WHAT IS TRANSPIRATION ?
The loss of water in the form of vapours from the aerial exposed parts of a plant is called transpiration process.
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.
What are the different types of Transpiration ?
Depending upon the aerial part of the plant through which transpiration process takes place, it can be categorized into three different types of transpiration i.e.,
1. Stomatal transpiration :
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.
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 the different types of transpiration.
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STOMATAL APPARATUS (STOMATA)
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.
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 affecting Transpiration
Rate of Transpiration is affected both by the Internal and External Factors.
(A) Internal factors :
- Leaf area : Larger the surface area of the leaves, more is the transpiration.
- 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.
- 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 :
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.
SIGNIFICANCE OF TRANSPIRATION
Transpiration has a great impact upon the metabolism of plants. It is both advantageous as well as disadvantageous.
Advantages of transpiration
- Ascent of sap : Transpiration exerts a tension or pull on water column in xylem which is responsible for the ascent of sap.
- Absorption of water : Transpiration helps passive absorption of water.
- Transport of minerals : Transpiration assists translocation of mineral salts through xylem.
- Cooling effect : Transpiration prevents excessive heating during hot summer days because of its cooling effect. The transpiring organs come to have temperature about 2-5 °C below that of the environment.
- Improvement in the quality of fruits : High rates of transpiration lead to the development of fruits with increased sugar and mineral contents.
Disadvantages of transpiration
- Wilting injury : Wilting results from the flaccid condition of leaf cells when the amount of water lost through transpiration exceeds the amount of water absorbed through roots. Metabolic processes, including photosynthesis, are retarded, checking growth. It greatly reduces the yield of crop plants. Permanent wilting, if not recovered, may lead to the death of the plants.
- Stunted growth : Excessive rate of transpiration leads to stunted growth of plants.
Agents or chemicals used to cut the rate of transpiration are called antitranspirants. Reduction in transpiration is desired in high priced crops, seedling transplantations in nurseries, etc.
To fulfil this requirement some. artificial means are used. These artificial means include certain antitranspirants like colourless plastic resin, silicon oils and low- viscosity waxes.
These antitranspirants, when sprayed on leaves, make a thin film on the transpiring surface. This film is permeable to CO, and O2 but not to water. In addition to these substances, phenyl- mercuric acetate (PMA, a fungicide) and abscisic acid (growth regulator) also serve as antitranspirants.
These two cause closure of stomatal pores. CO2 in higher concentration (0.05%) than in nature (0.03%) also working as antitranspirant causes natural closure of stomata. Further, higher concentration of CO, will close the stomata completely.
EXUDATION OF WATER OR GUTTATION
Guttation (Latin gutta = drop, exudation of drops) is the loss of water in the form of liquid, as droplets along the margins of the leaves through specialized structures known as hydathodes or water stomata.
Each hydathode is incompletely differentiated stoma or pore in the epidermis. It is followed by large intercellular spaces, loosely arranged parenchyma and blindly ending xylem elements.
One can see exudation of droplets of liquid water and also some solutes (inorganic or organic substances) from the margins and tips of leaves of potato, tomato, grasses, Nasturtium, Colocasia, in the morning.
When the water absorption exceeds that of transpiration, (i.e., root pressure is high and transpiration is low) hydrostatic pressure is built up in the xylem vessels. Due to this pressure water is pushed up the xylem ducts and out through the hydathodes.
In this article, we have discuss about what are the different types of Transpiration. The following points are covered briefly in this article :
- What is Transpiration ?
- What are the different types of transpiration ?
- Mechanism of Transpiration
- Factors affecting Transpiration
- Significance of Transpiration
- Antitranspirants etc.