# Transpiration - What Controls Rates of Transpiration?

How fast does water move through plants? Transpiration rates depend on two major factors: 1) the driving force for water movement from the soil to the atmosphere and 2) the resistances to water movement in the plant.

Driving force: The driving force for transpiration is the difference in water potential between the soil and the atmosphere surrounding the plant. This difference creates a gradient, forcing water to move toward areas with less water. The drier the air around the plant, the greater the driving force is for water to move through the plant and the faster the transpiration rate. The following section, FACTORS AFFECTING RATES OF TRANSPIRATION, expands on how changes in the environment alter this driving force and thus transpiration.

Resistances: There are three major resistances to the movement of water out of a leaf: cuticle resistance, stomata resistance and boundary layer resistance. These resistances slow water movement. The greater any individual resistance is to water movement, the slower the transpiration rate. The following section, FACTORS AFFECTING RATES OF TRANSPIRATION, expands on how changes in the plant alter these resistances and thus transpiration.

A simple equation describing how these factors alter transpiration is:

Transpiration = [Water potential(leaf)] – [Water potential(atmosphere)]

Resistance

The units for this equation are mols of water lost per leaf area per time (mol/cm2/s). This equation makes predicting rates of transpiration easy. For example, any time the numerator (the value for the driving force) is increased, the rate of transpiration becomes faster and vice versa. Similarly, if the denominator (the value for resistance) increases, this means there is greater resistance and thus, slower transpiration.

View the slideshow ’Water potential in plants and how to measure it’*NOTE: The PowerPoint may take several seconds to load.