What Wavelengths of Light Are Most Effective for Photosynthesis?

Red light in the range of roughly 600 to 680 nm drives photosynthesis more efficiently than any other part of the visible spectrum, with a secondary peak in the blue region around 440 nm. That much has been measured across dozens of crop species and remains the backbone of how we think about photosynthetically useful light. But the full picture is richer and stranger than “red and blue are best,” involving green light that outperforms red under certain conditions, far-red wavelengths that were once dismissed as useless, and organisms that photosynthesize with near-infrared radiation deep inside caves.

The Action Spectrum Across Crop Plants

The classic way to answer “which wavelengths work best” is to measure the action spectrum: shine light of each wavelength on a leaf and record how much carbon dioxide it fixes per photon. When this was done across 22 crop species over the range of 350 to 750 nm, the quantum yield curve showed two broad peaks, one centered at about 620 nm (orange-red) and a second at roughly 440 nm (blue). A shoulder appeared around 670 nm, right at the main absorption band of chlorophyll a. The blue peak reached about 70% the height of the red peak.¹

More recent work pinpointed the maximum quantum yield for CO₂ fixation on an absorbed-light basis at 620 to 640 nm, with a value of about 0.093 molecules of CO₂ fixed per absorbed photon. Between 427 and 560 nm, the yield held fairly steady at roughly 70% of that maximum. Above 680 nm, it fell quickly, reaching zero near 736 nm.² So on a per-photon basis, orange-red light wins. Blue photons carry more energy than red ones, though, which means a blue photon that does slightly less photosynthetic work per photon is “wasting” some of that extra energy as heat. Red photons are a better energy bargain.

Why Green Light Matters More Than Textbooks Suggest

The standard story says chlorophyll absorbs red and blue, reflects green, and therefore green light is useless for photosynthesis. That is a drastic oversimplification. Chloroplasts do absorb green light less strongly than red or blue, and carotenoid pigments are effective at absorbing shortwave radiation below about 520 nm, so there is a genuine dip in absorption in the green waveband.³ But “absorbs less” and “useless” are very different claims.

Green light penetrates deeper into a leaf than red or blue light does. The outer cell layers absorb most incoming red and blue photons, leaving the chloroplasts further inside relatively starved. Green photons slip past those upper layers and reach chloroplasts that would otherwise sit idle. In experiments with sunflower leaves under moderate to strong white light, adding green light increased whole-leaf photosynthesis more effectively than adding the same amount of red light.⁴ The key condition is that the leaf is already receiving substantial white light. Under those real-world circumstances, the surface chloroplasts are saturated, and the marginal gain from an extra red photon is small. An extra green photon, by contrast, reaches a deeper chloroplast that still has capacity to use it.

This matters practically because sunlight is rich in green wavelengths. If plants truly could not use green photons at all, they would be throwing away a large chunk of available solar energy. The reality is that leaves absorb a surprising amount of green light, just not quite as much as red or blue, and that absorption drives real photosynthetic work.

The Far-Red Surprise

For decades, wavelengths beyond 700 nm were considered outside the useful range for photosynthesis. The traditional definition of photosynthetically active radiation (PAR) runs from 400 to 700 nm. Far-red photons, those in the 700 to 750 nm range, barely drive photosynthesis when delivered alone. Their efficiency drops off sharply once you pass 685 nm.⁵

But here is where things get interesting. When far-red light is combined with shorter wavelengths, the total photosynthesis exceeds what you would get by adding the individual contributions together. This synergy, known as the Emerson enhancement effect (named for the botanist Robert Emerson, who first described it in the 1950s), occurs because the two photosystems in plant cells work best when both are being fed photons simultaneously. Shorter wavelengths preferentially excite one photosystem, and far-red light preferentially excites the other. Together they keep both running smoothly.⁶

Measurements using LEDs peaking at 711, 723, and 746 nm showed that far-red photons can be equally efficient at driving canopy photosynthesis when they act alongside traditional PAR wavelengths. The effect was consistent across diverse species, suggesting the mechanism is common in higher plants. These findings have prompted researchers to argue that far-red photons between 701 and 750 nm should be included in the definition of PAR.⁷

This is especially relevant in shaded environments. Under a canopy, the leaves above filter out most red and blue light but transmit and reflect far-red photons. In vegetation shade, far-red photons can make up more than half the total light between 400 and 750 nm. Studies in both C3 and C4 species found that far-red photons accounted for roughly a quarter of leaf gross photosynthesis when sunlight was filtered through another leaf, and 10 to 14% of gross photosynthesis for species growing in deep shade.⁸ For an understory plant, far-red is not marginal. It is survival.

How Carotenoids Extend the Useful Spectrum

Chlorophyll gets all the attention, but accessory pigments do real work. Carotenoids, the yellow and orange pigments in leaves, absorb strongly in the blue-to-green range (roughly 400 to 520 nm) where chlorophyll’s absorption dips. Once a carotenoid captures a photon, it transfers the energy to a nearby chlorophyll molecule remarkably fast. In higher-plant light-harvesting complexes, the fastest measured energy transfer times from carotenoid to chlorophyll are under 100 femtoseconds, with individual carotenoid species handing off energy in as little as 50 femtoseconds.⁹ That is astonishingly quick, faster than almost any competing loss process, which is why the overall transfer is so efficient.

In some photosynthetic complexes, the overall efficiency of carotenoid-to-chlorophyll energy transfer approaches nearly 100%.¹⁰ In a well-studied light-harvesting complex from a model plant, roughly 56% of energy transfer from carotenoids to chlorophylls proceeded through one excited state, with about 20% going through a second, lower-energy pathway.¹¹ The practical takeaway: blue and blue-green photons that miss chlorophyll often get caught by carotenoids and funneled into photosynthesis anyway. This is part of why the blue peak in the action spectrum is as strong as it is.

Underwater and Algal Photosynthesis Uses Different Rules

Everything discussed so far applies mainly to land plants. In water, the light environment changes dramatically. Water absorbs red light within the first few meters. By the time you are ten or twenty meters down, the available light is dominated by blue and green wavelengths. Marine algae and cyanobacteria have evolved pigment systems tuned to this shifted spectrum.

Cyanobacteria and red algae use large antenna complexes called phycobilisomes, which contain brightly colored proteins known as phycobiliproteins. These capture light energy across the 450 to 650 nm range and funnel it to the photosystems.¹² Phycobiliproteins come in several varieties, including phycoerythrins (which absorb green light and appear red), phycocyanins (which absorb orange-red and appear blue), and allophycocyanins. A single phycobilisome can contain hundreds of individual light-absorbing units, forming giant complexes weighing thousands of kilodaltons.¹³

The result is that for a red alga growing at depth, green light around 550 nm can be more effective for photosynthesis than red light, which never reaches it. The “best wavelength” depends entirely on the organism’s pigment toolkit and the light environment it inhabits. A universal answer for all photosynthetic life does not exist.

Life Beyond 700 nm

Some organisms push photosynthesis into wavelengths that most biology textbooks ignore entirely. Chlorophylls d and f, the most recently discovered chlorophylls, allow certain cyanobacteria to harvest near-infrared radiation between 700 and 780 nm for oxygen-producing photosynthesis. These pigments have been found in cyanobacteria living in the dim interiors of caves, where the only available light is far-red and near-infrared.¹⁴

Research on beachrock biofilms in subtropical coastlines has demonstrated that these chlorophyll-f-containing cyanobacteria carry out substantial near-infrared-driven photosynthesis in their natural habitat, not just in the lab. Using a combination of imaging techniques and oxygen sensing, researchers confirmed real oxygen production driven by near-infrared light within the rock.¹⁵ This suggests near-infrared-driven photosynthesis contributes meaningfully to primary production in shaded and endolithic habitats around the world.

At the extreme end of the spectrum, some purple bacteria use bacteriochlorophyll to absorb light well into the infrared. One bacterial antenna complex absorbs at 963 nm at room temperature, deep into the near-infrared range invisible to the human eye.¹⁶ These organisms carry out anoxygenic photosynthesis, meaning they do not split water or release oxygen, but they still use light energy to power their metabolism. The range of wavelengths that life has learned to exploit for photosynthesis runs from the ultraviolet all the way past 900 nm.

Why These Wavelengths and Not Others

The wavelengths that photosynthetic organisms use are not arbitrary. They correlate with features of the solar spectrum and Earth’s atmosphere. The peak photon flux density of sunlight reaching Earth’s surface falls near 685 nm, which is suspiciously close to the absorption peak of the reaction center pigment in Photosystem II (P680). The near-infrared cutoff of plant light use aligns with water absorption bands and the oxygen absorption band at 761 nm. Bacteriochlorophyll reaction center wavelengths in various photosynthetic bacteria correspond to local windows of atmospheric and water transparency.¹⁷

A broader evolutionary analysis has proposed that many fundamental biological pigments arose and diversified specifically to absorb and dissipate the prevailing solar spectrum over geological time. As the sun’s output and Earth’s atmospheric composition changed, organisms evolved pigments tuned to whatever wavelengths were most available at the surface.¹⁸ In other words, photosynthesis uses the wavelengths it does because those are the wavelengths the sun delivers most abundantly through our atmosphere. The pigments are tuned to the environment, not the other way around.

Practical Implications for LED Grow Lights

The action spectrum findings have shaped the indoor farming industry. Most horticultural LED fixtures use a combination of red and blue diodes, often in a ratio around 3 parts red to 1 part blue, because these correspond to the absorption peaks of chlorophyll and produce photosynthesis efficiently per watt of electricity. In trials with lettuce and basil grown under red-blue LEDs at various intensities, a light level of about 250 micromoles per square meter per second optimized both yield and resource use efficiency under those conditions.¹⁹

But the research on green and far-red light has complicated this “red and blue only” approach. Plants grown under purely red-blue light can look and taste different from those grown under broader spectra. They may develop abnormal leaf shape or reduced secondary metabolite content. The growing understanding that green light drives meaningful photosynthesis in thick canopies, and that far-red photons synergize with shorter wavelengths, has led many LED manufacturers to add green and far-red diodes to their fixtures. This produces a fuller spectrum that more closely mimics sunlight and can improve both photosynthetic performance and crop quality.

Light Quality and Plant Chemistry

Different wavelengths do more than just power photosynthesis. They also activate signaling pathways that affect what chemical compounds plants produce. Blue light, for example, triggers stomatal opening and chloroplast movement through specialized receptors called phototropins.²⁰ Different light qualities activate or suppress specific metabolic pathways through these photoreceptor signaling networks, influencing the production of compounds involved in defense, protection, and regulation.²¹

Blue light in particular tends to promote the accumulation of phenylpropanoid-based compounds, a group that includes many of the antioxidants and flavoring molecules people value in herbs, leafy greens, and fruits, without drastically altering plant shape compared to white light.²² This is why some indoor growers boost blue light levels in the final days before harvest: not to increase biomass, but to enhance flavor and nutritional content.

UV-A radiation, which sits just below the visible range, turns out to follow a continuous gradient with blue light rather than acting through a completely separate mechanism. Leaves acclimated under UV-A1 wavelengths and blue light both developed efficient photoprotection under high-intensity light, and when shifted to fluctuating conditions, UV-A1-grown leaves actually showed improvements in maximum quantum yield where blue-light-only leaves showed declines.²³ The boundary between “UV stress” and “useful blue light” is blurrier than textbooks often imply, with the acclimation responses growing stronger at longer peak wavelengths along a smooth gradient rather than switching on abruptly at 400 nm.

When the Red-to-Far-Red Ratio Becomes a Signal

Plants do not just use light for energy. They use the ratio of red to far-red light as information about their surroundings. Under an open sky, red and far-red arrive in roughly equal proportions. But when light passes through or reflects off leaves, the red wavelengths get absorbed and the far-red ones pass through, so the ratio drops. A plant detecting a low red-to-far-red ratio “knows” it is being shaded by neighbors and responds accordingly, often by elongating stems to reach above the canopy, redirecting resources from leaf growth to height.²⁴

This means that canopy shading involves two simultaneous changes: less total light and a shifted spectral ratio. The plant’s photosynthetic response to reduced light intensity and its architectural response to the altered red-to-far-red ratio are driven by different sensing pathways and produce different outcomes. A plant getting plenty of light but with a low red-to-far-red ratio, which you could engineer with LEDs, will still stretch as though it is shaded. Growers manipulating spectrum in controlled environments have to account for both the energy content of the light and the signaling information it carries.