Common sunflower
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 often eaten as a snack food. They are also used in the production of cooking oil, as food for livestock, as bird food, and as plantings 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.
Description
The plant has an erect rough-hairy stem, reaching typical heights of. The tallest sunflower on record achieved. Sunflower leaves are broad, coarsely toothed, rough and mostly alternate; those near the bottom are largest and commonly heart-shaped.Flower
The plant flowers in summer. What is often called the "flower" of the sunflower is actually a "flower head", wide, of numerous small individual five-petaled flowers. The outer flowers, which resemble petals, are called ray flowers. Each "petal" consists of a ligule composed of fused petals of an asymmetrical ray flower. They are sexually sterile and may be yellow, red, orange, or other colors. The spirally arranged flowers in the center of the head are called disk flowers. These mature into fruit.The prairie sunflower is similar in appearance to the wild common sunflower; the scales in its central disk are tipped by white hairs.
Heliotropism
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 direction throughout the day. This old misconception was disputed in 1597 by the English botanist John Gerard, who grew sunflowers in his famous herbal garden: " 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." The uniform alignment of sunflower heads in a field might give some people the false impression that the flowers are tracking the Sun.This alignment results from heliotropism in an earlier development stage, the young flower stage, before full maturity of flower heads. Young sunflowers orient themselves in the direction of the sun. At dawn, the head of the flower faces east and moves west throughout the day. When sunflowers reach full maturity, they no longer follow the sun and continuously face east. Young flowers reorient overnight to face east in anticipation of the morning. Their heliotropic motion is a circadian rhythm, synchronized by the sun, which continues if the sun disappears on cloudy days or if plants are moved to constant light. They are able to regulate their circadian rhythm in response to the blue-light emitted by a light source. 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 with the sun takes time.
When growth of the flower stalk stops and the flower is mature, the heliotropism also stops and the flower faces east from that moment onward. This eastward orientation allows rapid warming in the morning, and as a result, an increase in pollinator visits. Sunflowers do not have a pulvinus below their inflorescence. A pulvinus is a flexible segment in the leaf stalks of some plant species and functions as a 'joint'. It effectuates leaf motion due to reversible changes in turgor pressure which occurs without growth. The sensitive plant's closing leaves are a good example of reversible leaf movement via pulvinuli.
Image:SunflowerModel.svg|thumb|upright=.9|Vogel's model for n=1 ... 500
Floret arrangement
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; however, in a very large sunflower head there could be 89 in one direction and 144 in the other. This pattern produces the most efficient packing of seeds mathematically possible within the flower head.A model for the pattern of florets in the head of a sunflower was proposed by H. Vogel in 1979. This is expressed in polar coordinates
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 and gives a close packing of florets. This model has been used to produce computer generated representations of sunflowers.
Genome
The sunflower genome is diploid with a base chromosome number of 17 and an estimated genome size of 2,871–3,189 billion base pairs. Some sources claim its true size is around 3.5 billion base pairs.Etymology
In the binomial name Helianthus annuus, the genus name is derived from the Greek ἥλιος : hḗlios 'sun' and ἄνθος : ánthos 'flower'. The species name annuus means 'annual' in Latin.Distribution and habitat
The plant was first domesticated in the Americas. Sunflower seeds were brought to Europe from the Americas in the 16th century, where, along with sunflower oil, they became a widespread cooking ingredient. With time, the bulk of industrial-scale production has shifted to Eastern Europe, and Russia and Ukraine together produce over half of worldwide seed production.Sunflowers grow best in fertile, moist, well-drained soil with heavy mulch. They often appear on dry open areas and foothills. Outside of cultivation, the common sunflower is found on moist clay-based soils in areas with climates similar to Texas. In contrast, the related Helianthus debilis and Helianthus petiolaris are found on drier, sandier soils.
The precise native range is difficult to determine. According to Plants of the World Online, it is native to Arizona, California, and Nevada in the present-day United States and to all parts of Mexico except the Gulf Coast and southeast. Though not giving much detail, the Missouri Botanical Garden Plant Finder also lists it as native to the Western United States and Canada. The information published by the Biota of North America Program largely agrees with this, showing the common sunflower as native to states west of the Mississippi, though also listed as a noxious weed in Iowa, Minnesota, and Texas. Regardless of its original range, it can now be found in almost every part of the world that is not tropical, desert, or tundra.
Ecology
Threats and diseases
One of the major threats that sunflowers face today is Fusarium, a filamentous fungus that is found largely in soil and plants. It is a pathogen that over the years has caused an increasing amount of damage and loss of sunflower crops, some as extensive as 80% of damaged crops.Downy mildew is another disease to which sunflowers are susceptible. Its susceptibility to downy mildew is particularly high due to the sunflower's way of growth and development. Sunflower seeds are generally planted only an inch deep in the ground. When such shallow planting is done in moist and soaked earth or soil, it increases the chances of diseases such as downy mildew.
Another major threat to sunflower crops are broomrapes, a family of plants which parasitize the roots of various other plants, including sunflowers. Damage and loss to sunflower crops as a result of broomrape can be as high as 100%.
Cultivation
In commercial planting, seeds are planted apart and deep.History
The common sunflower was one of several plants cultivated by Native Americans in prehistoric North America as part of the Eastern Agricultural Complex, which also included goosefoot, little barley, squash, and a variety of other crops, most of which were replaced by maize and beans following their introduction. Although it was commonly accepted that the sunflower was first domesticated in what is now the southeastern US, roughly 5,000 years ago, there is evidence that it was first domesticated in Mexico around 2600 BCE. These crops were found in Tabasco, Mexico, at the San Andres dig site. The earliest known examples in the US of a fully domesticated sunflower have been found in Tennessee, and date to around 2300 BCE. Other very early examples come from rockshelter sites in Eastern Kentucky. 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. Of the four plants known to have been domesticated in eastern North America and to have become important agricultural commodities, the sunflower is currently the most economically important.Research of phylogeographic relations and population demographic patterns across sunflowers has demonstrated that earlier cultivated sunflowers form a clade from wild populations from the Great Plains, which indicates that there was a single domestication event in central North America. Following the cultivated sunflower's origin, it may have gone through significant bottlenecks dating back to ~5,000 years ago.
In the 16th century the first crop breeds were brought from America to Europe by explorers. Domestic sunflower seeds have been found in Mexico, dating to 2100 BCE. Native American people grew sunflowers as a crop from Mexico to Southern Canada. They then were introduced to the Russian Empire, where oilseed cultivators were located, and the flowers were developed and grown on an industrial scale. The Russian Empire reintroduced this oilseed cultivation process to North America in the mid-20th century; North America began their commercial era of sunflower production and breeding. New breeds of the Helianthus spp. began to become more prominent in new geographical areas. During the 18th century, the use of sunflower oil became very popular in Russia, particularly with members of the Russian Orthodox Church, because only plant-based fats were allowed during Lent, according to fasting traditions. In the early 19th century, it was first commercialized in the village of Alexeyevka in Voronezh Governorate by the merchant named Daniil Bokaryov, who developed a technology suitable for its large-scale extraction, and quickly spread around. The town's coat of arms has included an image of a sunflower ever since.