Helianthus

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.
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 to this species are industrial forestry and pine plantations in Alabama, Georgia, and Tennessee. They grow to and are primarily found in woodlands, adjacent to creeks and moist, prairie-like areas.
The common sunflower is the national flower of Ukraine, cultivated there for several centuries.

Description

Sunflowers are usually tall annual or perennial plants that in some species can grow to a height of 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, with bright yellow ray florets at the outside and yellow or maroon disc florets inside. Several ornamental cultivars of H. annuus have red-colored ray florets; all of them stem from a single original mutant. While the majority of sunflowers are yellow, there are branching varieties in other colors including, orange, red and purple.
The petiolate leaves are dentate and often sticky. The lower leaves are opposite, ovate, or often heart-shaped. The rough and hairy stem is branched in the upper part in wild plants, but is usually unbranched in domesticated cultivars.
This genus is distinguished technically by the fact that the ray florets are sterile, and by the presence on the disk flowers of a pappus that is of two awn-like scales that are caducous. 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.
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 Helianthus is driven by multiple biotic and abiotic factors and influences various floral morphology.
Helianthus species are used as food plants by the larvae of many lepidopterans.

Growth stages

The growth of a sunflower depends strictly on its genetic makeup and background. 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.

Facing the Sun (heliotropism)

Before blooming, Helianthus plant heads tilt upwards during the day to face the Sun. This movement is referred to as heliotropism, which continues for a short time when flower buds form and young Helianthus heads track the Sun. At night, the flower heads reorient their position and face east in anticipation of the sunrise. Sunflowers move back to their original position between the hours of 3am and 6am, and the leaves follow about an hour later.
By the time they are mature and reach anthesis, Helianthus generally stop moving and remain facing east, which lets them be warmed by the rising sun. Historically, this has led to controversy on whether or not Helianthus is heliotropic, as many scientists have failed to observe movement when studying plants that have already bloomed.
This is notably different from heliotropism in leaves, as the moving mechanism for leaves exists in the pulvinus. Since flowers do not have pulvini, the movement is caused by increased growth rate of the stems. The growth rate accumulation of the stem on the east side of the stem gradually pushes the flower from east to west during daytime. This matches with the Sun as it rises from the east and falls in the west. At night, the growth rate is higher in the west side of the stem that gradually pushes the flower from the west side back to the east side. In addition, it is not actually the whole plant that changes its direction to face the Sun, but the flower itself that bends to be illuminated by the Sun's rays.
The heliotropic movement is caused by growth on the opposite side of the flower, driven by accumulation of growth hormones during Sun exposure.
Heliotropism persists on cloudy days when the sun is not shining brightly, meaning that the movement is endogenous as a trained and continuous process. However, flower movement does not occur during long periods of rain or clouds. It also does not occur in a growth chamber when exposed to 16 hours of light or in greenhouses, suggesting that the plants require a directional, moving light source. Helianthus can also discriminate between different types of light. When exposed to different light frequencies, the hypocotyls will bend toward blue light but not red light, depending on the quality of the light source.
It is the circadian rhythms and the differences of the stem growth rate that work together and cause the heliotropism of the Helianthus. This is important for attracting pollinators and increasing growth metabolism. Future studies are required to identify the exact physiological basis and cellular mechanism for this behavior.

Taxonomy

Helianthus is derived from Greek ἥλιος hēlios "sun" and ἄνθος ánthos "flower", because its round flower heads in combination with the ligules look like the Sun.
There are many species recognized in the genus:

Helianthus agrestis Pollard – southeastern sunflower – Florida, Georgia
Helianthus ambiguus Britt. – Wisconsin, Michigan, Ohio, New York
Helianthus angustifolius L. – swamp sunflower – Texas, northern Florida to southern Illinois, Long Island, New York
Helianthus annuus L. – common sunflower, girasol – most of United States + Canada
Helianthus anomalus S.F.Blake – western sunflower – Nevada, Utah, Arizona, New Mexico
Helianthus argophyllus Torr. & A.Gray – silverleaf sunflower – Texas, North Carolina, Florida
Helianthus arizonensis R.C.Jacks. – Arizona sunflower – Arizona, New Mexico
Helianthus atrorubens L. – purpledisk sunflower – Louisiana, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Kentucky, Virginia
Helianthus bolanderi A.Gray – serpentine sunflower – California, Oregon
Helianthus × brevifolius E.Watson – Texas, Indiana, Ohio
Helianthus californicus DC. – California sunflower – California
Helianthus carnosus Small – lakeside sunflower – Florida
Helianthus ciliaris DC. – Texas blueweed – United States: Washington, California, Arizona, New Mexico, Nevada, Utah, Texas, Oklahoma, Colorado, Kansas, Illinois; Mexico: Tamaulipas, Coahuila, Chihuahua, Sonora
Helianthus cinereus Small – Missouri, Kentucky, Indiana, Ohio
Helianthus coloradensis Cockerell – prairie sunflower – Colorado, New Mexico
Helianthus cusickii A.Gray – Cusick's sunflower – Washington, Oregon, California, Idaho, Nevada
Helianthus debilis Nutt. – cucumberleaf sunflower – Texas to Maine, Mississippi
Helianthus decapetalus L. – thinleaf sunflower – eastern United States; Ontario, Quebec
Helianthus deserticola Heiser – desert sunflower – Arizona, Nevada, Utah
Helianthus devernii T.M.Draper – red rock sunflower – Nevada
†Helianthus diffusus Sims – Missouri†
Helianthus dissectifolius R.C.Jacks. – Chihuahua, Durango
Helianthus divaricatus L. – woodland sunflower or rough woodland sunflower – eastern United States; Ontario, Quebec
Helianthus × divariserratus R.W.Long Michigan, Indiana, Ohio, Connecticut
Helianthus × doronicoides Lam. – Texas, Oklahoma, Arkansas, Missouri, Iowa, Minnesota, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, Michigan, New Jersey, Virginia
Helianthus eggertii Small – Alabama, Kentucky, and Tennessee
Helianthus exilis A.Gray – California
Helianthus floridanus A.Gray ex Chapm. – Florida sunflower – Louisiana, Alabama, Georgia, Florida, South Carolina, North Carolina
Helianthus giganteus L. – giant sunflower – eastern United States; most of Canada
Helianthus glaucophyllus D.M.Sm – whiteleaf sunflower – Tennessee, South Carolina, North Carolina
Helianthus × glaucus Small – scattered locales in southeastern United States
Helianthus gracilentus A.Gray – slender sunflower – California
Helianthus grosseserratus M.Martens – sawtooth sunflower – Great Plains, Great Lakes, Ontario, Quebec
Helianthus heterophyllus Nutt. – variableleaf sunflower – Coastal plain of Texas to North Carolina
Helianthus hirsutus Raf. – hairy sunflower – central and eastern United States, Ontario
Helianthus × intermedius R.W.Long – intermediate sunflower – scattered locales in United States
Helianthus laciniatus A.Gray – alkali sunflower – United States: Arizona, New Mexico, Texas; Mexico: Coahuila, Nuevo León
Helianthus × laetiflorus Pers. – cheerful sunflower, mountain sunflower – scattered in eastern and central United States; Canada
Helianthus laevigatus Torr. & A.Gray – smooth sunflower – Georgia, South Carolina, North Carolina, Virginia, Maryland, West Virginia
Helianthus lenticularis Douglas ex Lindl. Minnesota to North Dakota, Idaho, Missouri, Texas
Helianthus longifolius Pursh – longleaf sunflower – Alabama, Georgia, North Carolina
Helianthus × luxurians E.Watson – Great Lakes region
Helianthus maximiliani Schrad. – Maximillian sunflower – much of United States and Canada
Helianthus membranifolius Poir. – Cayenne Island French Guiana
Helianthus microcephalus Torr. & A.Gray – eastern United States
Helianthus mollis Lam. – downy sunflower, ashy sunflower – Ontario, eastern and central United States
Helianthus multiflorus L. – manyflower sunflower – Ohio
Helianthus navarri Phil. – Chile
Helianthus neglectus Heiser – neglected sunflower – New Mexico, Texas
Helianthus niveus Brandegee – showy sunflower – United States: California, Arizona; Mexico: Baja California, Baja California Sur
Helianthus nuttallii Torr. & A.Gray – western and central United States, Canada
Helianthus occidentalis Riddell – fewleaf sunflower, western sunflower – Great Lakes region, scattered in southeastern United States
Helianthus × orgyaloides Cockerell – Colorado, Kansas
Helianthus paradoxus Heiser – paradox sunflower – Utah, New Mexico, Texas
Helianthus pauciflorus Nutt. – stiff sunflower – central United States, Canada
Helianthus petiolaris Nutt. – prairie sunflower, lesser sunflower – much of United States, Canada
Helianthus porteri Pruski – Porter's sunflower – Alabama, Georgia, South Carolina, North Carolina
Helianthus praecox Engelm. & A.Gray Texas sunflower – Texas
†Helianthus praetermissus – New Mexico sunflower – New Mexico†
Helianthus pumilus Nutt. – little sunflower – Colorado, Wyoming, Montana, Utah, Idaho
Helianthus radula Torr. & A.Gray – rayless sunflower – Louisiana, Mississippi, Alabama, Georgia, South Carolina, Florida
Helianthus resinosus Small – resindot sunflower – Mississippi, Alabama, Georgia, South Carolina, North Carolina, Florida
Helianthus salicifolius A.Dietr. – willowleaf sunflower – Texas, Oklahoma, Kansas, Missouri, Illinois, Wisconsin, Ohio, Pennsylvania, New York
Helianthus sarmentosus Rich. – French Guiana
Helianthus scaberrimus Elliott – South Carolina
Helianthus schweinitzii Torr. & A.Gray – Schweinitz's sunflower – South Carolina, North Carolina
Helianthus silphioides Nutt. – rosinweed sunflower – Lower Mississippi Valley
Helianthus simulans E.Watson – muck sunflower – southeastern United States
Helianthus smithii Heiser – Smith's sunflower – Alabama, Georgia, Tennessee
Helianthus speciosus Hook. – Michoacán
Helianthus strumosus L. – eastern and central United States, Canada
Helianthus subcanescens E.Watson – Manitoba, north-central United States
Helianthus subtuberosus Bourg.
Helianthus tuberosus L. – Jerusalem artichoke, sunchoke, earth-apple, topinambur – much of United States and Canada
Helianthus verticillatus Small – whorled sunflower – Alabama, Georgia, Tennessee