Nov 26, 2009


Transpiration is the loss of water vapour through evaporation in plants. The loss of water is replaced by the absorption of water from the soil by the plant roots.
Only 1% - used by plants for photosynthesis; 99% - evaporates
90% of transp. occurs through the stomata - also through lenticels in woody stems

-helps in absorption and transport of water and mineral ions from roots to different parts of plants
-produce cooling effect in plants
-helps to supply water to all plant cells for metabolic processs
-helps to prevent plants from wilting and maintaining cell turgidity

1. The surfaces of the mesophyll cells are covered by a thin layer of water.
2. Heat from the sun causes the water on the external surfaces of the mesophyll cells to evaporate, thus saturating the air spaces in the mesophyll with water vapour.
3. Outside the stomata, the air in the atmosphere is less saturated.
4. This means the that the concentration of water vapour in the atmosphere is lower than the concentration of water vapour in the air spaces of the leaves.
5. Hence, the water vapour in the air spaces evaporates and the water vapour diffuses from the plant cells through the stomata.
6. The movement of air carries water vapour away from the stomata.
7. The loss of water from mesophyll cell makes the cell hypertonic to an adjacent cell.
8. Water from the adjacent cell diffuses into the mesophyll cell by osmosis.
9. In the same way, water continues to diffuse from the neighbouring cells into the adjacent cells.
10. Eventually, water is drawn from the xylem vessels in the veins.
11. A pulling force is thus created to pull water up the xylem vessels as a result of the evaporation of water vapour from the mesophyll cells.
12. This pull is called the transpirational pull.

The external conditions that affect the rate of transpiration are
a) light intensity
b) temperature
c) relative humidity
d) air movement

Light intensity
-an increase in light intensity increases the rate of transp.
-light stimulates the opening of the stomata
-stomata open wider; more water vapour evaporates

-temperature increases; rate of transp. increases
-temp. increases; rate of evaporation of water from the surfaces of the mesophyll cells increases; rate of diffusion of water through the stomata increases

Air movement
-faster air movement helps removing the water vapour
-air movement increases the concentration gradient between the water vapour in the leaf and that outside the leaf; this increases the transp. rate.
-when the air is still, the transp. rate decreases/stops

Relative humidity
-high humidity surrounding the leaves reduces the evaporation of water from the stomata; transp. slows down
-a rise in temp. lowers the relative humidity of surrounding air; rate of transp. increases