Light affects plant growth by triggering the internal "clock" that all living things possess. That same biological response is what causes humans to have mood swings caused by a lack of sunlight in the winter. In plants, that biological response influences growth and ripening. There is more to light than what meets the eye when it comes to plant growth, however. Varying colors and degrees of light affect plants as well.
The biological trigger in plants that causes a reaction to light is the pigment protein "phytochrome." Phytochrome does not react equally to all light, however. Red light influences plants more than any other color inside the array of colors in a spectral field.
Shades of Red
Two shades of red exist in a typical spectrum: red and far red. The far red light is the strongest, but plants react to red light more than far red. The midday light begins the production of chlorophyll.
Chlorophyll and Light
Chlorophyll is the building block that begins photosynthesis. For some years, scientists thought that photosynthesis was the start of a plant's use of light. However, the discovery of phytochrome explained that the process of light in plant growth was the beginning, not the end, of the reaction. Once light produces chlorophyll it allows the plant to begin using light through photosynthesis to reproduce cells and grow.
Red is an important part of the equation, but other colors in the spectrum cause reactions in plants and effects their growth. Blue is responsible for leafy development. However, plants that are over-exposed to blue at the expense of red get thick and bushy without fruiting or producing flower or seed.
Green is one of the colors of the spectrum plants do not use. It is the reason foliage of most plants look green to the human eye. Plants reflect the color they are not using: green.
UV rays and plant grow lights are common ways to start plants early in the year in cold climates. They also make houseplants thrive better. These lights make use of red and blue rays to their advantage to improve on nature's spectrum and give plants what they prefer.