Water Relations in Soil and Plants
Water governs growth, metabolism, and survival in plants. Understanding how water is stored in soil, how it moves into and within plants, and how it supports cellular functions requires clarity on Field Capacity, Water Potential, Turgor Pressure, Diffusion Pressure Deficit (DPD/Suction Pressure), Available Soil Water, and the Permanent Wilting Point.
1) Field Capacity
Definition: Field capacity is the amount of water retained in a well-drained soil after excess gravitational water has percolated away and the downward movement has substantially slowed. It is typically reached within about 2–3 days after heavy rainfall or irrigation (actual time depends on soil texture and structure).
At saturation, both macro- and micropores are full. As drainage proceeds, water leaves the macropores, while water held by capillary and adsorptive forces remains in meso- and micropores. This retained water at field capacity is readily accessible to plant roots and coincides with a favorable air–water balance for roots and soil microbes.
Key points:
- Represents the upper limit of plant-available water in soil.
- Coarse (sandy) soils: low field capacity; Fine (clay) soils: high field capacity.
- Optimizes gas exchange, microbial activity, and root proliferation.
2) Water Potential (Ψ)
Definition: Water potential (Ψ) expresses the free energy status of water relative to pure water under standard conditions (assigned Ψ = 0). The presence of solutes, pressure/tension, and surfaces (matrices) lowers the free energy, so water potential in biological systems is generally negative.
Components of water potential:
- Solute (osmotic) potential, Ψs: Always negative; lowers Ψ as solute concentration increases.
- Pressure potential, Ψp: Hydrostatic pressure; positive in turgid cells, negative in xylem under tension.
- Matric potential, Ψm: Attraction of water to surfaces (soil particles, cell walls); usually negative.
General relationship: Ψ = Ψs + Ψp + Ψm
Direction of movement: Water moves spontaneously from higher (less negative) Ψ to lower (more negative) Ψ. This governs root water uptake and water transport through the plant body.
3) Turgor Pressure (P or Ψp)
Definition: Turgor pressure is the hydrostatic pressure of the cell sap against the cell wall that develops when a cell imbibes water osmotically. Expansion of the vacuole presses the cytoplasm against the wall; the wall resists expansion and exerts an equal and opposite wall pressure. At equilibrium: Turgor Pressure = Wall Pressure.
Physiological roles:
- Mechanical support: Maintains rigidity in non-woody tissues; loss of turgor causes wilting.
- Cell enlargement and growth: Drives irreversible cell expansion when coupled with wall loosening.
- Stomatal movement: Changes in guard-cell turgor regulate stomatal opening and closing.
4) Diffusion Pressure Deficit (DPD) / Suction Pressure
Definition: DPD (also called suction pressure) quantifies the tendency of a cell/solution to absorb water, defined as the deficit in diffusion pressure relative to pure water. In simple cellular terms:
DPD = Osmotic Pressure (OP) − Turgor Pressure (TP)
- Flaccid cell: TP ≈ 0 ⇒ DPD ≈ OP (maximum tendency to absorb water).
- Fully turgid cell: TP ≈ OP ⇒ DPD ≈ 0 (no further net water entry).
DPD helps explain water flow across tissues: water moves from regions of lower DPD (higher water potential) to regions of higher DPD (lower water potential) until equilibrium is approached.
5) Available Soil Water (ASW)
Definition: The portion of soil water that plants can readily absorb and use for growth and metabolism. Quantitatively, it is the water held in the soil between the upper and lower critical limits of plant availability.
Operational limits:
- Upper limit: Field Capacity.
- Lower limit: Permanent Wilting Point.
Thus, ASW = Water at Field Capacity − Water at Permanent Wilting Point.
Implications: Loams typically store the greatest ASW; sands store little; clays store much total water but a larger fraction is held too tightly to be plant-available. Scheduling irrigation aims to keep soil moisture within the ASW range.
6) Permanent Wilting Point (PWP)
Definition: The soil moisture content at which plants can no longer extract sufficient water to maintain turgor, resulting in permanent wilting. Even though some water remains in the soil, it is held by strong matric forces on soil colloids and is not physiologically available to roots.
Characteristics:
- Wilting is not reversed by a humid atmosphere alone; recovery requires water supply to the root zone.
- Fine-textured (clayey) soils generally have a higher PWP because of stronger water retention.
- PWP defines the lower limit of plant-available water and is fundamental for irrigation management.
Integrated Perspective: Soil–Plant–Atmosphere Continuum
Water movement is driven by gradients of water potential from soil (highest Ψ near field capacity) → root cortex → xylem (often under tension, negative Ψp) → leaf mesophyll → atmosphere (very low Ψ). Cell-level water status reflects the balance between Ψs (solute accumulation), Ψp (turgor), and Ψm (matrix effects). Agronomic practices that maintain soil near field capacity without waterlogging maximize available water and stabilize plant turgor.
Summary Table
| Concept | Definition | Key Point |
|---|---|---|
| Field Capacity | Water retained after gravitational drainage slows substantially. | Upper limit of plant-available water; good air–water balance. |
| Water Potential (Ψ) | Free energy status of water relative to pure water (Ψ = Ψs + Ψp + Ψm). | Water moves from higher (less −ve) Ψ to lower (more −ve) Ψ. |
| Turgor Pressure | Hydrostatic pressure of cell sap against the cell wall. | Maintains rigidity; drives growth; regulates stomata. |
| DPD / Suction Pressure | Tendency to absorb water; DPD = OP − TP. | High in flaccid cells; zero in fully turgid cells. |
| Available Soil Water | Water between Field Capacity and PWP. | Readily usable by plants; basis for irrigation scheduling. |
| Permanent Wilting Point | Moisture at which plants cannot regain turgor without added water. | Lower limit of plant-available water; higher in clayey soils. |
Exam-Oriented Notes (Quick Revision)
- At/near Field Capacity → optimal microbial activity, aeration, and root growth.
- Ψ becomes more negative with added solutes (↓Ψs) or increased matric binding (↓Ψm).
- Growth requires turgor + wall loosening; mere water entry without wall extensibility does not cause expansion.
- ASW is not the same as total soil water; part of total water is unavailable (below PWP).
