Theories of water translocation:
The translocation of water is explained by various theories:
1. Vital force theories
2. Root pressure theory
3. Capillary force theory
4. Cohesion force theory
Root pressure theory:
Root pressure, once believed to drive water up plant stems, faces objections. The force is weak (max 2 atm), insufficient for tall trees. Seasonal changes and water ascent without root pressure challenge its significance. Gymnosperms, despite lacking root pressure, still grow tall.
Capillary for theory:
Water rises in xylem vessels due to capillarity, where adhesion initially pulls water against the vessel walls. As it ascends, cohesive forces take over until gravity balances them. Objections include the lack of a free surface in xylem vessels, the low force magnitude limiting ascent, and the paradoxical relationship between tree height and vessel bore width.
Cohesion force theory:
The Transpiration Pull Theory, also known as Dixon’s theory of ascent of sap, proposes that there is a continuous water column from roots to leaves, maintained by cohesion forces between water molecules. This cohesion, due to hydrogen bonds, enables the water column to withstand tension up to 100 atmospheres. Adhesion forces exist between tracheary element walls and water molecules. Dixon and Joly measured cohesion force in tracheary elements to be 45 to 207 atmospheres, supporting the theory.