Transpiration

Under transpiration is understood ( sweating ) on the one hand the evaporation of water through the stomata in the leaves of plants, on the other hand, the visible perspiration on the sweat glands; excessive sweating is also called hyperhidrosis.

The amount of water transpired is expressed by the rate of transpiration.

There are two types of transpiration, in botany: stomatal and cuticular transpiration.

Stomatal transpiration

In plants, the water is generally issued by special adjustable openings, the stomata, the extent of release of water at different plant species can vary by more than two orders of magnitude and depends on the location of the plants.

The so-called stomata consist of two guard cells, which are provided with chloroplasts. The control of the gap apertures ( stoma ) is regulated by the turgor ( pressure of the cell ).

In this case, this regulation is particularly important as the plant would otherwise lose too much water and thus dry up. In warm regions, there are special adaptations for the transpiration of plants, because there is the plant still face the problem that they must accept on the one hand through the stomata CO2, but this again is losing water. To alleviate this problem, many plants have sunken stomata with special hairs.

Cuticular transpiration

Here transpiration over the entire cuticle of the leaf surface occurs. But it is in contrast to stomatal transpiration of little importance. However, it is of the plant can not be controlled, whereby it is only dependent on the Cuticuladicke. The thicker the layer of wax, the less water per unit of time transpiring unintentionally.

After one back in the 19th century by the plant physiologist Joseph Anton Böhm ( 1831-1893 ) represented hypothesis - the cohesion theory of water transport - transpiration is responsible for ensuring that as a result of water loss in the xylem of the plant as a result of cohesive forces, a negative pressure in developed the leaves, which is responsible for the transport of water and nutrients to the root system into the sheets.

See also: Photosynthesis - osmosis - diffusion

Factors of transpiration

Physiological factors

  • C3 plants
  • C4 plants
  • CAM plants

Environmental factors

  • Atmospheric humidity

With the increase of water content in the air will reduce the difference in water potential. The blade has a value of -15,000 hPa, the air at a relative humidity of 50 %, from about -1000000 hPa; at a relative humidity of 90 % there are 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 by a rapid removal of water vapor, so that the water potential is always high. In still air, however, can form a water-saturated atmosphere and hence reduce the potential difference around the stomata. Which holds close to saturation by recessed Stomataöffnungen into the leaf interior and / or fine hairs ( trichomes ) by means of the effect of the hydrodynamic boundary layer humidity in the vicinity of the stomata.

  • Light intensity

Since at elevated light increases the rate of photosynthesis, the CO2 demand increases in the plant. This is covered by the opening of the stomata, as now CO2 can flow into the inner leaf.

  • Temperature

In order to prevent overheating at high temperatures and therefore degeneration of the cell components, the plant utilizes the latent heat of evaporation. With increasing temperature, but decreases the amount of heat absorbed an evaporating water molecule. Thus, the plant has to evaporate more water to maintain its temperature. Another protective mechanism against overheating is a sunlight -reflecting waxy layer ( cuticle ).

  • Water availability

Can not get enough water from the soil, the plant, the transpiration is interrupted and the plant dries up.

Management factors

  • Plant density

Many plants have to share the same water. In addition, the susceptibility to disease increases.

  • Nutrient deficiency

Especially potassium, which is essential for the locking mechanism of the stomata.

  • Solidified soil

Reduced water availability.

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