Plant Movements
The major types of stimulus for plants
Phototropism
It is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, while roots usually exhibit negative phototropism, although geotropism may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The shoot is the part of their structure or body which causes the plant to respond to light. When you cut off the shoot of a plant it will not grow towards any sort of light, a once old myth that has been proved correct.
Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Different organs of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red light.
Phototropism is enabled by auxins. Auxins are plant hormones that have many functions. In this respect, auxins are responsible for expelling protons (by activating proton pumps) which decreases pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as expansins which break bonds in the cell wall structure, making the cell walls less rigid. In addition, the acidic environment causes disruption of hydrogen bonds in the cellulose that makes up the cell wall. The decrease in cell wall strength causes cells to swell, exerting the mechanical pressure that drives phototropic movement.
Short day plants Short-day plants flower when the night is longer than a critical length. They cannot flower under long days or if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering.
In general, short-day plants flower as days grow shorter (and nights grow longer) after 21 June in the Northern Hemisphere, which is during summer or fall. The length of the dark period required to induce flowering differs among species and varieties of a species.
Photoperiod affects flowering when the shoot is induced to produce floral buds instead of leaves and lateral buds. Note that some species must pass through a "juvenile" period during which they cannot be induced to flower; common cocklebur is an example of a plant species with a remarkably short period of juvenility and plants can be induced to flower when quite small.
Some short-day plants are as follows:
Chrysanthemum
Coffea
Euphorbia
Fragaria
Tobacco
Lemma
Day neutral plants such as cucumbers, roses and tomatoes, do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (see below for details), rather than in response to photoperiod.
Long day plants A long-day plant requires fewer than a certain number of hours of darkness in each 24-hour period to induce flowering. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. In the Northern Hemisphere, the longest day of the year is on or about 21 June. After that date, days grow shorter (until 21 December. This situation is reversed in the Southern Hemisphere (i.e. longest day is 21 December and shortest day is 21 June). In some parts of the world, however winter or summer might refer to rainy versus dry seasons, respectively, rather than the coolest or warmest time of year.
Some long-day obligate plants are:
· Dianthus
· Hyoscyamus
· Avena
· Lollium
· Trifolium
· Campanula carpatica
Gravitropism
Geotropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive geotropism and stems show negative geotropism. That is the roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behaviour can be easily demonstrated with a potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, bending upwards. Herbaceous stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth in a new direction.
Roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. In roots, an increase in the concentration of auxin will inhibit cell expansion; therefore the redistribution of auxin in the root can initiate differential growth in the elongation zone resulting in root curvature.
A “tropism” is a plant movement triggered by stimuli. The term geotropic refers to a plant whose roots grow down into the soil as a response to gravity. Plants commonly exist in a state of anisotropic growth, where roots grow downward and shoots grow upward. Anisotropic growth will continue even as a plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth's atmosphere, it will still grow roots down and stem up. The reason for this comes from the nature of a plant and its general response to gravity.
A similar mechanism is known to occur in plant stems except that the shoot cells have a different dose response curve with respect to auxin. In shoots, increasing the local concentration of auxin promotes cell expansion; this is the opposite of root cells.
The differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the gravity vector. In both roots and stems auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity. In stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion and results in the shoot curving up.
Vernalisation
Vernalisation is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter. In the much studied model species A. thaliana, the apical meristem, must be vernalized in order to promote flowering. Vernalisation of the meristem appears to confer competence to respond to floral inductive signals on the meristem. A vernalized meristem retains competence for as long as 300 days in the absence of an inductive signal. It is possible to de-vernalize a plant by exposure to high temperatures subsequent to vernalisation.
Vernalisation Many temperate plants have a vernalisation requirement and must experience a period of low winter temperature to initiate or accelerate the flowering process, or, as the case with many fruit tree species, to actually break dormancy, prior to flowering. Many plant species, including some ecotypes of Arabidopsis thaliana and winter cereals such as wheat, must go through a prolonged period of cold before flowering occurs. This ensures that reproductive development and seed production occurs at the optimum environmentally favorable time, normally following the passing of winter. The needed cold is often expressed in chill hours.
Following vernalisation, plants have acquired the competence to flower, although they may require additional seasonal cues or weeks of growth before they will actually flower. One of the most important influences that temperature has on the floral transition is the vernalisation response. Vernalisation activates a plant hormone called florigen present in the leaves which induces flowering at the end of the chilling treatment. Some plant species do not flower without vernalisation. Many biennial species have a vernalisation period, which can vary in period and temperature. Typical vernalisation temperatures are between 5 and 10 degrees Celsius.
Thigmotropism
Thigmotropism is a movement in which an organism moves or grows in response to touch or contact stimuli. Thigmotropism comes from the Greek for touch. Usually thigmotropism occurs when plants grow around a surface, such as a wall, pot, or trellis. Climbing plants such as vines, develop tendrils that coil around supporting objects. Touched cells produce auxin and transport it to untouched cells. Some untouched cells will then elongate faster so cell growth bends around the object. Some seedlings also inhibit triple response, caused by pulses of ethylene which cause the stem to thicken (grow slower and stronger) and curve to start growing horizontally.
Mimosa pudica is well known for its rapid plant movement. The leaves close up and droop when touched. However, this is not a form of tropism but a nastic movement, a similar phenomenon. The difference is that tropisms are influenced by the direction of their stimulus, while nastic movements are not.
Hydrotropism
Hydrotropism is a plant growth response in which the direction of growth is determined by a stimulus or gradient in water concentration. A common example is plant roots growing in humid air bending toward a higher relative humidity level. The process of hydrotropism is started by the root cap sensing water and sending a signal to the elongating part of the root. Hydrotropism is difficult to observe in underground roots, since the roots are not readily observable, and root geotropism is usually more influential than root hydrotropism. Also, water is not as strongly directional factor as gravity for geotropism, light for phototropism and touch for thigmotropism. Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.
Thus, root hydrotropism research has mainly been a laboratory phenomenon for roots grown in humid air rather than soil. Its ecological significance in soil-grown roots is unclear because so little hydrotropism research has examined soil-grown roots. Recent identification of a mutant plant that lacks a hydrotropic response may help to elucidate its role in nature. Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment
Misunderstandings
The greater growth of roots in moist soil zones than in dry soil zones is not usually a result of hydrotropism. Hydrotropism requires a root to bend from a drier to a wetter soil zone. Roots require water to grow so roots that happen to be in moist soil will grow and branch much more than those in dry soil.
Roots cannot sense water inside intact pipes via hydrotropism and break the pipes to obtain the water.
Roots cannot sense water several feet away via hydrotropism and grow toward it. At best hydrotropism probably operates over distances of a couple millimeters.