Plant a seed in the dark, and it may sprout and grow for a time, but once the food provided with the seed is consumed, the plant will wither and die. This is because one of the ways plants use light is to provide food energy. Light exposure also induces various plant functions. However, visible light consists of a range of wavelengths. Only some of these colors affect plants beneficially.
The Nature of Light
The sun emits electromagnetic energy, much of which enters the Earth's atmosphere. Visible light is just one small part near the middle of the spectrum, which includes ultraviolet light, infrared light, X-rays, radio waves, gamma rays and microwaves. Within the range of visible light, between 400 and 750 nanometers, the shorter wavelengths are blue. The longest visible light wavelengths are red. Plants react to the same range of light that the human eye perceives, but not in equal amounts for all wavelengths.
Plants and Blue Light
Blue light in particular is vital to food production. Recall the initial growth of a plant sprouted in the dark, which will grow thin and long. It will also be white. Once it is exposed to light, it turns green. The green color comes from chlorophyll, which is a photosensitive molecule that absorbs photons from blue light and uses this energy to drive a chemical reaction that converts carbon dioxide and water to glucose and oxygen. This chemical process is called photosynthesis. Plants can then use the glucose as stored energy to produce growth and foliage.
Plants and Red Light
Specific proteins, called phytochromes, react to light in the longer wavelength range. When phytochromes absorb a certain quantity of red light, they trigger hormones that initiate flower formation and fruit production. Other factors, like day length, night length and temperature all play a role, but without sufficient red light waves, many plants will never bloom.
Plants and Green Light
Green light wavelengths fall between the red and blue ends of the visible light spectrum. Plants absorb green least of all the colors. Because green is the color most reflected, human eyes pick up that color, making plants therefore appear green. In reality, they appear green because they're anything but green.
Artificial Light and Plant Growth
Plants can grow from artificial sources so long as those sources include the wavelengths that plants need for growth and development. "Cool white" fluorescent bulbs emit blue light; placed about 6 inches above a plant, they will encourage foliage and growth. Incandescent bulbs are a good source of red light for bloom.