Helianthus

Helianthus ()[3] is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae.[4][5] Except for three South American species, the species of Helianthus are native to North America and Central America. The common names "sunflower" and "common sunflower" typically refer to the popular annual species Helianthus annuus, whose round flower heads in combination with the ligules look like the sun.[6] This and other species, notably Jerusalem artichoke (H. tuberosus), are cultivated in temperate regions and some tropical regions as food crops for humans, cattle, and poultry, and as ornamental plants.[7] The species H. annuus typically grows during the summer and into early fall, with the peak growth season being mid-summer.[8]

Several perennial Helianthus species are grown in gardens, but have a tendency to spread rapidly and can become aggressive. On the other hand, the whorled sunflower, Helianthus verticillatus, was listed as an endangered species in 2014 when the U.S. Fish and Wildlife Service issued a final rule protecting it under the Endangered Species Act. The primary threats are industrial forestry and pine plantations in Alabama, Georgia, and Tennessee. They grow to 1.8 m (6 ft) and are primarily found in woodlands, adjacent to creeks and moist, prairie-like areas.[9]

The disk of a sunflower is made up of many little flowers. The ray flowers here are dried
Sunflower florets are arranged in a natural spiral having a Fibonacci sequence

Sunflowers originate in the Americas. They were first domesticated in what is now Mexico and the Southern United States.[10][11] 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.[11] In the 16th century the first crop breeds were brought from America to Europe by explorers.[11]

Sunflowers are thought to have been domesticated 3000–5000 years ago by Native Americans who would use them primarily as a source for edible seeds. They were then introduced to Europe in the early 16th century and made their way to Russia. In Russia, where oilseed cultivators were located, these flowers were developed and grown on an industrial scale. Russia then reintroduced this oilseed cultivation process to North America in the mid-20th century; North America began their commercial era of sunflower production and breeding.[12] New breeds of the Helianthus spp. began to become more prominent in new geographical areas.

This species' geographical history accounts for its evolutionary history, with its levels of genetic variation across its gene pool increasing as new hybrids are created both for commercial use and in the wild. Subsequent to this, sunflower species are also experiencing the bottle neck effect in their gene pool as a result of selective breeding for industrial use.[12]

Before blooming, sunflower plants tilt during the day to face the sun in order to gain more sunlight for photosynthesis. This heliotropism continues for a short time when the plant blooms, young sunflower heads tracking of the sun. This is thought to help attract pollinators, as many are more attracted to warm flowers. By the time they are mature, though, sunflowers generally stop moving to remain facing east. which lets them be warmed by the rising sun.[13] The movement of sunflowers through heliotropism happens as the sunflower follows the sun, the opposite side of the sunflower stem begins to accumulate growth hormones and this causes growth which redirects the sunflower.[13][14] The rough and hairy stem is branched in the upper part in wild plants, but is usually unbranched in domesticated cultivars.[13]

Sunflowers are usually tall annual or perennial plants that in some species can grow to a height of 300 cm (120 in) or more. Each "flower" is actually a disc made up of tiny flowers, to form a larger false flower to better attract pollinators. The plants bear one or more wide, terminal capitula (flower heads made up of many tiny flowers), with bright yellow ray florets (mini flowers inside a flower head) at the outside and yellow or maroon (also known as a brown/red) disc florets inside. Several ornamental cultivars of H. annuus have red-colored ray florets; all of them stem from a single original mutant.[15] While the majority of sunflowers are yellow, there are branching varieties in other colours including, orange, red and purple.

The petiolate leaves are dentate and often sticky. The lower leaves are opposite, ovate, or often heart-shaped.

This genus is distinguished technically by the fact that the ray florets (when present) are sterile, and by the presence on the disk flowers of a pappus that is of two awn-like scales that are caducous (that is, easily detached and falling at maturity). Some species also have additional shorter scales in the pappus, and one species lacks a pappus entirely. Another technical feature that distinguishes the genus more reliably, but requires a microscope to see, is the presence of a prominent, multicellular appendage at the apex of the style. Further, the florets of a sunflower are arranged in a natural spiral.[16]

Variability is seen among the perennial species that make up the bulk of those in the genus. Some have most or all of the large leaves in a rosette at the base of the plant and produce a flowering stem that has leaves that are reduced in size. Most of the perennials have disk flowers that are entirely yellow, but a few have disk flowers with reddish lobes. One species, H. radula, lacks ray flowers altogether.

Overall, the macroevolution of the Heliabthus is driven by multiple biotic and abiotic factors and influences various floral morphology.[17]

Helianthus species are used as food plants by the larvae of many lepidopterans. The seeds of H. annuus are used as human food.

The growth of a sunflower depends strictly on its genetic makeup and background.[18] Additionally, the season it is planted will have effects on its development; those seasons tend to be in the middle of summer and beginning of fall. Sunflower development is classified by a series of vegetative stages and reproductive stages that can be determined by identifying the heads or main branch of a single head or branched head.[18]

Researchers have analyzed the impact of various nitrogen-based fertilizers on the growth of sunflowers. Ammonium nitrate was found to produce better nitrogen absorption than urea, which performed better in low-temperature areas.[19]

In Brazil, a unique system of production called the soybean-sunflower system is used: sunflowers are planted first, and then soybean crops follow, reducing idle periods and increasing total sunflower production and profitability. Sunflowers are usually planted in the extreme southern or northern regions of the country. Frequently, in the southern regions, sunflowers are grown in the beginning of rainy seasons, and soybeans can then be planted in the summer.[20] Researchers have concluded that the soybean-sunflower method of plantation could be further improved through changes in fertilizer use. The current method has been shown to have positive environmental impacts.[21]

Sunflowers have been proven to be excellent plants to attract beneficial insects, including pollinators. Helianthus spp. are a nectar producing flowering plant that attract pollinators and parasitoids which reduce the pest populations in nearby crop vegetation. Sunflowers attract different beneficial pollinators (e.g., honey bees) and other known insect prey to feed on and control the population of parasitic pests that could be harmful to the crops.[23] Predacious insects are first attracted to sunflowers once they are planted. Once the Helianthus spp. reaches six inches and produces flowers it begins to attract more pollinators. Distance between sunflower rows and crop vegetation plays an important role in this phenomenon, hypothesizing that closer proximity to the crops will increase insect attraction.[23]

In addition to pollinators of Helianthus spp., there are other factors such as abiotic stress, florivory, and disease which also contribute to the evolution of floral traits. These selective pressures, which stem from several biotic and abiotic factors are associated with habitat environmental conditions which all play a role in the overall morphology of the sunflowers’ floral traits.[24]

An ecosystem is composed of both biotic (which are living elements of an ecosystem such as plants, animals, fungi, protists, and bacteria), and abiotic factors (non-living elements of an ecosystem such as air, soil, water, light, salinity and temperature).[25]

It is thought that two biotic factors can explain for the evolution of larger sunflowers and why they are present in more drier environments.[24] For one thing, the selection by pollinators is thought to have increased the sunflower’s size in a drier environment.[24] This is because in a drier environment, there are typically less pollinators.[24] As a result, in order for the sunflower to be able to attract more pollinators, they had to increase the morphology of their floral traits in that they had to increase their display size.[24] Another biotic factor that can explain for the evolution of larger sunflowers in drier environments is that the pressure from florivory and disease favors smaller flowers in habitats that have a more moderate supply of moisture (mesic habitat).[24] Wetter environments usually have more dense vegetation, more herbivores, and more surrounding pathogens.[24] As larger flowers are typically more susceptible to disease and florivory, smaller flowers may have evolved in wetter environments which explains the evolution of larger sunflowers in more drier environments.[24]

Regarding the phylogeographic relations and population demographic patterns across sunflowers, earlier cultivated sunflowers formed a clade from wild populations from the Great Plains, which demonstrates 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 ~5000 years ago.[26]

The seed and sprouts of the common sunflower (Helianthus annuus L.) have many medicinal uses. The edible seed and the sprout have an abundance of nutrients and biological activities and have many antioxidants such as phenolic acids, flavonoids and vitamins.[27] The common sunflower has many antioxidant effects which serve as a protective function for cellular damage.[27] Their phytochemical constituents, which include phenolic acids, flavonoids and tocopherol (vitamin E), have many potential benefits. The sunflower seed and sprout also have high concentrations of vitamins A, B, and C and are high in niacin.[27] They also have many minerals such as calcium, potassium and iron. Sunflower seed extract contain antidiabetic effects where secondary metabolites within those extracts are able to control glucose levels effectively.[27] The bioactive peptides of the common sunflower are known to have antihypertensive effects.[27] Sunflower oil also helps in anti-inflammatory activity, prevents gastric damage and is a therapeutic alternative in the healing process for microscopical and clinical wounds.[27]

The following species were previously included in the genus Helianthus.[28]