Water stress

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As drought stress or water stress is called a stress which is caused by lack of water to living organisms, and particularly to plants.

• 2.2.1 leaf fall
• 2.2.2 drought stress avoidance
• 2.2.3 Drought Tolerance

Survey

Drought stress is especially important in arid climates ( desert belt, subtropics ), but also in cold areas ( tundra, alpine regions).

The important factors that result in arid regions to drought stress are: low rainfall, high temperatures, low humidity and unfavorable soil properties (water retention). They cause plant transpiration is greater than the water absorption. In cold districts, there is a risk that the Wassernachlieferung is insufficient by the freezing of the ground water, thus leading to drought stress.

Water shortages and drought stress are the main limitations for agricultural production. The losses caused by drought stress are far superior to those that are due to other abiotic or biotic factors. Most developing countries are located in arid regions and are therefore particularly affected.

Adaptation strategies in plants

Stable Hydro plants differ from hydro -labile plants in their ability to adapt to drought stress. They have their anatomy and their metabolism so far adapted to the problem of water shortage that they can respond to drought and maintain the water content in their tissues for a period of time. This stable hydro plants can be subdivided according to their strategy of adaptation in turn, water-saving and water- consuming plants:

Water-saving plants avoid water uptake by the root system and let it initially in the ground, they share it is really to be able to use it longer one. Thus it can be overcome with less water consumption a longer drying phase.

Water consuming plants follow the opposite strategy. When occurring drought they deprive the soil of all the water and store it, so that it can be dispensed with more wasted water uptake during the dry phase. An example of this kind of customization is Prosopis sp., Of Mesquite. Its root system ranges up to 100 meters deep. Once the available water in the soil is less, it extracts all the soil layers, the residual water and stores it in its trunk. Thus it is not only securing the water supply during the dry phase, he gains a competitive advantage also nearby plants to which therefore can not absorb more water from this soil.

Short-term adjustments

When drought stress ( abscisic acid ABA ) is the reduction of turgor pressure due to water loss abscisic acid formed in the root and the leaf. Abscisic acid influence on the target cells and target tissues physiological and biochemical reactions in a way that facilitates the organism 's survival under drought stress or in the first place. One of the most important tasks of abscisic acid is the induction of Stomataschlusses. The regulation of the stomata happens already after a few minutes. ABA can therefore be used in two ways:

Thus ABA allows precise and reversible control of the water balance of a plant or even a single sheet. Under light conditions, the stomatal closure leads to a decrease in the internal CO2 concentration and thus to a strong inhibition of photosynthesis. This results in the C3 plant to an increased activity of photorespiration and thus to the formation of reactive oxygen species (H2O2).

The closing of the stomata in detail

For drought stress, abscisic acid is formed in the root and also in the leaf. It is guided through the transpiration stream to the guard cells. In these cells, it induces the release of calcium ions. The increased level of calcium results in inhibition of the proton ATPase, resulting in a reduction of the transmembrane gradient H , which leads to depolarization. In addition, calcium ions bind to chloride channels. This flow of chloride ions passively from the cell, resulting in a further depolarization result. However, the exact individual steps are still the subject of research.

This depolarization has two consequences. Firstly, close the potassium channels that are open only during hyperpolarization. In addition, the outward potassium channels are the most active, which leads to a strong outflow of potassium ions. The chloride and malate anions flow to it by coupled, causing the cell to lose water osmotically and the stoma closes. The ions are stored in adjacent cells.

The regulation of the stomata happens already after a few minutes. Thus it allows a precise and reversible control of the water balance of a plant or even a single sheet. The molecular mechanisms of abscisic acid are not yet known in detail. However, only about 10 % of the usual abscisic acid concentration were found in wilting tomato mutants. However, after exogenous stomata functioned properly again.

Long-term adaptations

By slight lack of water over a longer period, the average growth of root and shoot is affected. If a plant less water available, which sprout growing more slowly or stops throughout its growth. The Assimilate actually used for this purpose are now the root concentrates made ​​available, it grows on and on and can tap into deeper areas of fresh water, while the upper soil layers dry out slowly. This plant is characterized in moist soils with a shallow root system in dry soil by a very deep root system. This is only in vegetative plants of the case. Reproducing Plants store the remaining assimilates more in fruits and inhibit the growth of the whole plant, hence the root. ABA also causes an increase of hydraulic conductivity of the water, the root.

Leaf fall

In this strategy, perennial crops water loss is minimized by not Transpirationsfläche is present during periods of poor water availability (drought or winter). Leaf-fall can only be partially or completely, as with many plants of the temperate zone ( Winterdormanz ) in certain plants ( arid zone). This form reserve storage in roots or in above-ground plant organs, which allow rapid expulsion in better conditions. An example of deciduous plants from arid zones is the African baobab tree. It drives completely new until after the next rain again. The entire leaf loss is called Blattabscission.

Drought stress avoidance

Dürremeidende plants close their life cycle of an impending drought. The most of the year survived the plant in the form of storage organs below the earth ( cryptophytes ) or in the form of seeds that germinate only when the next rainfall. This phenomenon can be observed especially in deserts and steppes. After a heavy rain to germinate the seeds in the ground, grow up within a few weeks, flower, form fruits and seeds, and die in just 6 weeks from again. The seeds survive in the soil again until the next rainy season.

Drought tolerance

Some perennial desert plants have a very deep root system. The baobab tree, for example, is up to 80 m deep taproots, with whom he gets to groundwater. Others have an extremely ausgeweitetes root system, the roots of which can be in some species up to 30 m long. Therefore acacias are very far apart. Here the root system competes for the rare rainfall. Due to extreme reduction of the leaf surface or conversion to thorns and narrow growth is the surface that is exposed to direct sunlight, reduced and thus reduced the transpiration rate. Visually, this can best be seen from a large, narrow cactus. Photosynthesis takes no longer leaves instead, but in the shoots of the plant. The leaves can also be thickened, rolled or hairy. Other plants have their offspring regressed completely. This stocky, rosette -like growth habit, the air circulation is reduced and a kind of microclimate is created. Thus also evaporation is avoided. The forms and variations are endless.

The succulents (eg cacti) are particularly well adapted to water scarcity. They have great water reserves in their tissues and have no leaves. They carry out photosynthesis with the tribe and thus optimize the ratio of volume to Transpirationsfläche. The water is stored in most succulents in a fan-like stem formed by the absorption of water it expands. Consumes the plant, the water, the subjects move in together again. Between these longitudinal ribs morning dew collects, she runs down and is passed directly to the roots where the plant can absorb the water. To minimize transpiration they have the CAM mechanism.

• Botany
• Plant Physiology