Transpiration is the process by which water vapour leaves the living plant body, principally the leaves, and enters the atmosphere.
During the growing period of a crop there is a continuous movement of water from the soil into the roots, through the stem and out through the leaves to the atmosphere.
The process includes cuticular transpiration, or direct evaporation into the atmosphere from moist membranes through the cuticle, and stomatal transpiration, or outward diffusion into the atmosphere through the stomate and lenticels, of water vapour previously evaporated from imbibed membranes into intercellular spaces within the plant.
Transpiration is basically an evaporation process. However, unlike evaporation from a water or soil surface, transpiration is modified by the plant structure and stomatal behaviour operating conjunction with the physical principles governing evaporation.
The atmosphere provides the energy required by the plant to withdraw water from the soil. Under conditions of unlimited soil moisture and when the stomata are fully open, the atmospheric conditions control the rate of transpiration.
= Weight of Water Transpired/Weight of dry matter produced
The transpiration ratio for most crops varies from 300 to 800.
FACTORS AFFECTING TRANSPIRATION
The factors affecting transpiration may be summarised as follows:
1) HUMIDITY :
When the humidity of the air surrounding the leaf is increased, there will be a decrease in the vapour pressure difference between the leaf and the surrounding air. This will reduce the rate of transpiration.
2) WATER VAPOUR :
The water vapour surrounding the leaves is swept away by wind. If the wind brings in cooler and/ or humid air the rate of transpiration will decrease.
3) RADIATION :
Radiation influences transpiration substantially. It raises the leaf’s temperature above the surrounding air, resulting in an increase in the rate of transpiration. This is because the saturation vapour within the leaf increases as the leaf’s temperature rises. Leaf temperatures in the sunlight are usually 5 to 10°C above the air temperature. The presence of sunlight stimulates the opening or closing of stomata. In most plants the stomata are open during the day and closed at night.
4) TEMPERATURE :
As the temperature increases, there is an increase in aqueous vapour pressure of leaves and the transpiration increases.
5) WIND :
As the wind velocity increases, the water vapour in the leaf surface are quickly removed and the rate of transpiration increases.
4) ATMOSPHERIC PRESSURE :
If the atmospheric pressure is more, naturally there will be lesser transpiration. Thus, decrease in atmospheric pressure can increase transpiration.
5) PHYSIOLOGIC FACTORS :
Transpiration depends upon the physiologic factors of the plants, such as density and characteristics of the stomata, extent and character of protective coverings and leave structure. Stomata contains numerous pores. Water escapes through stomata as air enters and it results in transpiration. Stomata open with day light, and close with darkness or when the moisture supply is insufficient. When stomata are fully opened, the transpiration rate is a maximum and at that stage, it depends upon the same factors on which free water evaporation depends.
6) SOIL MOISTURE :
Transpiration depends upon the water content of the soil. The water content of the soil after the gravitational water has drained out is called the field capacity. The water in the soil between the field capacity and the wilting coefficient is available for transpiration. The rate of transpiration decreases as the water content reduces.