6.1 Plant & Water Relationship

Water is often referred to as life because life cannot exist without it. Protoplasm, the physical basis of life, consists of approximately 90% water. This is why water is known as the “fluid of life.” Without water, protoplasm becomes constricted and may die. Additionally, all the metabolic reactions in plants cease without sufficient water.

Importance of Water in Plants

1. Maintaining Protoplasm:

• Water is irreplaceable in maintaining the living nature of protoplasm.

• To prevent protoplasm from becoming constricted, an adequate water supply is essential.

2. Transpiration and Photosynthesis:

• Water is critical for transpiration and photosynthesis.

• In the dry season, irrigation ensures that large plants receive the necessary water.

3. Solvent for Metabolic Reactions:

• Water acts as an important solvent, facilitating numerous metabolic reactions.

4. Cellular Growth and Movement:

• Water plays vital roles in plant cell growth and movement.

Water Absorption in Plants

Plants absorb water primarily through their roots from the soil. This process involves three mechanisms: imbibition, diffusion, and osmosis.

1) Imbibition

• Definition: The absorption of water by dry or semi-dry colloidal substances.

• Example: A dry piece of wood absorbs water when placed in it.

• Mechanism:

• Hydrophilic substances like cellulose, starch, and gelatin absorb water.

• These substances swell when water is absorbed and shrink when water is lost.

• Since cell walls and protoplasm are colloidal in nature, they absorb water and swell.

• Significance: Imbibition is a crucial process for water absorption in plants.

2) Diffusion

• Definition: The movement of molecules from a region of higher concentration to a region of lower concentration.

• Examples:

• Fragrance from incense spreading in a room.

• Sugar dissolving in water and making it sweet.

• Key Terms:

• Diffusion Pressure: The potential energy of a substance to diffuse from a higher concentration to a lower concentration at constant temperature and pressure.

• Diffusion Pressure Deficit (DPD): The difference between the diffusion pressure of a solution and that of a solvent.

• Role in Plants:

• Due to the diffusion pressure deficit in mesophyll tissues, water-deficient cells absorb water from adjacent cells.

• Diffusion plays a significant role in water absorption in plants.

3) Osmosis

• Definition: The movement of a solvent (water) through a selectively permeable membrane from a solution of lower solute concentration to one of higher solute concentration.

• Examples:

• Raisins swelling in water due to osmosis.

• Swollen raisins shrinking when placed in a concentrated sugar solution.

• Mechanism:

• Two solutions with the same solute and solvent but different concentrations are separated by a selectively permeable membrane.

• The solvent flows from the solution with a lower solute concentration (higher water content) to the one with a higher solute concentration (lower water content).

• Significance:

• Osmosis allows plants to absorb water from the soil.

• It is a natural phenomenon essential for plant survival.

In osmosis:

• Solvent moves from high solvent density (low solute concentration) to low solvent density (high solute concentration).

• Solute does not cross the selectively permeable membrane.

These processes—imbibition, diffusion, and osmosis—work together to ensure plants absorb and utilize water effectively for survival and growth.