Plants use light across a broad range of wavelengths, but red light (roughly 600 to 700 nm) and blue light (roughly 400 to 500 nm) have traditionally been considered the most important for photosynthesis. That two-color shorthand, while useful, is increasingly out of date. Research over the past two decades has shown that far-red, green, and even ultraviolet wavelengths play roles in growth, development, and photosynthetic performance that the old model underestimates or misses entirely.
Red and Blue Are the Workhorses, but the Story Has Gotten Complicated
The standard definition of photosynthetically active radiation, or PAR, covers photons from 400 to 700 nm. Within that band, chlorophyll a and chlorophyll b absorb most strongly in the blue and red regions, which is why those wavelengths dominate conversations about grow lights and plant productivity. A leaf held up to the sun looks green precisely because it absorbs red and blue photons and reflects or transmits much of the green.
For decades, 450 nm (blue) and 660 nm (red) have been treated as something close to ideal wavelengths for driving photosynthesis. Recent work complicates that picture. A study updating the classic McCree photosynthetic action curve found that quantum yields were actually lowest at 450 and 660 nm in both tomato and lettuce, with an unexpected finding that blue light was more efficient than red under the conditions tested.1Journal of Photochemistry and Photobiology B: Biology. Updates to McCree’s photosynthetically active radiation curve — 55 years later That result does not mean red and blue are unimportant. It means the neat peak-wavelength story from introductory biology glosses over real variation between species, leaf structures, and light conditions.
Part of the reason isolated chlorophyll measurements do not predict whole-leaf behavior is that a leaf is not a thin film of pigment. Light bounces around inside the leaf’s internal structure, lengthening the path that each photon travels and changing which wavelengths get absorbed most efficiently. Studies comparing absorption by isolated chloroplasts versus intact leaves show considerable differences, because the leaf’s architecture effectively scrambles the light.2PubMed. Light absorption by isolated chloroplasts and leaves: effects of scattering and ‘packing’ The practical takeaway is that “what chlorophyll absorbs in a test tube” and “what a plant uses in a field” are not the same question.
Far-Red Light and the Emerson Enhancement Effect
Far-red light, wavelengths from about 700 to 750 nm, sits just outside the official PAR definition. On its own, far-red drives photosynthesis poorly. But when it arrives alongside shorter-wavelength light, something striking happens: overall photosynthesis increases beyond what you would expect by simply adding the two light sources’ individual contributions together. This synergy, known as the Emerson enhancement effect, has been documented for more than half a century and is now being revisited with modern tools.3PubMed Central. Greater Than the Sum of the Parts: Revisiting the Enhancement Effect in Photosynthesis Using Simulated Sun- and Shade-Light
The mechanism behind this enhancement involves the two photosystems that work in series during photosynthesis. Far-red photons preferentially excite one of the two photosystems, and shorter-wavelength photons excite the other. When both receive energy simultaneously, the whole chain runs more efficiently than when either works alone. Researchers have argued that far-red photons in natural sunlight are used efficiently enough for photosynthesis that the 400 to 700 nm PAR definition should be expanded to include them.4PubMed. Photosynthesis in sun and shade: the surprising importance of far-red photons
One study found that adding far-red light to standard PAR did boost photosynthesis, but the increase was only about 40 to 64 percent of what the same number of additional PAR photons would have achieved, largely because leaves absorb far-red less efficiently. Once absorbed, though, far-red photons drove photosynthesis with similar efficiency to PAR photons.5PubMed Central. Incident Far-Red Photons Drive Leaf Photosynthesis Less Efficiently Than PAR Light, but Are More Effective in Promoting Growth So far-red is less effective per photon delivered to the leaf surface, but not per photon that actually gets absorbed. The bottleneck is getting it through the door, not what happens once it is inside.
Far-Red Light and Plant Shape
Beyond photosynthesis itself, far-red light is a powerful signal that reshapes how a plant grows. Plants exposed to more far-red tend to grow taller, develop larger leaf areas, and adjust leaf angles to intercept more light.6The Crop Journal. Far-red light: A regulator of plant morphology and photosynthetic capacity In lettuce, far-red treatment significantly increased plant height, leaf area, and overall biomass, while reducing leaf thickness.7Food Production, Processing and Nutrition. Effects of far-red light on growth, endogenous hormones, antioxidant capacity and quality of Lettuce
When far-red and blue light were combined for lettuce, plants developed both elongated and expanded leaves, resulting in larger total leaf area than either high-red or high-blue treatments alone.8PubMed Central. Blue and Far-Red Light Affect Area and Number of Individual Leaves to Influence Vegetative Growth and Pigment Synthesis in Lettuce For growers interested in leafy greens or ornamentals, far-red can be a tool for manipulating plant architecture without necessarily changing the total amount of light energy supplied.
The Red-to-Far-Red Ratio as an Environmental Signal
Plants do not just absorb red and far-red light passively. They actively monitor the ratio between the two using a family of photoreceptors called phytochromes. A phytochrome molecule flips between two forms: one absorbs red light, and the other absorbs far-red.9Nature. Structural basis for the photoconversion of a phytochrome to the activated Pfr form The balance between these two forms tells the plant how much of each color is reaching its tissues.
This matters because neighboring vegetation changes the ratio. Leaves absorb red light efficiently but transmit and reflect far-red. So a plant growing near competitors receives light that is depleted in red and enriched in far-red. The resulting drop in the red-to-far-red ratio acts as an early warning system, triggering what researchers call the shade-avoidance response: rapid stem elongation, upward leaf angling, and earlier flowering, all aimed at outgrowing competitors to reach unfiltered sunlight.10PubMed. Shade avoidance This response can begin before a plant is actually shaded, because the spectral signal arrives as reflected light from neighbors, giving the plant a head start.
For anyone growing plants under artificial lights, the red-to-far-red ratio is one of the more consequential things to get right. Too little far-red relative to red, and shade-loving species may struggle with compact, stunted growth. Too much far-red, and plants may stretch excessively, wasting energy on stem elongation at the expense of leaf production or fruit set.
Green Light Penetrates Where Red and Blue Cannot
The old textbook claim that plants “don’t use green light” is wrong. Green wavelengths are absorbed less efficiently by chlorophyll than red or blue, which is why leaves look green to our eyes. But less efficiently absorbed does not mean unused. Green photons penetrate deeper into thick leaves and into lower canopy layers, driving carbon fixation in cells that red and blue photons never reach.
Classic work on spinach showed that about 90 percent of blue and red light is absorbed in the top fifth of a leaf. Green light, by contrast, drives carbon fixation much deeper, reaching the middle layers of leaf tissue where fixation rates are naturally highest.11Plant and Cell Physiology. Green Light Drives CO2 Fixation Deep within Leaves In dense canopies, green light is often the only wavelength that makes it through the upper leaves to reach the ones below. This gives green light a role in whole-canopy photosynthesis that single-leaf measurements dramatically understate.
A meta-analysis looking across multiple studies found that green light was similarly effective at increasing plant biomass as red and blue light. It also increased intrinsic water-use efficiency by about 15 percent and leaf area by about 7 percent, while decreasing stomatal conductance by roughly 15 percent.12Journal of Experimental Botany. Green light is similarly effective in promoting plant biomass as red/blue light: a meta-analysis Those are not trivial effects. Green light also seems to produce thinner leaves with greater specific leaf area, which can be useful for leafy crops where texture and tenderness matter.
What Blue Light Does Beyond Feeding Photosynthesis
Blue light is absorbed by chlorophyll and feeds directly into photosynthesis, but it also activates an entirely separate set of receptors that regulate how a plant behaves. Phototropins, which respond specifically to blue and UV-A light, control phototropism (bending toward light), stomatal opening, chloroplast movement within cells, and leaf expansion.13PubMed Central. Phototropins Promote Plant Growth in Response to Blue Light in Low Light Environments All of these responses serve to optimize how the plant captures and uses light energy. Stomatal opening, in particular, is critical because open stomata let in the carbon dioxide the plant needs for photosynthesis.14PubMed Central. Blue light activates calcium-permeable channels in Arabidopsis mesophyll cells via the phototropin signaling pathway
A second class of blue-light receptors, the cryptochromes, influences stem elongation, flowering time, and the plant’s internal clock.15PubMed Central. Cryptochrome-mediated light responses in plants Plants grown under red light alone often develop abnormally: poor stomatal function, weak photosynthetic capacity, and oddly shaped leaves, a condition sometimes called “red light syndrome.” Adding blue light prevents these problems, which is why most serious horticultural lighting includes at least some blue.
That said, blue and purple light at high intensities can actually suppress photosynthesis. In tomato, blue and purple LED treatments reduced both the net photosynthetic rate and electron transport compared to white light, while triggering stronger photoprotective responses. The plants essentially detected excess blue-light energy and activated safety mechanisms that dissipated the surplus as heat rather than using it for growth.16Environmental and Experimental Botany. Response of photosynthetic capacity of tomato leaves to different LED light wavelength The lesson: more blue is not always better. Like most things in plant physiology, the dose and the balance with other wavelengths matter.
Ultraviolet Light as Both Threat and Tool
Ultraviolet radiation sits below 400 nm, outside the PAR range entirely. UV-B (280 to 315 nm) can damage DNA and proteins, and plants have evolved a dedicated receptor for detecting it. This receptor, called UVR8, triggers a signaling cascade that leads to the production of UV-absorbing compounds, particularly flavonols, that act as a kind of natural sunscreen.17PubMed Central. The UVR8 UV-B Photoreceptor: Perception, Signaling and Response Recent research has found that this receptor can even be activated in the absence of UV-B light through certain biochemical intermediates, suggesting the pathway is more versatile than originally thought.18Nature Communications. Flavonoid pathway intermediates implicate UVR8 in functions beyond canonical UV-B signaling
UV-A radiation (315 to 400 nm) occupies a gray zone. It can promote growth or inhibit it depending on the dose and the species. In tomato, replacing blue light with UV-A in a red-light background caused plants to develop larger leaf areas, steeper leaf angles, and longer stems, while maintaining photosynthetic function similarly to blue light.19Environmental and Experimental Botany. UVA radiation promotes tomato growth through morphological adaptation leading to increased light interception A broader review of UV-A effects found a messy landscape: some studies report growth promotion, others report inhibition, and the effects can differ between roots and shoots within the same plant.20Plant Science. UV-A radiation effects on higher plants: Exploring the known unknown For practical purposes, small amounts of UV exposure can help produce more compact plants with higher concentrations of protective pigments and antioxidants, which is why some indoor growers add UV LEDs to their setups.
Photoprotection and Why Plants Waste Light on Purpose
Not all absorbed light energy goes into making sugars. When the incoming light exceeds what a plant can use, the surplus energy becomes dangerous. It can generate reactive molecules that damage the photosynthetic machinery itself. Plants have evolved several ways to dump excess energy safely, and light wavelength plays a role in triggering these defenses.
Blue light activates a specific protective mechanism at the level of the main light-harvesting antenna complex. This mechanism, driven exclusively by blue light, causes the antenna to convert excess absorbed energy into heat rather than passing it along to the reaction centers.21Journal of Plant Physiology. Blue-light-controlled photoprotection in plants at the level of the photosynthetic antenna complex LHCII It is an elegant solution: the same wavelength that drives high rates of photosynthesis also activates the safety valve when things get too intense.
Chlorophyll itself may serve a dual purpose. Under non-saturating light conditions, chlorophyll a and b absorb light to power photosynthesis. Under overwhelming light, a significant portion of absorbed energy can be safely dissipated as heat. And because green light penetrates deeper into the leaf, the lower cell layers can keep working productively even while the upper layers are in full protective mode. This may be part of why plants evolved to keep green as the “leftover” color rather than evolving pigments that absorb everything equally. The partial transparency to green allows the bottom of the leaf to continue fixing carbon under conditions that would overload a leaf absorbing all wavelengths at its surface.
What This Means for Indoor and Greenhouse Growing
The rise of LED technology has made it possible to dial in almost any combination of wavelengths. Early horticultural LEDs were dominated by red and blue chips, producing the purple-pink glow familiar to anyone who has walked past an indoor farm. These narrow-band fixtures are efficient at driving photosynthesis per watt of electricity, but the research suggests the story is not as simple as “red plus blue equals optimal.”
In lettuce, adding white LEDs (which supply green, yellow, and other wavelengths alongside red and blue) to a red-blue combination improved shoot and root growth, crispness, sweetness, and reduced nitrate content compared to red-blue alone.22Scientia Horticulturae. The effects of red, blue, and white light-emitting diodes on the growth, development, and edible quality of hydroponically grown lettuce (Lactuca sativa L. var. capitata) Broader-spectrum white LEDs have also been shown to produce seedlings with similar growth characteristics and energy consumption as blue-plus-red setups in begonia, petunia, and geranium, with the added benefit of making the growing environment pleasant for the humans working in it.23PLOS ONE. Spectral effects of light-emitting diodes on plant growth, visual color quality, and photosynthetic photon efficacy: White versus blue plus red radiation
The choice is not always straightforward, though. In eggplant, red-blue supplemental lighting produced taller plants and more fruit in early production stages compared to broader-spectrum options like pink or warm-white. But those broader spectra, which included more green light, increased leaf number and leaf area. Over a full growing season, cumulative yield actually decreased as the light spectrum broadened and green light increased.24HortScience. The Effect of Light Spectrum on Growth, Yield, and Spectral Transmission in Greenhouse Eggplant: Red-blue versus Broad Spectrum The best spectrum depends on what you are growing, what stage it is in, and whether you are optimizing for vegetative bulk, fruit yield, speed, or nutritional content.
Light Quality and Flavor, Aroma, and Nutritional Compounds
Plants produce an enormous range of secondary compounds, from the anthocyanins that make blueberries blue to the terpenes that give basil its fragrance. Many of these pathways respond to light quality, which opens the door to tuning flavor, aroma, and nutritional value with lighting choices.
A review of 40 studies on medicinal and aromatic plants, microgreens, and edible flowers found that blue light, either alone or combined with red, was the most commonly used and often the most effective spectrum for boosting concentrations of anthocyanins, carotenoids, phenols, and terpenes. Most studies used intensities above 200 µmol per square meter per second and a 16-hour photoperiod.25PubMed Central. Beyond vegetables: effects of indoor LED light on specialized metabolite biosynthesis in medicinal and aromatic plants, edible flowers, and microgreens The variation between species was large, however, so there is no single recipe that works for everything.
In sweet basil, narrowband blue and red LED supplements altered the volatile compounds responsible for flavor, shifting the secondary metabolic profile in measurable ways regardless of the season.26Frontiers in Plant Science. Narrowband Blue and Red LED Supplements Impact Key Flavor Volatiles in Hydroponically Grown Basil Across Growing Seasons The implication for chefs, herbalists, and specialty growers is that “more light” and “the right color of light” are different levers. You can potentially grow a basil plant that is perfectly healthy but tastes flat under one spectrum, and grow a smaller but vastly more aromatic plant under another.
Why Aquatic Photosynthesis Plays by Different Rules
Everything discussed so far applies mainly to land plants. Underwater, the game changes because water itself filters light. Red wavelengths are absorbed within the first few meters of water, which is why deep ocean environments are bathed in blue and blue-green light. Marine algae and seaweeds have evolved additional pigments, like fucoxanthin in diatoms and phycobilins in red algae, that capture wavelengths chlorophyll alone cannot reach.
Diatoms, one of the most productive groups of photosynthetic organisms in the ocean, use fucoxanthin chlorophyll a/c-binding proteins that harvest blue-green light efficiently and can also dissipate excess energy when conditions demand it.27PubMed. Structural basis for blue-green light harvesting and energy dissipation in diatoms Red seaweeds, with their phycobilin pigments, were long assumed to live in deep water precisely because they could capture the blue-green wavelengths that penetrate the deepest, a concept called complementary chromatic adaptation. But experimental tests of that theory have not held up well. When researchers measured photosynthetic performance of brown, green, and red seaweeds under different light colors, performance depended on overall light intensity, not spectral distribution.28Journal of Phycology. A PHYSIOLOGICAL TEST OF THE THEORY OF COMPLEMENTARY CHROMATIC ADAPTATION. II. BROWN, GREEN AND RED SEAWEEDS The depth distribution of seaweeds appears to be driven more by how much total light reaches them than by the color of that light, a finding that surprised many ecologists when it first emerged.
If you are setting up a planted freshwater aquarium or cultivating marine algae, the practical point is that spectrum still matters for growth form and pigment production, but total light intensity and duration tend to matter more for photosynthetic output than precise wavelength tuning. The elaborate color-matching intuition, that red plants need blue-green light because that is what is “left over” underwater, turns out to be less reliable than it sounds.