2.3 - Main Organ for Transpiration

Necessity of Transpiration in Plants

Transpiration is the loss of water in the form of water vapour through evaporation from plants to the atmosphere.
Although transpiration also occurs through stems and flowers, about 90% of water is lost through stomatal pores in leaves.
Roots continuously absorb water by osmosis.
Water absorbs heat energy from leaves and evaporates, producing a cooling effect.
Transpiration produces a pulling force that moves water and mineral salts continuously through xylem vessels from the roots to all plant cells.

As light intensity increases, the rate of transpiration increases until it becomes constant.
It becomes constant when relative air humidity, temperature, or air movement becomes a limiting factor.

Low relative air humidity causes water vapour to evaporate faster from stomata, increasing the rate of transpiration.
As relative air humidity increases, the rate of transpiration decreases.

An increase in temperature increases the kinetic energy of water molecules and the rate of transpiration.

A potometer measures water uptake by a leafy shoot as an estimate of its transpiration rate.
Manipulated variable: light intensity, temperature, air movement, or relative air humidity.
Responding variable: distance moved by the air bubble per unit time.
Constant variable: type of plant.
Cut the leafy shoot under water, fit it into a water-filled potometer, and seal all connections with petroleum jelly to make them airtight.
Introduce an air bubble into the capillary tube, mark its initial position, expose the shoot to one factor at two different levels for the same duration, and measure the bubble distance.
Change only one environmental factor at a time and reset the apparatus before each reading.

$Rate of transpiration = \frac{distance moved by air bubble (cm)}{time (min)}$

Higher air movement, light intensity, or temperature increases the rate; higher relative air humidity decreases it.