What is perspiration Why is it important


transpiration, in the case of plants, reference for the release of water vapor to the environment due to diffusion, which, in contrast to evaporation, predominantly via the stomatastomatal T.) and to a lesser extent over the cuticle (cuticular T.) takes place and can be controlled by their opening state or structure (stomata opening movements). For plants, T. is not only a disadvantage to be taken into account at the location, which in certain circumstances can become a growth-limiting factor, but also offers leaves and shoots protection against overheating (evaporation cold).

In higher plants, T. occurs primarily as a release of steam into the intercellular areas of the leaves, with the Transpiration rate depends on the difference between the water potential or vapor pressure (water vapor concentration) inside the leaf and the atmosphere and the sum of the diffusion resistances. In analogy to the flow of electrons in a circuit, the Stomal resistance (rs) and the boundary layer resistance (rb; boundary layer) to more or less large Transpiration currents. The Transpiration rate E can thus be described as follows:

The driving force of the T. is the low water potential of the air that is not saturated with water vapor, whereby the plant is “clamped” between the high water potential of the soil and the low water potential of the air. The existing potential gradient, i.e. the difference in the water concentration inside (ci) and outside (ca), thus generating a transpiration current without using its own energy. It not only supplies the above-ground parts of the plant with water via the xylem, but also allows nutrients from the soil to enter the plants. Factors that increase this potential gradient lead to an increase in T. Temperature increases, for example, reduce the water vapor pressure in the air if the absolute humidity remains the same and thus increase the T. An increase also occurs with wind, as changes in rb comes. In the same way, factors that control the opening and closing of the stomata affect the T. The amount of water released by T. can be substantial in higher plants depending on the surface area of ​​the leaves that act as the main organs of T. A birch tree can evaporate up to 70 l of water on a sunny day, and even up to 400 l of water on hot, dry days. This usually results in a typical daily cycle in which the T. increases in the morning due to the light-induced opening of the stomata and in the morning due to the warming of the leaf surface. At noon the T. usually falls off again (stoma closure) and then comes to a standstill by evening. If the T. comes to a standstill due to high humidity, the water flow within the plant is maintained by guttation. (Drought stress, wilting)

Transpiration: Overview of the diffusion resistances that exist during the gas exchange of a leaf. The exchange of O2 is not included for reasons of clarity. rs stomatal resistance, rc cuticular resistance, ra Boundary layer resistance, rm Mesophyll resistance (only with O2)