What Happens When You Shine Three Lights on Sunflowers? The Plants Surprisingly Grow Toward the Dimmest One

Young sunflower seedlings exposed to three lights bent toward the dimmest one, showing that plants combine light signals in ways that can distort their sense of direction.

Researchers grew week-old sunflower seedlings, each one to three centimetres tall, under blue LED lights and photographed their stems over at least 21 hours. With one lamp, the seedlings leaned farther as the light grew brighter, but doubling the intensity did not double the bend. With two lamps on opposite sides, they leaned toward the brighter one, and the bend grew with the brightness gap.

In the final test, two equally bright lamps faced each other and a third, weaker lamp was placed at a right angle. Across 34 seedlings, the average growth direction pointed almost exactly toward the dimmer lamp, although individual plants varied considerably. The two stronger signals appeared to cancel each other out, leaving the weaker sideways signal to steer growth.

The plants sense light with phototropins, proteins concentrated near the stem surface, and their responses are compressed rather than proportional to brightness. A mathematical model from the team, published in the Proceedings of the National Academy of Sciences, predicts that a plant may sometimes bend slightly away from the strongest light, by up to six degrees. That effect has not yet been observed, because young stems sway and circle as they grow.

Key points

• Seedlings bent toward the dimmest of three lamps on average, not the brightest.
• Doubling light intensity produced less than double the bending response.
• Competing signals from the two stronger lamps appeared to cancel each other out.
• The model predicts a bend of up to six degrees away from the strongest light.
• The predicted effect has not been observed yet and is hard to detect.
• The study was led with Yasmine Meroz of Tel Aviv University as co-author.