Loss of water in the form of vapours from plant surface. The loss of water due to transpiration is quite high. Rather 98-99% of the water absorbed by a plant is lost in transpiration. Hardly 0.2% is used in photosynthesis while the remaining is retained in the plant during growth.
TRANSPIRATION IS DRIVEN BY DIFFERENCES IN VAPOR
Although a small amount of water vapor may be lost through small openings (called lenticels) in the bark of young twigs and branches, the largest proportion by far (more than 90%) escapes from leaves. Indeed, the process of transpiration is strongly tied to leaf anatomy. The outer surfaces of a typical vascular plant leaf are covered with a multilayered waxy deposit called the cuticle.
The integrity of the epidermis and the overlying cuticle is occasionally interrupted by small pores called stomata (sing. stoma). Each pore is surrounded by a pair of specialized cells, called guard cells. These
guard cells function as hydraulically operated valves that control the size of the pore.
The interior of the leaf is comprised of photosynthetic mesophyll
cells. The somewhat loose arrangement of mesophyll cells in most leaves creates an interconnected system of intercellular air spaces. This system of air spaces may be quite extensive, accounting for up to 70 percent of
the total leaf volume in some cases.
Stomata are located such that, when open, they provide a route for the exchange of gases (principally carbon dioxide, oxygen, and water vapor) between the internal air space and the bulk atmosphere surrounding the leaf. Because of this relationship, this space is referred to as substomatal space. The cuticle is generally impermeable to water and open stomata provide the primary route for escape of water vapor from the plant.
Transpiration may be considered a two-stage process:
(1) the evaporation of water from the moist cell walls into the substomatal air space and
(2) the diffusion of water vapor from the substomatal space into the
It is commonly assumed that evaporation occurs primarily at the surfaces of those mesophyll cells that border the substomatal air spaces.
Types of Transpiration
The 5 to 10 percent is accounted for by cuticular transpiration. Although the cuticle is composed of waxes and other hydrophobic substances and is generally impermeable to water, small quantities of water vapor can pass through. The contribution of cuticular transpiration to leaf water loss varies considerably between species. It is to some extent dependent on the thickness of the cuticle. Thicker cuticles are characteristic of plants growing in full sun or dry habitats, while cuticles are generally thinner on the leaves of plants growing in shaded or moist habitats. Cuticular transpiration
may become more significant, particularly for leaves with thin cuticles, under dry conditions when stomatal transpiration is prevented by closure of the stomata.
THE RATE OF TRANSPIRATION IS INFLUENCED BY ENVIRONMENTAL FACTORS
The rate of transpiration will naturally be influenced by factors such as humidity and temperature, and wind speed, which influence the rate of water vapor diffusion between the substomatal air chamber and the ambient atmosphere.
WHAT ARE THE EFFECTS OF HUMIDITY?
Humidity is the actual water content of air.
Relative humidity is the ratio of the actual water content of air to the maximum amount of water that can be held by air at that temperature.
Relative humidity is most commonly expressed as RH×100, or percent relative humidity.
With increase in relative humidity the rate of transpiration becomes decrease.
WHAT IS THE EFFECTS OF TEMPERATURE?
Temperature modulates transpiration rate through its effect on vapor pressure, which in turn affects the vapor pressure gradient.
With the increase in atmospheric temperature, the rate of transpiration also increases. This is not only because evaporation occurs quickly in warmer air but also because warm air is capable of holding more water vapours than the cold air.
Soil Water Content:
Availability of soil water affects the rate of transpiration. If there is little water available, the resulting tendency for dehydration of the leaf causes stomatal closure and a consequent fall in transpiration. Such a condition usually occurs during periods of drought and when the soil is frozen or at a temperature so low that water is not absorbed by roots.
The velocity of wind greatly affects the rate of transpiration. Fast moving air currents continually bring fresh, dry masses of air in contact with leaf surfaces and thus maintain a high rate of transpiration.
ADVANTAGES OF TRANSPIRATION:
1. Ascent of Sap:
Ascent of sap mostly occurs due to transpiration pull exerted by transpiration of water. This pull also helps in absorption of water.
2. Removal of Excess Water:
It has been held that plants absorb far more amount of water than is actually required by them. Transpiration, therefore, removes the excess of water.
3. Cooling Effect:
Radiant heat falling on the plants increases their temperature which may be dangerous to the plants. Transpiration, by evaporating water, lowers down their temperature by 10°—15°C.
4. Mechanical Tissue:
The development of mechanical tissue, which is essential for providing rigidity and strength to the plant, is favoured by the increase in transpiration.
5. Distribution of Mineral Salts:
Mineral salts are mostly distributed by rising column of sap.
6. Increasing Concentration of Mineral Salts:
The sap absorbed from the soil contains low concentration of mineral salts. The loss of water through transpiration increases the concentration of mineral salts in the plant.
7. Root System:
Transpiration helps in better development of root system which is required for support and absorption of mineral salts.
8. Quality of Fruits:
The ash and sugar content of the fruit increases with the increase in transpiration.
Excessive transpiration induces hardening and resistance to moderate drought.
Transpiration maintains the shape and structure of plant parts by keeping cells turgid.
Transpiration supplies water for photosynthesis. As water evaporates through the stomata, it results in pulling of water, molecule by molecule into the leaf from the xylem.
DISADVANTAGES OF TRANSPIRATION:
Wilting or loss of turgidity is quite common during noon due to transpiration being higher than the rate of water absorption. Wilting reduces photosynthesis and other metabolic activities.
2. Reduced Growth:
Transpiration reduces availability of water inside the plant. Water deficit decreases growth and hence the plant gives a stunted appearance.
3. Reduced Yield:
A single wilting reduces growth by 50%. It is because decreased availability of water inside the plant checks meristematic activity and hence the formation of flowers, fruits and seeds.
4. Abscisic Acid:
Water stress produces abscisic acid. Abscisic acid prevents several plant processes and promotes abscission of leaves, flowers and fruits.
5. Wastage of Energy:
Since 98-99% of absorbed water is lost through transpiration, the energy used in absorption and conduction of water goes waste.
In order to reduce transpiration during critical periods, the plants produce several types of modifications— thick cuticle, hair, prickles, spines, thorns, sunken stomata, phylloclades, cladodes, etc. Nevertheless transpiration cannot be checked.
Stomatal transpiration will always occur whenever stomata are open for gaseous exchange (so essential for photosynthesis and respiration). Similarly cuticular and lenticular types of transpiration cannot be checked as there is no method of their control. Hence transpiration is regarded as a necessary evil or unavoidable evil.