Transport in Plants - Notes | Class 11 | Part 4: Transpiration



It is the evaporative loss of water by plants through the stomata in the leaves.

Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. The remaining is lost by transpiration.

Transpiration can be studied using cobalt chloride paper. It turns colour (blue to pink) on absorbing water.

During transpiration, exchange of O2 & CO2 in the leaf also occurs.

Stomata are open in day time and close during night.

Opening or closing of stomata is due to change in the turgidity of the guard cells.

The inner wall of guard cell lining stomatal aperture is thick and elastic and the outer wall is thin.

When turgidity of guard cells increases, the outer walls bulge out and pull the inner walls into a crescent shape.

Cellulose microfibrils in the guard cells are oriented radially rather than longitudinally making it easier for the stoma to open.

The guard cells lose turgidity due to water loss (or water stress) and the inner walls regain their original shape. As a result, the stoma closes.

Usually lower surface of a dicot leaf has more stomata. In monocot leaf, they are about equal on both surfaces.

Factors affecting transpiration:
  • External factors: Temperature, light, humidity, wind etc.
  • Plant factors: Number & distribution of stomata, number of stomata open, water status of plant, canopy structure etc.
The transpiration-driven ascent of xylem sap depends on the following physical properties of water:
  • Cohesion: Mutual attraction between water molecules.
  • Adhesion: Attraction of water molecules to polar surfaces (e.g. surface of tracheary elements).
  • Surface Tension: In liquid phase, water molecules are more attracted to each other than in gas phase.
These properties give water high tensile strength (ability to resist a pulling force) and capillarity (ability to rise in thin tubes). Capillarity is aided by small diameter of the tracheary elements – tracheids & vessel elements.

Xylem vessels supply the water from the root to leaf vein. There is a continuous thin film of water over the cells. So, as water evaporates through the stomata, water pulls into the leaf from the xylem. In atmosphere, concentration of water vapour is lower than that in substomatal cavity and intercellular spaces. This also helps water to diffuse into the surrounding air. This creates a ‘pull’.

The forces generated by transpiration can create pressures to lift a xylem sized column of water over 130 m high.

Transpiration & Photosynthesis – a Compromise

Photosynthesis is limited by available water which is swiftly depleted by transpiration.

The humidity of rainforests is mainly due to the cycling of water from root to leaf to atmosphere and back to the soil.

C4 photosynthetic system helps to maximise the availability of CO2 and minimise water loss.

C4 plants are twice as efficient as C3 plants in fixing carbon (making sugar). However, C4 plants lose only half as much water as a C3 plant for the same amount of CO2 fixed.

Uses of Transpiration:
  • Creates transpiration pull for absorption and transport.
  • Supplies water for photosynthesis.
  • Transports minerals from soil to all parts of the plant.
  • Cools leaf surfaces, sometimes 10 - 15°, by evaporation.
  • Maintains shape & structure of plants by keeping cells turgid.

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