Transpiration: Mechanism, Types, Factors affecting  transpiration, Biological significance and  Disadvantages

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  • Plants absorb a large quantity of water from the soil by root hairs and only a small part (1-2%) of this water is utilized by them in their life processes.
  • The remaining large part (98-99%) of water is lost in the form of vapor from internal tissues.
  • The process of loss of water in the form of vapor through the exposed aerial parts of the plants is called transpiration.
  • Transpiration is regulated by vital activities of protoplasm and some structural peculiarities of the transpiring organs.

Why transpiration is called a necessary evil?

  • Transpiration facilitates the upward movement of water and minerals absorbed by roots but at the same time if water loss through transpiration is more than the absorption of water, it can cause wilting and even death of the plant. Hence, transpiration is called a necessary evil.

Mechanism of transpiration:

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  • The mesophyll cells of the leaf withdraw water from the xylem tissue by osmotic diffusion and become turgid.
  • From the surface of the turgid cells, water gets evaporated into the intercellular spaces.
  • This vapor present in intercellular spaces becomes saturated and diffuses outwardly to the environment where the air is unsaturated.
  • Water vapor diffuses through stomata, cuticle and lenticel.
  • The rate of this water loss is reduced just after heavy rainfall as the atmospheric air is usually saturated with vapor in such cases.

Types of Transpiration:

  • Depending upon the plant parts involved, transpiration is classified into the following three types:
    1. Stomatal Transpiration:
      • Transpiration occurring through stomata is called stomatal transpiration.
      • The stomata constitute the major pathway through which 90% of water vapor is lost by the aerial parts of the plant.
      • Stomata are minute pores found on the epidermis between two guard cells.
      • These stomata allow water vapor to escape through them into the atmosphere.
      • Stomata are found on leaves, young stem, flowers and fruits.
      • In isobilateral leaves, the stomata are equally distributed on both upper and lower surfaces, whereas, in dorsiventral leaves, they are found on the lower surface.
    2. Cuticular Transpiration:
      • The loss of water vapor through the cuticle or cuticular surface of the plant parts is called cuticular transpiration.
      • It constitutes about 3-10% of the total transpiration.
      • In herbaceous plants growing in moist shady places, the cuticle is very thin and constitutes up to 50% of transpiration.
    3. Lenticular Transpiration:
      • Transpiration occurring through lenticel is called lenticular transpiration.
      • A lenticel is a porous tissue consisting of cells having large intercellular spaces.
      • Lenticels are formed in the periderm of secondarily thickened organs and on the bark of perennial dicot stems and roots during secondary growth.
      • It constitutes only about 0.5% of the total transpiration.

Factors affecting rate of Transpiration:

  • There are various factors affecting the rate of transpiration in plants. They are:
    1. Environmental factors
    2. Plant factors

Environmental factors:

  1. Light:
    • It is the most essential factor since it plays an important role in opening and closing of stomata.
    • Stomata remains open in the light and closed in the dark.
    • Hence, the rate of transpiration is higher in stronger intensity of light and lower in the dark.
  2. Temperature:
    • Rate of transpiration is directly proportional to temperature.
    • High temperature increases the rate of transpiration up to a certain limit.
    • The stomata are found to remain open at high temperature even in the dark.
    • Furthermore, high temperature causes heating and lowers the humidity of the environment enhancing transpiration. Low temperature on the other hand, decreases the rate of transpiration.
  3. Wind:
    • Wind increases the rate of transpiration by removing saturated air from around the transpiring organs.
    • However, high wind velocity (40-50 km/hr) decreases the rate of transpiration by closing the stomata.
  4. Humidity of surrounding air:
    • High humidity of surrounding air decreases the rate of transpiration as it reduces the diffusion of vapor from intercellular spaces to atmosphere.
    • Low humidity increases the rate of transpiration.
  5. Soil condition:
    • Since transpiration depends on the rate of absorption of water from soil, the condition of soil (dry, moist, wet) affects the rate of transpiration.
    • Absorption of water from soil is directly influenced by many factors like aeration in soil, water, temperature etc.

Plant factors:

  1. Area of Transpiring surface:
    • A plant with large transpiring area shows more transpiration than the plant with smaller transpiring area.
  2. Thickness of cuticle:
    • The rate of transpiration depends on the thickness of the cuticle.
    • A plant with a thin layer of cuticle shows more transpiration.
  3. Location of stomata:
    • The rate of transpiration is reduced if stomata are sunken deep and minimizes direct contact with the environment.
    • The rate of transpiration increases if stomata are distributed superficially.
  4. Number of stomata:
    • Plant parts with large number of stomata show more transpiration.
  5. Root/Shoot ratio:
    • It is the ratio of dry weight of root biomass to dry weight of shoot biomass.
    • A high root/shoot ratio increases the rate of transpiration while low ratio decreases the rate of transpiration.

Biological significance of Transpiration:

  • Transpiration removes the excess quantity of water from the plant body.
  • It lowers the temperature of the leaves and plant as a whole by evaporating water.
  • Transpiration exerts suction force on the leaves surface, which helps in the ascent of sap.
  • With the increase in transpiration, the rate of absorption of water and minerals increases and the concentration of minerals increases inside the plant.
  • The ash and sugar content of the fruit increases with the increase in transpiration.
  • Transpiration enhances the extensive development of the root system that supports the heavy shoot growth.

Disadvantages of Transpiration:

  • High rate of transpiration causes the wilting of plants.
  • Increased rate of transpiration reduces photosynthesis which stops the growth of the plants.
  • It inhibits the production of flowers, fruits and seeds.
  • A plant loses a huge amount of water through transpiration which otherwise may be required for other life processes.

Transpiration: Mechanism, Types, Factors affecting  transpiration, Biological significance and  Disadvantages