Definition of Water Relations

• Water relations refer to the absorption, movement, and loss of water within plants and its relation to physiological processes.
• It involves water potential, osmotic potential, turgor pressure, diffusion, osmosis, and transpiration.

Importance of Water in Plants

• Constituent of protoplasm (70–90%).
• Medium for biochemical reactions.
• Required for photosynthesis (source of electrons in photolysis).
• Maintains cell turgidity for growth.
• Acts as transport medium for minerals and organic solutes.
• Regulates temperature through transpiration.

WATER POTENTIAL (Ψw)

Concept

• Water potential (Ψw) is the potential energy of water compared to pure water.
• Term “Water potential” introduced by Slatyer and Taylor (1960).
• Units: Megapascal (MPa) or bars (1 MPa = 10 bars).
• When Ψw of a cell = Ψw of solution → no net movement of water.
• Water moves from higher Ψw to lower Ψw.
• Pure water has Ψw = 0.

Equation: Ψw = Ψs + Ψp

where,

• Ψw = Water potential
• Ψs = Solute potential (always negative)
• Ψp = Pressure potential (usually positive)

Diffusion

• The passive movement of molecules from high to low concentration.
• Does not require energy (ATP).
• Example: movement of gases (CO₂, O₂) and water vapor in leaves.
• Term “Diffusion” given by Thomas Graham (1829).
• Fick’s law explains the rate of diffusion.

Osmosis

• Movement of water molecules through a semipermeable membrane from low solute to high solute concentration.
• The driving force is osmotic gradient.
• Osmosis continues until equilibrium is achieved.

• Term “Osmosis” coined by De Vries (1877).
• The semipermeable membrane is often the plasma membrane or tonoplast.
• Reverse osmosis (RO) – external pressure applied to move water opposite to osmotic flow.

Types of Osmosis

1. Endosmosis: Water enters the cell (cell swells).
2. Exosmosis: Water leaves the cell (cell shrinks).

Plasmolysis

• Shrinkage of protoplasm from the cell wall due to loss of water.
• Occurs when cells are placed in hypertonic solution.
• Deplasmolysis is the reverse process.
• ·First demonstrated by Wilhelm Pfeffer (1877) using Tradescantia leaf cells.
• ·Plasmolysis stages: Incipient → evident → complete plasmolysis.

Turgor Pressure (TP)

• The pressure exerted by the cell contents against the cell wall.
• Maintains cell rigidity and supports herbaceous plants.
• TP is positive pressure, increases with water uptake.
• Maintains opening and closing of stomata via guard cells.

Osmotic Pressure (OP)

• The pressure required to stop osmosis.
• It increases with solute concentration.
• Measured using Pfeffer’s osmometer.
• Van’t Hoff’s law: OP = CRT
• where, C = concentration, R = gas constant, T = absolute temperature.

Relationship Between Potentials; TP = OP – DP where DP = Diffusion Pressure Deficit.

WATER ABSORPTION

Absorbing Organ

• Root hairs (unicellular extensions of epidermal cells).
• Maximum absorption occurs in root hair zone (just above root tip).
• Absorption mainly by young roots (older roots are suberized).

Pathways of Water Movement

1. Apoplast Pathway: Through cell walls and intercellular spaces (non-living route). Dominant in cortex region. Blocked by Casparian strip in endodermis.
2. Symplast Pathway: Through cytoplasm via plasmodesmata (living route). Water crosses plasma membrane once.
3. Transmembrane Pathway: Water moves across cell membranes and vacuoles Common in all tissues.

FORCES RESPONSIBLE FOR WATER MOVEMENT

• Osmotic and imbibition forces in roots.
• Cohesion (attraction between water molecules).
• Adhesion (attraction between water and cell walls).
• Surface tension – water column continuity.
• Transpiration pull – creates negative pressure at leaf end → upward movement of water.
• Cohesion-Tension theory proposed by Dixon and Joly (1894).
• Root pressure: positive pressure developed in xylem due to osmotic absorption (e.g., guttation).
• Imbibition: absorption of water by colloidal particles (like cell wall, seeds, dry wood).

Summary Points:

• Water potential concept → Slatyer & Taylor (1960)
• Plasmolysis → Wilhelm Pfeffer (1877)
• Osmosis → De Vries (1877)
• Cohesion-tension theory → Dixon & Joly (1894)
• Root pressure mainly occurs in herbaceous plants at night.
• Guttation occurs through hydathodes.
• Imbibition is a physical adsorption process.
• Highest water potential = pure water (0 MPa).
• Lowest water potential = highly concentrated solution (very negative).
• Root hair zone = zone of maximum absorption.
• Casparian strip made of suberin (blocks apoplast pathway).
• Turgor pressure is necessary for growth and stomatal movement.