SIGNIFICANCE OF TRANSPIRATION (Transpiration as a necessary evil)
SIGNIFICANCE OF TRANSPIRATION Plant physiologists have argued about whether transpiration is advantageous or harmful to the plants. It is now well known that transpiration is the most important excretory mechanism in the plants but at the same time it is not of much ad- vantage as most of the water absorbed by the plant is lost through this process. Many workers believe that since transpiration happens it must be good for the plants, as though no function can occur if it is not advantageous in living beings. However, most philosophers reject this idea and do not agree that all things in the universe must have their purpose. It seems that transpiration must be advantageous to the plants because natural selection operates very strongly in the living beings and only those features are survived and retained by their possessers which are the fittest.
The advantages and disadvantages of transpiration are discussed below:
Advantages of Transpiration
Proponents of the advantages of transpiration have suggested three possible roles of the process in plants:
(1) It controls the rate of absorption of water.
Rapid rate of transpiration indirectly increases the rate of water absorption by roots. In a rapidly transpiring plant the xylem cells are generally in a state of negative tension or reduced pressure. With the result, water is pulled into the root from the soil, the process is termed passive water absorption.
(2) It helps in the mineral absorption and ascent of sap.
SIGNIFICANCE OF TRANSPIRATION
Usually water and minerals, absorbed by the plant from the soil, move upward in the transpiration stream. This has led many people to believe that transpiration helps in the absorption of minerals. Actually, the absorption of minerals is a complex process. It follows the law of diffusion and occurs consistently when the transpiration is stopped. Moreover, the absorption of minerals requires expenditure of metabolic energy. However, when soil minerals are abundant, its absorption may be related to the rate of transpiration.
Rapid rate of transpiration helps the upward movement of water and minerals through the xylem ducts. There is no doubt that water and minerals move upward in the transpiration stream but still, it is not an essential factor. SIGNIFICANCE OF TRANSPIRATION
(3) It regulates the plant temperature.
Transpiration prevents the heating of
plant leaves (heat burns) during excessive
day temperature. It causes a cooling effect
by removing a certain amount of heat nearly
600 calories per gram of evaporated water, from the leaf.
Disadvantages of Transpiration
SIGNIFICANCE OF TRANSPIRATION The above mentioned advantages oftranspiration are not of much significance to the plants as compared to its disadvantages. These dis- advantages are burden on the plant’s economy. Some of them are listed below:
(1) Nearly 97% of the total water absorbed by the plants is lost in the form of vapours, only 3% or less than that is actually utilized in growth and plant metabolism.
(2) Transpiration often results in waterdeficits which causes injury to the plants by desiccation
(3) Rapid transpiration causes midway leaf water deficits (temporary wilting). Under certain conditions when the rate of transpiration exceeds the rate of absorption, the plants face internal water deficit. If such condition continues for some time, permanent water deficit (wilting) may develop which causes injury to the plants.
(4) Many xerophytes have to develop their structural modifications in order to reduce transpiration viz., thick hypodermis, reduction of leaves, reduction of stomatal number, sunken stomata etc. These modifications are extra burden on the plants.
(5) Deciduous trees have to shed their leaves to check transpiration.
SIGNIFICANCE OF TRANSPIRATION Besides all the about mentioned disad-vantages, the process of transpiration is un- avoidable because of the anatomical structure of the leaves. Since stomata are required for gaseous ex-change in photosynthesis and respiration, the loss of water vapors through them cannot be avoided. Therefore, Curtis (1926) truly called ‘transpiration as a necessary evil’.
