Factors Affecting the Rate of Transpiration
Environmental factors such as intensity of light, temperature, atmospheric humidity, wind velocity, availability of soil water, etc., affect the rate of transpiration.
External Factors Affecting The Rate of Transpiration
Light affects the rate of transpiration as it is directly involved in the opening and closing of stomata. Stomata open in light and close in darkness. Therefore, the rate of transpiration is high during the daytime and at night, only some amount of water is lost by cuticular and lenticular transpiration.
Light also affects the rate of transpiration indirectly by affecting the temperature and membrane permeability of the cell. Long duration of light and high intensity of light increases the temperature, thereby increasing the rate of transpiration. High intensity of light also increases the permeability of cell membrane, resulting into diffusion of water vapours into the atmosphere.
The rate of transpiration increases with the increase in atmospheric temperature. This is because at higher temperature, there is more evaporation of water from the mesophyll cells and therefore, there is greater saturation of the leaf atmosphere with water vapour.
Rise in temperature increases the rate of water evaporation from the cell surface which opens the stomata and decreases the relative humidity of the atmosphere. As a result, there is rapid diffusion of water vapour from humid atmosphere inside the leaf to outer dry atmosphere.
The rate of transpiration depends upon the difference in vapour pressure between the internal atmosphere of the leaf and air outside the leaf. If outer air is more humid, it will reduce the diffusion of water vapour from leaf to the outer atmosphere. In other words, rate of transpiration decreases with the increase in relative humidity of atmosphere and the rate of transpiration increases by lowering atmospheric humidity.
Lowering of atmospheric pressure reduces the density of the environment which allows rapid diffusion of water vapour .In other words the rate of transpiration increases with the decrease in atmospheric pressure.
Transpiring leaf continuously adds water vapour to the atmosphere. Air around the leaf becomes saturated and the rate of transpiration decreases. The wind removes the air of that area and is replaced by fresh air and results in the increase in the rate of transpiration.
Hence, the increase in wind velocity increases the rate of transpiration but very high wind velocity decreases the rate of transpiration because it leads to stomatal closure by lowering the temperature of transpiring surface.
Availability of soil water
The rate of transpiration depends upon the rate of absorption of soil water by roots. This is further influenced by a number of soil factors like soil water, soil particles, soil temperature, soil air etc. The rate of transpiration increases with the increase in the availability of water in the soil.
In a dry soil, the soil solution becomes more concentrated, therefore, there is less tendency of water to enter by osmosis. It means there is less uptake of water by the roots, hence, the rate of transpiration is also lowered.
Plants show many external and internal factors which enable them to reduce the rate of transpiration. Such features are called xeromorphic and are found in xerophytes, the plants growing in dry habitats. Plants growing in soil with adequate water supply are known mesophytes. Such plants can, however, also show some xeromorphic features.
Structure of leaf surface
The following anatomical features reduces the rate of transpiration:
(a) Presence of thick cuticle on the surface of the leaf: Cuticular transpiration decreases with the thickness of cuticle and cutinisation of epidermal walls.
(b) Presence of wax, resin and sugar on the surface of the leaf: Presence of wax layer or trichomes on the leaf reduce the rate of transpiration.
(c) Presence of compact mesophyll cells: Compact mesophyll cells reduces the rate of transpiration
(d) Reduction in the number of stomata: Less no of stomata reduces the rate of transpiration
(e) Presence of sunken stomata: The sunken stomata reduce the rate of transpiration by providing an area where little air movement occurs.
Area of transpiring surface
Leaves are the main organ of transpiration. Therefore, any decease in leaf surface area will reduce transpiration and conserve water absorbed by roots. Many grasses roll up their leaves during dry condition, thus exposing less surface to the air, thereby lowering the rate of transpiration.
Reduction in the leaf surface is achieved when leaves are reduced to needles, for example, Pinus or spines for example cacti. A corn plant loses 3-4 litres of water per day, whereas a tree-sized desert cactus with no foliage loses less than 25 ml of water per day. The shedding of leaves in dry or cold season by deciduous plants is a xeromorphic adaptation.
The number, distribution and structures of stomata affect the rate of transpiration. The greater number of stomata per unit area, the greater is the rate of stomatal transpiration. However, the distribution of stomata is also important.
For example, in dicotyledons, the lower surface has more stomata than the upper surface whereas monocotyledonous leaves, which are held vertically, have equal distribution of stomata on both the surfaces. Loss of water is more from the leaves that have more stomata on the lower surface than those having equal number on both the surfaces.
In xerophytic plants, stomata occur below the general level of leaf surface. Such stomata are known as sunken. In this condition, water vapour diffusing through stomata accumulates in the pit where stoma is located. It reduces the diffusion rate and this helps in reducing the rate of transpiration.
Age of plant
Rate of transpiration is slow at seedling stage, maximum at maturity and gradually decrease near senescence