Unit 2

Plant Physiology

Chapter 2.1

Water Relation

Class 12 Biology – Water Relation Notes PDF

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Introduction

Water relation is an important part of plant physiology dealing with the movement, absorption, transport and loss of water in plants. The major processes studied in this chapter are diffusion, osmosis, plasmolysis, ascent of sap, transpiration and guttation.

1. Diffusion

Definition Diffusion is the spontaneous movement of molecules or ions from a region of higher concentration to a region of lower concentration as a result of their kinetic energy. The movement continues until a state of equilibrium is reached.

Diffusion is a passive process; therefore it does not require metabolic energy from the cell. Molecules move randomly, but the net movement is from the region where they are more concentrated toward the region where they are less concentrated.

Fig. 1 – Diffusion of Molecules

Diffusion Pressure

The tendency or potential ability of molecules or ions to diffuse from a region of higher concentration toward a region of lower concentration is called diffusion pressure.

Diffusion Pressure Gradient (DPG)

The difference in concentration or diffusion pressure between two regions is called the diffusion pressure gradient. A greater gradient generally produces a faster net rate of diffusion.

Fig. 2 – Diffusion Pressure Gradient

Factors Affecting the Rate of Diffusion

1. Temperature

Increase in temperature increases kinetic energy, so the rate of diffusion generally increases.

2. Size / Molecular Mass

Smaller and lighter particles usually diffuse faster than larger and heavier particles.

3. Nature of Medium

Diffusion is faster through a less dense medium and slower through a denser or more concentrated medium.

4. Diffusion Pressure Gradient

A greater difference between the two regions generally increases the rate of net diffusion.

Graham’s Law: Rate of diffusion ∝ 1 / √(density or molar mass)

Importance of Diffusion in Plants

Carbon dioxide enters leaves for photosynthesis mainly by diffusion.
Oxygen and carbon dioxide are exchanged during respiration.
Water vapour diffuses from moist leaf surfaces to the atmosphere during transpiration.
Diffusion assists movement of dissolved substances over short distances within tissues.

2. Osmosis

Definition Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution).

Essential Conditions for Osmosis

Two solutions of different concentration must be present.
A selectively permeable membrane must separate the two solutions.
A water-potential difference must exist between the two sides.

Fig. 3 – Osmosis Through a Selectively Permeable Membrane

Endosmosis and Exosmosis

Endosmosis

Movement of water into a cell or osmotic system. A plant cell placed in a hypotonic solution gains water and becomes turgid.

Exosmosis

Movement of water out of a cell or osmotic system. A plant cell placed in a hypertonic solution loses water and may become plasmolysed.

Hypotonic, Isotonic and Hypertonic Solutions

Fig. 4 – Effect of Different Solutions on a Plant Cell

Osmotic Pressure

Osmotic pressure is the pressure required to prevent the entry of water into a solution through a selectively permeable membrane. A more concentrated solution generally has a higher osmotic pressure.

Turgor Pressure and Wall Pressure

Turgor pressure (TP): pressure exerted by the cell contents against the cell wall after water enters the cell.
Wall pressure (WP): counter-pressure exerted by the cell wall on the cell contents.
In a fully turgid cell, wall pressure balances turgor pressure.

In traditional terminology: DPD = OP − TP
DPD = Diffusion Pressure Deficit, OP = Osmotic Pressure, TP = Turgor Pressure

Importance of Osmosis in Plants

Helps root hairs absorb water from the soil.
Maintains turgidity and mechanical support in soft plant parts.
Assists movement of water from cell to cell.
Contributes to stomatal movement.
Supports growth by cell enlargement.

3. Plasmolysis

Definition Plasmolysis is the shrinkage of the protoplast away from the cell wall when a living plant cell loses water by exosmosis after being placed in a hypertonic solution.

Stages of Plasmolysis

The normal cell is initially turgid or partly turgid.
When placed in a hypertonic solution, water leaves the cell by exosmosis.
The vacuole and protoplast shrink.
The plasma membrane begins to separate from the cell wall.
With further water loss, the protoplast contracts strongly and the cell becomes plasmolysed.

Fig. 5 – Plasmolysis and Deplasmolysis

Deplasmolysis

When a plasmolysed cell is placed in water or a hypotonic solution, water enters by endosmosis and the protoplast expands again. This recovery is called deplasmolysis, provided the cell has not been irreversibly damaged.

Significance of Plasmolysis

Demonstrates that the plasma membrane is selectively permeable.
Helps explain preservation of some foods by concentrated salt or sugar solutions.
Can be used experimentally to study osmotic behaviour of cells.
Excessive plasmolysis causes wilting and may injure plant tissues.

4. Ascent of Sap

Definition The upward movement of water and dissolved mineral salts from roots to the aerial parts of a plant through the xylem is called the ascent of sap.

Path of Water

Soil water → root hair → cortex → endodermis → pericycle → root xylem → stem xylem → leaf xylem → mesophyll cells.

Fig. 6 – Path of Water Through a Plant

Forces Proposed for Ascent of Sap

Root Pressure

Positive pressure generated in roots can push xylem sap upward for a limited distance. It is particularly evident when transpiration is low.

Capillarity

Adhesion of water to xylem walls and cohesion among water molecules contribute to capillary rise, but capillarity alone cannot explain water transport in tall trees.

Cohesion–Tension / Transpiration Pull Theory

The most widely accepted explanation is that evaporation of water from leaf surfaces generates negative pressure or tension in the xylem. Because water molecules cohere to one another, a continuous column of water is pulled upward from roots toward leaves.

Cohesion: attraction among water molecules maintains a continuous water column.
Adhesion: attraction between water and xylem walls helps stabilize the column.
Transpiration pull: evaporation from leaves creates the major upward pulling force.

Fig. 7 – Cohesion–Tension Mechanism

5. Transpiration

Definition Transpiration is the loss of water in the form of water vapour from the aerial parts of a living plant, mainly through stomata.

Types of Transpiration

1. Stomatal Transpiration

Water vapour is lost through stomata. It accounts for the major part of transpiration in most plants.

2. Cuticular Transpiration

Water vapour is lost directly through the cuticle and epidermal surface.

3. Lenticular Transpiration

A small amount of water vapour is lost through lenticels of woody stems.

Main Site

In most leaves, stomata are the principal adjustable pathway controlling gaseous exchange and water loss.

Fig. 8 – Stomatal Transpiration

Factors Affecting Transpiration

External Factors

Light: generally promotes stomatal opening and increases transpiration.
Temperature: higher temperature usually increases evaporation and transpiration.
Humidity: high atmospheric humidity reduces the water-vapour gradient and lowers transpiration.
Wind: moderate air movement usually increases transpiration by removing the humid boundary layer.
Soil water: water shortage causes stomatal closure and reduces transpiration.

Internal Factors

Number, distribution and opening of stomata.
Thickness of cuticle.
Leaf area and orientation.
Presence of hairs or sunken stomata.
Water status of the plant.

Importance of Transpiration

Creates transpiration pull for ascent of sap.
Helps transport mineral salts from roots to leaves.
Produces a cooling effect on the plant body.
Helps maintain continuous movement of water through the plant.
Supports distribution of water needed for photosynthesis and other metabolic processes.

Excessive Transpiration If water loss becomes greater than water absorption, cells lose turgidity and the plant may show wilting.

Wilting

Wilting is the drooping of leaves and young shoots caused by loss of cell turgor. Temporary wilting may disappear when water balance is restored, whereas prolonged severe water deficit can produce permanent injury.

6. Guttation

Definition Guttation is the loss of liquid water, often containing dissolved salts, from the tips or margins of leaves through specialized structures called hydathodes.

Guttation is commonly observed when soil moisture is high, root pressure is positive and transpiration is low, such as during humid nights or early mornings.

Fig. 9 – Guttation Through Hydathodes

Transpiration vs Guttation

Feature	Transpiration	Guttation
Form of water lost	Water vapour	Liquid water
Main structure	Mostly stomata; also cuticle and lenticels	Hydathodes
Driving force	Mainly evaporation and vapour-pressure gradient	Mainly positive root pressure
Common time	Usually stronger during daytime under favourable conditions	Often at night or early morning when transpiration is low
Dissolved salts	Generally not carried away with pure water vapour	Droplets may contain dissolved mineral salts

7. Quick Revision & Exam Points

Important Questions

Define diffusion and explain the factors affecting diffusion.
What are diffusion pressure and diffusion pressure gradient?
Define osmosis. Differentiate endosmosis and exosmosis.
Explain the effect of hypotonic, isotonic and hypertonic solutions on plant cells.
Define plasmolysis and explain its stages and significance.
What is ascent of sap? Explain the cohesion–tension theory.
Define transpiration and describe its types and factors.
Write the importance and disadvantages of transpiration.
Define guttation and explain the role of hydathodes.
Differentiate transpiration and guttation.

One-Minute Revision

Diffusion → high concentration to low concentration.
Diffusion is passive.
Higher temperature generally increases diffusion.
Osmosis involves water and a selectively permeable membrane.
Endosmosis → water enters.
Exosmosis → water leaves.
Hypotonic solution → plant cell becomes turgid.
Hypertonic solution → plasmolysis may occur.
Plasmolysis → protoplast shrinks from cell wall.
Ascent of sap occurs through xylem.
Transpiration pull is a major force for ascent of sap.
Stomatal transpiration is the major type.
High humidity usually lowers transpiration.
Guttation releases liquid water.
Hydathodes are associated with guttation.

Diagram Practice Practice drawing: diffusion, osmosis, plant cells in different solutions, plasmolysis, ascent of sap, stomatal transpiration and guttation. These diagrams are intentionally kept simple so students can reproduce them in exams.

Study note: This page is a typed, student-friendly companion to the original embedded handwritten PDF. The diagrams are redrawn as responsive SVG figures so they remain clear on phones, tablets and desktop screens.