Photosynthesis is the biological process by which plants convert light energy into chemical energy for growth. The type of light—specifically its wavelength or color—is a determining factor in how efficiently a plant carries out this conversion. The quality of light a plant receives directly influences its growth rate, physical structure, and ultimate yield.
The Light Spectrum
Light is a form of electromagnetic radiation, and the visible portion is only a small part of this spectrum. Wavelengths are measured in nanometers (nm), ranging from violet (shorter) to red (longer). The light range plants use for photosynthesis is called Photosynthetically Active Radiation (PAR). The PAR spectrum spans from 400 nm to 700 nm, encompassing violet, blue, green, yellow, orange, and red light. Violet and blue light have the shortest wavelengths and highest energy per photon, while red light has the longest wavelength and lowest energy.
Plant Pigments and Light Absorption
Photosynthetic pigments, located within the plant’s chloroplasts, intercept light energy. The two primary pigments are Chlorophyll a and Chlorophyll b, which are responsible for the green color visible in most leaves. These pigments absorb light across the spectrum, but they are particularly effective at capturing photons in the blue and red regions.
Chlorophyll a, the molecule central to the light-dependent reactions, absorbs light most strongly in the blue-violet range (around 430 nm) and the red range (near 662 nm). Chlorophyll b acts as an accessory pigment, broadening the plant’s light-harvesting capacity by absorbing slightly different wavelengths, such as blue light (around 455 nm) and orange-red light (near 642 nm).
Plants appear green because chlorophyll molecules reflect or transmit the green light range (500 nm to 600 nm), which they do not absorb efficiently. Accessory pigments, such as carotenoids, absorb light in the blue-green and violet regions.
Carotenoids also help protect the photosynthetic apparatus from damage by dissipating excess energy. The specific pattern of wavelengths absorbed by a plant’s pigments is known as its absorption spectrum.
The Action Spectrum: Which Colors Drive Efficiency
While the absorption spectrum shows which colors a plant catches, the action spectrum shows the actual rate of photosynthesis—the efficiency with which captured light is used to produce sugars. Measuring oxygen output or carbon dioxide uptake under different colors reveals two peaks of maximum efficiency corresponding to the blue (400–500 nm) and red (600–700 nm) wavelengths. These are confirmed as the most photosynthetically effective colors. The relative dip in efficiency between these two peaks is often referred to as the “Green Gap.”
Roles of Blue and Red Light
Red light is highly efficient at driving the light-dependent reactions, maximizing energy production and yield. Blue light is also highly effective for photosynthesis, and it regulates plant development. Blue light promotes compact vegetative growth and plays a role in the opening of stomata for gas exchange.
The Role of Green Light
The idea that green light is useless is an oversimplification based on pigment extracts. In a living leaf, green light (500–600 nm) is absorbed and used with efficiency, sometimes up to 70%. Unlike red and blue light, which are heavily absorbed by the top cell layers, green light penetrates deeper into the leaf tissue and throughout the plant canopy. This deeper penetration allows it to drive photosynthesis in shaded lower leaves, making it nearly as efficient as red light at high light intensities.
Practical Applications of Customized Light
Understanding the action spectrum has revolutionized modern controlled-environment agriculture, including vertical farms and greenhouses. Growers customize light recipes for specific crops and growth phases using this knowledge. Light-Emitting Diode (LED) technology is ideal because it emits precise, narrow bands of light.
Specialized LED fixtures concentrate light in the highly photosynthetically active red and blue wavelengths. For instance, a young plant may receive more blue light to encourage compact growth, while a plant nearing harvest receives more red light to promote flowering and fruiting. This targeted approach conserves energy compared to traditional broad-spectrum lighting. Tuning the light spectrum optimizes growth, morphology, and the nutritional content of the final product.