page_title: Helianthus annuus
section_title: Heliotropism in Helianthus annuus
hierarchical_section_title: Helianthus annuus / Heliotropism in Helianthus annuus
caption_attribution_description: 日本語: 2017-07-15 Ono-himawarino-oka-park (小野市立ひまわりの丘公園)
context_page_description: Helianthus annuus, the common sunflower, is a large annual forb of the genus Helianthus grown as a crop for its edible oil and edible fruits. This sunflower species is also used as wild bird food, as livestock forage, in some industrial applications, and as an ornamental in domestic gardens. The plant was first domesticated in the Americas. Wild Helianthus annuus is a widely branched annual plant with many flower heads. The domestic sunflower, however, often possesses only a single large inflorescence atop an unbranched stem. The name sunflower may derive from the flower's head's shape, which resembles the sun.
  Sunflower seeds were brought to Europe from the Americas in the 16th century, where, along with sunflower oil, they became a widespread cooking ingredient.
context_section_description: 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. 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." 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 (anthesis). 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 by 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 (petiole) 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.
caption: Sunflowers just before sunset
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