Transpiration & its types, mechanism [Asked in JRF SRF NET ARS]

Transpiration

  • Transpiration is the evaporation of water from cell surfaces and its loss through the anatomical structures of the plant (stomata, lenticels and cuticles). 
  • The total water loss by transpiration may be very great. 
  • The daily water loss of large, well watered tropical tress may run as high as 500 litres. 
  • A corn plant may loose 3 to 4 litres of water per day. [Source: https://www.agricoachingdelhi.in/]
  • Whereas a tree sized desert cactus may loose water less than 25 ml per day. 
  • In general about 99 per cent of water absorbed by a plant during the growth is lost in transpiration (necessary evil). [BHU 2017] 
  • Water lost by a growing field of corn would be about 8-11 inches of water per acre during the growing season. 
  • Transpiration is mostly taking place through the stomata
  • Although large quantities of water are absorbed by plant from the soil, only a small amount is utilized. The excess of water is lost from the aerial parts of plants in the form of water vapour and this process is called as transpiration. [Source: https://www.agricoachingdelhi.in/]
  • Nearly >95 per cent of water absorbed by the plants is lost through transpiration and only <5 per cent is utilized by the plant. 

Types of transpiration
  • In general, there are 3 types of transpiration i.e. stomatal, cuticular and lenticular transpirations.
  • 1. Stomatal transpiration
    • Most of the transpiration takes place through stomata.
    • Stomata are usually confined in more numbers on the lower sides of the leaves.
    • In most of the monocots, they are equally distributed on both sides of leaves, while in aquatic plants, stomata are present on the upper surface of the floating leaves.
    • Stomatal transpiration accounts for 80-90 per cent of the total water loss from the plants. [JRF 2016] [Source: https://www.agricoachingdelhi.in/]
  • 2. Cuticular transpiration
    • The loss of water through the impervious cuticle is called as cuticular transpiration.
    • It may contribute a maximum of about 10 ten per cent of the total transpiration.
  • 3. Lenticular transpiration
    • The loss of water through the lenticels of woody stems is called as lenticular transpiration.
    • It accounts for about 0.1 per cent of the total transpirational loss of water.
The mechanism of stomatal transpiration can be studied in 3 steps.
  • 1. Osmotic diffusion of water in the leaf, from xylem to intercellular space above the stomata through the mesophyll cells. [Source: https://www.agricoachingdelhi.in/]
  • 2. Opening and closing of stomata (stomatal movement).
  • 3. Simple diffusion of water vapour from intercellular spaces to outer atmosphere