Factors Affecting the Rate of Transpiration

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Temperature

An increase in temperature increases the kinetic energy of molecules, increasing the rate of evaporation of water molecules from the mesophyll cells and accelerating the rate of diffusion through the stomal pores and into the atmosphere. The greater the temperature, the more rapidly the water molecules will diffuse away from the stomata, decreasing the water potential of the atmosphere. As water molecules always move down a concentration gradient, this increases the transpiration rate. 

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Humidity

In more humid air, the water potential of the atmosphere is greater, reducing the water potential gradient between inside and outside the leaf, decreasing the transpiration rate.

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Air Movement

The movement of the surrounding air blows away the layers of humid air surrounding the stomata, decreasing the water potential of the atmosphere and so increasing the water potential gradient between inside and outside the leaf. This causes water molecules to diffuse out of the stomata more rapidly, increasing the transpiration rate. 

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Light Intensity

Light intensity affects the rate of transpiration by controlling the degree of stomal opening. In most plants, the stomata open wider when the light intensity increases, increasing the rate of transpiration. 

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