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Water is an inorganic, transparent liquid that has no colour, taste, or smell. It is the main constituent of the earth’s hydrosphere and fluid of most living organisms. A vital component for all known forms of life.

IMPORTANCE OF WATER TO THE PLANT

• EXCELLENT SOLVENT.
• MAINTAINS TURGITY.
• UNIFORM TEMPERATURE.
• HELPS IN PHOTOSYNTHESIS AND RESPIRATION.
• TRANSPORT OF ORGANIC, INORGANIC MATERIALS, AND INTERNAL COMMUNICATION.

WATER POTENTIAL– In pure water, the difference between the free energy of the water molecule and the energy of water in any other system. It is termed as water potential. The water potential helps in determining the direction of water in which it will flow i.e from one part of the plant to another even from one cell to another. The water potential is measured in Bars. The water potential is affected by two factors- Amount of solutes and External pressure.

AVAILABILITY OF WATER IN SOIL

Soil is a combination of various minerals particles and organic matter. The small particles of soil have spaces between them through which the water percolates and reaches the groundwater table.

FIVE PRINCIPAL WATER IS AVAILABLE IN THE SOIL

• GRAVITATIONAL WATER– When water enters the soil from the surface either by irrigation or by rain it percolates through the spaces between the soil particles and reaches the water table.
• CAPILLARY WATER-AVAILABLE TO PLANTS-In minute pores of soil water against the force of gravity is held by the capillary forces. This water is readily available to the plant.
• HYGROSCOPIC WATER-The soil particles are covered by a thin film of water. This film of water is held by a strong force of attraction between soil particles and water molecules.
• RUNAWAY WATER- The major part of this form of water is rainwater. That flows on the surface.
• CHEMICALLY COMBINED WATER- Some water molecules chemically combined with soil minerals.

MEANS OF TRANSPORT IN PLANTS

The water is transported by the following means-

• DIFFUSION.
• OSMOSIS.
• PLASMOLYSIS AND DEPLASMOLYSIS.
• IMBITITION.

DIFFUSION– The movement of molecules or ions of a solute or solvent, be it a solid, liquid, or gas from the region of its higher concentration to that of its lower concentration.OR The net movement of a substance from an area of its high chemical potential to an area of lower chemical potential, as a result of the random translational kinetic motion of molecules, ions or atoms. Or The net movement of a substance from an area of its high chemical potential into the area of its lower chemical potential as a result of the random kinetic motion of molecules, ions and atoms.

DIFFUSION PRESSURE– During the process of diffusion, the diffusing ions or molecules exert pressure is called as diffusion pressure. It is directly proportional to the concentration or the number of diffusing particles.

FACTORS AFFECTING THE RATE OF DIFFUSION

• TEMPERATURE– Diffusion is directly proportional to the temperature.
• DENSITY– The rate of diffusion is inversely proportional to the square root of the density of the diffusing substance. This is also known as Graham’s Law of diffusion.
• PERMEABILITY OF MEDIUM– The rate of diffusion decreases with density of the medium.
• DIFFUSION PRESSURE GRADIENT– The rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends of a system and inversely proportional to the distance between the two.

IMPORTANCE OF DIFFUSION

• By absorption and distribution, it keeps the cell walls of the internal plant tissue moist.
• Spreading of ions and other substance throughout the protoplast.
• Transpiration, the exchange of gases between the plant and its surrounding.
• The aroma of flowers is due to the diffusion of volatile aromatic compounds to attract pollinating animals.

OSMOSIS– Diffusion of water from its pure state or dilute solution into a solution or stronger solution when two are separated by a semi-permeable membrane. Or The movement of solvent or water molecules from the region of their higher diffusion pressure or free energy to the region of their lower diffusion pressure or free energy across a semi-permeable membrane. The osmosis is of two types- Exosmosis and Endosmosis. In exosmosis, the entry of water is into the cell whereas, in endosmosis, it is the exit of water from a cell.

SEMI-PERMEABLE MEMBRANE	SELECTIVE/ DIFFERENTIALLY PERMEABLE MEMBRANE
The membrane which allows the movement of the solvent and not the solute through them.	They are also a semi-permeable membrane but allow selective passage of the solutes in the solution.

TYPES OF OSMOTIC CONCENTRATIONS

• HYPERTONIC– A solution having concentration such that it gains water or solvent by osmosis.
• HYPERTONIC– A solution having concentration such that it losses water or solvent by osmosis.
• ISOTONIC– A solution having concentration such that it neither gains nor loses water or solvent by osmosis.

OSMOTIC PRESSURE– The osmotic pressure of the solution is equivalent to the pressure which must be exerted upon it to prevent the osmosis. Or– It is a maximum amount of pressure that can be developed in a solution separated from pure water by a semi-permeable membrane is called osmotic pressure. The unit of osmotic pressure is Pascal.

REVERSE OSMOSIS– It is the process that allows removal of water molecules from various contaminants through a semi-permeable membrane. This process is also called as Hyperfilteration.

IMPORTANCE OF OSMOSIS

• The entry of soil water into the roots is carried out by osmosis.
• Cell to cell water movement.
• Living cells remains turgid only by the osmotic entry of water.
• Leaves become turgid and expanded due to their osmotic pressure.
• The resistance of the plant to drought and frost.
• Opening and closing of stomata are affected by osmosis.
• Plays an important role in growth of radicle and plumule during germination.
• The folding and droopings of leaves in Mimosa is due to osmosis.

PLASMOLYSIS– If a plant cell is placed in a hypertonic solution, water from the cell sap flows out due to exosmosis. The cell shrinks. Or The shrinkage of the protoplast of a cell from its cell wall under the influence of a hypertonic solution is called plasmolysis.

DEPLASMOLYSIS– The swelling of the plasmolysed cell when placed in a hypotonic solution or water is called as deplasmolysis.

INCIPIENT PLASMOLYSIS	EVIDENT PLASMOLYSIS
The stage at which the shrinkage of cell contents from the cell wall is detectable.	The stage when the cell wall has reached its limit of contraction and cytoplasm has detached from the cell wall attaining a spherical shape.

IMPORTANCE OF PLASMOLYSIS

• It proves that the cell membrane is permeable and the cell wall is elastic.
• Osmotic Pressure can be measured.
• It is shown by living cells.
• By salting tennis lawns, the weeds can be killed due to permanent plasmolysis.
• Salting of pickles and sweeping of jam and jellies with sugar kill the spores of the fungi and bacteria.

IMBIBITION– The imbibition is the phenomenon of adsorption. It is the absorption of water by the solid particles of a substance without forming a solution is called imbibition. The solid particles which absorb water or any other liquid are called Imbibants. The liquid which is imbibed is known as Imbibate. The imbibate is held in between and over the surface of particles of the solid substance through the process of Capillary and Adsorption. The adsorption is a surface phenomenon whereby substance in solution migrates to a surface building up to a higher concentration than in the rest of the solution.

IMBIBITION PRESSURE– The pressure that an imbibant develops when submerged in pure imbibing liquid is called imbibition pressure. The new terminology for imbibition pressure is Matric Potential.

IMPORTANCE OF IMBIBITION

• Absorption of water.
• Absorption of water by germinating seed in through imbibition.
• The breaking of the seed coat is by inhibition.

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