Drought stress
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As drought or water stress is called a stress , which is caused by lack of water on living organisms and especially on plants.
Overview
Drought stress is particularly relevant in arid climates (desert belts, subtropics ), but also in cold areas ( tundra , alpine regions).
Plants open the pores of the leaves to absorb CO 2 from the air. The CO 2 flows into the leaf from the air. On the other hand, when the water potential of the air is lower than the water potential of the leaf, the water flows out of the leaf. This leads to a decrease in the internal pressure (turgor) in the leaf cell, which creates water stress or drought stress .
The important influencing variables that lead to drought stress in arid areas are: low precipitation, high temperatures, low air humidity and unfavorable soil properties (water retention capacity). They cause the plant to transpire more than it absorbs water. In cold regions there is a risk that the water in the ground will freeze in insufficient supply, which leads to drought stress. In hot years, even in a temperate climate (Switzerland), the foliage can suffer damage, trees shed their leaves prematurely, or entire branches fall off due to branch breakage (summer break). Forest damage can occur in the forest .
Water scarcity and drought stress are the most important limitations for agricultural production. The losses due to drought stress far exceed those that can be attributed to other abiotic or even biotic factors. Most developing countries are located in arid areas and are therefore particularly affected.
Adaptation strategies in plants
Hydrostable plants differ from hydrolable plants in their ability to adapt to drought stress. They have adapted their anatomy and metabolism to the problem of water shortage so that they can respond to drought and maintain the water content in their tissues for a period of time. These hydrostable plants can in turn be divided into water-saving and water-consuming plants based on their strategy of adaptation:
Water-saving plants avoid water absorption through the roots and initially leave it in the soil, they literally divide it up in order to be able to use it longer. In this way, a longer dry phase can be overcome with less water consumption.
Plants that consume water follow the opposite strategy. When drought occurs, they withdraw all water from the soil and store it so that further wasted water absorption during the dry phase can be dispensed with. An example of this type of adaptation is Prosopis spec., The mesquite tree . Its roots reach up to 100 meters deep. As soon as the water available in the soil becomes less, it removes the remaining water from all soil layers and stores it in its trunk. This not only ensures the water supply during the dry phase, it also gives itself a competitive advantage over nearby plants, which can no longer absorb water from this soil.
Short-term adjustments
In the case of drought stress, abscisic acid (ABA ) is formed in the roots and also in the leaves due to the reduction in turgor pressure due to water loss . Abscisic acid influences physiological and biochemical reactions in the target cells and target tissues in a way that facilitates or even enables the organism to survive under drought stress. One of the most important tasks of abscisic acid is to induce stomatal closure. The regulation of the opening of the stoma occurs after a few minutes. ABA can therefore be used in two ways:
- As an early warning system. A water deficit in the soil triggers the increased production of ABA in the roots. It is distributed in the plant through assimilation currents and ultimately also reaches the leaves with their stomata, which then close.
- In direct response to water loss from the leaf. The falling turgor pressure in the cells of leaves causes the local release of ABA, which causes the stomata to close.
ABA thus enables precise and reversible control of the water balance of a plant or even of an individual leaf. Under light conditions, the closure of the stomata leads to a decrease in the internal CO 2 concentration and thus to a strong inhibition of photosynthesis . In C 3 plants, this leads to increased photorespiration activity and thus to the formation of reactive oxygen compounds (H 2 O 2 ).
Closing the stomata in detail
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Abscisic acid is formed in the roots and also in the leaves during drought stress . It is brought to the guard cells by the current of perspiration . It induces the release of calcium ions in these cells. The increased calcium level causes an inhibition of proton ATPase , which leads to a decrease in the transmembrane H + gradient, which leads to depolarization . Calcium ions also bind to chloride channels. This causes chloride ions to passively escape from the cell, which results in further depolarization. However, the exact individual steps are still the subject of research.
This depolarization has two consequences. On the one hand, the potassium channels, which are only open when hyperpolarized , close . In addition, the outward-facing potassium channels are particularly active, which leads to a strong outflow of potassium ions. The chloride and malate anions flow in coupled to it, whereby the cell loses water osmotically and the stoma closes. The ions are stored in neighboring cells.
The regulation of the opening of the stoma takes place after a few minutes. Thus, it allows precise and reversible control of the water balance of a plant or even an individual leaf. The molecular modes of action of abscisic acid are not yet known in detail. However, only about 10% of the usual abscisic acid concentration was found in wilting tomato mutants. After exogenous addition, however, the stomata functioned correctly again.
Long-term adjustments
A slight lack of water over a longer period of time influences the proportional growth of roots and shoots . If a plant has less water available, the shoot grows more slowly or stops growing altogether. The assimilates actually used for this are now made available in a concentrated manner to the root, it grows on and on and can thus tap new water in deeper areas, while the upper soil layers slowly dry out. As a result, plants in moist soils are characterized by a shallow root system and in dry soils by a very deep root system. However, this is only the case with vegetative plants. Reproductive plants tend to store the remaining assimilates in fruits and inhibit the growth of the entire plant, including the roots. ABA also increases the hydraulic conductivity of the roots.
Leaf fall
This perennial planting strategy minimizes water loss by leaving no area for transpiration during periods of poor water availability ( drought or winter). The fall of leaves can only be partial in certain plants ( arid zone ), in low-precipitation years with heat waves it can wilt the leaves prematurely or completely as in many plants in the temperate zone ( winter dormancy ). These form reserve stores in roots or in aboveground plant organs , which allow them to sprout quickly under better conditions. An example of deciduous plants from arid zones is the African baobab tree . It only sprouts completely again after the next rain. The total leaf loss is called leaf absission .
Avoidance of drought stress
Drought-resistant plants complete their life cycle before an impending drought . Most of the year the plant survives in the form of storage organs beneath the ground ( cryptophytes ) or in the form of seeds that only germinate when the next rain falls. This phenomenon can be observed especially in deserts and steppes. After heavy rain, the seeds in the soil germinate, grow within a few weeks, bloom, develop fruits and seeds and die again after just 6 weeks. The seeds survive in the soil again until the next rainy season.
Drought tolerance
Some perennial desert plants have extremely deep roots. Baobab trees, for example, form tap roots up to 80 m deep, which they use to access groundwater. Others have extremely extensive roots, the roots of which can be up to 30 m long in some species. Therefore, umbrella acacias are very far apart. Here the roots compete for the rare rainfall. By extreme reduction of the leaf surface or conversion to thorns and narrow growth, the surface that is exposed to direct sunlight is reduced and thus the transpiration rate is lowered. Visually, this can best be seen from a large, narrow cactus . Photosynthesis no longer takes place in leaves, but in the shoot of the plant. The leaves can also be thickened, curled, or hairy. Other plants have completely receded their shoots. This compact, rosette-shaped growth reduces air circulation and creates a kind of microclimate . This also prevents evaporation. There are no limits to the shapes and variations.
The succulents (e.g. cacti) are particularly well adapted to water scarcity . They have large reserves of water in their tissues and have no leaves. They perform photosynthesis with the trunk and thus optimize the ratio of the volume to the transpiration area. In most succulents, the water is stored in a fan-like trunk, and it expands when it absorbs water. When the plant uses up the water, the compartments are closed again. Morning dew collects between these longitudinal ribs, runs down and is conducted directly to the roots, where the plant can absorb the water. To minimize perspiration, they have the CAM mechanism .
Individual evidence
- ↑ Shrawan Singh, DR Singh, Ayyam Velmurugan, Iyyappan Jaisankar, TP Swarnam: Coping with Climatic Uncertainties Through Improved Production Technologies in Tropical Island Conditions . In: Biodiversity and Climate Change Adaptation in Tropical Islands . Elsevier, 2008, ISBN 978-0-12-813064-3 , pp. 623-666 , doi : 10.1016 / b978-0-12-813064-3.00023-5 ( elsevier.com [accessed October 25, 2019]).
- ^ Smith, Robert L .: Ecology. 6., update Edition Pearson Education, Munich 2009, ISBN 978-3-8273-7313-7 , pp. 148-149 .
- ↑ a b How the forest suffers from drought , Neue Zürcher Zeitung, August 2, 2018.
- ^ J. Wery, SN Silim, EJ Knights, RS Malhotra, R. Cousin: Screening techniques and sources of tolerance to extremes of moisture and air temperature in cool season food legumes . In: Expanding the Production and Use of Cool Season Food Legumes . Springer Netherlands, Dordrecht 1994, ISBN 94-010-4343-4 , pp. 439-456 , doi : 10.1007 / 978-94-011-0798-3_26 .
- ↑ a b M. Farooq, A. Wahid, N. Kobayashi, D. Fujita, SMA Basra: Plant stress drought: effects, mechanisms and management . In: Agronomy for Sustainable Development . tape 29 , no. 1 , March 1, 2009, ISSN 1773-0155 , p. 185-212 , doi : 10.1051 / agro: 2008021 .