explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere

Transport Mechanisms

Transpiration

Transpiration is the process by which water is lost from a plant to the atmosphere. It occurs in two consecutive stages:

1. Evaporation – Water moves from the internal leaf surfaces (mesophyll cells, intercellular spaces, and the epidermal layer) to the leaf surface and evaporates into the thin boundary layer that surrounds the leaf.
2. Diffusion – The water vapour that has evaporated diffuses from the leaf surface through the boundary layer and into the surrounding air until the vapour concentration in the air equals that at the leaf surface.

Because the leaf surface is exposed to the atmosphere, the concentration gradient of water vapour drives the diffusion step. The overall rate of transpiration depends on the balance of water potential between the leaf interior and the atmosphere.

The water potential of a leaf can be expressed as:

\$\Psi{\text{leaf}} = \Psis + \Psip + \Psim + \Psi_g\$

where:

• \$\Psi_s\$ – Solute potential (negative, due to dissolved solutes)
• \$\Psi_p\$ – Osmotic potential (negative, due to solute concentration)
• \$\Psi_m\$ – Matric potential (negative, due to adhesion to cell walls)
• \$\Psi_g\$ – Gravitational potential (positive, increases with height above the soil)

When \$\Psi_{\text{leaf}}\$ is lower (more negative) than the water potential of the surrounding air, water will move outward, leading to transpiration.

Factors Influencing Transpiration