In a newly-published article in Science, the researchers say the young plant's sun-tracking (also called heliotropism) can be explained by circadian rhythms – the behavioral changes tied to an internal clock that humans also have, which follow a roughly 24 hour cycle. A young flower faces east at dawn and greets the sun, then slowly turns west as the sun moves across the sky. During the night, it slowly turns back east to begin the cycle again. The researchers found that the plant's turning is actually a result of different sides of the stem elongating at different times of day. Growth rates on the east side were high during the day and very low at night, whereas growth rates on the west side were low during the day and higher at night. The researchers tied plants up so they couldn't move or turned them away from the sun – and they found those flowers eventually had "decreased biomass and less leave area" than flowers that could move with the sun. And in support of the circadian rhythm theory, plants exposed to artificial light at different intervals "could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours. Mature sunflowers respond differently to the sun. According to the research, as overall growth slows down, the circadian clock ensures that the plant reacts more strongly to light early in the morning than in the afternoon or evening, so it gradually stops moving westward during the day. The researchers compared mature flowers facing east with those they turned to face west, and found that the east-facing blooms attracted five times as many helpful pollinators, because the east-facing flowers heat up faster. http://www.scpr.org/news/2016/08/06/63325/the-mystery-of-why-sunflowers-turn-to-follow-the-s/
