Water transport in plants
The water transport in plants is a process in which plants through their roots absorb water and minerals on the vascular tissue in the xylem forward and the water by transpiration through the guard cells on the underside of leaves leave as vapor. The exact processes involved in water transport are the subject of ongoing research.
Research history
Stephen Hales , a student of Isaac Newton , was the first to investigate the movement of water in plants experimentally ( Vegetable Staticks , 1727, German: Statick der Gewächse , 1748). He realized that it was not primarily - as had been assumed until then - that the pressure from the root drives the flow of sap, but rather the transpiration of the leaves. As the root of water and mineral nutrients receives the study was available only in the second half of the 19th century, when Julius Sachs , the hydroponic introduced. He determined which chemical elements are necessary for plant growth and which are absorbed by the roots. He discovered that the water and nutrients are absorbed through the fine root hairs .
What actually went on in the plant, however, was largely only the subject of speculation until the end of the 19th century. In 1891 Eduard Strasburger showed that the rise of the juice in the xylem can be explained purely physically and does not require living cells. In the following years Strasburger's former assistant Henry Horatio Dixon and others developed the cohesion theory to explain the rise of the juice, according to which a suction tension caused by perspiration drives the juice. However, this theory remained controversial for a long time because many botanists rejected a purely physical explanation and could refer to phenomena such as guttation , which cannot be explained in this way. With the discovery of active ion transport through cell membranes around 1930, another mechanism was added that can cause a slight flow of sap without perspiration.
Water absorption into the root
Of the three basic organs leaf, shoot axis and root, the root specializes in absorbing water and ions from the soil and is therefore normally located underground. In particular, this task is performed by the root hairs in the root hair zone, which provide a large surface for mass transfer in this area. In addition to water absorption, the absorption of ions is also a central function. These are very diluted (10 −4 mol / L) and also not in the ratio required by the plant. The concentration of the ions (read: dissolved nutrient salts) is an energy-consuming process.
Water comes in various forms in the soil. The groundwater is not accessible for many plants because their roots do not go deep enough. In the upper soil layers, water is present as adhesive water (adsorbed on soil particles, also called spring water, derived from the swelling of clay minerals ), as capillary water ( transported upwards by capillary forces and bound to soil pores) and as moisture (water vapor) in the air in the soil . The matrix potential of the retained water is usually so negative that it is not available to plants. Plants use capillary water to meet their needs.
The soil and its pores are in a moisture equilibrium with the environment. The humidity at the groundwater level is 100%, in the air space above the earth's surface it corresponds to the respective air humidity (for example 40%). In between there is a humidity gradient , due to which diffusion or capillary suction can take place (this is based, among other things, on the process of thin-layer chromatography in chemistry). The compensation is based on the temperature dependence of the water vapor saturation vapor pressure. The essential moisture transport mechanisms are gravitation, water vapor diffusion and liquid transport by capillary forces, to a lesser extent the effect of electrical fields and ion concentration gradients.
Water uptake by the root is possible if the water potential Ψ of the root is lower (ie more negative) than that of the surrounding soil, as water moves from places with high water potential to places with low water potential.
The water potential of the soil is not determined from the osmotic potential, since ions are usually too diluted (see above), but primarily from the matrix potential. The drier soils become, the further the water potential decreases, i.e. i.e., water uptake by the root becomes more difficult. The water potential of the soil is typically between Ψ = −0.01 MPa and Ψ = −1.5 MPa. Ψ = −1.5 MPa is called the permanent wilting point , because most plants can no longer draw water from the soil at this value. In salt soils it can be less than −0.2 MPa, in dry soils it can be −2 MPa and in deserts and salt steppes it can be much lower. On the other hand, it can be around 0 after precipitation or in the vicinity of the groundwater. The water potential of the root can fluctuate greatly due to osmotically active substances and depending on the species. This allows plants to adapt to their environment in order to continue to absorb water. The osmotic potential is based on the one hand on ions absorbed from the soil, in particular potassium ions, and on the other hand on organic compounds dissolved in the cell. The water potential of a normal root is Ψ = −0.2 MPa to Ψ = −0.5 MPa, with halophytes below −2 MPa and with desert plants even below −10 MPa. The water potential in the tracheal juice is usually between −0.5 and −1.5 MPa, in the leaves between −0.5 and −2.5 MPa. The water passes from the leaves into the air, as this has a water potential of −94 MPa at a humidity of 50%, for example.
Water can penetrate the root in three ways: via the apoplast , via the symplast and transcellularly (i.e. both via the apoplast and via the symplast). First, water passes from the apoplast into the symplast. This leads to a decrease in the water potential in the apoplast and water flows in from the directly adjacent soil. This also reduces its water potential and water also flows in from the surrounding area. However, due to the limited water conductivity of the soil, this process is limited to a maximum of a few cm. As soon as the water supplies are exhausted in one place, the roots follow the retreating water through growth in other regions and exploit them - the same applies to ions. At low temperatures (for many species this includes temperatures just above freezing point) the transport resistance of the water in the soil increases, the permeability of the plasma membrane for water decreases and root growth decreases. At temperatures below freezing point, the retained water even freezes. The resulting lack of water, known as frost drought , is often misinterpreted as freezing.
Since the water potential decreases inwards, towards the central cylinder, the water diffuses in this direction. However , the apoplastic route is blocked by the Caspary streak in the endodermis and the water is forced into the symplasts. If there is plenty of water and no water is removed by transpiration, a positive, hydrostatic pressure can build up in the central cylinder, the root pressure. Like a seal, the Caspary strip prevents the pressure from being equalized by allowing the water to flow back into the cortical parenchyma . Thus the water rises. Exactly how the root pressure is built up has not yet been clarified; in any case, energy is required for its generation. It probably arises from the secondary active storage of inorganic ions in the main vessels of the xylem from the xylem parenchyma. The root pressure is normally 0.1 MPa, but in some species, such as the tomato, it is also over 0.6 MPa. In the case of insufficient water supply or strong transpiration, however, there is a negative hydrostatic pressure in the root area, which primarily determines the negative water potential and no longer the osmotic term. Normally, the root pressure predominates at night, but the perspiration suction quickly after sunrise.
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Plant roots can also absorb water and nutrients via mycorrhiza from symbiotic fungi.
Water transport and delivery
While the root absorbs water, the rest of the plant loses water to its surroundings through transpiration. This transpiration is inevitable when the plant has a higher water potential than its surroundings. Transpiration only does not take place if the plant and the external medium are in equilibrium with each other, i.e. have an identical water potential. At 20 ° C, this only happens from a relative humidity of 99 to 97.5%. Such high relative humidity is rarely reached, e.g. B. as a result of cooling down at night, shortly before reaching the dew point . During the day, the relative humidity is usually 40–60%. If the plant is not in equilibrium with its ambient air, it will permanently lose water. It only loses a small part (up to 10%) of this via the cuticle , but mainly via the stomata . If the stomata are open for the necessary CO 2 absorption, the evaporation of water through these stomata cannot be prevented. Since carbon dioxide is only present in traces in the air (0.037%), the plant loses several hundred water molecules for every CO 2 molecule ingested .
The water that the epidermis loses to its surroundings sucks it in from the inner parts of the plant. This suction continues via the conduction pathways of the xylem to the roots and thus draws the water from the roots to the tips of the leaves. According to the cohesion theory, this effect is called perspiration suction . The perspiration suction is not solely responsible for the water flow in plants. In order to raise a column of water by ten meters against gravity, a negative pressure of about 0.1 MPa must be applied, plus another 0.2 MPa to overcome the friction forces of the water in the xylem.
In the xylem of the tallest trees in the world (more than 110 m) (individual specimens of the coastal redwood Sequoia sempervirens in California), a negative pressure of more than 3 MPa would have to be applied in order to ensure water flow to the tips. However, if the negative pressure is stronger than the cohesive force of the water, it begins to boil. The boiling of the water is not only countered by the cohesive forces between the individual molecules, but also the forces of adhesion to the wall of the guide vessel. In experiments in a glass tube, gas-free water withstood pressures of up to −30 MPa before it ruptured. In the conductive tissue of plants, the negative pressure seldom falls below −4 MPa, but embolisms still occur due to the formation of gas bubbles, as the transported water is contaminated with dissolved gases and ions. These emboli pose a serious problem for the plant as they block further water transport. The water transport must then take place via the detour via the horizontally running pits . Certain repair mechanisms do exist. But these are not yet understood. Emboli caused by frost are usually irreparable. To compensate for this, the plant has to build new lines at the beginning of the growing season.
The loss of water through transpiration leads to perspiration suction and this leads to a flow of water from the root to the tip, the transpiration current. The transpiration benefits the plant in several ways: on the one hand, the leaves are cooled by the evaporative cooling, on the other hand, ions (dissolved nutrient salts) are transported in the xylem. However, plants showed no growth problems in experiments even with a 15-fold reduced transpiration. Even without transpiration, there is an internal flow of water that is completely sufficient for ion transport. This comes about through the root pressure, growth water and the internal water cycle in the phloem and xylem as well as guttation in special cases. Growth water is water that is used to increase the volume of the plant. This may account for a significant share especially in herbaceous plants, especially in phases of growth, while corn for. B. 10-20% of the transpiration water. The internal water cycle in the phloem and xylem is created by the fact that water in the xylem flows upwards and to the tips, but in the phloem for the transport of assimilates in the opposite direction and both systems are connected to one another. Thus, transpiration is not primarily used for transport, but is simply inevitable, especially due to the inevitable uptake of carbon dioxide .
See also
Web links
Individual evidence
- ^ Karl Mägdefrau: History of Botany . Gustav Fischer, Stuttgart 1973. pp. 81-84.
- ^ Karl Mägdefrau: History of Botany . Gustav Fischer, Stuttgart 1973. pp. 207f.
- ↑ Ilse Jahn (Ed.): History of Biology . 3rd edition, Nikol special edition, Hamburg 2004, pp. 511f.
- ↑ a b c d Peter Schopfer, Axel Brennicke: Plant Physiology . Founded by Hans Mohr . 6th edition. Elsevier, Spektrum, Munich / Heidelberg 2006, ISBN 3-8274-1561-6 , pp. 313–315 ( limited preview in Google Book search).
- ↑ a b c d e f g h i Elmar Weiler , Lutz Nover: General and molecular botany . Thieme, Stuttgart 2008, ISBN 978-3-13-147661-6 , p. 238–248 ( limited preview in Google Book search).
- ↑ Martin Krus: Moisture transport and storage coefficients of porous mineral building materials. Theoretical basics and new measuring techniques. Doctoral thesis at the Faculty of Civil Engineering and Surveying at the University of Stuttgart, Stuttgart 1995, PDF file.
- ↑ Ulrich Kutschera: Short textbook of plant physiology , Quelle & Meyer, UTB 1861, Wiesbaden 1995, p. 63
- ↑ a b c d e Andreas Bresinsky , Christian Körner , Joachim W. Kadereit , Gunther Neuhaus , Uwe Sonnewald : Textbook of Botany . Founded by Eduard Strasburger . 36th edition. Spektrum Akademischer Verlag, Heidelberg 2008, ISBN 978-3-8274-1455-7 , p. 263-265 .