Light and Plants
Light is one of the essential elements of plant life. It provides the energy for photosynthesis---the process that breaks up water molecules and uses the hydrogen to form carbohydrates, their main food. The color of light affects the growth of plants because they use light to produce food in a process called photosynthesis. This process is based on light from a mass of burning hydrogen and helium located 93 million miles from earth.
Proteins and pigments in chloroplasts, the structures that process sunlight for use in photosynthesis, react differently to light. The reactions of these chloroplasts send energy to different processes, controlling the maturation, reproduction and death of the organism. All plants must adapt to conditions of atmospheric composition, available light and climate over generations.
Chlorophylls and Carotenoids
Our sun produces light of all colors in a unique spectral mixture. It burns with a fairly constant intensity that is modified only by the distance it travels and our atmosphere. Chlorophylls and carotenoids, both present in chloroplasts, absorb light of differing wavelengths. The balance of chloroplasts depends on the plant's characteristics and its native climate. Chlorophylls absorb light most efficiently in the red and violet areas of the spectrum---they reflect green light. Carotenoids absorb light most proficiently in the blue area---they reflect red and orange light. Carotenoids, in particular the protein beta carotene, reflect red and yellow light. A large population of beta carotene marks the ripening stage of vegetables like tomatoes and carrots.
Understanding Color and Growth
Chlorophylls dominate in growing plants, absorbing reds and blues and reflecting greens and yellows. The color of light signals plants how to grow by activating chloroplasts. As the spring sun rises in the sky, it brightens---the result of more blue in its spectrum. According to the Arizona University Master Gardener's book, blue light encourages vegetative growth.
As the sun rises high in the summer sky, red and blue light begin to balance, triggering "inflorescence" or flowering. This means that fluorescent light, which tends toward the blue end of the light spectrum, is a good choice for starting seedlings. As fall approaches, the lowering sun turns yellow and plants begin dying back, because their chloroplasts are "blind" to the color. The chlorophylls fade and the carotenoids become more dominant to grab every ounce of blue light left---reflecting reds and yellows instead of chlorophyll's green.