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Sunflowers

For other uses, see Sunflower (disambiguation).

Helianthus annuus
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Asterales
Family: Asteraceae
Subfamily: Helianthoideae
Tribe: Heliantheae
Genus: Helianthus
Species: H. annuus
Binomial name
Helianthus annuus
L.

The sunflower (Helianthus annuus) is an annual plant native to the Americas. It possesses a large inflorescence (flowering head), and its name is derived from the flower's shape and image, which is often used to capture the sun. The plant has a rough, hairy stem, broad, coarsely toothed, rough leaves, and circular flower heads. The heads consist of many individual flowers which mature into seeds, often in the hundreds, on a receptacle base. From the Americas, sunflower seeds were brought to Europe in the 16th century, where, along with sunflower oil, they became a widespread cooking ingredient. Leaves of the sunflower can be used as cattle feed, while the stems contain a fibre which may be used in paper production.

Structure

The root of a sunflower reflects its characteristic of being a dicot. Out of the seed, the first root, radicle, pushes through and develops into a taproot. It continues to expand through primary and secondary tissues. Primary roots develop from primary tissues of the apical trimester that increase the length of the plant. Secondary roots, from secondary tissues of the lateral trimesters give rise to the girth of the plant. Both structures are vital for the growth and strength of the stem.

The stem of a sunflower grows from the plume found inside the seed. The plumule is an embryo shoot with a hypocaust stem structure below the point where the plume was attached and an epicycloid stem structure above this attachment point.[1] Since a sunflower is a divot, the cross-section of the stem organizes the vascular bundles in an away to separate the cortex and create a pith. This is opposite of its root structure, which does not include a pith. The vascular bundles consisting of xylem and phloem transport water, mainly acquired from the roots, and food, mainly developed in the leaves, throughout the plant.

The leaf of a sunflower is considered a simple leaf, being one which consists of a single blade.[2] The plumule gives rise to the first leaves of the plant that will go on to grow into organs for transpiration, with the opening and closing of the stomata found within the cell structure of leaves; for photosynthesis, and for other metabolic activities.[3] As far as structure, a sunflower blade is heart-shaped, has pinnate venation, and alternates along the stem. Its green color accents the vibrant green or other variety of leaves the flower has.

The flower of a sunflower is actually several flowers, which is why it is considered an inflorescence. An inflorescence is a group of several flowers.[4] Therefore, the many individual packets at the center of the head are the fruits of the plant, not the seeds. Each flower of the sunflower consists of the typical structures of a flower: receptacle, peduncle, sepal, petals, stamen, and a pistil. Consequently, every flower is able to develop fruit, or the ripened ovary, with the ovule (seed) inside.

Description

What is usually called the "flower" on a mature sunflower is actually a "flower head" (also known as a "composite flower") of numerous florets (small flowers) crowded together. The outer petal-bearing florets (ray florets) are sterile and can be yellow, red, orange, or other colors. The florets inside the circular head are called disc florets, which mature into seeds

The flower petals within the sunflower's cluster are always in a spiral pattern. Generally, each floret is oriented toward the next by approximately the golden angle, 137.5°, producing a pattern of interconnecting spirals, where the number of left spirals and the number of right spirals are successive Fibonacci numbers. Typically, there are 34 spirals in one direction and 55 in the other; on a very large sunflower there could be 89 in one direction and 144 in the other.[5][6][7] This pattern produces the most efficient packing of seeds within the flower head.[8][9][10]

Sunflowers commonly grow to heights between 1.5 and 3.5 m (5–12 ft.). The tallest sunflower confirmed by Guinness World Records is 8.0 m (2009, Germany). In 16th-century Europe the record was already 7.3 m (24 ft., Spain).[11] Most cultivars are variants of H. annuus, but four other species (all perennials) are also domesticated. This includes H. tuberosus, the Jerusalem Artichoke, which produces edible tubers.

Mathematical model of floret arrangement

A model for the pattern of florets in the head of a sunflower was proposed by H. Vogel in 1979.[12] This is expressed in polar coordinates

r = c \sqrt{n},
\theta = n \times 137.5^{\circ},

where θ is the angle, r is the radius or distance from the center, and n is the index number of the floret and c is a constant scaling factor. It is a form of Fermat's spiral. The angle 137.5° is related to the golden ratio (55/144 of a circular angle, where 55 and 144 are Fibonacci numbers) and gives a close packing of florets. This model has been used to produce computer graphics representations of sunflowers.[13]

Genome

The sunflower, Helianthus annuus, genome is diploid with a base chromosome number of 17 and an estimated genome size of 2871–3189 Mbp.[14][15] Some sources claim its true size is around 3.5 billion base pairs (slightly larger than the human genome).[16]

Cultivation and uses



To grow best, sunflowers need full sun. They grow best in fertile, moist, well-drained soil with heavy mulch. In commercial planting, seeds are planted 45 cm (1.5 ft.) apart and 2.5 cm (1 in) deep. Sunflower "whole seed" (fruit) are sold as a snack food, raw or after roasting in ovens, with or without salt and/or seasonings added. Sunflowers can be processed into a peanut butter alternative, sunflower butter. In Germany, it is mixed with rye flour to make Sonnenblumenkernbrot (literally: sunflower whole seed bread), which is quite popular in German-speaking Europe. It is also sold as food for birds and can be used directly in cooking and salads. American Indians had multiple uses for sunflowers in the past, such as in bread, medical ointments, dyes and body paints.[17]

Sunflower oil, extracted from the seeds, is used for cooking, as a carrier oil and to produce margarine and biodiesel, as it is cheaper than olive oil. A range of sunflower varieties exist with differing fatty acid compositions; some 'high oleic' types contain a higher level of monounsaturated fats in their oil than even olive oil.

The cake remaining after the seeds have been processed for oil is used as a livestock feed. Some recently developed cultivars have drooping heads. These cultivars are less attractive to gardeners growing the flowers as ornamental plants, but appeal to farmers, because they reduce bird damage and losses from some plant diseases. Sunflowers also produce latex, and are the subject of experiments to improve their suitability as an alternative crop for producing nonallergenic rubber.

Traditionally, several Native American groups planted sunflowers on the north edges of their gardens as a "fourth sister" to the better known three sisters combination of corn, beans, and squash.[18] Annual species are often planted for their allelopathic properties.[19]

However, for commercial farmers growing commodity crops, the sunflower, like any other unwanted plant, is often considered a weed. Especially in the Midwestern US, wild (perennial) species are often found in corn and soybean fields and can have a negative impact on yields.

Sunflowers can be used in phytoremediation to extract toxic ingredients from soil, such as lead, arsenic and uranium, and used in rhizofiltration to neutralize radionuclides and other toxic ingredients and harmful bacteria from water. They were used to remove caesium-137 and strontium-90 from a nearby pond after the Chernobyl disaster,[20] and a similar campaign was mounted in response to the Fukushima Daiichi nuclear disaster.[21][22]

Heliotropism misconception

A common misconception is that flowering sunflower heads track the Sun across the sky. Although immature flower buds exhibit this behaviour, the mature flowering heads point in a fixed (and typically easterly) direction throughout the day.[23][24] This old misconception was disputed in 1597 by the English botanist John Gerard, who grew sunflowers in his famous herbal garden: "[some] have reported it to turn with the Sun, the which I could never observe, although I have endeavored to find out the truth of it."[11] The uniform alignment of sunflower heads in a field might give some people the false impression that the flowers are tracking the sun.

The uniform alignment results from heliotropism in an earlier development stage, the bud stage, before the appearance of flower heads (anthesis).[25] The buds are heliotropic until the end of the bud stage, and finally face East.[26][27] Their heliotropic motion is a circadian rhythm, synchronized by the sun, which continues if the sun disappears on cloudy days. If a sunflower plant in the bud stage is rotated 180°, the bud will be turning away from the sun for a few days, as resynchronization by the sun takes time.[28] The heliotropic motion of the bud is performed by the pulvinus, a flexible segment just below the bud, due to reversible changes in turgor pressure, which occurs without growth.

History

Although it was commonly accepted that the sunflower was first domesticated in what is now the Southeastern US, roughly 5000 years ago,[29] there is evidence that it was first domesticated in Mexico[30] around 2600 BC. These crops were found in Tabasco, Mexico at the San Andre's dig site. The earliest known examples in the United States of a fully domesticated sunflower have been found in Tennessee, and date to around 2300 BC.[31] Many indigenous American peoples used the sunflower as the symbol of their solar deity, including the Aztecs and the Otomi of Mexico and the Incas in South America. In 1510 early Spanish explorers encountered the sunflower in the Americas and carried its seeds back to Europe.[32] Of the four plants known to have been domesticated in what is now the eastern continental United States [33] and to have become important agricultural commodities, the sunflower is currently the most economically important.

During the 18th century, the use of sunflower oil became very popular in Russia, particularly with members of the Russian Orthodox Church, because sunflower oil was one of the few oils that was allowed during Lent, according to some fasting traditions.[34]

Among the Zuni people, the fresh or dried root is chewed by the medicine man before sucking venom from a snakebite and applying a poultice to the wound.[35] This compound poultice of the root is applied with much ceremony to rattlesnake bites.[36] Blossoms are also used ceremonially for anthropic worship.[37]

Culture

  • The sunflower is the state flower of the US state of Kansas, and one of the city flowers of Kitakyūshū, Japan.
  • The sunflower is often used as a symbol of green ideology. The sunflower is also the symbol of the Vegan Society.
  • During the late 19th century, the flower was used as the symbol of the Aesthetic Movement.
  • Subject of Van Gogh's series of paintings, Sunflowers
  • The sunflower is the national flower of Ukraine.
  • The sunflower was chosen as the symbol of the Spiritualist Church for many reasons, but mostly because it turns toward the sun as "Spiritualism turns toward the light of truth". As stated earlier in the article, this is in fact, not true. Modern Spiritualists often have art or jewelry with sunflower designs.[38]
  • Starting from February 2010, Kuban Airlines of Russia painted part of its fleet in a new livery featuring enormous sunflowers.[39]

Cultivars

The following are cultivars of sunflowers (those marked agm have gained the Royal Horticultural Society's Award of Garden Merit):-

  • American Giant
  • Arnika
  • Autumn Beauty
  • Aztec Sun
  • Black Oil
  • Chianti Hybrid
  • Claret agm[40]
  • Dwarf Sunspot
  • Evening Sun
  • Florenza
  • Giant Primrose
  • Gullick's Variety agm[41]
  • Incredible
  • Indian Blanket Hybrid
  • Irish Eyes
  • Italian White
  • Kong Hybrid
  • Large Grey Stripe
  • Lemon Queen agm[42]
  • Loddon Gold agm[43]
  • Mammoth Russian
  • Miss Mellish agm[44]
  • Mongolian Giant
  • Orange Sun
  • Pastiche agm[45]
  • Peach Passion
  • Peredovik
  • Prado Red
  • Red Sun
  • Ring of Fire
  • Rostov
  • Skyscraper
  • Solar Eclipse
  • Soraya
  • Strawberry Blonde
  • Sunny Hybrid
  • Sunshine
  • Taiyo
  • Tarahumara
  • Teddy Bear
  • Thousand Suns
  • Titan
  • Valentine agm[46]
  • Velvet Queen
  • Yellow Disk

Other species

There are many species in the sunflower genus Helianthus, and many species in other genera that may be called sunflowers.

See also

References

Footnotes
General
  • Pope, Kevin; Pohl, Mary E. D.; Jones, John G.; Lentz, David L.; von Nagy, Christopher; Vega, Francisco J.; Quitmyer Irvy R. (18 May 2001). "Science, 292(5520):1370–1373.
  • Shosteck, Robert (1974) Flowers and Plants: An International Lexicon with Biographical Notes. New York: Quadrangle/The New York Times Book Co. ISBN 9780812904536.

External links

  • National Sunflower Association
  • Sunflowerseed—USDA Economic Research Service. Summary of sunflower production, trade, and consumption and links to relevant USDA reports.
  • Sunflower cultivation—New Crop Resource Online Program, Purdue University
  • —Home garden cultural information on growing sunflowers
  • Sunflower seed vigor—Relationship between seed size and NaCl on germination, seed vigor and early seedling growth of sunflower (Helianthus annuus L.)
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