some of the physical force theories are capillary force theory, atmospheric pressure theory, transpiration pull-cohesive force theory.

1. Capillary force theory

Christian Wolf proposed the capillary force theory in 1873. He suggests that the water rises up in the narrow tubes of xylem vessels by surface tension. However, ascent of sap in tall trees is not possible by capillary force.

2. Atmospheric pressure theory

Atmospheric pressure theory suggests that the atmospheric pressure is responsible for the ascent of sap. As water rises only up to 10.4 meters due to atmospheric pressure, this theory is not applicable to tall trees.

3. Transpiration pull-cohesive force theory

This widely accepted theory was proposed by Henry H. Dixon and John H. Jolly in 1895. It is based on the following notions:

1. There is a continuous water column right from the vein endings of the leaves down to root hairs. 
2. There is a great mutual attraction among the water molecules, called cohesive force. 
3. There is a great adhesive force between the water column and the wall of xylem vessels. 
4. The energy for the upward movement of water column is provided by transpiration from the leaf tissues. This creates a more negative water potential in the leaf cells, causing water to move from xylem ducts to these cells.

Figure 8 Mechanism of ascent of sap

Transpiration

Transpiration is defined as the process of the loss of water vapour from the internal tissues of living plants through the aerial parts such as the leaves, green stems, and flower, etc. It is measured by potometer.

Transpiration is not a purely physical process like an evaporation but is a vital phenomenon controlled and regulated by the living cells and structural peculiarities of the transpiring organs. Transpiration is a necessary evil because it is a vital phenomenon that provides a force for the ascent of sap and is unavoidable.

Figure 9 A diagram showing the transpiration in plants

On the basis plant parts where the transpiration occurs, it is of four types – stomatal, lenticular, cuticular, and bark transpiration.

Stomatal transpiration

The loss of water vapour through the stomata, which are present on the epidermal surfaces of leaves and green stem etc., is called stomatal transpiration. Stomata are the microscopic pores surrounded by the specialized guard cells (see Figure 10 & 14 ). The Stomatal transpiration accounts for more than 90% of total transpiration.

Figure 11 Stomata of Tradescantia
Image source: https://www.flickr.com/photos/yersinia/4457678322

Lenticular transpiration

 It is the transpiration in which the water vapour is lost through the lenticels. Lenticels are the areas on the bark which are filled with loosely arranged cells known as complementary cells (see Figure 12). Since lenticels have no closing mechanism, lenticular transpiration occurs continuously during day and night. Lenticular transpiration occurs in woody stems and certain fruits. Lenticular transpiration accounts for about 0.1% of total transpiration.

Figure 12 Many lenticels on a branch of Wax Myrtle
Image source: https://www.flickr.com/photos/13389908@N03/1420020650

Cuticular transpiration

The transpiration in which water vapour is lost through the cuticle is cuticular transpiration. Cuticle is a layer of wax-like covering on the epidermis of leaves and herbaceous stems (see Figure 13). Cuticular transpiration occurs during day and night. As cuticle is impervious to water, water lost through this transpiration accounts for 3-10% of the total loss of water by transpiration.

Figure 13 Vertical section of leaf showing cuticle
Image source: Wikimedia

Cuticular transpiration depends on the thickness of cuticle. When cuticle is thin, the cuticular transpiration is more. The increased thickness of cuticle causes the reduced loss of water vapour.

Bark transpiration

When the water vapour is lost through the bark, the transpiration is known as bark transpiration. The surface of the bark is generally covered by the cork. Cork is generally impermeable to water. It occurs continuously day and night. Bark transpiration accounts for about 0.5% of total transpiration.

Figure 14 A trunk of Thuja plicata showing barks
Image source: https://www.picturethisai.com/wiki/Thuja_plicata.html

Structure of stomata

Stomata are the minute pores of elliptical shape present on the epidermal surfaces of leaves, green stem, etc (see Figure 15). Each stoma is surrounded by a pair of specialized epidermal cells called guard cells. The guard cells are kidney-shaped in dicot plants and dumb-bell shaped in monocots. The guard cells control the size of stomata by changes in their turgidity.

The wall of the guard cells surrounding the pore is thickened and inelastic due to the presence of secondary layer of cellulose but rest of the wall is thin, inelastic and permeable. Each guard cell has a cytoplasmic lining and a central vacuole containing cell sap. Its cytoplasm contains a nucleus and a number of chloroplasts. The epidermal cells surrounding the guard cells are, in some species, specialized and called subsidiary cells. Subsidiary cells support in the movement of the guard cells. The stomata are the site of gaseous exchange for respiration and transpiration.

Figure 15 Structure of stomata: A. kidney-shaped stomata of dicot leaf; B. dumb-bell-shaped stomata of monocot

Factors affecting transpiration

The factors affecting transpiration are light, temperature, carbon dioxide concentration, water deficits and abscisic acid (ABA), Relative humidity, air movement, and plant factors (cuticle, transpiring area, hair, arrangement of mesophylls, number of stomata, and root/shoot ratio).

Light

•       Stomata opens in presence of light and closes in its absence.

•       The intensity of light is proportional to the rate of transpiration up to a certain limit.

Temperature

•       Increase in temperature causes an increase in stomatal opening.

•       The rate of transpiration increases with the increase in temperature.

•       However, in plants like onion and cotton, stomatal opening declines at temperature above 30 degree C; the increased temperature beyond this limit decreases the rate of transpiration.

Carbon dioxide concentration

•       An increase in CO₂ concentration over a normal level causes stomata to close at night; decreases the rate of transpiration.

•        The CO₂ present in the intercellular spaces controls the opening and closing of stomata.

•       If the closed stomata, in which the intercellular CO₂ is consumed due to photosynthesis, are exposed to sunlight, those stomata open and cause transpiration.

•       The rate of transpiration is inversely proportional to the concentration of CO₂.

Water deficit and abscisic acid (ABA)

•       During water deficits, plants are in the incipient wilting of leaves and they are called water-stressed plants.

•       During water deficit, stomata close and reduce the rate of transpiration.

•       The accumulated ABA in water stressed plants causes stomata to close.

•       When the water potential of water stressed plants is restored, then the stomata reopen and ABA disappears.

Relative humidity

•       The rate of transpiration is inversely proportional to the relative humidity.

Air movement

•       When air is stagnant, the rate transpiration is low

•       When air blows smoothly, the air current increases the rate of transpiration.

•       The wind with high velocity decreases the rate of transpiration as it causes the closing of stomata.

Plant factors

a)       Cuticle: The rate of transpiration is inversely proportional to the thickness of cuticle.

b)      Transpiring area: The size of leaf and number of leaves in plants define the transpiring area. The rate of transpiration increases with transpiring area.

c)       Hair: Hairs decrease the rate of transpiration because they insulate the leaf surface from temperature and air current.

d)      Arrangement of mesophylls:  In a leaf loosely arranged mesophylls, the rate of transpiration is higher than the one with tightly arranged.

e)      Number of stomata: An increased stomata number causes an increased rate of transpiration. However, sunken stomata decline the rate of transpiration.

a)       Root/shoot ratio: Low root surface area and increased shoot surface area causes the increasing transpiration because the water transpiring from the shoot will be more than the water absorbed by the root. 

Significance of transpiration

Transpiration has both the advantages and disadvantages.

Advantages 

1. It creates the suction force and helps in the ascent of sap. 
2. It helps in keeping the temperature of the plant low even when the plant is exposed to bright sunlight. 
3. It helps in the distribution of water throughout the plant. 
4. It helps in the rapid translocation of minerals and water through the xylem, once they enter the plant through the root hairs. 
5. It helps in evaporating excess amount of water. 
6. It affects the diffusion pressure deficit, thereby directly helping in diffusion through the cells.

Disadvantages

1. Since most of the water absorbed by plants is transpired, the transpiration involves wasteful expenditure of energy. 
2. Excessive transpiration injury to plants due to loss of water and desiccation of plant.
3. Excessive transpiration leads to the stunted growth of plants.
4. Excessive transpiration causes wilting and drying of plants.

Transpiration is necessary evil

Transpiration is a dominant process in mesophytes. About 95% of water absorbed by plants is lost by this process. It does not produce any useful influence on the plant. It produces water deficit and dehydration in plants. Still, Plants cannot avoid this process.

Anti-transpirants

The chemical substances that are used to reduce the rate of transpiration are called anti-transpirants. Anti-transpirants are sprayed on crop plants during dry season to avoid wilting when the rate of transpiration is high. Anti-transpirants are of two types – metabolic inhibitors and film foaming substances. Metabolic inhibitors include phenyl mercuric acetate (PMA) and abscisic acid (ABA) which reduce the opening of stomata. Film foaming substances include silicon emulsions which form a film on the surface of leaves.

Guttation

Guttation is the process of loss of water in the form of water droplets along with minerals (cell sap) from the tip of the leaf. It was discovered by Burgerstein in 1887. The amount of loss of water by this process is negligible in comparison to loss by transpiration. It occurs only in certain herbs. About 345 genera belonging to 115 families are reported to have this process. Garden Nasturtium, Oat and other cereals, Balsam, Tomato, Cucurbits are the best-known plants to have this phenomenon.  These plants have the water accumulated at the tip of the leaves.

Figure 16 A rose twig showing guttation on its leaves
Image source: https://www.flickr.com/photos/martinlabar/5852497916

This process is observed frequently during warm humid nights. The plants showing guttation have specialized pores, called hydathode, in the epidermal layer of leaf. So, the water droplets are deposited on margin and tips of the leaves of some grasses and dicotyledons.

Figure 17 An electron micrograph showing hydathode in Brassica leaf
Image source: https://www.flickr.com/photos/138014579@N08/36234398741/

The probable cause of guttation is the root pressure. The conditions reducing the root pressure such as cold and dry aerated soil reduce the rate of guttation. The water due to guttation contains various kinds of enzymes, sugars, amino acid, organic acids, vitamins, other organic compounds, and mineral salts.

Significance of guttation

1. It helps the plants to dispose off the unwanted solutes. 
2. It improves the acquisition of nutrients in plants. 
3. It helps in maintaining water balance for the proper growth and the development of the plant body. 
4. It helps in the progressive development of hydrostatic pressure that helps to pump water up to the leaves. 
5. Guttation fluid helps for non-invasive measurement and organic and inorganic chemical quantification.