Factors affecting the rate of transpiration in plants are divided into two i.e. external factors and internal factors which are discussed below:
A. External Factors :
1. Light : Light is very important factor of transpiration . In most plants stomata open during the day. Also light helps in increasing the temperature of atmosphere. Stomata open in dim sunlight (50-100 ft. candles) or in moonlight.Red light (660 mu)is most effective in causing stomatal opening followed by blue light (445 mu) like photosynthesis.
2. Relative Humidity (R.H.) of atmosphere : Relative humidity is the ratio of actual amount of water vapour present in the atmosphere and the amount of water vapour required to saturate it completely (before condensation) at that temperature and pressure. If R.H. of a place is low there is high rate of transpiration and if it is high the transpiration is low. R.H. decreases with increasing temperature and increases with decreasing temperature.
3. Wind :Wind velocity effects transpiration. If wind velocity is high the rate of transpiration is also high. It is due to the reason because wind removes humid air from around the leaf due to which a difference between the inner water vapour and outside is maintained.
4. Temperature :Temperature directly effects the transpiration. As the temperature rises so does the transpiration. Temperature in its turn is effected by light.
5. Available water :If available water in the soil is low the transpiration is also low. With the deficiency of available water leaves wilt and stomata are closed thus decreasing the rate of transpiration.
B. Internal Factors :
6. Structure of leaves :Thick cuticle, waxy coating on leaves, sunken stomata, covering of dead hair on leaves decreases transpiration. That is why these adaptations are found in xerophytes.