Searching for the Sun
At night, in the sun's absence, the sunflowers reorient and face east again, anticipating the sun's return.
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Its not love. Its heliotropism, and sunflowers are not the only plants that track the sun. But how sunflowers do it is a mystery to most people who notice the phenomenon.
Researches have found 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. When fully grown, as tall as people or taller in some cases, plants that always face east get a head start, warming up early to attract pollinators.
Researchers study sunflowers in fields, pots and growth chambers to understand their amazing evolutionary adaptations.
First, to find out what the evolutionary advantage of this solar tracking might be, researchers 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, suggests an internal clock at work. 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 proves that a 24-hour circadian rhythm guides sunflowers' movement. But without muscles, how do sunflowers move?
The answer is 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. By collecting samples of the opposite sides of stems from sunflowers periodically, researchers have found that different genes, related to light detection and growth, appear active on opposite sides of sunflowers' stems.
Next, researchers needed to know why mature sunflowers wind up facing east when they are finished growing. They found that east-facing flowers in pots, as opposed to flowers they forced to face west at dawn, were warmer and attracted more pollinators. Heating up the west-facing flowers brought in more pollinators as well. The researchers think the plants develop an eastward preference when young, and continue it as mature plants because being warm in the morning when insects are more active offers a reproductive advantage.
Many unknowns remain: For example, just how does the young sunflower weave together light signals, the circadian clock and growth rates to reorient its head every night? While scientific inquiry continues, some space remains to imagine sunflowers and their romance with the sun.
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