1. Two theories to explain water + minerals transport in plants
- root pressure theory
- cohesion-tension theory.
2. Root-pressure theory:
- accumulation of mineral ions in the xylem
- enhances water molecules to move into root hairs (by osmosis).
- water pressure ↑ builds up in the root
- pressure pushes up water + dissolved minerals
- through the xlem
- toward the top of the plant.
- but
not strong enough to push up water to the top of
tall trees.
 |
| Root pressure |
3. In small plants:
- root pressure can build high enough
- to force water and minerals completely out of the tips of the leaves
- the process = guttation.
4. Cohesion-tension theory suggests that:
- water inside the xylem is pulled upward
- by the -ve pressure (or tension)
- that extends all the way from leaves to roots.
5. In the leaf xylem:
- -ve pressure (tension) builds
- as water evaporates during transpiration.
- evaporated water is continually replaced
- thus cohesive bond pull the string of water molecules up
- to create a transpiration pull.
- transpiration pull is relayed
- molecule by molecule
- down the entire column of water in the xylem.
 |
| Transpirational pull in the leaf |
6. In the stem, water molecules:
- exist as a long unbroken chain in the xylem.
- are pulled upwards by tensions produced (during transpiration).
- are
held by
cohesion + adhesion forces
 |
| Cohesion and adhesion forces in the xylem |
7. Transpiration pull:
- can extend down to the roots
- only through an unbroken chain of water molecules.
8. At the cellular level:
- the gradients of water potential
- drive the osmostic movement of water
- from cell to cell
- within the roots up
to the leaves.
 |
| Water potential in leaf, stem and root |
