Photosynthesis is the foundational process by which plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of sugars. Light is not a uniform source of energy, however, and the specific color, or quality, of the light profoundly influences how efficiently a plant can perform this biological work.
Light as Energy: The Electromagnetic Spectrum
Light travels as electromagnetic radiation, which can be described as both waves and particles called photons. The colors we perceive are a small fraction of this spectrum, known as visible light, which ranges in wavelength from approximately 400 to 700 nanometers (nm). This specific range is often referred to as Photosynthetically Active Radiation, or PAR, because it is the portion plants utilize for food production.
The color of the light is determined by its wavelength, and wavelength is inversely related to the energy carried by its photons. Shorter wavelengths, such as violet and blue light (around 400–500 nm), contain higher energy per photon. Conversely, longer wavelengths, like red light (around 600–700 nm), have lower energy per photon.
The Specialized Machinery: Photosynthetic Pigments
Plants possess various pigment molecules that serve as light-harvesting antennae within their cells. The most prominent of these are the chlorophylls, specifically chlorophyll a and chlorophyll b, which are directly involved in converting light energy. Chlorophyll a acts as the primary reaction center pigment, while chlorophyll b serves as an accessory pigment. These pigments absorb light across the visible spectrum, but they are particularly effective at capturing blue-violet and red wavelengths. The reason plants appear green to the human eye is that chlorophyll molecules reflect or transmit the green light portion of the spectrum, which is then perceived by us. Accessory pigments, such as carotenoids, absorb light mainly in the blue-green and violet regions, broadening the range of light the plant can utilize for photosynthesis. They also serve a protective function, helping to shield the photosynthetic apparatus from damage caused by excess light energy, a process known as photo-oxidation.
Mapping Efficiency: Absorption and Action Spectra
The relationship between light color and plant function is quantified through two concepts: the absorption spectrum and the action spectrum. The absorption spectrum shows which wavelengths of light are taken up by the pigments in the leaf. The action spectrum, however, measures the actual rate of photosynthesis, demonstrating the efficiency of each color in driving sugar production.
The action spectrum peaks sharply in the blue-violet region (around 430–470 nm) and the red region (around 640–670 nm), confirming these colors are the most effective for photosynthesis. Photons in these specific wavelengths are optimally aligned with the energy levels required to excite electrons in the reaction centers, initiating the chemical conversion process.
The traditional view held that green light (500–600 nm) was largely ineffective due to the low absorption rate of isolated chlorophyll molecules. While chlorophyll absorbs less green light than red or blue, recent studies show that green light is not entirely wasted in a whole leaf or plant canopy. Because green light is poorly absorbed by the upper cell layers, it can penetrate deeper into the leaf tissue and even reach lower leaves in a dense canopy. This deeper penetration means that green light is often used with similar efficiency to red light in the deeper cell layers of a leaf, leading to a significant overall contribution to whole-plant carbon fixation.
Real-World Applications of Light Color
The detailed understanding of the action spectrum has revolutionized controlled environment agriculture, particularly through the use of Light Emitting Diode (LED) technology. Growers can now create specific light recipes that optimize plant growth by delivering precise wavelengths.
For instance, blue light is often emphasized during the initial vegetative growth stages because it encourages strong, compact leaf growth and robust root development. Conversely, an increased proportion of red light is used later in the plant’s life cycle to stimulate flowering and fruit production. By manipulating the ratio of red and blue light, often with the addition of some green light for deeper canopy penetration, indoor farms can maximize yield and control plant morphology.