At dawn, whole fields of sunflowers stand at attention, all facing east, and begin their romance with the rising sun. As that special star appears to move across the sky, young flowers follow its light, looking up, then over and westward, catching one final glance as the sun disappears over the horizon.
At night, in its absence, the sunflowers face east again, anticipating the sun’s return.
They do this until they get old, when they stop moving. Then, always facing east, the old flowers await visits from insects that will spread their pollen and make new sunflowers. Those flowers too, will follow the sun.
It’s not love. It’s heliotropism, and sunflowers are not the only plants that track the sun. But until now, how sunflowers do it has been a mystery.
In a study published Friday in Science, researchers revealed that the sunflower’s internal clock and ability to detect light work together, turning on genes related to growth at just the right time to allow the stems to bend with the arc of the sun. The research team also showed that when fully grown, as tall as people in some cases, plants that always face east get a head start, warming up early to attract pollinators.
To get to the bottom of sunflowers’ pursuit of the sun, Stacey Harmer and Hagop Atamian, plant biologists at the University of California, Davis, and their colleagues studied sunflowers in fields, pots and growth chambers.
First, to find out what the advantage of this solar tracking might be, they prevented outdoor potted sunflowers from tracking the sun. As a result, the plants grew smaller than those that followed the sun. Chasing the sun promoted growth. But what triggered it?
The fact that sunflowers switch directions at night to face east again, with no apparent cue, suggested an internal clock at work. The researchers put sunflower plants in a room with lights rigged to mimic the sun’s path on different light and dark cycles. The plants behaved as expected on a 24-hour cycle. But during a 30-hour day, they were confused. And when plants that had learned a 24-hour cycle outdoors were placed under a fixed light indoors, they continued to bend from east to west for a few days, as if following the sun. This meant that a 24-hour circadian rhythm was guiding the sunflowers’ movement. But without muscles, how did they move?
The answer was in their stems. Like those of other plants, the stems of young sunflowers grow more at night — but only on their west side, which is what allows their heads to bend eastward. During the day, the stems’ east side grows, and they bend west with the sun. Dr. Atamian collected samples of the opposite sides of stems from sunflowers periodically, and found that different genes, related to light detection and growth, appeared active on opposite sides of the stems.