Young sunflowers follow the sun, then settle facing east for good
Before they bloom, sunflower heads swing from east to west during the day and turn back overnight to greet the dawn. Once mature, they stop and stay pointed east, warmed by the rising sun. That switch explains why scientists studying adult plants long argued over whether sunflowers track the sun at all.
The motion has no hinge. Leaves that track light use a joint-like organ called a pulvinus, but flowers lack one. Instead the stem grows faster on its eastern side during the day, gradually tipping the head westward, with growth hormones building up on the side away from the light. The behaviour continues on cloudy days, showing it is an internal, trained rhythm, yet it fails under constant greenhouse lighting, so the plants need a light source that moves. Circadian clocks and uneven stem growth work together, though the exact cellular mechanism is still unknown.
Helianthus, from Greek words for sun and flower, holds about 50 species in the daisy family, all native to North America. What looks like one bloom is a head of many tiny florets: sterile yellow rays around the rim and yellow or maroon disc florets inside, arranged in a natural spiral. Every red-rayed ornamental cultivar descends from one original mutant. Some species reach 300 cm or more.
Two species became crops. The common sunflower, H. annuus, supplies oil, meal and seeds, and most cultivated varieties derive from it; the Jerusalem artichoke, H. tuberosus, is grown for its edible tubers. The common sunflower is Ukraine's national flower and has been grown there for centuries. The Land Institute and other breeders are testing perennial species such as the Maximilian sunflower as potential seed crops.
Sunflowers also help farmers indirectly by attracting pollinators and parasitoid insects that cut pest numbers in nearby fields. Researchers think scarce pollinators in dry habitats may have favoured bigger flowers there. Not every species thrives: the whorled sunflower, H. verticillatus, was listed as endangered in 2014 by the U.S. Fish and Wildlife Service, its woodland and creekside habitat in Alabama, Georgia and Tennessee squeezed by commercial forestry and pine planting.
Source: Helianthus
