Heliotropism
Encyclopedia
Heliotropism is the diurnal motion
of plant parts (flowers or leaves) in response to the direction of the sun.
It is found in some members of family Malvacea e.g Malva or Lavetara
Heliotropism was first described by Leonardo da Vinci
(along with gravitropism
) in his botanical studies. The term "heliotropism," though, was introduced in the early 19th century by A. P. de Candolle
, for the movement of the stem tip towards light, which is different from phototropism
. The term heliotropism is for movement not growth
. The motor cells are specialized in pumping potassium
ion
s into nearby tissues, changing their turgor pressure
. The segment flexes because the motor cells at the shadow side elongate due to a turgor rise. Heliotropism is a response to blue light.
Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
of plant parts (flowers or leaves) in response to the direction of the sun.
It is found in some members of family Malvacea e.g Malva or Lavetara
Heliotropism was first described by Leonardo da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...
(along with gravitropism
Gravitropism
Gravitropism 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 gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems...
) in his botanical studies. The term "heliotropism," though, was introduced in the early 19th century by A. P. de Candolle
A. P. de Candolle
Augustin Pyramus de Candolle also spelled Augustin Pyrame de Candolle was a Swiss botanist. René Louiche Desfontaines launched Candolle's botanical career by recommending him at an herbarium...
, for the movement of the stem tip towards light, which is different from phototropism
Phototropism
Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, 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 term heliotropism is for movement not growth
Floral heliotropism
Heliotropic flowers track the sun's motion across the sky from east to west. During the night, the flowers may assume a random orientation, while at dawn they turn again toward the east where the sun rises. The motion is performed by motor cells in a flexible segment just below the flower, called a pulvinusPulvinus
A pulvinus is a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent movement. It consists of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma cells...
. The motor cells are specialized in pumping potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s into nearby tissues, changing their turgor pressure
Turgor pressure
Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...
. The segment flexes because the motor cells at the shadow side elongate due to a turgor rise. Heliotropism is a response to blue light.
Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.