Heliotropism

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Daisies (Bellis perennis) face the Sun after opening in the morning and will follow the Sun through the day Bellis perennis - Gaenseblume4.jpg
Daisies ( Bellis perennis ) face the Sun after opening in the morning and will follow the Sun through the day

Heliotropism, a form of tropism, is the diurnal or seasonal motion of plant parts (flowers or leaves) in response to the direction of the Sun.

Contents

The habit of some plants to move in the direction of the Sun, a form of tropism, was already known by the Ancient Greeks. They named one of those plants after that property Heliotropium , meaning "sun turn". The Greeks assumed it to be a passive effect, presumably the loss of fluid on the illuminated side, that did not need further study. [1] Aristotle's logic that plants are passive and immobile organisms prevailed. In the 19th century, however, botanists discovered that growth processes in the plant were involved, and conducted increasingly in-depth experiments. A. P. de Candolle called this phenomenon in any plant heliotropism (1832). [2] It was renamed phototropism in 1892, because it is a response to light rather than to the sun, and because the phototropism of algae in lab studies at that time strongly depended on the brightness (positive phototropic for weak light, and negative phototropic for bright light, like sunlight). [3] [4] A botanist studying this subject in the lab, at the cellular and subcellular level, or using artificial light, is more likely to employ the more abstract word phototropism, a term which includes artificial light as well as natural sunlight. The French scientist Jean-Jacques d'Ortous de Mairan was one of the first to study heliotropism when he experimented with the Mimosa pudica plant. The phenomenon was studied by Charles Darwin and published in his penultimate 1880 book The Power of Movement in Plants , a work which included other stimuli to plant movement such as gravity, moisture and touch.

Floral heliotropism

Heliotropic flowers track the Sun's motion across the sky from east to west. Daisies or Bellis perennis close their petals at night but open in the morning light and then follow the sun as the day progresses. 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 pulvinus. The motor cells are specialized in pumping potassium ions into nearby tissues, changing their turgor pressure. The segment flexes because the motor cells at the shadow side elongate due to a turgor rise. This is considered to be turgor-mediated heliotropism. For plant organs that lack pulvini, heliotropism can occur through irreversible cell expansion producing particular growth patterns. This form of heliotropism is considered to be growth-mediated. [5] Heliotropism is a response to light from the Sun.

Explanations

Several hypotheses have been proposed for the occurrence of heliotropism in flowers:

In general, flower heliotropism could increase reproductive success by increasing pollination, fertilization success, and/or seed development, [9] especially in the spring flowers.

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.

Tropical convolvulaceous flowers show a preferred orientation, pointing in the general direction of the sun but not exactly tracking the sun. They demonstrated no diurnal heliotropism but strong seasonal heliotropism. If solar tracking is exact, the sun’s rays would always enter the corolla tube and warm the gynoecium, a process which could be dangerous in a tropical climate. However, by adopting a certain angle away from the solar angle, this is prevented. The trumpet shape of these flowers thus acts as a parasol shading the gynoecium at times of maximum solar radiation, and not allowing the rays to impinge on the gynoecium. [10]

Sunflowers in full bloom are not heliotropic, so they do not follow the Sun. The flowerheads face east all day, so in the afternoon, they are backlit by the Sun. SunflowerSunset2.jpg
Sunflowers in full bloom are not heliotropic, so they do not follow the Sun. The flowerheads face east all day, so in the afternoon, they are backlit by the Sun.

In the case of sunflowers, a common misconception is that sunflower heads track the Sun across the sky throughout the whole life cycle. The uniform alignment of the flowers does result from heliotropism in an earlier development stage, the bud stage, before the appearance of flower heads. The apical bud of the plant will track the Sun during the day from east to west, and then will quickly move west to east overnight as a result of the plant's circadian clock. [11] The buds are heliotropic until the end of the bud stage, and finally face east. Phototropic bending can be catalyzed in the hypocotyls of juvenile sunflower seedlings while heliotropic bending in the shoot apex does not start occurring until the later developmental stages of the plant, showing a difference between these two processes. [11] The flower of the sunflower preserves the final orientation of the bud, thus keeping the mature flower facing east.

Leaf heliotropism

Leaf heliotropism is the solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicularly to the sun's rays in the morning ( diaheliotropism ), and others have those that orient themselves parallel to these rays at midday ( paraheliotropism ). [12] Floral heliotropism is not necessarily exhibited by the same plants that exhibit leaf heliotropism.

Related Research Articles

<span class="mw-page-title-main">Common sunflower</span> Species of flowering plant in the family of Asteraceae

The common sunflower is a species of large annual forb of the daisy family Asteraceae. The common sunflower is harvested for its edible oily seeds which are used in the production of cooking oil, as well as other uses such as food for livestock, bird food, and planting in domestic gardens for aesthetics. Wild plants are known for their multiple flower heads, whereas the domestic sunflower often possesses a single large flower head atop an unbranched stem.

<span class="mw-page-title-main">Auxin</span> Plant hormone

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.

<span class="mw-page-title-main">Tropism</span> Directed growth of an organism in response to environmental stimuli

In biology, a tropism is a phenomenon indicating the growth or turning movement of an organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus. Tropisms are usually named for the stimulus involved; for example, a phototropism is a movement to the light source, and an anemotropism is the response and adaptation of plants to the wind.

<span class="mw-page-title-main">Plant physiology</span> Subdiscipline of botany

Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants.

<span class="mw-page-title-main">Coleoptile</span> Protective sheath in certain plants

Coleoptile is the pointed protective sheath covering the emerging shoot in monocotyledons such as grasses in which few leaf primordia and shoot apex of monocot embryo remain enclosed. The coleoptile protects the first leaf as well as the growing stem in seedlings and eventually, allows the first leaf to emerge. Coleoptiles have two vascular bundles, one on either side. Unlike the flag leaves rolled up within, the pre-emergent coleoptile does not accumulate significant protochlorophyll or carotenoids, and so it is generally very pale. Some preemergent coleoptiles do, however, accumulate purple anthocyanin pigments.

<i>Helianthus</i> Genus of flowering plants, the sunflowers

Helianthus is a genus comprising around 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of Helianthus are native to North America and Central America. The best-known species is the common sunflower. This and other species, notably Jerusalem artichoke, are cultivated in temperate regions and some tropical regions, as food crops for humans, cattle, and poultry, and as ornamental plants. The species H. annuus typically grows during the summer and into early fall, with the peak growth season being mid-summer.

<span class="mw-page-title-main">Gravitropism</span> Plant growth in reaction to gravity and bending of leaves and roots

Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. 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 grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

Thermotropism or thermotropic movement is the movement of an organism or a part of an organism in response to heat or changes from the environment's temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures. Mimosa pudica also show thermotropism by the collapsing of leaf petioles leading to the folding of leaflets, when temperature drops.

<span class="mw-page-title-main">Nastic movements</span> Undirected movement in response to external stimuli

In biology, nastic movements are non-directional responses to stimuli that occur more rapidly than tropisms and are usually associated with plants. The movement can be due to changes in turgor. Decrease in turgor pressure causes shrinkage, while increase in turgor pressure brings about swelling. Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position. The tropic movement is growth movement but nastic movement may or may not be growth movement. The rate or frequency of these responses increases as intensity of the stimulus increases. An example of such a response is the opening and closing of flowers, movement of euglena, chlamydomonas towards the source of light. They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:

Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall.

<span class="mw-page-title-main">Thigmonasty</span> Undirected movement in response to touch or vibration

In biology, thigmonasty or seismonasty is the nastic (non-directional) response of a plant or fungus to touch or vibration. Conspicuous examples of thigmonasty include many species in the leguminous subfamily Mimosoideae, active carnivorous plants such as Dionaea and a wide range of pollination mechanisms.

<span class="mw-page-title-main">Plant perception (physiology)</span> Plants interaction to environment

Plant perception is the ability of plants to sense and respond to the environment by adjusting their morphology and physiology. Botanical research has revealed that plants are capable of reacting to a broad range of stimuli, including chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch. The scientific study of plant perception is informed by numerous disciplines, such as plant physiology, ecology, and molecular biology.

Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria. They mediate light responses as varied as visual perception, phototropism and phototaxis, as well as responses to light-dark cycles such as circadian rhythm and other photoperiodisms including control of flowering times in plants and mating seasons in animals.

<span class="mw-page-title-main">Nutation (botany)</span> Term in botany

Nutation refers to the bending movements of stems, roots, leaves and other plant organs caused by differences in growth in different parts of the organ. Circumnutation refers specifically to the circular movements often exhibited by the tips of growing plant stems, caused by repeating cycles of differences in growth around the sides of the elongating stem. Nutational movements are usually distinguished from 'variational' movements caused by temporary differences in the water pressure inside plant cells (turgor).

Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal embryo will very early produce all of the body parts that it will ever have in its life. When the animal is born, it has all its body parts and from that point will only grow larger and more mature. However, both plants and animals pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.

<span class="mw-page-title-main">Phototropism</span> Growth of a plant in response to a light stimulus

In biology, phototropism is the growth of an organism in 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 contain a hormone called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the furthest 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 called positive phototropism, while growth away from light is called negative phototropism. Negative phototropism is not to be confused with skototropism, which is defined as the growth towards darkness, whereas negative phototropism can refer to either the growth away from a light source or towards the darkness. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.

<span class="mw-page-title-main">Cholodny–Went model</span> Botany model

In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards the light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

<span class="mw-page-title-main">John Z. Kiss</span> American space plant biologist

John Z. Kiss is an American biologist known for his work on the gravitational and space biology of plants. Kiss is senior vice president for academic affairs and provost at the Florida Institute of Technology. Previously, he was dean of the College of Arts & Sciences at the University of North Carolina Greensboro. He has also served as dean of the Graduate School at the University of Mississippi. and distinguished professor and chair of the botany department at Miami University. He has worked with NASA since 1987 and served as principal investigator on eight spaceflight experiments on the Space Shuttle, the former Russian space station Mir, and on the International Space Station. His research focuses on the sensory physiology of plants in space. He received the NASA Outstanding Public Leadership Medal in 2014. In 2021, Asteroid Kiss 8267 was named in his honor, a recognition that coincided with his receipt of the 2021 COSPAR International Cooperation Medal. His international collaboration on a spaceflight project with NASA and the European Space Agency has led to the discovery of novel sensory mechanisms in plants.

Peter Boysen Jensen was a Danish plant physiologist. His research was fundamental to further work on the auxin theory of tropisms.

Stacey Harmer is a chronobiologist whose work centers on the study of circadian rhythms in plants. Her research focuses on the molecular workings of the plant circadian clock and its influences on plant behaviors and physiology. She is a professor in the Department of Plant Biology at the University of California, Davis.

References

  1. ↑ Whippo, Craig W. (2006). "Phototropism: Bending towards Enlightenment". The Plant Cell. 18 (5): 1110–1119. doi:10.1105/tpc.105.039669. PMC   1456868 . PMID   16670442.
  2. ↑ Hart, J.W. (1990). Plant Tropisms: And other Growth Movements. Springer. p. 36. ISBN   9780412530807 . Retrieved 2012-08-08.
  3. ↑ "Phototropism and photomorphogenesis of Vaucheria". Archived from the original on 2016-03-04. Retrieved 2012-08-21.
  4. ↑ Donat-Peter Häder; Michael Lebert (2001). Photomovement. Elsevier. p. 676. ISBN   9780080538860 . Retrieved 2012-08-08.
  5. ↑ Vandenbrink, Joshua P.; Brown, Evan A.; Harmer, Stacey L.; Blackman, Benjamin K. (July 2014). "Turning heads: The biology of solar tracking in sunflower". Plant Science. 224: 20–26. doi:10.1016/j.plantsci.2014.04.006. ISSN   0168-9452. PMID   24908502. S2CID   887356.
  6. ↑ Hocking B.; Sharplin D. (1965). "Flower basking by arctic insects". Nature. 206 (4980): 206–215. Bibcode:1965Natur.206R.215H. doi: 10.1038/206215b0 . S2CID   4209695.
  7. ↑ Kevan, P.G. (1975). "Sun-tracking solar furnaces in high arctic flowers: significance for pollination and insects". Science. 189 (4204): 723–726. Bibcode:1975Sci...189..723K. doi:10.1126/science.189.4204.723. PMID   17792542. S2CID   20984125.
  8. ↑ Lang A.R.G.; Begg J.E. (1979). "Movements of Helianthus annuus leaves and heads". J Appl Ecol. 16 (1): 299–305. Bibcode:1979JApEc..16..299L. doi:10.2307/2402749. JSTOR   2402749.
  9. ↑ Kudo, G. (1995). "Ecological Significance of Flower Heliotropism in the Spring Ephemeral Adonis ramosa (Ranunculaceae)". Oikos. 72 (1): 14–20. Bibcode:1995Oikos..72...14K. doi:10.2307/3546032. JSTOR   3546032.
  10. ↑ Patiño, S.; Jeffree, C.; Grace, J. (2002). "The ecological role of orientation in tropical convolvulaceous flowers" (PDF). Oecologia. 130 (3): 373–379. Bibcode:2002Oecol.130..373P. doi:10.1007/s00442-001-0824-1. PMID   28547043. S2CID   512722.
  11. 1 2 Vandenbrink, Joshua P.; Brown, Evan A.; Harmer, Stacey L.; Blackman, Benjamin K. (2014). "Turning heads: The biology of solar tracking in sunflower". Plant Science. 224: 20–26. doi:10.1016/j.plantsci.2014.04.006. PMID   24908502. S2CID   887356.
  12. ↑ Häder, D.-P.; Lebert, M. (19 June 2001). Photomovement. Elsevier. ISBN   9780080538860 – via Google Books.
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