All gardeners know that plants need light in order to grow. What many don't know is the complex ways in which plants use light to not only increase in size but tell time and judge and respond to growing conditions. Red light plays a central role in many of these intriguing plant processes.
If you've ever shined light into a prism, you've seen how it fractures into many different colors of light. What you are observing are the wavelengths or colors of light. Sunlight consists of a full array of colored light, beginning with red light and ending at violet. Different wavelengths of light serve different functions in regulating plant growth and behavior.
Red light occurs at one extreme of the spectrum of visible light. Plants are capable of detecting different forms of red light that provide them with clues about their environment. Red light occurs in sunlight, and when plants detect red light, they know that they are in the sun. Far red light occurs in the shade, when seeds are lightly covered with soil and at twilight. Perception of far-red light indicates that plants are growing in the shade or in another environment with limited access to full sun.
Phytochrome occurs in plant cells and absorbs red light. Phytochrome occurs in two forms: phytochrome red (PR) and phytochrome far red (PFR). As the Missouri Botanical Garden explains, when phytochrome absorbs the type of light for which it is named, it changes structurally to the other form. For example, as phytochrome red absorbs red light from the sun, it converts to phytochrome far red. If exposed to far-red light, PFR changes back to PR. In addition, exposing PFR to darkness causes it to revert to PR form.
Phytochrome serves two important functions for plants. It allows plants to perceive their growing environment and germinate when conditions are ideal for survival. For example, tiny lettuce seeds have few food reserves and need to germinate when they know sunlight -- and, therefore, energy -- is immediately available. Seeds with high levels of PR, indicating shaded conditions or heavy soil coverage, will not germinate.
Phytochrome also acts as a timekeeper. As levels of the two types of phytochrome change with differing seasonal amounts of light and darkness, plants can withhold flowering until a particular time of year.
As a gardener, understanding the effects of red light helps you to make the best choices for your plants. Red light stimulates flowering and fruit production, according to the University of Missouri Extension, but doesn't encourage leafy growth; in fact, red light-grown plants quickly become spindly. Providing the type of light that matches the plant's seasonal requirements helps ensure full, healthy growth.