Water potential and absorption

Water Potential

Definition: Water potential (Ψw) is the potential energy of water in a system compared to pure water, under reference conditions. It determines the direction of water movement.

Units: It is expressed in pressure units, usually megapascals (MPa).

Pure water at standard temperature and pressure has a water potential of zero.

Principle: Water always moves from a region of higher water potential (less negative) to a region of lower water potential (more negative).

Components of water potential (Ψw):

1. Solute potential (Ψs or Ψπ): Always negative; more solute → more negative value.
2. Pressure potential (Ψp): Positive when turgid, negative when under tension (e.g., xylem).
3. Matric potential (Ψm): Due to adhesion of water to surfaces (important in soils).

           So, Ψw = Ψs + Ψp + Ψm

Plant–Water Relationship (from absorption onwards)

1. Absorption of Water

• Root hairs are the main absorbing units.
• Water enters root hair cells by osmosis because their cell sap has lower (more negative) water potential than the surrounding soil solution.
• From there, water moves inward through the root cortex by three pathways:
  • Apoplast pathway (through cell walls, passive).
  • Symplast pathway (through cytoplasm via plasmodesmata).
  • Transmembrane/vacuolar pathway (through vacuoles).
• At the endodermis, Casparian strips block apoplast flow, forcing water into the symplast before it reaches the xylem.

2. Ascent of Water

• Water moves up the xylem mainly by transpiration pull (cohesion–tension theory).
• Cohesion (between water molecules) and adhesion (with xylem walls) maintain a continuous water column.
• Root pressure may also push water upwards but is minor compared to transpiration pull.

3. Transport to Tissues

• Once in the xylem, water is distributed to leaves, flowers, fruits, etc.
• Movement is governed by gradients of water potential.

4. Utilization in Leaves

• In mesophyll cells, water is used for photosynthesis, cell growth, and maintaining turgidity.
• Excess water evaporates into intercellular spaces and diffuses out through stomata (transpiration).

5. Water Potential Gradient in Whole Plant

Soil (highest Ψw) → Root hair → Cortex → Xylem → Mesophyll → Atmosphere (lowest Ψw).
This continuum of decreasing water potential drives water flow.

✅ In summary:

• Water potential explains the passive movement of water in plants.
• Water absorption begins in roots, then water ascends via xylem due to cohesion-tension, and finally moves to mesophyll where it is used or lost by transpiration.
• The entire process depends on a gradient of water potential from soil to atmosphere.