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Factors Affecting the Rate of Transpiration
The rate transpiration is affected by a number of environmental (external) and plant (internal) factors. These factors also affect the stomatal movements.
Effect of Environmental factors
The environmental factors such as humidity (vapour pressure), temperature, air movements (wind), atmospheric pressure, light and availability of soil water influence the rate of transpiration most.
Effect of Humidity (Vapour Pressure)
The humidity of the atmosphere the gradient the sub-stomatal cavity and air outside the leaf. Low humidity outside the leaf favors transpiration because it increases the gradient.
Effect of Temperature
The higher the temperature the greater the rate of transpiration since high temperature provides latent heat of vaporization and lowers the relative humidity of air. Both these factors increase the vapour gradient between sub-stomatal cavity and external atmosphere. Temperature also helps opening of stomata since stomata remain open up to about 30° C. in fact, in the presence of light, the temperature has the greatest effect on the rate of transpiration.
Effect of Air Movements (Wind)
Moving air, especially the low speed wind, sweeps away the water vapour in the air outside the stomata, speeding up diffusion of water vapour by making a steeper gradient between stomatal cavity and the air. Thus, windy conditions lead to an increased rate of transpiration. On the other hand, still air builds up a shell around the leaf reducing the steepness of vapour gradient. High winds may result in closure of stomata.
Effect of Atmospheric Pressure
Water vapour pressure decreases as the atmospheric pressure decreases with increasing altitude. The lower the atmospheric pressure, the greater is the rate of evaporation of water from leaves. Most alpine plants show xeromorphic features which help reduce water loss.
Effect of Light
Light affects transpiration indirectly. Stomata tend to be open in the light, therefore, the rate of transpiration increases in the light than in the dark. However, in CAM plants the stomata open in dark and remain closed in light. Light is also the chief source of heat energy for the plant, enhancing evaporation.
Effect of Availability of Soil Water
Storage of water in the soil leads to wilting of plants and closure of the stomata. As the soil dries out the remaining soil water becomes more concentrated solution and becomes bound to soil particles more tightly. The water movement into the root cells by osmosis is reduced. This reduced uptake of water slows down the rate of transpiration. When the water potential of the soil water is lower than the cells of the root the permanent wilting, point has been reached and plants die quickly.
Effect of Plant factors
The root-shoot ratio, leaf surface area and surface area to volume ratio and leaf structure are the major internal factors affecting the rate of transpiration.
Root-Shoot Ratio
The efficiency of absorbing (roots) and evaporating surfaces (shoots) affect the rate of transpiration. If rate of transpiration exceeds rate of absorption, a water stress condition develops and rate of transpiration is reduced. It has been observed that rate of transpiration increases with increase in root shoot ratio
For example, the rate of transpiration is higher in Sorghum as compared to Corn since, it possesses secondary roots which increase its absorption surface.
Leaf Surface Area & Surface Area to Volume Ratio
The rate of transpiration is directly proportional to leaf surface area and surface area to volume ratio. Transpiration of a plant increases with its total leaf surface area and with leaf surface area to volume ratio.
For example, conifers, the leaves are reduced to needles and the rate of transpiration is reduced, while the shade plants with large leaf surface area lose much water vapour.
Structure of Leaf
Leaf shape, thickness of cuticle and number and distribution of stomata influence the rate of transpiration.
Cuticle: if the cuticle is thin the rate of transpiration is higher, for example in ferns 30%-45% water loss is cuticular since the cuticle is thin. In xerophytes cuticle is extremely thick and prevent transpiration through it.
Stomata: the number of stomata per unit area affect the stomatal transpiration. The higher the number of stomata, the greater the rate of transpiration. In xerophytes, the number of stomata are reduced and sunken, thus the rate of transpiration is lesser.
Distribution of stomata plays an important role in preventing loss of water vapour. In dicots, the stomata are confined to lower epidermis usually, therefore not in, contact with environment directly and the rate of transpiration is lesser. On the other hand, in monocots, the leaves are vertical and stomata are distributed on both surfaces equally, thus the rate of transpiration is higher.