Cell Water Relations

Introduction

• Cell water relations explain how plant cells gain, lose, and maintain water balance.
• Governed by water potential (Ψw) and its components.
• Directly related to turgor, growth, plasmolysis, and wilting.

Key Terms

• Water Potential (Ψw): Determines direction of water movement. Water moves from higher (less negative) Ψw → lower (more negative) Ψw.
• Osmotic Potential (Ψs): Always negative. Higher solutes → more negative Ψs.
• Turgor Pressure (TP or Ψp): Positive pressure exerted by protoplast against cell wall. Maintains rigidity and growth.
• Wall Pressure (WP): Equal and opposite pressure exerted by cell wall on protoplast. At equilibrium: TP = WP.

Types of Cells Based on Water Relations

• Flaccid Cell: Normal cell placed in isotonic solution. Ψw inside = Ψw outside → no net water movement.
• Turgid Cell: Cell in hypotonic solution (Ψw outside > Ψw inside). Water enters → protoplast pushes cell wall → positive turgor pressure. Essential for rigidity, stomatal opening, and growth.
• Plasmolysed Cell: Cell in hypertonic solution (Ψw outside < Ψw inside). Water exits → protoplast shrinks away from cell wall. Example: Rheo leaf experiment in salt/sugar solution.

Plasmolysis

• Incipient Plasmolysis: Point where protoplast just begins to shrink.
• Full Plasmolysis: Protoplast completely detaches from cell wall.
• Deplasmolysis: Reversal when plasmolysed cell is placed in hypotonic solution.
• Useful in determining osmotic potential (Ψs) of cell sap.

Important Relationships

Water Potential Equation: Ψw = Ψs + Ψp (Matric potential Ψm usually negligible in single cells).

• At Incipient Plasmolysis: Ψp = 0. Hence, Ψw = Ψs
• In Fully Turgid Cell: Ψw (inside) = Ψw (outside). Net water movement = 0

Graphical Representation

Typical exam diagram (easy to sketch):

• X-axis → Osmotic potential (Ψs)
• Y-axis → Water potential (Ψw), Pressure potential (Ψp)
• Show transitions: Plasmolysis → Flaccid → Turgid

Agronomic Importance

• Wilting: Loss of turgor pressure in cells leads to drooping of leaves.
• Stomatal Movement: Guard cells gain turgor → stomata open; lose turgor → stomata close.
• Growth & Expansion: Turgor-driven cell enlargement.
• Drought Tolerance: Cells with ability for osmotic adjustment maintain turgor under stress.
• Irrigation Management: Knowledge of cell water status helps schedule irrigation.