Phototropism

Phototropism is a directional growth response to light. Shoots usually show positive phototropism, curving toward light so that their leaves can capture more of it for photosynthesis, while some roots and a few climbing plants grow away from it, a negative response. The bending comes from uneven growth, not from movement: cells on the shaded side of a stem elongate faster than those on the lit side.

The response is triggered mainly by blue light, detected by photoreceptor proteins called phototropins, which carry a light-absorbing flavin. Their activation causes the hormone auxin to be shifted toward the shaded side, where it loosens cell walls and speeds elongation. This idea of sideways auxin redistribution is called the Cholodny-Went hypothesis, after two scientists who proposed it independently in the 1920s, and later research showed that carrier proteins of the PIN family redirect the flow of auxin.

Studies of phototropism launched the science of plant hormones. In 1880 Charles Darwin, working with his son Francis, showed that grass seedlings sense light at their tips yet bend lower down, implying that a signal travels between the two. Peter Boysen-Jensen and Frits Went then proved the signal was a chemical, later identified as auxin.