Question
Compare root pressure and transpiration pull as mechanisms for water movement in plants. Which mechanism is primarily responsible for moving water to the top of tall trees?
Solution — Step by Step
Root pressure is a positive pressure generated in the xylem at the root level due to the active uptake of mineral ions into the root xylem by root cells. As mineral ions accumulate in the xylem, the water potential inside the xylem drops. Water flows in by osmosis from surrounding soil (which has higher water potential), creating a slight pressure that pushes water upward.
Root pressure is measurable — if you cut a plant near the stem base, liquid oozes out. This is called guttation — water droplets at leaf margins in the morning when transpiration is minimal but root pressure is active. Typical root pressure values: 0.1–0.5 MPa.
Transpiration pull (cohesion-tension theory, proposed by Dixon and Jolly) is the mechanism where water lost by evaporation from leaf surfaces creates a tension (negative pressure) that pulls water up through the xylem.
The process:
- Water evaporates from mesophyll cells in leaves → exits through stomata (transpiration)
- This creates a water deficit in leaf cells → water potential decreases in leaves
- Water is pulled from xylem into leaf cells by osmosis
- This creates tension (negative pressure) in the xylem column
- Because water molecules are strongly cohesive (hydrogen bonding), they are pulled upward as a continuous column
- Water enters roots from the soil to replace the water pulled upward
| Feature | Root Pressure | Transpiration Pull |
|---|---|---|
| Direction of force | Push (positive pressure, upward) | Pull (negative pressure/tension, upward) |
| Magnitude | Weak (0.1–0.5 MPa) | Strong (up to −3 MPa in tall trees) |
| Responsible for | Short plants, guttation, early morning water movement | Main mechanism in tall trees |
| When active | Night, early morning (when stomata closed) | Day (when stomata open and transpiration occurs) |
| Mechanism | Active ion transport → osmosis | Transpiration → cohesion of water column |
Root pressure alone cannot explain water rising to the tops of tall trees. Consider: the tallest trees (coast redwoods, Sequoia sempervirens) reach ~115 m. To push water to that height by pressure would require about 1.1 MPa just to overcome gravity (0.01 MPa per metre × 115 m). Root pressures of 0.1–0.5 MPa are far too weak.
Transpiration pull (cohesion-tension mechanism) is the primary driver in tall trees. Measurements of xylem pressure in tall trees show tensions of −2 to −3 MPa — negative pressures far exceeding root pressure values. Water rises as a continuous, cohesive column under this tension.
The cohesion of water (strong hydrogen bonding between molecules) is essential — the water column doesn’t break apart under tension because the hydrogen bonds between water molecules are stronger than the tensile stress placed on the column.
Root pressure contributes primarily to:
- Water movement in herbaceous (non-woody) plants
- Water movement at night when transpiration is absent (refilling of xylem vessels that have air-embolised during the day)
- Guttation — visible as water droplets on leaf margins in the morning
It is a supplementary, not primary, mechanism for tall tree water transport.
Why This Works
The cohesion-tension theory works because of water’s unique physical properties:
- High cohesion: Water molecules attract each other (hydrogen bonding) — the water column is like a long, flexible chain
- High adhesion: Water adheres to xylem cell walls (also via hydrogen bonding) — helps prevent the column from pulling away from the walls
- High tensile strength: The water column can withstand negative pressures (tensions) without breaking under normal conditions
Think of the xylem as a very thin straw. The leaves at the top are constantly “sucking” (via transpiration), and the cohesive water column transmits this suction all the way to the roots — which then pull in water from the soil.
Alternative Method — Analogy
Imagine a chain of water molecules in the xylem as a long rope. Transpiration at the top pulls the rope up; root pressure gives a small push at the bottom. For a 10-metre rope, the push barely helps — the pull does all the work. For a 100-metre rope, you need a very strong pull and the push is negligible.
Common Mistake
Students often state that both mechanisms are “equally important” or that “root pressure drives water in tall trees.” This is incorrect — root pressure is too weak by an order of magnitude. For tall trees (over 30 m), transpiration pull (cohesion-tension) is the only viable mechanism.
Also, don’t confuse transpiration (water loss from leaves) with translocation (food transport in phloem). Both involve upward or bidirectional transport, but through different tissues: xylem (water, transpiration pull) and phloem (sucrose, pressure flow hypothesis).
NEET and CBSE Class 11 ask: (1) which theory explains water transport in tall trees — cohesion-tension theory / transpiration pull; (2) what is guttation and which pressure causes it — root pressure; (3) what properties of water are essential for cohesion-tension — cohesion + adhesion + tensile strength; (4) who proposed cohesion-tension theory — Dixon and Jolly (1894).