Leaf Diffusive Resistance (LDR)

Definition; Leaf Diffusive Resistance (LDR) refers to the resistance offered by the leaf to the diffusion of water vapor and gases (mainly CO₂ and O₂) through the stomata and leaf boundary layers.

• It is the reciprocal of leaf diffusive conductance (stomatal conductance).
• R =1/ gr Where:

• • r = resistance (s cm⁻¹ or s m⁻¹)
  • g = conductance

Components of Leaf Resistance

Total leaf resistance is the sum of three resistances:

• Stomatal resistance (rs): Resistance to diffusion through the stomatal pore. The most important and dynamic component (regulated by guard cells).
• Cuticular resistance (rc): Resistance to diffusion through the cuticle. Generally very high; cuticular transpiration is usually < 10% of total transpiration.
• Boundary layer resistance (rb): Resistance due to still air layer surrounding the leaf. Depends on wind speed, leaf size, and leaf shape.

Thus: rleaf = rs + rc + rbr

Factors Affecting LDR

• Stomatal behavior:
  • Light (blue light promotes opening)
  • CO₂ concentration (high CO₂ → closure)
  • Water status (low Ψw → closure, ↑ resistance)
  • Abscisic acid (ABA) → induces closure under drought
• Environmental conditions: Temperature, humidity, vapor pressure deficit (VPD), wind speed. Higher wind → reduces boundary layer resistance
• Leaf characteristics: Leaf size, thickness, waxy cuticle, pubescence

 Points to br remember

Plant Water Potential

• Term introduced by: Slatyer & Taylor (1960).
• Units: Megapascal (MPa) or bar (1 MPa ≈ 10 bar).
• Pure water at standard state → Ψw = 0.
• In most plants, leaf Ψw under normal conditions = –0.2 to –1.5 MPa.
• During drought stress → Ψw may fall to –3 to –5 MPa.
• Soil Ψw:
  • Field capacity: around –0.03 MPa
  • Permanent wilting point: –1.5 MPa

Components of Water Potential

• Solute potential (Ψs): Always negative. Approx. Ψs of mesophyll cell sap = –0.5 to –1.5 MPa.
• Pressure potential (Ψp): Can be positive (turgid cells) or negative (xylem under tension). In xylem during transpiration: –0.5 to –2.0 MPa.
• Matric potential (Ψm): Highly significant in soils. Water tightly adsorbed in soil colloids has Ψm ≈ –10 MPa (unavailable to plants).

Cell Water Relations

• Incipient plasmolysis: Occurs when Ψp = 0 → used to determine Ψs of cell sap.
• Plasmolysis experiment: First demonstrated in Tradescantia leaf epidermal cells.
• Wilting types: Temporary wilting: Midday stress, recovers at night. Permanent wilting: Occurs when soil Ψw ≤ –1.5 MPa.
• Turgor pressure in healthy cells: Around 0.5 to 1.0 MPa.

Osmotic Adjustment (OA)

• First reported: In barley under drought (Jones & Turner, 1978).
• Magnitude of OA: Sorghum, barley, chickpea: 1.0–2.0 MPa. Wheat: 0.5–1.0 MPa Rice, maize: <0.5 MPa (low OA capacity).
• Compatible solutes:
  • Proline: accumulates up to 100-fold during drought.
  • Glycine betaine: common in sugar beet, barley, wheat.
  • Mannitol/sorbitol: in celery, apple, pear.
  • Sucrose: major osmoticum in maize, sorghum.
• Breeding relevance: OA is a selection criterion in dryland crops (ICAR dryland breeding programs).
• Agronomy fact: Crops with high OA maintain yield under drought better than non-accumulators.