How Do Sunflowers Turn To Face The Sun?

It may not surprise you that sunflowers have something of a thing for the sun. OK, that’s true of all plants, which is why they orient their flowers and leaves to follow the sun as it moves across the sky. But it’s sunflowers that are the most obvious about it, craning their big yellow heads atop their long green stalks toward the east in the morning and due west by sundown. Even at night they’re hankering for the light, turning their faces back east in the dark so they’ll be ready to catch the first rays the moment dawn breaks.

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What botanists have long wondered is just how the sunflower and similar types of flowers manage this mobility. In a study published in Science, a team of researchers at last answer those questions, finding the solutions partly in genes, partly in hormones and partly in the sunflower’s need to make friends with its favorite pollinator—the bee.

What makes the sunflower such a puzzle is that it belongs to a group of flowers that lack what is known as a pulvinus—a thickening at the base of a leaf or other structure that changes its rigidity in response to light. As the sun moves across the sky, different amounts of water flow into different parts of the pulvinus, nudging the leaf in the sunniest direction. The sunflower, with no such structure, should be immobile.

To determine how it moves all the same, a team of investigators headed by plant biologist Hagop Atamian at the University of California began by messing with the sunflower’s mind—or at least its rhythms. Potted sunflowers growing outside were turned 180 degrees as soon as the sun went down. Having bent to face west throughout the day, the flowers would now be facing east and ready for the morning. If they remained pointed that way it would mean that something in the environment was telling them how they were oriented and that they could remain still. If they bent the other way—effectively the wrong way for morning—it would mean that their circadian clock was dictating their motion and that the plants had no idea where they were.

That second possibility was what played out, as the confused plants worked all night to change their direction like they ordinarily would, only to wake up, as it were, and find they were facing the wrong way. So score one for the circadian system.

But light sensitivity—or heliotropism—plays a role too. At the summer solstice, sunflowers move more slowly than they do all year, taking the full 16 hours of available daylight to follow the sun from east to west. In winter, sunflowers grown in greenhouses speed through their daily calisthenics, going east to west in just the eight hours the sun is out. When it’s cloudy, the entire system slows down.

That awareness of the light has limits, though. Under artificial lighting, the researchers found that they could effectively train plants to follow the peak summer pattern indefinitely, bending east to west over 16 hours to follow a moving light source and then reorienting themselves east during the eight hours of darkness. When the plants were switched from a 24- to a 30-hour cycle, however, they lost their way, unable to track properly until they were returned to a shorter, natural day. In this case, the researchers concluded, circadian rhythms and light sensitivity were in competition, and neither one won.

Determining the mechanism of the motion was the most complex part of the study. Plant genes are known to regulate the level of a hormone called auxin, which causes stems and shoots of plants to lengthen. During the part of a day that a sunflower is tracking west, the auxin-promoting genes are more active on the east side of the stem, pushing the plant the other way. At night, as the plant shifts east, the pattern reverses itself.

The investigators, however, found that auxin does not work alone. Another hormone, called gibberlin, is also in play. One species of sunflower is known to be deficient in gibberlin, leaving it with short stems and unable to track the sun effectively. When the investigators extracted the hormone from full-sized sunflowers and applied it to the dwarf variety, they grew normally and became good sun-trackers.

And how do the bees figure into this? For them it’s mostly about the heat and less about the light. Thermal readings of sunflowers revealed that those that faced the sun in the morning were, no surprise, warmer than those that had been rotated the other way. Bees preferentially landed on those warm sides. When the investigators used space heaters to warm up the cooler flowers, the bees began to land there too—though still not in the same numbers they did on the sun-facing flowers.

All of this, it’s fair to guess, is bigger news to botanists than it is to the rest of us. Still, it was millions of years ago that the sun, the flowers and the bees began their cyclic dance. It’s nice, at last, to be able to follow their story.