Transpiration
The process of movement of water molecules through plant and its evaporation (or loss) from aerial parts of the plant (such as leaves) is called transpiration.
Plants absorb water from soil by their roots and only a small amount of this water is utilised by plant for their growth and metabolism and the remaining excess water is released out of their body through evaporation by a process called transpiration.
The significance of transpiration is given below:
(i) Transpiration helps in the conduction of water and different minerals to different parts of the plant.
(ii) Since due to transpiration water gets continuously eliminated from the plant body so a balance of water is maintained within the plant.
(iii) It maintains osmosis in the plant and keeps plant cells rigid.
(iv) Transpiration helps in accumulation of certain hydrophilic salts on the leaf surface, which keeps the leaves moist.
(v) It also maintains the turgidity of the plant cells and helps in cell division.
(vi) An optimum level of transpiration helps in the proper growth of the plants.
(vii) The cooling effect from a plant is due to the evaporation of water from its leaves.
10 different factors that affects the rate of transpiration are given below:
1. Relative Humidity:
The relative humidity can be defined as the percentage of water vapour present in the air at a given time and temperature relative to the amount required to be present to make the air saturated at that temperature. The rate of transpiration is inversely proportional to the relative humidity, which means that if the rate of transpiration is higher then the relative humidity will be lower and if the relative humidity is higher then the rate of transpiration will be lower.
It is so because the interior portion of the leaf has a nearly saturated air in its intercellular spaces. Relative humidity of the atmospheric air governs its vapour pressure deficit or DPD (Diffusion Pressure Deficit) or water potential. Since DPD of atmospheric air is higher at low relative humidity, so more amount of water vapour will diffuse out of the interior part of the leaf as compared to the high relative humidity when DPD is lower.
2. Atmospheric Temperature:
A high temperature opens stomata (present on the leaf surface of plant) even in darkness. Besides producing a heating effect, it also lowers the relative humidity of the air and increases the vapour pressure inside transpiring organ. For every 10°C rise in atmospheric temperature, vapour pressure inside the leaves gets doubled and the relative humidity decreases by 50%. Thus, the rate of transpiration increases.
Whereas, very high temperature may cause desiccation and closure of the stomata. Very low temperature also closes the stomatal opening and hence decreases the rate of transpiration.
3. Light:
Usually in most of the plants stomata opens in the presence of light and closes in darkness. So, a high intensity light increases the rate of transpiration. As the transpiration occurs mostly through stomata of the leaves. Whereas, the rate of transpiration falls down in the absence of light.
4. Wind:
The rate of transpiration is less in the still air as the water vapour accumulates around the transpiring organs and reduces the DPD (Diffusion Pressure Deficit) of the air.
The continuous flow of the air increases the rate of transpiration by removing the saturated air around the leaves. An optimum wind velocity increases the rate of transpiration. While an excessive wind velocity decreases the rate of transpiration by closing the stomata, which occurs due to mechanical effect, drying and cooling of the transpiring organs.
5. Atmospheric Pressure:
Low atmospheric pressure increases the rate of transpiration by enhancing the rate of evaporation and producing air currents.
6. Availability of Water:
The rate of transpiration depends mostly upon the availability of water in the soil. As the water that plants absorb from the soil (through their roots) is the water which is lost through the process of transpiration. Decrease in the availability of water in the soil causes partial dehydration of the leaf cells which results in closure of stomata and decrease in the rate of transpiration.
7. Leaf Area (Transpiring Area):
The rate of transpiration is higher in plants with large leaf area, than in plants with less leaf area. As the large surface area of the leaf provides maximum space for large number stomata. Thus, increases the rate of transpiration.
8. Leaf Structure:
(A)Thickness of Cuticle:
A thick cuticular layer in the epidermal wall of the leaf decreases the rate of transpiration. As it covers the stomata of the leaf and not exposes the stomata to the atmosphere.
(B) Number and Position of Stomata:
Since most of the transpiration takes place through the stomata, so their number influences the rate of transpiration. Most of the dicot plants have stomata restricted to the lower surface of their leaves, while the monocot leaves have equal number of stomata on both the surfaces.
(C) Sunken Stomata:
The sunken stomata decreases the rate of transpiration as it reduces the surface area of the stomata. Thus, only a little movement of air occurs.
(D) Mesophyll:
Compact mesophyll decreases the rate of transpiration as it contains more palisade tissues and few intercellular spaces, while a loose mesophyll increases the rate of transpiration as it contains more number of spongy tissues and larger intercellular spaces.
(E) Leaf Modifications:
Formation of prickles, leaf spines, scaly leaves, phyllodes phylloclades are all different leaf modifications found in xerophytes which decreases the rate of transpiration.
9. Root/Shoot Ratio:
An increase in root/shoot ratio decreases the rate of transpiration while decrease in root/shoot ratio increases the rate of transpiration.
10. Mucilage and Solutes:
Increase in the amount of mucilage and solutes decreases the rate of transpiration by holding water tenaciously.
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