Photosynthesis is the fundamental process by which plants convert light energy into chemical energy, enabling their growth and the production of organic compounds. Light, therefore, serves as the primary energy source for plants, making its characteristics, particularly its color, a central factor in their ability to thrive.
The Spectrum of Light
What humans perceive as “white light,” such as sunlight, is not a single color but rather a composite of various colors, each corresponding to a different wavelength. This phenomenon is known as the electromagnetic spectrum, and the portion visible to the human eye ranges from approximately 400 to 700 nanometers (nm). Within this visible spectrum, colors are arranged in a specific order: red, orange, yellow, green, blue, indigo, and violet.
Colors Driving Photosynthesis
Not all colors within the visible light spectrum are equally effective in powering photosynthesis. The primary pigment responsible for light absorption in plants is chlorophyll, which primarily absorbs light in specific regions of the spectrum. Chlorophyll a, the most abundant photosynthetic pigment, strongly absorbs blue-violet light, with a peak around 430 nanometers, and red light, with a peak around 662 nanometers. Chlorophyll b also absorbs light in the blue and red regions, specifically around 453 nm and 642 nm, expanding the range of light a plant can utilize.
Scientists use an “action spectrum” to illustrate the rate of photosynthesis at different wavelengths of light, showing that maximum photosynthesis occurs in the blue-violet and red regions. This strong correlation between the light absorbed by chlorophyll and the rate of photosynthesis highlights the importance of these specific colors.
Why Plants Appear Green
Plants appear green to our eyes because chlorophyll, the dominant pigment in their leaves, largely reflects green light rather than absorbing it. While plants absorb almost all blue and red photons, they absorb only about 85-90% of green photons, reflecting the remainder. This reflected green light is what reaches our eyes, giving plants their characteristic color.
Because green light is mostly reflected, it is the least effective color for photosynthesis compared to blue and red light. Despite this, some green light is still absorbed by plants and can contribute to photosynthesis, especially as it penetrates deeper into the plant canopy.
Optimizing Light for Plant Growth
Understanding which light colors are most effective for photosynthesis has practical applications in horticulture, particularly in the design of artificial grow lights. Many modern grow lights, especially LED systems, are engineered to emit light primarily in the blue and red spectra. This targeted approach maximizes photosynthetic efficiency, as these wavelengths are most readily absorbed by chlorophyll.
Blue light (450-495 nm) is important for leaf development, overall plant morphology, and promoting compact growth. Red light (620-750 nm) plays a significant role in flowering, fruiting, and stem elongation. While blue and red light are crucial for energy production, other wavelengths also contribute to plant development. For instance, far-red light (700-800 nm) can increase the rate of photosynthesis and promote stem extension. Adding some green light to a red-blue LED spectrum can also increase plant growth and biomass.