Gardeners and botanists have known for a long time that when a green plant is shaded by neighboring plants it will often react by elongating its stem. This so-called shade-avoidance reaction raises the plant's leaves out of the shade, toward the light.

Seeing The Future

Now three scientists at the University of Buenos Aires have found evidence that plants can sense the presence of neighbors that might cast shadows on them at some future time.

The scientists transferred seedlings of white mustard into soil already occupied by similar seedlings of the same height. Within three days, the stems of the newly transferred seedlings began growing faster than before, even though the neighboring plants were too far away and still too short to cast shade on them.

Characteristic Colors

The scientists guessed that the new seedlings were sensing the presence of neighboring plants by detecting the characteristic color of light reflected sideways from leaves of the neighboring plants.

That characteristic color contains a very small amount of red light and a larger amount of what is known to botanists as far-red light.

The Competition

To test their guess, the scientists in effect blinded some new seedlings to that far-red light by placing clear blue cylinders around their stems. The accelerated growth was reduced or abolished.

This striking result suggests that light scattered sideways from adjacent leaves provides an early signal of competition to young green plants. The young plants react to the signal by accelerating their stem growth to get a head start in the race for light.