Rooted in place and without a brain or nervous system, plants seem insensate organisms, yet if you've ever observed how plants grow, their behavior suggests that they do possess the ability to respond to the world around them. For example, plants will bend or grow toward light, some seedlings will not germinate unless light conditions are ideal and flowers open every year around the same time. Many plant behaviors are driven by light, especially red light.
Light occurs in wavelengths. Each range of wavelengths corresponds to a different color. Certain plant responses occur only in the presence of a particular color of light. To plants, red and blue light are the most important--green light is not absorbed and reflects back, giving plants their green coloration--driving germination, growth, circadian rhythms and other life processes.
Red light induces several behaviors in plants. Red light controls flowering, according to the Oregon State University Extension. The Missouri Botanical Garden's website also points out the role red light plays in making plants aware of their environment. Special molecules called phytochrome absorb red light. Red light absorption by phytochrome controls germination and stem elongation in some circumstances.
There are two types of phytochrome, which change from one to the other according to exposure to light. Phytochrome red absorbs red light and, as absorption occurs, converts to the second form, phytochrome far-red. Phytochrome far-red absorbs far-red light, the type of light found in shaded sites, such as under trees. In the darkness, phytochrome far-red begins converting back to phytochrome red. The ratio of the two types of phytochrome acts as a clock, allowing plants to determine the amount of light and darkness they are exposed to daily.
If you've ever grown a houseplant in a sunny window, you know the plant will eventually lean towards the light. Parts of the plant that receive little or no light contain high levels of phytochrome red, produced by darkness, that stimulates growth, allowing the plant to stretch toward the light. As the Missouri Botanical Garden points out, small seeds with few energy reserves need seedlings to emerge into light, where they can begin producing energy from sunlight right away. Phytochrome far-red--indicative that the seed lies in the shade--inhibits germination in these species.
When providing artificial lighting for plants, understanding the role of light wavelength or color helps you to encourage certain types of growth. In order for plants to flower, for example, they must receive the correct amount of red light each day, so artificial lighting should provide this. Likewise, the light requirements of outdoor plants take into account the types and amounts of phytochrome needed for growth and flowering. Placing plants in improper light means that they won't grow or produce flowers.