August 9, 2016 | David F. Coppedge

Sunflower Motion Is Complex

They follow the sun, but how? Scientists are just finding out the mechanisms behind light tracking in sunflowers.

According to New Scientist, the mystery of why some sunflowers track the sun across the sky has been solved. But what a solution! Notice the observations needing to be explained:

  • Growing sunflower plants face the sun at sunrise and point the opposite direction at sunset.
  • During the night, they turn around, ready for the next sunrise.
  • When mature, the flowers stop tracking the sun and just face the sunrise.
  • You can’t trick them to follow a 30-hour cycle with artificial lights. They refuse to move.
  • When artificial lights keep a 24-hour cycle, the sunflowers again move with them.

I’m continually astonished at how sophisticated plants are,” says Stacey Harmer of UC Davis, lead author of experiments. “They’re really masters of coping with the environment.” Sun tracking is called heliotropism. But how do plants know direction without eyes? How do they keep time without clocks?

Actually, sunflowers have clocks. All organisms do. In plants, they consist of proteins that interact on a 24-hour cycle in a feedback loop with genes that express them. That’s why a different light cycle doesn’t work. Somehow, they can tell if the artificial day is too long, and they’ll stop moving. You can turn potted plants around, but you can’t fool them. The research is published in Science.

How is the light linked to growth? Using markers on the stems, Harmer’s team observed the shaded side of stems growing faster than the sunlit side. Sunlight apparently breaks down growth hormones, causing the stem to bend toward the light. That doesn’t explain, though, how the plant turns back to the east during the night. Gene expression tied to the circadian clock takes care of this in the absence of light cues. PhysOrg says,

The plant anticipates the timing and the direction of dawn, and to me that looks like a reason to have a connection between the clock and the growth pathway,” Harmer said. This behavior of sunflowers had been described by scientists as far back as 1898, but no one had previously thought to associate it with circadian rhythms.

It’s astonishing that something everyone has noticed for centuries is just getting explained in 2016. Millions of students have played with plants and light in school for a long time. Observing something, though, is not the same as explaining it. Even so, it doesn’t appear Harmer’s team has it all figured out. They mainly established two things:

Here we show that heliotropism in the common sunflower, Helianthus annuus, is generated by the coordinate action of light-signaling pathways and the circadian clock and enhances plant performance in the natural environment.

But why does the mature sunflower face east at sunrise and stop tracking the sun all day? The researchers found that east-facing flowers got about five times more pollinators than flowers they turned to face west. Insects seem to like the warm flowers, too, even when they were heated artificially. So it makes sense for the flowers to catch the morning swarm, but why not all day? Two possible reasons were found: one is that the plant reaches its maximum height, so that stems stop growing. Another is that the plant shifts its sensitivity to certain wavelengths of light when mature.

The authors summarize their findings:

Circadian oscillators enhance fitness by coordinating physiological processes with predictable changes in the environment. Our findings demonstrate that such effects accrue in part through the coordinate regulation of directional growth by environmental response pathways and the circadian oscillator. Such coordination generates the heliotropic movement of young sunflowers, enhancing plant growth, and also leads to the eastward orientation of blooming sunflower disks, promoting a key component of reproductive performance.

They credit Charles Darwin with being the first to recognize phototropism. He published a book on The Power of Movement in Plants in 1880. Certainly many a farmer or observant person had noticed the phenomenon long before Darwin did. “Heliotropism, or solar tracking, is a more dynamic form of phototropism, with aerial portions of the plant following the Sun’s movement throughout the day.”

Genes, clocks, enzymes — that’s a lot of complexity to regulate this wonder of nature. Sunflowers could do just as well by pointing to the sunrise and staying put. That would be simpler. “Ah,” the Darwinian will say, “but those that follow the sun would be warmer and attract more pollinators.” Look; observing a benefit does not explain how the benefit arrived. How did a blind plant without a brain originate proteins and enzymes that can keep time? How is the circadian rhythm calibrated? How do the enzymes respond to certain wavelengths of light, and not others? Why does the plant follow the sun before the flowers open to be pollinated? Why doesn’t the mature flower track the sun all day to stay warm for pollinators? Heliotropism is costly to the plant. Things don’t just happen; they need to be explained at a detailed level.

The simplistic Dar-wine story dulls the senses. It gives a warm feeling of having explained something when it explains nothing. In fact, none of the 3 articles cited here referred to evolution or natural selection at all. Harmer’s team just did good old-fashioned experimentation on the plants. They figured out a few things, but not everything. A simple sunflower is enough to astonish a PhD with its sophistication. That’s the real finding in this story.

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