Measurement of Transpiration

 Following four methods are usually used for measuring transpiration in plants :

a. Weight method

The method consists of weighing the potted plants at regular intervals. The loss of weight over a

the prescribed period of time represents the weight of transpired water.

b. Cobalt Chloride method

This method is based on the fact that the dry cobalt chloride is blue in colour while the moist cobalt

chloride is prink in clour. Actually, in this method, the weight of transpired water is not measured. But, it

measures only the rate of transpiration.

A small piece of filter paper is soaked in3% cobalt chloride solution and thoroughly dried. A single

cobalt chloride paper is clipped under a glass coverslip along with an ordinary standard pink and ordinary

standard blue coloured paper on the surface of the transpiring leaf. The colour of the cobalt chloride will

gradually change from blue to pink. The time taken for this change is noted. The rate of colour change

is an indication of the rate of transpiration.

c. Collecting and Weighing Transpired Water (Porometry)

This method consists of passing the air of known moisture content over a potted plant kept under a

closed glass chamber through an opening. The air containing transpired water is passed out over anhydrous

calcium chloride whose initial weight is already known. The increase in the weight of anhydrous

calcium chloride will be equal to the weight of transpired water plus the original moisture content of the air.

The weight of the original moisture content of the air being known already, the weight of the transpired

water is easily calculated.

Based on this principle, the transpiration rate in crop plants is quickly measured using the modern

equipment, Steady State Porometer.

d. Potometer Methods

These methods are based on the assumption that the rate of absorption of water is approximately

equal to the rate of transpiration. The apparatuses used for measuring the transpiration are called as

Potometers. Following three types of potometers are very common :

i. Drawin’s Potometer

ii. Farmer’s Potometer and

iii. Ganong’s Potometer

More recently, Steady State Porometer is also available in the market to measure the rate of transpiration

in plants.