In botany, transpiration is the evaporation of water through the leaves of the plants , especially through their adjustable stomata ( stomata , hence “stomatal perspiration”), but also through the rest of the outer skin ( cuticle , hence “cuticular perspiration”).
The amount of water transpired over a certain period of time is the rate of transpiration .
In the case of plants, the water is generally released through special adjustable openings, the stomata , whereby the extent of the water release can vary by more than two orders of magnitude in different plant species and is dependent on the respective location of the plants.
The so-called stomata consist of two guard cells that are provided with chloroplasts. The control of the stomata (stomata) is regulated by the turgor (pressure of the cell).
This regulation is particularly important because otherwise the plant would lose too much water and thus dry out. In warm regions there are special adaptations for the transpiration of the plants, because there the plant is always faced with the problem that on the one hand it has to absorb CO 2 through the stomata , but it also loses water again. To reduce this problem, many plants have sunken stomata with special hairs.
Here transpiration takes place over the entire cuticle of the leaf surface. In contrast to stomatal perspiration, however, it is of little importance. However, it cannot be controlled by the plant, which means that it is only dependent on the thickness of the cuticle. The thicker the wax layer, the less water unwantedly perspires per unit of time.
According to a hypothesis put forward by the plant physiologist Joseph Anton Böhm (1831-1893) as early as the 19th century - the cohesion theory of water transport - transpiration is partly responsible for the fact that, as a result of the loss of water in the xylem of the plants as a result of cohesive forces, a negative pressure in developed in the leaves , which is responsible for the transport of water and nutrients from the root system into the leaves.
Factors of perspiration
- Atmospheric humidity
As the water content in the air increases, the difference in water potential decreases . The leaf has a value of −15,000 hPa, the air at a relative humidity of 50% of approx. −1,000,000 hPa; at a relative humidity of 90% it is only −130,000 hPa. Since water always flows to the more negative potential, the force is much higher in dry air.
- Wind speed
With an increased wind speed, the water vapor is transported away more quickly, so that the water potential is always high. When the air is stagnant, however, a water-saturated atmosphere can form around the stomata and thus the potential difference decreases. Through sunk stomata openings in the leaf interior and / or fine hairs ( trichomes ), which by means of the effect of the hydrodynamic boundary layer, keep the humidity in the vicinity of the stomata close to saturation.
- Light intensity
Since the photosynthesis rate increases with increased light, the CO 2 requirement in the plant increases. This is covered by opening the stomata, as CO 2 can now flow into the inside of the leaf.
In order to prevent overheating at high temperatures and thus a degeneration of the cell components, the plant uses the evaporative cooling. However, as the temperature rises, the amount of heat absorbed by an evaporating water molecule decreases. So the plant needs to evaporate more water to maintain its temperature. Another protective mechanism against overheating is a layer of wax ( cuticle ) that reflects sunlight .
- Water availability
If the plant can no longer absorb enough water from the soil, the aspiration of perspiration is interrupted and the plant dries up.
- Plant density
Many plants have to share the same water. In addition, the susceptibility to disease increases.
- Nutritional deficiency
Especially potassium, which is essential for the closing mechanism of the stomata.
- solidified soil
reduces water availability.
- DVWK leaflet 238: Determination of evaporation from land and water surfaces , 1996, 134 pages, DIN A4, ISBN 3-935067-84-4
- Leaflet ATV-DVWK-M 504: Evaporation in relation to land use, vegetation and soil , September 2002, 144 pages, DIN A4, ISBN 3-936514-03-8
- Expiration (English: expiration)
- Evaporation (evaporation from non-vegetated surfaces)
- Interception (evaporation from the surface of plants)