ABSORPTION OF WATER IN PLANTS
Water absorption is the entry of water from soil into roots and its movement to the xylem. It sustains cell turgor, photosynthesis, nutrient transport, growth, and temperature regulation. The root system—especially the thin-walled root hairs—is the primary interface with soil water.
MODES OF WATER ABSORPTION
Plants use two principal modes:
- Passive absorption – Driven by transpiration pull; roots mainly serve as conduits. Dominant under normal field conditions.
- Active absorption – Depends on root metabolism (ATP), ion uptake and osmotic adjustment; important when transpiration is low.
PATHWAYS WITHIN ROOT
- Apoplast: through cell walls and intercellular spaces up to the endodermis (blocked by Casparian strip).
- Symplast: through the cytoplasm via plasmodesmata after crossing a membrane once.
- Transmembrane: repeated crossings of membranes/tonoplasts from cell to cell.
PASSIVE ABSORPTION (SEQUENCE & FEATURES)
Definition: Water entry into roots due to transpiration pull generated in leaves; little direct energy expenditure by roots.
Proper Sequence of Events:
- Evaporation at leaf mesophyll lowers water potential in leaf cell walls.
- Tension in xylem develops and extends as a continuous negative pressure (cohesion–tension).
- Pull transmitted downward through an unbroken water column in xylem to the root stele.
- Water potential gradient forms: soil solution > root epidermis > cortex > endodermis > pericycle/xylem.
- Water enters root hairs osmotically and moves apoplastically/symplastically to xylem.
- Bulk flow in xylem carries water upward to the transpiring leaves.
Key Characteristics:
- Rate tightly coupled to transpiration (light, VPD, wind).
- Minimal dependence on root respiration; can proceed when root metabolism is reduced (to a limit).
- Accounts for the major share of daily water uptake in most crops.
ACTIVE ABSORPTION (SEQUENCE & FEATURES)
Definition: Metabolism-dependent water uptake by roots, usually via ion accumulation that lowers root water potential and/or via positive root pressure.
Proper Sequence of Events (Osmotic type):
- Root respiration produces ATP.
- Active ion uptake into epidermal/cortical cells increases cellular solute concentration.
- Root cell water potential becomes more negative than soil solution.
- Osmosis into root hairs occurs, increasing xylem sap pressure.
- Root pressure develops (notable at night/low transpiration) and pushes water upward.
Proper Sequence of Events (Non-osmotic type):
- Membrane carriers and ATP-driven pumps directly facilitate water entry against small gradients.
- Water crosses membranes repeatedly (aquaporins may assist) into stele and xylem.
Key Characteristics:
- Important in humid, cool nights, waterlogged or low-transpiration conditions.
- Manifestations include guttation and exudation from cut stems.
- Strongly inhibited by respiratory poisons or oxygen deficiency.
ROLE OF ROOT HAIRS IN WATER ABSORPTION
- Enormous surface area: Thousands per mm of root; intimate contact with moist soil micro-pores.
- Short diffusion path: Thin primary walls (cellulose/pectin), minimal barrier to water entry.
- High osmoticum: Solutes in root-hair cytoplasm favor water influx.
- Mucilage secretion: Improves adhesion to soil particles, maintains a moist rhizosphere film.
- Selective uptake sites: Ion transporters/aquaporins modulate water and mineral flow to cortex and stele.
- Turnover and plasticity: New hairs form near the growing tip to track fresh, less-depleted soil volumes.
FACTORS AFFECTING WATER ABSORPTION
External (Soil–Atmosphere)
- Soil water content: Maximal near field capacity; too dry → steep resistance; too wet → anoxia.
- Soil aeration: Oxygen supports respiration and membrane transport; compaction/waterlogging reduces uptake.
- Soil temperature: Low temp slows diffusion and metabolism; very high temp damages membranes and proteins.
- Soil solution concentration (salinity): High salts lower soil water potential → physiological drought.
- Soil texture & structure: Fine textures hold more water but may have poor aeration; structure controls pore continuity.
- Atmospheric demand: High VPD, wind, light → ↑ transpiration → ↑ passive uptake (if soil supply is adequate).
Internal (Plant)
- Root system size & architecture: Deeper/branched roots explore larger soil volumes.
- Root hairs: Density, length, and longevity directly raise absorption capacity.
- Endodermis/Casparian strip: Forces symplastic entry, maintaining selectivity and preventing apoplastic backflow.
- Transpiration rate: Sets the strength of the xylem tension (passive uptake).
- Metabolic activity: Respiration fuels active ion pumping and aquaporin regulation.
- Aquaporin gating & hormones: ABA, pH, Ca²⁺, and phosphorylation rapidly alter membrane water permeability.
COMPARISON: PASSIVE VS ACTIVE ABSORPTION
| Parameter | Passive Absorption | Active Absorption |
| Primary driver | Transpiration pull (xylem tension) | Root metabolism (ATP), ion accumulation |
| Root energy requirement | Low/indirect | High/direct |
| Dominance | Major under normal field conditions | Notable in low-transpiration conditions |
| Typical indicators | Strong daytime flow; linked to stomatal status | Night exudation, guttation, measurable root pressure |
| Sensitivity | Atmospheric demand, soil hydraulic conductivity | O₂ availability, temperature, metabolic inhibitors |
| Path emphasis | Bulk flow through xylem; apoplast significant | Membrane transport; symplast/transmembrane emphasized |
FLOW SEQUENCES (AT-A-GLANCE)
Passive Absorption:
Leaf evaporation ↓ψleaf → xylem tension → tension transmitted to root xylem → ψ gradient soil→epidermis→cortex→stele → water entry via root hairs → bulk flow to shoot.
Active Absorption:
Root respiration → active ion uptake → root cell ψ becomes more negative → osmotic inflow at root hairs → root pressure development → upward push (not sufficient for tall trees alone).
PRACTICAL SIGNIFICANCE
- Irrigation timing: Aim near field capacity; avoid prolonged waterlogging to preserve aera
