Sunflowers’ sun tracking tied to circadian rhythms and varying growth rates

NPR : Sunflowers are known for their ability to turn to follow the sun throughout the day. Now Stacey Harmer of the University of California, Davis, and her colleagues have determined the mechanism that allows the plants to do so. The researchers found that the circadian rhythm of plants is closely tied to the 24-hour day. The flowers start each morning facing east. During the day the stem grows primarily on the east side, twisting the flower’s head toward the west. During the night, growth occurs on the other side, turning the flower back toward the east. Harmer’s group found that plants that were restricted from moving grew significantly less than unbound plants. Plants exposed only to artificial light demonstrated the same 24-hour rotational pattern independent of whether the artificial light was on a 24- or 30-hour cycle.