Factors Affecting the Rate of Transpiration

Factors Affecting the Rate of Transpiration

There is a close inter-relationship between transpiration and leaf structure. The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period. Plants regulate the rate of transpiration by opening and closing of stomata (see figure below). There are, however, a number of external factors that affect the rate of transpiration, namely: temperature, light intensity, humidity, and wind.

The opening and closing of stomata. Different environmental conditions trigger both the opening and closing of stomata.

Temperature

Temperature affects the transpiration rate in two ways. Firstly, at warmer temperatures water molecules move faster, and the rate of evaporation from stomata is therefore much faster. Secondly, the water-holding capacity of warm air is greater than that of cold air. Assuming that cold air and warm air contain the same amount of water, the cold air may be saturated, and therefore have a shallow water concentration gradient, while the warm air may will be able to hold more water vapour, and will therefore have a steeper water concentration gradient.

Temperature vs transpiration rate.

Light intensity

At high light intensity, the rate of photosynthesis increases. As photosynthesis increases, the amount of stored glucose in the guard cells increases. This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute). As the water potential decreases, more water enters the guard cells making them more turgid. The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration.

Transpiration vs light intensity.

Relative humidity

The amount of water vapour in the air is referred to as the humidity. Water always moves down a concentration gradient. Therefore when the humidity is high (lots of water vapour in the air) the water potential gradient between the inside of the leaf stomata and the atmosphere is shallow and the rate of transpiration will be low. However, if the atmosphere is dry, there will be a steep water concentration gradient between the humid inside of the stomata and the outside air and the rate of transpiration will therefore be fast.

Transpiration vs humidity.

Wind

When water is lost from the leaf it forms a thin layer outside the leaf. This reduces the water potential between the leaf and the atmosphere outside. When there is wind, this layer is blown away, thus maintaining the water potential gradient across the leaf.

Wind speed vs transpiration.