Following factors affect the rate of transpiration:
(a) External factors
- Humidity of air
Water is evaporated through the stomata. This follows the simple law of diffusion. ‘Ibis diffusion can take place only if the water vapor content of the outer atmosphere is less than that of the inter-cellular spaces of the leaf. Transpiration is negligible in an atmosphere saturated with water. ‘Me drier the air. the more rapid is the transpiration. Following factors affect the humidity of air:
- Saturation deficit: Saturation deficit is the difference between the
amount of Ysater vapor actuall) present in the air and the amount necessary to completely saturate it. The rate of transpiration depends upon the saturation deficit.
- Absolute humidity: ‘Hu: amount of moisture actually present in the air is called its absolute humidity
- Relative humidity: The percentage of the amount of moisture
necessary for saturation at a particular temperature is called relative humidity. The relatfte humidity decreases (or increases) with every rise in the temperature. But the absolute humidity remains unaffected. The rate of transpiration increases at lower relative humidity of the air and vice versa.
Temperature affects the saturation deficit of the air. So it has an indirect influence on the rate of transpiration. Rise in temperature decrease the relative humidity of the air. It increases the rate of transpiration. Low temperatures decrease the capacity (lithe air to hold moisture. Therefore, it increases the. relative humidity. It decreases the rate of transpiration. ligh temperatures also open the stomata widely. It increases the
The moving air is called n intl. Wind has a [ton erlid effect on hum idit) Dry nind rea ‘es the moist air from the immediate %kinks of the
plants. It lour, • amount of air moisture near plant. As a result, the
humidity of the air isitinered. It promotes transpiration.
The rate of transpitation increases in light and decreases in the dark. Light affects transit ration in tuo nays. It raises the temperature of the lea es and increases the transpiration. Secondly. shun:Ha are opened in the presence of light. It increases transpiration.
- Atmospheric pressure
Low air pressure reduces the densitn of the air.’Ibtis it increases the rate of transpiration. Air pressure is lonered at high altitudes.
- Availability of Soil Water
‘there should be enough uater absorbed by the roots from the soil. It n ill compensate the stater lost by transpiration. When there is loner absorption of water. there n ill be to rate of transpiration. the factors hie!’ affect the absorption of %%titer also ailed the rate of transpiration.
(b) hrtentel hatees
I. Role of stomata: Most of the transpiration takes place through stomata. transpiration depends on the opening anti closing of stomata. Di Ilerent factor affect the opening and closing of stomata.
- Water relation of the Parenchyma cell: The parench) ma mesophyll cells also control the rate of iranspimtion. they become – saturated oith ;safer. Thus their nails easily • lose nater into the internal atmosphere of the leaf ibis loss of nater is compensated b) absorption of mato- from the root. If root cells do not absorb much %% titer then the %%titer content of the mesophyll cells decreases. .Fherelbre. mesophyll cells loss turgor. Their cell nails become tip. thus the evaporation trout their surfaces is reduced although the stomata remain open. As a result osmotic pressure of the memmhyll cells increases. thus they n ithdrann ater from the guard cells. the guard cells lose their turgor. Ilms stomata are closed Oen in the presence tn. HOB. Thus the inter nil %%Mel’ relation of the leaf are self regulating mechanism for the control of transpiration
- Number of stomata per unit area: the number of stomata per mitt area of leaf surlitce and their position also affect the rate of
transpiration. The stomata are sunken in depression bet. the epidermis in some plants like oleander and Pinus. It reduces the rate of transpiration.
4. Thickness of cuticle: Cuticular transpiration depends upon the thickness of cuticle. If cuticle is thin. there is more transpiration and vice versa.