The rate of transpiration is affected by a number of internal (plant factor) and external factors.
I. Internal or Plant Factors:
(i) Root – shoot ratio:
If all other conditions are favourable for transpiration, the water absorbing capacity of roo surface and transpiring capacity of the leaf suface regulate the rate of transpiration if transpiration is greater than absorption, a water deficit results, causing reduction in transpiration rate. In other words a low root/shoot ratio decreases the rate of transpiration.
(ii) Leaf area:
The greater the leaf area, the higher will be the water loss due to transpiration.
(iii) Stomatal frequency:
Stomatal frequency means the total number of stomata per unit leaf area. Stomatal frequency varies with different species. Greater the frequency of stomata, faster is the rate of transpiration.
(iv) Structure of leaf:
Presesncc of thick cuticlc, wax layers and trichomes on the surface of leave reduce the rate of transpiration.
II. External or Environmental Factors
There is a close relationship between the opening of stomata and presence of light. Light affects the rate of transpiration in two ways. Firstly, light causes stomata to open.
As a result of wide opening of stomata, the saturated interior cells of leaf arc exposed to the outer atmosphere. Consquently, the rate of transpiration is increased in bright sunlight. Secondly, it increases the temperature of leaf and thus affects the rate of transpiration. In a nut shell, the combined affect of light causes opening of stomata and incerases the rate of vaperisation of water.
The increase in the wind velocity increases the rate of transpiration by remouing the water vapour of the atmosphere from the vicinity of transpiring surface and lowering relative humidity. The transpiration is faster in mild wind. The winds of much higher velocity retard the rate of transpiration.
The increase in temperature increases the rate of transpiration. This is due to increase in the rate of evaporation of water from cell surface and decrease in the humidity of the external atmosphere. However, there is a limit in rise of temperature in relation to loss of water by transpiration. At very high temperature, usually beyond 35°C the rate of transpiration gradually falls due to inactivity of the protoplasm.
(iv) Humidity of the air:
Humidity is expressed as the percentage of water vapour present in the atmosphere. The relative humidity of the atmosphere affected the rate of transpiration to a great extent because it influences the DPD gradient between the intercellular spaces and outside atmosphere.
The higher the relative humidity of the outside atmosphere the lower will be the rate of transpiration. Conversely the lower the relative humidity of the outside atmosphere the higher will be the rate of transpiration.
(v) Atmospheric pressure:
The reduction of atmospheric pressure reduces the density of the external atmosphere. This allows more rapid diffusion of water. The plants growing on hills show higher rate of transpiration because of low atmospheric pressure and thus they develop xerophytes characters
(vi) Water Supply:
Deficiency of water in soil decreases the rate of transpiration. This is due to low absorption of water from the soil.