Long-Distance Water Transport — Complete Pathway

Three Mechanisms of Water Ascent (in order of importance):

1. Transpiration Pull (Cohesion-Tension Theory — Dixon & Joly): PRIMARY mechanism

   • Transpiration creates negative pressure (tension) in leaf xylem
   • Cohesion (H-bonds) maintains unbroken water column
   • Adhesion (water-to-xylem wall) prevents detachment
   • Dominant mechanism for all tall plants

2. Root Pressure: SECONDARY mechanism

   • Active mineral absorption lowers root Ψw → water enters osmotically
   • Generates positive pressure (1-2 atm) in xylem
   • Insufficient alone for tall trees
   • Responsible for guttation

3. Capillary Action: MINIMAL contribution

   • Surface tension of water in narrow xylem vessels
   • Maximum height: ~1 m in typical xylem