Plants use the energy from photons of light to convert water and carbon dioxide molecules into glucose and oxygen. This chemical reaction is known as photosynthesis. Light also triggers hormonal responses to initiate reproduction, including flower formation. But not just any part of the light spectrum will do. Flowering plants require a certain amount of exposure to specific colors to grow and bloom.
Flowering Plants and Visible Light
The visible light band is just a narrow section of the whole electromagnetic spectrum. What the human eye perceives as color are waves of specific sizes ranging from 400 to 700 nanometers in length. Flowering plants utilize light best in two specific bands of visible light: from 400 to 450 nm and 650 to 680 nm. To the naked eye, the shorter wavelengths look blue and the longer ones appear red.
How Blue Light Affects Flowering Plants
For basic energy needed for growth, nothing beats blue light, at least for plants. Structures in plants called chloroplasts contain chlorophyll, which are photoreceptive molecules that capture the photons from blue light wavelengths to carry out photosynthesis. Other wavelengths of light, which humans perceive as other colors, are not as optimal for use by the chlorophyll. Plants are the least receptive to green, which is why stems and leaves appear that color. This is true for all plants, whether they flower or not.
How Red Light Affects Flowering Plants
Blooming and fruiting require a complex hormonal response to trigger the flowering plants' reproductive mechanisms. Plant structures called phytochromes are proteins that react to red light wavelengths. A certain amount of red light exposure is required by a flowering plant before bloom formation can begin. In nature, the atmosphere, in conjunction with its angle to the sun, allows more blue light penetration than red light, but as the Earth tilts more toward the sun as summer approaches, more red light is able to get through. Thus, flowers tend to appear in the middle and late spring and into the summer months for outdoor flowering plants.
Phototropism and Flowering Plants
Many flowers seem to bend toward the sun, and some even move throughout the day to follow the light, even as the sun's position changes in the sky. This is a plant phenomenon called "phototropism." In reality, the stem, not the flower, is moving. And it's not the part of the stem facing the light that is changing, but the part of the stem farthest from the light. The dark side of the plant grows faster, by hormonal inducement, than the part that had optimal light exposure. Once again, the wavelength of that light matters. Only blue wavelengths trigger a phototropic response in plants.
Photoperiodism and Flowering Plants
The photoperiod is a term that describes the amount of light in a day. Some flowering plants will not bloom until there are a certain number of hours of daylight. Spinach is one such example. It remains green and leafy in short-day weather and grows flower stems once the days grow longer. Other greens bolt because of temperature changes. Other flowering plants, like rice, won't bloom if the nights are not long enough. Some flowering plants are not day-length sensitive, or photoperiodic, at all.