Transpiration is the loss of water due to evaporation that occurs through stomata in the leaves. It helps in transporting water and minerals, cools leaf surfaces by evaporative cooling and maintains the shape and structure of plants by keeping cells turgid.
Transpiration is affected by external factors, like temperature, light, humidity, etc. and plant factors like the number and distribution of stomata. Typically, the lower surface of a dicot leaf has a greater number of stomata, while a monocot leaf has an equal number of stomata on both surfaces. The opening and closing of the stomata is directly impacted by the turgidity of guard cells present near the stomatal aperture.
Transpiration from the stomata causes a ‘pull’ that causes the upward ascent of water in xylem cells. This transpiration pull is made possible by three physical properties of water – cohesion, adhesion and surface tension. Cohesion is the mutual attraction between water molecules. Adhesion is the property of water to be attracted to polar surfaces like tracheids and vessel elements. Finally, surface tension is the property of water molecules to be extra attracted to each other.
While water evaporates through the stomata during transpiration, a thin film of water appears over the cells, which pulls water, molecule by molecule, into the leaf from the xylem. The concentration of water vapour in the atmosphere is lower than that in the substomatal cavity and intercellular spaces. This creates a water potential gradient, which makes the water diffuse into the surrounding air, creating a pull.
Photosynthesising plants require a lot of water. Since water swiftly depletes due to transpiration, the lack of available water may limit photosynthesis. The plant C4 photosynthetic system solves this by increasing the availability of carbon dioxide and decreasing water loss.
This system is more efficient than the C3 system since it not only fixes carbon twice as a C3 system, but also loses half the amount of water lost by the C3 system while fixing the same amount of carbon dioxide.