From a tropism is called in botany when a plant to a stimulus reacts with a movement. Plant movements help the individual plant to make the best possible use of the habitat or to develop it, or to avoid dangers. Plant movements can be taxies , nastias , tropisms or autonomous movements .
Plant movements are triggered by stimuli. Most of the stimuli take place in the vicinity of the plant and induce processes in the plant. However, there are also autonomous or endogenous stimuli that arise inside the plant and are still little known.
Plants receive stimuli from appropriate recipients, e.g. B. a pigment that reacts to a certain wavelength range of light. The stimulus leads to different reactions in plants. Some plant cells build up action potentials , in others the stimulus starts or inhibits a chemical reaction or sequence of reactions. In all cases the stimulus is only the triggering signal , not a substrate or energy source.
Stimulus strength and presentation time
In order for a stimulus to have an effect, it must exceed a stimulus threshold , whereby a permanent stimulus has a deadening effect. The minimum length of time that a stimulus has to act in order to induce a reaction is called the presentation time. The success of a stimulus depends on both the strength of the stimulus and the duration. The greater the stimulus strength, the shorter the presentation time can be. This is defined in the stimulus quantity law: R = I × t , where R stands for the success of the stimulus, I for the stimulus strength and t for the stimulus duration. This only applies in the vicinity of the stimulus threshold and has no influence on stimulus quantities that are far above the threshold value.
In some reactions, the strength of the reaction depends on the strength of the stimulus. If the strength of the reaction is always the same, regardless of how strongly the stimulus threshold is exceeded, one speaks of an all-or-nothing reaction .
Taxis are free local movements that are determined by an external factor. In the plant kingdom these are only possible in flagellated or amoeboid unicellular organisms , in mobile colonies and in unicellular developmental stages of more highly organized forms ( meiospores , zoospores , gametes ).
Positive taxis serve the respective forms of life to orient themselves towards the optimal conditions. Photosynthetically active organisms strive for light, heterotrophic or mixotrophic organisms for the substrate . Many of these movements are therefore photo- or chemotaxies . To prevent harm to the individual there are also negative taxis that lead away from a stimulus. These stimuli are, for example, high radiation intensities or toxic chemical concentrations .
If an organism moves specifically towards a source of stimulus, it is a topo- or strophotaxis. If the movement is in the opposite direction of the stimulus, it is a question of phobotaxis or a phobic reaction.
In some plants, the gametes specifically search for the sexual partner by chemotaxis, with z. Sometimes highly specific attractants ( pheromones ) work. The direction of movement is determined either by a local (topical reaction) or a temporal (phobic reaction) concentration gradient .
Especially photosynthetically active organisms, the phototaxis helps in the search for the optimal light intensity . Phototaxis can also be topical or phobic. The targeted approach or removal from a light source (topo-phototaxis) requires that both temporal changes in intensity and different levels of exposure of the flanks can be perceived (as long as Euglena and Chlamydomonas were still included in the plants, they gave good examples of this, they are able to perceive the direction of light through an eye spot ).
Magnetotaxis is known from individual plant protozoa.
- Hydro-, hygro-, xerotaxis: reaction to moisture differences (dryness)
- Barytaxis: reaction caused by pressure
- Geotaxis or Gravitaxis: reaction to the gravitational pull
- Rheotaxis: reaction caused by flowing water
- Thigmo-, haptotaxis: reaction to touch stimuli
- Stereotaxis: response to rigid touch stimuli
- Thermotaxis: reaction to temperature changes
- Electric, galvanotaxis : response to electrical stimuli
- Tono-, Osmotaxis: The reaction to changes in the osmotic pressure of the surrounding fluid
- Car taxis: self-regulation
Nastias and tropisms
Plants tied to one location carry out various movements with their organs . A distinction is made between nastia and tropisms . In most cases, it is easy to differentiate between the movements of individual organs, but there are also mixed forms of some plant movements.
From Nastien , curvature movement is used when the movement direction is determined by the construction of the moving organ. The direction from which the stimulus comes is therefore not decisive. The stimulus only serves as a signal for a fixed movement. Nastias occur relatively quickly and can be reversible. Often nastias are based on turgor changes . With some nastia more or less strong growth movements are also involved.
Pure nasties are z. B. the movements of stomatal lock cells, which are predominantly photonastic and hydronastic. However, they are also thermonastically sensitive. Many nastias are turgor movements .
Overview of different nastias
Examples of nasties:
- Seismonasty or thigmonasty: Reaction to vibration or contact ( mimosa , Venus flytrap , some tendrils)
- Chemonasty: reaction to chemical stimuli, nutrients (dorsiventral marginal tentacles of the sundew )
- Aeronasty: movement responses to oxygen
- Thermonasty: reaction to heat ; Temperature difference (opening / closing e.g. of crocus or tulip flowers )
- Photonasty: response to light ; Light intensity (closing and opening of flowers with different light intensities)
- Nyctinastia: Autonomous change in the position of plant organs that coincides with the day-night rhythm, mostly caused by light and temperature stimuli
- Autonyctinasty, (autonyctitropic): Sleep movement always occurs in the same way, regardless of the position
- Gravi-, geonyctinastia (gravi-, geonyctitropic): Nyctinastia only occurring with gravity
- Stomatonasty: opening and closing of the stomata regulated by the movements of the guard cells
- Seismonasty: movement reactions to vibratory stimuli
- Thigmo, haptonastia: movement response triggered by touch stimuli
- Trauma atonasty: Non-directional movement caused by injury
- Hydro-, hygro-, xeronastia: Movement triggered by changing the (air) humidity (dryness)
- Psychronasty: Movement reaction caused by cooling
- Hyponastia: Curvature movement due to increased growth of the underside (abaxial) compared to the upper side (adaxial) of a plant part
- Epinasty : Curvature movement due to increased growth of the upper side (adaxial) compared to the lower side (abaxial) of a plant part
- Paranastia: The nastic curvature of a plant organ which is not caused by the increased length of the upper or lower side (epinastia, hyponastia), but rather by a lateral flank.
- Diplonasty: That form of eccentric growth in thickness of rungs in which the growth in thickness is promoted on two opposite flanks.
- Autonasty: After a previous nasty, the return to its starting position independent of stimuli or external factors. With complete constancy of the external conditions expiring nastia, z. B. many unfolding movements.
- Aitionastie: Are all those forms of nastia that are caused by a change in the external conditions affecting the plant.
The stimulus that leads to a nastia is also passed on within a plant, so that e.g. B. in the case of mimosa, also neighboring leaves react. This happens on the one hand through chemical messenger substances, on the other hand through electrical impulses.
The opening and closing movements of some flowers are an example of thermonastia. This is the case with tulips , crocuses or daisies . The top of the petals in these plants has a higher temperature optimum than the bottom. This affects the growth of the sides, that is, with a rise in temperature, the top of the petals grows faster than the bottom. This opens the flower. This process repeats itself over and over again, which leads to repeated opening and closing. Since these movements are dependent on the growth of the petals, the petals of a tulip elongate by 7% during a thermonastic movement and by over 100% during an entire flowering cycle.
There are also thermonastic flower stalks (for example in wood sorrel ) and tendrils. Leaves, on the other hand, are seldom thermonastically active, but some plants whose leaf axils are equipped with “joints” (for example mimosa) perform thermonastic turgor movements.
There are also a large number of plants whose flowers make photonastic movements. In the case of sensitive plants (e.g. some types of gentian ), even the short-term decrease in light caused by a cloud is sufficient to close the flowers. The effect is reversed in night-flowering plants (e.g. nodding catchfly ).
The leaves of some plants also react photonastically. Some of them (e.g. some spring herbs ) lower their leaves in the dark by accelerating the growth of the upper side of the leaves. Fully grown leaves can only react photonastically with turgor changes (e.g. "joints" of the mimosa). This also includes the most important photonastic movements, those of the stomata . The opening width of the stomata is not only controlled by the intensity and quality of light, but also by the CO 2 concentration (chemonasty) and by the phytohormones auxin and abscisic acid .
The movement of the stomata has already been mentioned as a partially chemonastic reaction. Another example are the dorsiventral marginal tentacles on a leaf of the insectivorous genus sundew . While the central tentacles are built radially and show chemotropism , the edge tentacles are able to curve nastically towards the middle of the leaf when stimulated. The chemical stimulus emanating from the prey is stronger than the thigmic touch stimuli ( thigmonastia ). With the help of protein receptors, the plant recognizes the protein particles released by the prey. The edge tentacles can also be stimulated by other tentacles in the middle of the leaf via conduction. Even then they curve, which is not a nastic, but a tropistic movement towards the source of the stimulus.
Seismonasties take place after tremors. This can e.g. Sometimes very fast movements that are achieved not through growth, but through turgor changes. The direction of movement is determined by the structure of the reacting parts. Seismonasties are mostly all-or-nothing reactions , where a shock from a raindrop or gust of wind is usually sufficient.
The best-known example of a seismonasty are the movements of the leaves of the mimosa , whose leaves first collapse in pairs after being shaken, then the individual leaflets approach and finally the petiole folds down. This reaction can also be triggered by other stimuli (including injury, heat), but in nature the most common vibration is caused by roaming animals. If the irritation is severe, the reaction may progress to other parts of the plant that were not directly affected.
The flower organs of many plants are also seismonastically active, e.g. B. the stamens of the barberry , which are folded inward when irritated.
Thigmonastia (haptonastia) occurs as a reaction to a touch stimulus (thigmic stimuli). This effect can be clearly observed in the tendrils of the bryony . In these tendrils, a distinction can be made between an upper and a lower side , depending on the morphogenesis . When young, the tendrils are rolled up like a spiral, then stretch and begin to circle ( nutation ). The tendrils react when the top or bottom is irritated, but they always curve towards the bottom due to increased growth on the top ( auxin-controlled ). The first reaction to a stimulus, however, is initially a loss of turgor on the flank that is becoming concave and an increase in turgor on the opposite side. In addition, there is an increase in wall elasticity due to auxin . If a support has been grasped, it is wrapped several times by the tendril end. In the middle of the tendril, curls also form due to increased growth on the top, which attracts the plant to the support. To avoid breaks, these turns have one or more reversal points between left and right turns. The fact that there are also changes in places that were not directly stimulated proves that excitation is also passed on in thigmonastia.
The stimulus that leads to a twist in these tendrils must meet special requirements. It must not be uniform and smooth, that is, a smooth rod that is pressed against the tendril with even pressure would not be wound around it. A temporal and spatial change in the pressure is required.
In tropisms ( Greek trope , turn '), the stimulus directional reactions , the stimulus determines the movement properties (sequence, orientation) of the reaction of the plant organ . If the organ turns towards the stimulus, it is a positive tropism (pro, anatropism) - if it turns away, it is a negative tropism (apotropism). If the organ is oriented at a certain angle to the direction of stimulation, the tropism is called plagiotropic ( e.g. side branches), at a 90 ° angle dia- , homalotropic or transversal tropism and orth- , parallelotropic if it grows almost vertically upwards or downwards , clinotropic if the setting is at an oblique angle, whereas cataclinotropic here means negative clinotropism. As ageotrop is called a growth, which is not aligned with the force of gravity.
Tropisms take place relatively slowly and mean long-lasting changes for the plant, as they are mostly growth processes. Since a curvature occurs through unilateral elongation growth, shoots with long growth zones curve with a large radius, while roots with short growth zones curve with a small radius. With a positive curvature, there is increased growth on the side facing away from the stimulus during extension growth. This applies not only to higher plants, but also to some unicellular systems. If, however, these have pronounced tip growth instead of elongation, the stimulus can inhibit the tip growth and initiate a new apex at the side facing the stimulus , so that the side facing the stimulus grows more strongly.
Overview of different tropisms
- Phototropism: reaction to light (positive: shoots; negative: roots; diaphototropic: leaves)
- Scototropism: Growing towards darkness, reaction to shadows (lianas)
- Heliotropism : reaction to the course of the sun
- Barytropism: Growth movement directed towards pressure stimulus
- Gravitropism (formerly also geotropism): reaction to gravity (roots, bananas)
- Rheotropism (Stromwendigkeit): the property of growing parts of plants to take a certain direction to a flowing liquid
- Thigmo-, pieso-, haptotropism: reaction to touch (some tendrils)
- Stereotropism: reaction to contact with a solid body or rough surface; rigid touch stimuli
- Mechanotropism: reaction to mechanical forces (wind, snow forces)
- Anemotropism (wind maneuverability): orientation in response to a current of air, wind
- Chemotropism: reaction to chemical stimuli, nutrients (roots)
- Trophotropism: Determined by the supply of nutrients
- Oxytropism: Reaction caused by acids
- Alk (c) aliotropism: By reaction to alkaline substances
- Aerotropism: The growth towards or away from a region with higher oxygen levels.
- Sucrose chemotropism: growth caused by sugar
- Protein chemotropism: growth caused by proteins
- Hydro-, hygro-, xerotropism: reaction to (air) moisture, dryness (liverworts; roots)
- Thermotropism: reaction to heat
- Autotropism: The tendency of the plant organs to grow in a position of equilibrium (proper direction) if it is not influenced by external stimuli (self-organization).
- Autoorthotropic: grows crooked
- Autoskoliotrope: growing straight
- Aitiotropisms: Tropisms caused by external stimuli as opposed to autropism
- Somatotropism: Direct substrate influence on growth
- Galvano- or electrotropism: reactions to electrical stimuli
- Magnetotropisums: Influence of an electromagnetic field on growth
- Traumatropism: Change in growth caused by injury
- Tono-, osmotropism: the reaction to changes in the osmotic pressure of the surrounding fluid
- Camptotropism: A caused curvature appears in the same direction in another place as well; geotropic induction .
The phototropism (also known as light turning ) causes with one-sided exposure that almost all above-ground trunks and branches turn towards the light and continue to grow in the direction from which the rays are incident. At the same time, the leaf surfaces are often more or less perpendicular to the incidence of light.
Positive photochromic organs turn to light , especially for optimal photosynthesis. Positive phototropic are z. B. mostly the stem axes and many petioles. With negative phototropism, however, the organs turn away, such. B. the roots of the ivy , the hypocotyl of the germinating mistletoe and the radicles of some plants and the leaves of the lettuce . Most roots, however, are not affected by light, so they are aphototropic . Side branches often show plagiophototropism, while leaf blades usually even show transverse phototropism, i.e. That is, they are at an angle of 90 ° to the incident light.
Some organs can also switch between positive and negative phototropism over time. So the flowers of the wall cymbal initially turn towards the light, but after fertilization they turn away from the light in order to reach a suitable place for the seed to germinate. The seeds spread through the plant itself ( autochory ).
The decisive factor for perception is not the direction of light, but the difference in brightness between the light and the shadow side. The responsible photoreceptors are not in the cells involved in the curvature ; Instead, stimuli are transmitted from the receptors to the deeper cells by phytohormones (especially auxins ).
Scototropism means growth towards shadow, towards darkness. This was mainly found in lianas (e.g. window leaves ), which find their support tree in this way. Once the support tree has been reached, the scototropism changes to a positive phototropism. Scototropism is not to be equated with negative phototropism, since turning towards the darkest sector does not mean turning away from light.
The gravitropism is partly geotropism (German also Erdwendigkeit called). Gravitropic plants are able to bring their organs to the acceleration of gravity in a certain direction by curving their growth . This enables z. B. Plants on a slope to take an upright posture. Organs that move towards the center of the earth (e.g. main roots) are positive gravitropic organs, while negative gravitropic organs move away from it. In contrast, transversal or plagiotropism, also called plagiogravitropism, is found in the first-order lateral roots, i.e. That is , they grow horizontally or at a certain angle downwards, while the lateral roots of the second order are mostly gravitropically insensitive, i.e. agravitropic .
As with phototropism, gravitropism can alternate between positive, negative and transversal tropism in an organ. The buds of the poppy , for example, are positive gravitropic and only become negative gravitropic when they bloom.
In gravitropic reactions, as in phototropic reactions, the curvature is achieved by the different growth of two halves of the organ. The change takes place in the main growth zones, whereby fully-grown organs can also be stimulated to grow again. When a horizontally placed shoot is bent up, it often first leads to an overbending, i.e. That is, the shoot bends too far, so that there is a backward bend in order to reach the optimal position.
The presentation time for gravitropism can be very short, e.g. B. two minutes at the shepherd's purse . This can only be measured by removing any gravity effect from the plant after the stimulus (e.g. by turning a clinostat ). The reaction time can be between a few minutes (e.g. oat coleoptile) and several hours (e.g. Grasnodi ). The plants also perceive small and short changes in their position, but only react with a bend when the heavy stimulus has acted on them for a longer period of time. The perception of the gravitational stimuli takes place in the roots and in the case of coleoptiles in the tips, while in sprouts it takes place in the extension zones of all internodes that are still growing. Perception is associated with the displacement of statoliths in the cytoplasm of certain cells ( statocytes ). Amyloplasts in particular come into question as statoliths .
If chemical substances are unevenly present in solution or in gaseous form in the vicinity of a plant , the plant can react chemotropically based on the concentration gradient of this substance. Chemotropically acting substances are often attractive in low concentrations (positive chemotropic) and in high concentrations repellent (negative chemotropic).
Roots can also react chemotropically. B. after aerotropism ( O 2 as stimulus) or hydrotropism (moisture as stimulus). In the case of hydrotropic roots, cell elongation is inhibited on the moist side of the root, so that the root curves towards moisture. Aerotropic roots seek out oxygen-rich soil layers to ensure root respiration .
Chemotropism in leaves has been demonstrated relatively rarely. An example, however, are the central tentacles of the sundew leaves. As with all chemotropisms, growth processes are responsible for the movements, the frequency of leaf curvature and backward curvature are limited. However, various nastias are also involved in the movement of the sundae leaves.
Differentiation Nastie vs. Tropism
Difficulties in differentiating between nastic and tropic reaction types can arise. Such is found e.g. B. in the tendrils . In some tendrils, a distinction can be made between an upper and a lower side , depending on the morphogenesis . If the same side always reacts with growth, no matter which side has been stimulated, then it is a thigmonastia (e.g. bryony ). If it is not possible to distinguish between the top and the bottom, and the irritated side always reacts, then it is a question of thigmotropia (e.g. colorful Klimme ).
Often there are also movements of plant organs, which represent a mixture of nasty and tropism. The movements of the sun leaf are, for example, a mixture of the chemotropism of the central tentacles and the chemo- and thigmonasty of the marginal tentacles. On the other hand, the edge tentacles can also be stimulated to a movement by an irritated central tentacle via excitation conduction, but this is then also a tropism.
Endogenous movements, i.e. movements not controlled by external factors, are referred to as autonomous. These movement mechanisms can be divided into passive and active mechanisms.
Metabolism-dependent processes are involved in the active mechanisms. These can be contractions of fibrillary proteins (movement caused by flagella ), one-sided promotion or inhibition of growth or local lower or higher turgor levels .
Nutation is an example of an active autonomous movement . Seedlings, young tendrils and, above all, bindweed plants perform circular movements by promoting growth on one side. Most plants move counterclockwise (viewed from above) - but there are exceptions such as B. in hops ; some plants even change the direction of the wind.
Other active autonomous movements are movements caused by changes in the turgor. The leaves of the meadow clover are swung up and down by turgor changes in the leaf joints in order to increase the rate of transpiration . The sleep movements of many legume leaves are also due to changes in the turgor. These are reversible mechanisms. However, there are also turgor changes that lead to irreversible spinning or splashing movements. Slingshot movements are due to tissue tension . Here, a tissue expands against a resistance due to an increase in turgor until a certain limit value is exceeded, so that the organ tears open along pre-formed tears. This is e.g. B. the case with the fruit walls of spring herbs . The spring herbs throw their seeds several meters away. The spray cucumber , which is one of the so-called juice pressure spreaders , is under a high pressure of up to 6 bar. During the ripening process, a separating tissue forms at the base of the stem, and when it is torn, the fruit shoots away half a meter like a champagne cork. At the same time, the 50 or so seeds are thrown in the opposite direction up to 12 meters from the inside of the fruit. They reach a speed of 10 m / s (36 km / h). This form of spreading is a form of the so-called ballochorie , the spread of plant seeds through centrifugal mechanisms. In addition to exploding fruits, there are exploding stamens in some genera.
Movements caused by swelling are called hygroscopic movements. These, too, are primarily used to spread spores, pollen, seeds and fruit. When swollen, dead cells expand almost only perpendicular to the direction of the microfibrils . Since the alignment of the microfibrils in cell walls often rotates 90 ° per layer for stability, curvatures or torsions occur . The outer peristomal teeth of the spore capsules of the moss also curve hygroscopically when they dry out, so that they eventually detach the capsule lid.
Another example is the pine cones ' scales , which are closed in moist air and open when it is dry. The cell walls on the outside of the scales swell when the humidity is high, expand and the scales curve inward. When dry, the walls dry up, shrink and the scale curves outward so that the seed can fall out.
In plants, movements can also be caused by cohesive forces, which result in the curvature of dead, and rarely also living cells. This is e.g. B. the case with the fern sporangia.
The fastest plant movement
The fastest plant movement observed so far is the Canadian dogwood . After the extremely fast, explosive opening of the petals, the exposed stamens unfold explosively in just 0.3-0.5 milliseconds. Which corresponds to 2400 times the acceleration of gravity . They hurl the pollen upwards at a speed of around 3 meters per second (see above spray cucumber: 10 m / s).
- U. Lüttge, M. Kluge, G. Bauer: Botany. 5th edition. Wiley-VCH, Weinheim 2005, ISBN 3-527-31179-3 .
- N. Campbell et al .: Biology. Spektrum Akademischer Verlag, Heidelberg 1997, ISBN 3-8274-0032-5 .
- P. Sitte, EW Weiler, JW Kadereit, A. Bresinsky, C. Körner: Strasburger - textbook of botany for universities. 34th edition. Spektrum Akademischer Verlag, Heidelberg 1999, ISBN 3-8274-0779-6 .
- C. Correns, Alfred Fischel, E. Küster: Terminology of the development mechanics of animals and plants. Engelmann, 1912, archive.org , Forgotten Books, 2016, ISBN 978-1-334-46936-7 .
- S. Venugopal: Biology. Part II, Saraswati House, 2016, ISBN 978-81-7335-871-5 (reprint), chap. 15.1-15.7.
- Erwin Bünning : Development and movement physiology of the plant. 3rd edition, Springer, 1953, ISBN 978-3-642-87329-4 , p. 515.
- Wilhelm Zopf : The mushrooms. Trewendt, 1890, p. 210.
- Gordon Gordh, David Headrick: A Dictionary of Entomology. 2nd. Edition, CABI, 2011, ISBN 978-1-84593-542-9 , pp. 29, 72, 1388.
- Eleanor Lawrence: Henderson's Dictionary of Biology. 14th. Edition, Pearson Education, 2008, ISBN 978-0-321-50579-8 , p. 689.
- Peter W. Barlow: Differential Growth in Plants. Pergamon Press, 1989, ISBN 0-08-036841-7 , p. 54.
- M. Eichhorn: German Dictionary of Biology / Dictionary Biologie Englisch. Volume / Band 1, Routledge, 1999, ISBN 0-415-17129-6 , p. 727.
- E. Korschelt among other things: Concise dictionary of natural sciences . 8th volume, Gustav Fischer, 1913, p. 256, archive.org .
- Otto Schmeil (edited by Wilhelm J. Fischer): botany . tape 2 . Quelle & Meyer, Heidelberg 1951.
- World's Fastest Plant: New Speed Record Set on Live Science, May 12, 2005.