Transpiration in leaves

Transpiration is the loss of water vapour from leaves. It happens in two connected stages: water first evaporates inside the leaf, then water vapour diffuses out.

Water on the surfaces of mesophyll cells evaporates into the air spaces between the cells. During evaporation, liquid water changes into water vapour.

The water vapour then diffuses through the air spaces and leaves through small openings called stomata. Diffusion is the net movement of water vapour from a region of higher concentration to a region of lower concentration. Thus, evaporation supplies water vapour inside the leaf, and diffusion carries it out.

Effects of temperature and wind speed on transpiration rate

Transpiration is the loss of water vapour from a plant, mainly through the leaves. Water evaporates from the surfaces of mesophyll cells into the leaf air spaces, then water vapour diffuses out of the leaf through the stomata.

Effect of temperature on transpiration

A higher temperature generally increases the transpiration rate. Water evaporates more rapidly from mesophyll cell surfaces, and water vapour diffuses out of the leaf more rapidly. Together, these changes increase the rate at which the leaf loses water.

Effect of wind speed on transpiration

Greater wind speed generally increases the transpiration rate. Wind carries water vapour away from the leaf, preventing it from building up in the surrounding air. This maintains a difference in water vapour concentration between the leaf air spaces and the air outside, so water vapour continues to diffuse out of the leaf.

Investigating temperature or wind speed

To investigate one factor, vary either temperature or wind speed and keep the other conditions as constant as possible. For example, when testing temperature, keep wind speed, light and humidity as constant as possible. When testing wind speed, keep temperature, light and humidity as constant as possible.

Measure water loss over equal time intervals and compare the results. For example, a plant and its pot can be weighed before and after each interval, with the soil surface covered to reduce direct evaporation from the soil. A greater decrease in mass over the same time indicates a higher rate of water loss by transpiration.

Leaf structure and water vapour loss

Inside a leaf, the mesophyll cells have moist surfaces next to air spaces. These spaces connect with one another and expose a large internal surface area. Water can evaporate from these surfaces into the air spaces.

The water vapour then moves through the connected spaces towards the stomata: small pores in the leaf surface. When stomata are open, water vapour can diffuse out of the leaf into the surrounding air.

The number and size of open stomata affect the opportunity for water vapour to escape. When other conditions are similar, more open stomata, or larger open stomata, provide more space for diffusion and allow a greater rate of water vapour loss.

Transpiration pull

Water evaporates from surfaces inside leaves and is lost from the plant as water vapour. As water is lost, it draws more water from the leaf’s xylem, the tissue that carries water upwards through the plant. This drawing force is called transpiration pull.

Water molecules attract one another, so they remain connected as a continuous column within the xylem. When water is drawn from the top of the column into the leaf, the attraction between molecules transmits the pull down the column. This draws water upwards through the plant to replace the water lost from the leaves.

Effect of humidity on transpiration rate

Water evaporates inside a leaf and diffuses as water vapour from the leaf’s air spaces into the surrounding air, mainly through openings called stomata. This loss of water vapour is called transpiration.

Humidity describes how much water vapour is in the surrounding air. It affects the difference in water vapour concentration between the leaf air spaces and the outside air:

• High humidity: The outside air contains more water vapour, so the concentration difference is smaller. Water vapour diffuses out of the leaf more slowly, decreasing the transpiration rate.
• Low humidity: The outside air contains less water vapour, so the concentration difference is greater. Water vapour diffuses out of the leaf more rapidly, increasing the transpiration rate.

These comparisons apply when other conditions remain the same.

Wilting

A plant wilts when it loses water faster than it can replace it. Water is lost from the plant through transpiration, mainly from its leaves, and replacement water is taken up by the roots.

Water inside a plant cell pushes the cell contents against the cell wall. This pressure, called turgor pressure, makes the cell firm. When the plant loses water faster than it takes it up, its cells lose water and their turgor pressure decreases. The cells become less firm.

The less-firm cells provide less support to leaves and other soft parts of the plant. These parts can no longer remain upright and begin to droop: this is wilting.