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Photo: AFP
Photo: AFP

Here comes the sun

Tuned to the 24-hour cycle, the sunflower must have its own 'circadian' rhythms that drive growth and movement

Many scientific papers I run across carry nearly opaque titles. Usually that makes me just move on. But every now and then, something about the title makes my brow furrow, and I go back to read a little more. This essay, then, is about a paper in the Science a month ago: Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits.

If your eyes glazed over, mine did too. Yet, something about that title...

Some years ago, a journalist friend and I spent several days driving through Vidarbha, meeting the families of farmers who had committed suicide. As you can imagine, it was a generally depressing journey. About the only thing I remember from those days that made me smile was sunflowers. Fields of them that we passed one day, back and forth as we retraced our route to return to a particular village. Fields of large gorgeous yellow sunflowers, like some mildly preposterous army, all facing one way.

Towards the sun, of course. This little fact about sunflowers, we all know, and my memory of those fields is just that. Wherever we saw them, whatever the time of day, the flowers faced sun-wards.

Only, that impression got pricked last month. Not that I’m complaining, for what was in that paper about sunflowers might actually be even more captivating. “What [the researchers] did," the University of Southern California biologist Steven Kay told the Los Angeles Times, “is take a dusty old scientific curiosity, and did really great science on it."

The researchers are at the Universities of Virginia and California (Davis and Berkeley). One thing they realized was that mature sunflowers actually don’t track the sun. They simply face east, and thus welcome the sun each morning, but they don’t turn during the day. Young sunflowers, on the other hand, are—for flowers—wildly active. They turn towards the west during the day, tracking the sun’s motion across the sky. At night, they slowly turn to the east again, finishing their motion just in time for sunrise, just in time to greet the sun again.

As the flowers reach maturity, a UC-Davis press release says, their “overall growth slows down (and) the plant reacts more strongly to light early in the morning than in the afternoon or evening, so it gradually stops moving westward during the day."

Sun-worshippers as kids, mere sun-greeters as adults. How can you not be intrigued?

But for me, the really fascinating part of all this is how these scientists went about their research. For example, one of the paper’s co-authors, Hagop Atamian, used a marker to put several regularly-spaced dots on the stem of each sunflower plant. Some dots faced east, some faced west. Time-lapse photography then showed that during the day, the spacing between dots on the east side increases more than on the west—and the result is that the flower turns to the west. Come night, and it’s the opposite: the plant’s growth favours the west side, which turns the flower back towards the east.

Tuned as it is to the 24-hour cycle of the day, the plant must contain some kind of clock mechanism—its own “circadian" rhythms—that drives this growth and consequent movement. Atamian showed this with some more experiments. He moved some young sunflowers indoors, under a fixed light source. For some days, the plants’ circadian rhythms continued to kick in, moving the flowers back and forth. Then they stopped moving.

But when Atamian replaced the stationary light with apparent movement—switching lights on and off in sequence across the room through the day—the plants began moving again, tracking this apparent movement. Still, and crucially, they were only able to do this when the cycle of this artificial “day" was about 24 hours. Increase it to near 30 hours, and the sunflowers were figuratively baffled.

That internal clock, again.

Not just that: Atamian and his colleagues also tied some plants to stakes so they couldn’t move, and rotated others to face the wrong way. These tortured specimens ended up relatively stunted compared to their sun-facing peers. So, there is a definite reason to follow the sun.

And yes—as the flowers grow older, the circadian clock changes priorities, to make them more responsive to light in the morning than later in the day. Eventually, they stop turning and remain steadfastly east-facing. Why is this useful? Because the sun warms the flowers in the morning, and, well, “bees like warm flowers", said Stacey Harmer, biology professor at UC-Davis and one of Atamian’s co-authors. Indeed. The paper reports that east-facing flowers attract five times as many bees and other pollinating insects than other flowers. This got underlined when the scientists exposed the flowers they had turned west to a heater—the bees then flocked there too.

Fabulous stuff, if you ask me. So, when I’m next in Vidarbha, you bet I will be looking at those sunflowers for more than just their pretty faces.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers explores the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at dilip@livemint.com. To read Dilip D’Souza’s previous columns, go to www.livemint.com/dilipdsouza

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