Water Movement Through Soil–Plant–Atmosphere Continuum (SPAC)

Concept

• SPAC = continuous pathway of water flow from soil → roots → xylem → leaves → atmosphere.
• Movement is unidirectional (soil to air) and driven by water potential (Ψw) gradient.
• Atmosphere has extremely low Ψw → creates a strong suction force.

Water Potential Gradient in SPAC

Typical Ψw values:

Water moves from higher Ψw (less negative) → lower Ψw (more negative).

Steps of Water Movement in SPAC

(a) Soil → Root (Absorption): Roots absorb water mainly in the root hair zone (just behind root tip).

• Pathways of movement:
  • Apoplast: cell walls + intercellular spaces; blocked by Casparian strip.
  • Symplast: through cytoplasm and plasmodesmata.
  • Transmembrane: across cell membranes, cell-to-cell.
• At endodermis, water must pass through the symplast → selective uptake.

(b) Root → Xylem (Radial Transport): Water crosses root cortex → endodermis → pericycle → xylem. Casparian strip ensures regulation of ions and water.

(c) Xylem Transport (Long-Distance): Water moves upward in xylem vessels and tracheids.

• Main driving force: Transpiration pull.
• Explained by Cohesion–Tension Theory (Dixon & Joly, 1894):
  • Cohesion between water molecules (H-bonds).
  • Adhesion to xylem walls.
  • Tension created by transpiration in leaves.
• Root pressure contributes only at night or in small plants (guttation).

(d) Leaf → Atmosphere (Transpiration)

• In leaves, water evaporates from mesophyll cell walls into intercellular spaces.
• Diffuses out via stomata (stomatal transpiration ~90%).
• Atmosphere has very low Ψw → maintains steep gradient.

Driving Forces in SPAC

• Soil moisture tension → matric & osmotic forces in soil.
• Osmotic potential in roots → solutes lower Ψs in cells.
• Transpiration pull → evaporation lowers Ψw in mesophyll.
• Cohesion–tension mechanism → maintains continuous column.
• Atmospheric demand (VPD) → vapor pressure deficit drives diffusion.

Agricultural Significance of SPAC

• Basis of irrigation scheduling (when Ψw or RWC drops too low).
• Explains critical stages of irrigation (maximum transpiration + nutrient demand).
• Guides breeding for drought tolerance (deep roots, osmotic adjustment).
• Helps improve water use efficiency (WUE) in crops.
• Used in ET estimation models (Penman–Monteith, FAO CROPWAT).

Key Facts

• Plant water content = 70–90% of fresh weight.
• Cohesion–tension can sustain water columns up to ~120 m in tall trees.
• Root pressure: 0.1–0.3 MPa (not sufficient for tall trees).
• Relative Water Content (RWC):
  • Healthy = 80–95%.
  • Wilting = <50%.
• Transpiration ratio = 200–1000 g H₂O per g dry matter produced.
• C₄ plants (maize, sorghum) have higher WUE than C₃ plants (wheat, rice).

Facts on SPAC (Soil–Plant–Atmosphere Continuum)

• Plant body contains 70–90% water (fresh weight basis).
• Atmosphere has the lowest water potential (~ –100 to –200 MPa), creating a strong gradient.
• Soil water potential at field capacity = –0.01 to –0.03 MPa.
• Leaf water potential usually = –1.0 to –3.0 MPa.
• Cohesion between water molecules allows a continuous water column up to ~120 m height (Sequoia trees).
• Root pressure = 0.1–0.3 MPa, enough to push water only 2–3 m (important in guttation, not in tall trees).
• Relative Water Content (RWC):
  • 80% = fully turgid (healthy).
  • <50% = permanent wilting.
• Transpiration ratio = 200–1000 g water lost per g dry matter produced.
• Stomatal transpiration accounts for ~90% of total water loss in plants.
• Root hairs are the main absorbing structures (increase surface area ~20x).
• In India, agriculture uses ~80–85% of available freshwater, mostly lost through transpiration & evaporation.
• Cohesion–Tension Theory (Dixon & Joly, 1894) is the most accepted explanation of water ascent.