Taken together, these observations demonstrate that the movement is guided by circadian rhythms rather than photoperiodism, which is driven solely by light changes.
Atamian filmed the plants in action and demonstrated that the east side of the stem grew faster in the daytime, causing the plant to twist around, while the west side put on a spurt at night to return the plant to its original position. Sunflowers that don't produce the growth hormone gibberellin, which lengthens their stalks, were unable to track the Sun in this way, unless injected with gibberellin.
These observations were matched by the observation that genes associated with growth are expressed more strongly on one side of the plant during daylight and the other at night.
Once the flowers reach a certain point, they abandon their youthful sun-worshiping and face east throughout the day, which Harmer and Atamian suggest is driven by a stronger reaction to blue light in the mornings than afternoons.
In Science, Atamian and Harmer report that when they rotated pots containing mature sunflowers so that the flowers faced west, pollinators abandoned them. “Bees like warm flowers,” Harmer explained. When seeking flowers in the morning, bees want the warmest they can get, and naturally these are the ones facing east.
The difference was so stark that five times as many bees visited east-facing sunflowers as those looking west, making a huge difference in the chances of successful reproduction.