Factors affecting transpiration

Factors affecting Transpiration

    The factors affecting rate of transpiration can be categorized under two groups:
    1. External or Environmental factors (eg. atmospheric humidity, temperature, wind velocity, light, water supply, atmospheric pressure, sprays and dusts and vital activities)
    2. Internal or Structural factors (eg. stomatal apparatus and its frequency, water content of mesophyll cells and structural peculiarities of the leaf).

    A. External factors
    1. Atmospheric humidity
    • When the atmosphere is humid, the rate of transpiration is reduced.
    • It is because atmosphere is more saturated with moisture and retards the diffusion of water vapour from the intercellular spaces of the leaves to the outer atmosphere through stomata.
    • In dry atmosphere, the RH is low and the air is not saturated with moisture and hence, the rate of transpiration increases.
    2. Temperature
    • An increase in temperature brings about an increase in the rate of transpiration by lowering the relative humidity and wider opening of stomata.
    3. Wind
    • When wind is stagnant (not blowing), the rate of transpiration remains normal
    • When the wind is blowing gently, the rate of transpiration increases as it removes moisture from the vicinity of the transpiring parts of the plant thus facilitating the diffusion of water vapour from the intercellular spaces of the leaves to the outer atmosphere though stomata.
    • When the wind is blowing violently, the rate of transpiration decreases as it creates hindrance in the outward diffusion of water vapours from the transpiring part and it may also close the stomata.
    4. Light
    • Light increases the rate of transpiration as stomata remain open under light conditions coupled with increased temperature.
    • In dark, due to closure of stomata, the stomatal transpiration is almost stopped.
    5. Available soil water
    • The rate of transpiration will decrease if the available soil water is not sufficient enough for easy absorption by the roots.
    6. CO2
    • An increase in the atmospheric CO2 concentration and the concentration inside the leaf causes stomatal closure and reduced transpiration.

    B. Internal factors
    1. Internal water conditions
    2. Structural features
    • The number, size, position and the movement of stomata affect rate of transpiration.
    • In dark, stomata are closed and stomatal transpiration is checked.
    • Sunken stomata help in reducing the rate of stomatal transpiration.
    • In xerophytes, the leaves are reduced in size or may even fall to check transpiration.
    • Thick cuticle and the presence of wax coating on exposed parts reduce cuticular transpiration.
    (a) Leaf area
    • Greater the leaf area greater will be the magnitude of transpiration.
    • On the per unit basis smaller plants often transpire at a greater rate than do larger plants.
    • Larger plants loose more but on per unit area basis smaller plants loose more.
    (b) Leaf structure
    Leaf structure determines the rate of transpiration in three ways.
    (1) Thickness of cuticle
    (2) Number, density and thickening of the epidermal hairs.
    (3) The ratio of internal exposed surface area to the external exposed surface area of the leaf. Cuticular transpiration depends on the degree of its thickness. The epidermal hairs increase the thickness of the adherent stationary air. They reduce the rate of transpiration. If more of the leaf cells are exposed to the intercellular spaces the internal air of the leaf will tend to become saturated rapidly losing more water in transpiration.
    (c) Stomata
    • The Rte of transpiration is influenced by the number, spacing, distribution structural peculiarities, size of the stomatal aperture and the periodicity of the stomatal opening.
    • The rate of transpiration in is little in xerophytes because their stomata remain open during the night and closed during the day.
    • Sunken stomata reduce the rate of transpiration.
    (d) Leaf orientation
    • Solar radiations cause more heating when the flat surface of the leaf lies perpendicular to the incident light.
    • The effect is minimum when it lies parallel to it as found in compass paints Lactuca.
    • Leaves of Eucalyptus hang downwardly to avoid overheating during the hot periods of the day.
    (e) Leaf size and shape
    • With the decrease in the leaf size the rate of transpiration decreases.
    • It is very little in needle shaped leaves
    (f) Leaf modifications
    • Like spines, thorns and scales show reduced rate of transpiration.
    (g) Root shoot ratio
    • Parker found that transpiration increases with increase in root shoot ratio.
    • Sorghum typically transpires at higher rate than corn plant per unit of leaf surface.
    • Muller has pointed out that the secondary root development is much more advanced in Sorghum than that in corn.
    (h) Mucilage and solutes
    • They decrease the rate of transpiration by holding water tenaciously.
    (i) Water content of the leaves
    • Optimum transpiration continues only when the leaves have sufficient moisture.
    • Low water content of the leaf generally brings down the rate of transpiration by decreasing water vapor pressure inside the leaf and closure of the stomata.
    (j) Diseases
    • The rate of transpiration is generally higher in the diseased plants as compared to healthy ones.


Last modified: Tuesday, 5 June 2012, 7:05 PM