Transpiration : Process, Types, Factors affect Transpiration

WHAT IS TRANSPIRATION ?

Plants absorb large quantities of water as well as mineral salts from the soil with the help of their roots. This water is conducted in the upward direction upto the tips of stem, branches and leaves by a process called ascent of sap. We are going to discuss about Transpiration process, transpiration types and factor affect transpiration.
Transpiration

From the absorbed water only 1-2% is utilized by the plants for the purpose of photosynthesis and other processes. The remaining 98-99% water is lost by the transpiration process through the exposed surfaces of the plants.

Major part of water is lost by way of transpiration from the leaves which have large surface area. Transpiration process is very useful for plants for two reasons. One to get rid of excess water absorbed by roots (about 98%) and two for cooling the plant in hot weather.

WHAT IS TRANSPIRATION PROCESS ?

The loss of water in the form of vapours from the aerial exposed parts of a plant is called transpiration process.

TRANSPIRATION TYPES

Depending upon the aerial part of the plant through which transpiration process takes place, it can be categorized into three transpiration types i.e.,

1. Stomatal transpiration : 

Loss of water vapours through microscopic pores (stomata surrounded by specialized guard cells is called stomatal transpiration types. Stomatal transpiration accounts for upto 90% of total water loss from the plants through transpiration process. It js thus, most important physiologically. 

2. Cuticular transpiration : 

Some transpiration takes place by the direct evaporation of water from the outer walls of the epidermal cells. This layer is impermeable to water. But some loss does take place through this layer. Water loss due to evaporation through transpiration.

It constitutes upto 10% of total water loss from the plants through transpiration. It should be noted that greater the thickness of cuticle, lesser will be the rate of transpiration. It was another among transpiration types.

3. Lenticular transpiration : 

Water loss through the lenticels in woody stems and fruits is called lenticular transpiration. It constitutes about 1-2% of total water loss through transpiration process. Lenticels never close. They remain open all the times.
It was all about transpiration types.
Transpiration process

STOMATAL APPARATUS (STOMATA) 

Stomata (sing. stoma) are microscopic pores surrounded by two specialized kidney. shaped or dumb-bell shaped guard cells. They are present in the epidermis of leaves and young green stems. They are chiefly concerned with gaseous exchange during photosynthesis, respiration and water loss through transpiration process

Transpiration is an inevitable process through open stomata. The guard cells surrounding a pore are highly specialized epidermal cells with chloroplasts. They are placed side by side (parallel) and are kidney shaped in dicot leaves. They are, however, dumb-bell shaped in monocots. 

Their walls are differentially thickened. The opposing inner walls of guard cells are thickened crescentically in the middle and are, thus, inelastic. The remaining walls are thin and elastic.

MECHANISM OF STOMATAL TRANSPIRATION 


Water absorbed by the roots of a plant moves upwards through the xylem vessels of roots, stems and reaches up to the leaves. In the leaves are present a large number of spongy mesophyll cells. They are exposed to numerous intercellular spaces present between them. The turgidity of the mesophyll cells is maintained by osmotic diffusion of water from the xylem vessels of the leaves.

The water from these cells keep on evaporating through their exposed cell walls. The air inside the intercellular spaces gets saturated. Water potential of the air in the intercellular spaces of the leaf becomes higher than that of the air present outside the leaf.

As a result, water vapours from the substomatal spaces move to outside through open stomata. The entire movement of water vapours from the surface of mesophyll cells into the outside atmosphere is the result of diffusion.

The cell walls that are losing water due to transpiration process, replace it by drawing more of it s from the adjoining cells. Most of this water - travels along the cell wall by imbibition. Whereas, a very small amount of water enters a by osmosis .

Thousands of cells of - the leaves lose water due to transpiration. It - causes pulling of more water from below e through the xylem vessels. The transpiration pull thus created can draw up water up to the height of the plant which can be 50 metres or even more.

More transpiration process occurs from the lower surface of a dorsiventral leaf as compared to upper surface. This is because of the fact that more stomatal openings are present on the under surface.

Factors affect TRANSPIRATION

Rate of Transpiration is affected both by the Internal and External Factors.

(A) Internal factors : 

(1) Leaf area : Larger the surface area of the leaves, more is the transpiration.
(2) Leaf structure : Presence of thick cuticle, coating of wax, sunken stomata, compact mesophyll and reduction in the number of stomata reduce the rate of transpiration.
(3) Water content of leaves: Due to insufficient absorption of water by roots, if the water content of the leaves decreases, the leaves wilt and transpiration is reduced.

(B) External factors : 

The external factors which influence the rate of transpiration are :
(1) Light : It may be considered as one of the most important environmental factors. It causes stomata to open. Besides, the light increases the temperature of the leaf which in turn increases transpiration process. When it is cloudy during the day, the stomata are partially closed and transpiration is reduced.
(2) Temperature : An increase in the temperature brings about a decrease in the relative humidity of the air. It increases the rate of transpiration: Generally, an increase in temperature up to certain limit causes an increase in the rate of transpiration. Very low and very high temperatures cause closure of - stomata and hence, decline in transpiration.
(3) Humidity of the atmosphere : The rate of transpiration is directly related to the vapour pressure of the atmosphere. The rate of transpiration decreases with the rise in vapour pressure in the atmospheric air and it increases with the fall in vapour pressure.
(4) Wind : In quiet atmosphere water vapours often accumulate in the vicinity of the transpiring leaves. This brings about a decrease in the rate of transpiration. But, when the leaves are exposed to winds of moderate velocity, the water vapours will not be allowed to accumulate in the vicinity of the leaves.
It will lead to increased rate of transpiration process. Wind of high velocity causes closure of stomata, drying and cooling of the surface of leaves. It causes decrease in the rate of transpiration.
(5) Atmospherie pressure : Lowering of atmospheric pressure reduces the density of the air. This permits more rapid diffusion of water vapours.
(6) Soil water : If the amount of soil water available is not sufficient the rate of transpiration decreases. This affects the opening and closing of stomata. A decrease in water uptake by plants causes partial or complete closure of stomata. It again causes decrease in transpiration.
(7) Carbon dioxide : Increase in CO, level in outside air over normal 0.03% causes closure of stomata and results in decrease in transpiration.
Transpiration types

ADAPTATIONS IN PLANTS TO REDUCE TRANSPIRATION 

Many plants specially xerophytes have evolved a variety of permanent adaptations to cut down transpiration. Some of these external adaptations are : -
(a) Thick cuticle : The leaves may be covered by very thick cuticle e.g., Banyan.
(b) Loss of leaves : In some plants leaves may be dropped or may be absent as in most cacti.
(c) Narrow leaves : To reduce the surface area for transpiration, leaves in some plants become narrower, e.g., Nerium.
(d) Fewer stomata : In some plants, the number of stomata may be reduced.
(e) Sunken stomata : In some plants, the stomata are sunken (stomata depressed below the adjoining epidermal cells) or covered by hair, e.g., Nerium.

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