Read this article to learn about the meaning, factors affecting and measurement of transpiration.
Transpiration is the process by which water vapour leaves the living plant body and enters the atmosphere. It involves continuous movement of water from the soil into the roots, through the stem and out through the leaves to the atmosphere.
The process includes particular transpiration or direct evaporation into the atmosphere from moist membranes through the cuticle and stomatal transpiration from the leaves. Transpiration is basically an evaporation process. However, unlike evaporation from a water surface, transpiration is modified by plant structure and stomatal behaviour operating in conjunction with the physical principles governing evaporation.
There are three main factors which influence transpiration.
a. Climate factors;
b. Soil factors; and
c. Plant factors.
The important climatic factors are light intensity, atmospheric vapour pressure, temperature and wind. The soil factors are those governing the water supply to the roots and the plant factors include the extent and efficiency of root systems in moisture absorption, leaf area, leaf arrangement and structure and stomatal behaviour.
The rate of transpiration depends on the supply of energy to vapourize water, the water vapour pressure or concentration gradient in the atmosphere which constitutes the driving force and the resistance to diffusion in the vapour pathway.
Unlike evaporation from free water surface vegetation virtually ceases to transpire at night due to stomatal closure (A water surface continues to evaporate, though at a slower rate). Thus, over a period of say a month, evaporation from a free water surface may exceed the transpiration from a crop. Over a few hours during the day, however, the crop may evaporate more than a free water surface.
In the field transpiration can be measured by an instrument called phytometer. It is a vessel of big size filled with soil in which plants are transplanted. The soil surface is sealed to prevent direct evaporation. Thus, the water is allowed to escape only by way of transpiration. The amount of water transpired is determined by weighing the vessel after decided interval of time.