The rate of water loss from the leaves of a plant may be affected by a variety of environmental and internal factors. The following are the affecting factors:
i) Environmental factors
ii) Internal factors.
Environmental Affecting Factors of Transpiration
Following are the environmental affecting factors of traspration:
Affecting Factors of Transpiration: Light
Light is the most important factor that affects the rate of transpiration through stomatal mechanism. Normally, stomata open in light for photosynthetic gas exchange and close in dark. In general, transpiration rate increases with light intensity until all the stomata are open and transpiration into a maximum . Light effects may be related to its involvement in photosynthesis. This decrease CO2 concentration in the sub-stomatal cavity and increase soluble solutes in the guard cells. Besides, light also provides ATP for K+—transport into the guard cells.
Affecting Factors of Transpiration: Temperature
Temperature is the next major affecting factor of transpiration after light. At a given limited intensity, an increase in temperature increases the amount of evaporation from the mesophyll cell and also increases the amount of water vapor the air can take before it becomes saturated. Both light and temperature increase the water potential gradient between the air inside and outside the leaf increasing the rate of transpiration. However, temperature above 30-35°C brings about stomatal closing and decreased rate of transpiration in a number of plants. This is one of the reasons for mid-day close of stomata in many plants. Temperature also modifies the effect of light and Carbon dioxide on stomatal mechanism.
Affecting Factors of Transpiration: Carbon dioxide
Increase in the concentration of CO2 over the normal level (0.03%) causes stomata closure and hence decrease in the rate of transpiration. Decrease in CO2 concentration below the normal level causes stomatal opening in most plants.
Affecting Factors of Transpiration: Atmospheric humidity
The relative humidity is an expression of the ratio of the actual vapor pressure to the vapor pressure of the atmosphere when saturated at the same temperature. When relative humidity of the atmosphere is high, the rate of transpiration is low due to reduced water potential gradient between the leaf and air.
Affecting Factors of Transpiration: Atmospheric pressure
The transpiration rate increases with the decrease in atmospheric pressure. The lowering of atmospheric pressure enhances diffusion of water vapors.
Affecting Factors of Transpiration: Wind
Strong winds remove moist saturated air from the vicinity of plants and replacing it with drier unsaturated air. This promotes transpiration rate. Faster winds often induce closure of stomata due to rapid water loss from the guard cells and thus causing a reduction in the transpiration rate. Moderate winds, however, have a significant cooling effect and lower the rate of transpiration.
Internal Affecting factors of transpiration
Several structural features of plants like leaf area, structure of leaf and its orientation, root-shoot ratio, age of the plant, etc. influence the rate of transpiration. Rolling, twisting and curling of leaves also reduce transpiration because the amount of radiation received by the leaf is decreased. Several anatomical features of leaves like the presence of a thick cuticle, coating of wax on the leaf surface, reduction in the number of stomata, the presence of sunken stomata and epidermal hairs reduce the rate of transpiration. Usually, the rate of transpiration increases with the increase in shoot-root ratio.