The following points highlight the six main factors affecting transpiration in plants. The factors are: 1. Humidity of Air 2. Light or Illumination 3. Temperature 4. Wind 5. Atmospheric Pressure 6. Soil Factors.
Factor # 1. Humidity of Air:
As transpiration involves diffusion of water vapour from regions of high concentration (intercellular spaces of leaves) to that of low concentration (outside air), humidity or amount of water vapour in the atmosphere, surrounding the plant has considerable influence on the process.
On damp foggy days the rate of transpiration decreases as the outer air remains saturated with water vapour. The less moisture there is in air, the greater will be the rate of transpiration.
Factor # 2. Light or Illumination:
It has marked effect on transpiration. The opening and closing of the stomata, through which by far the maximum amount of water is lost, depend on light. Moreover, due to absorption of radiant energy and its transformation into heat, temperature of the leaf is raised bringing about an increase in transpiration rates.
Factor # 3. Temperature:
It increases the rate of transpiration as it hastens transformation of water into water vapour.
Factor # 4. Wind:
By wind or air current water vapour given off during transpiration is removed; thus saturation of the surrounding air is avoided which otherwise would retard the rate of transpiration. Winds of high however, retard transpiration, because the stomata close up due to high winds. Moreover, winds of high velocity bring about a reduction in temperature which undoubtedly affects transpiration.
Factor # 5. Atmospheric Pressure:
Atmospheric pressure affects transpiration, because it has influence on the rate of vaporization. When atmospheric pressure is high, the rate of transpiration is low. However, in any given locality variations in the atmospheric pressure are too slight to have any considerable effect upon transpiration rates.
Plants growing in high altitudes have distinctly lower atmospheric pressures, and as such, those plants have significantly high rates of transpiration, if other environmental factors are not limiting.
Factor # 6. Soil Factors:
As all necessary water is absorbed from the soil, factors like water content, composition, temperature, concentration of soil solution, etc., indirectly influence the rate of transpiration.