“I turn to you like a flower leaning towards the light … Oh wait – I AM a flower!”
Almost all plants can photosynthesize, and photosynthesis is key to these plants’ survival: it lets them make sugar molecules that serve as fuel and building materials. But plants respond to light—sometimes, to specific wavelengths of light—in other ways as well. These non-photosynthesis-related responses allow plants to adjust to their environment and optimize growth.
For instance, some types of seeds will germinate only when they receive a sufficient amount of light — along with other cues. Other plants have ways to detect if they are in the shade of neighboring plants based on the quality of light they receive. They can increase their upward growth to outcompete their neighbors and get a bigger share of sunshine.
Plant responses to light depend, logically enough, on the plant’s ability to sense light. Light sensing in plants involves special molecules called photoreceptors, which are made up of a protein linked to a light-absorbing pigment called a chromophore. When the chromophore absorbs light, it causes a change in the shape of the protein, altering its activity and starting a signaling pathway. The signaling pathway results in a response to the light cue, such as a change in gene expression, growth, or hormone production.
Source: Phototropism & photoperiodism