Key takeaways in 30 seconds
- → Plants absorb primarily in blue (430–450 nm) and red (640–680 nm)
- → Far-red (700–780 nm) controls flowering and elongation via phytochrome
- → Green (500–600 nm) penetrates foliage — it is not useless
- → UV stimulates aromas, anthocyanins and natural defences
- → Full spectrum vs targeted spectrum: the choice depends on your objective
What plants see — and what you think they see
The human eye vs the plant: two spectral realities
The human eye is sensitive to light between 380 and 700 nm, with a photopic sensitivity peak at 555 nm (yellow-green). That is why our spaces are lit with green-rich sources — it is what our brain perceives as "bright".
Plants, on the other hand, have evolved to capture the energy of sunlight via specific pigments: chlorophylls A and B, carotenoids, and for non-photosynthetic responses, phytochromes, cryptochromes and phototropins. Each absorbs in precise spectral ranges.
White light does not exist for a plant:
Sunlight is a continuum of photons at different wavelengths. A "white" LED lamp is in reality a blue LED coated with a phosphor that converts some blue photons into longer wavelengths. The result is not "white" — it is a discontinuous spectrum with peaks and troughs that your plants process photon by photon.
Key wavelengths in horticulture
Blue — 400 to 500 nm
Cryptochromes & PhototropinsBlue is absorbed by chlorophyll A (peak at 430 nm) and B (peak at 453 nm), as well as by cryptochromes and phototropins. It plays several critical roles:
- • Compactness and elongation control (blue dominance = low, dense plant)
- • Stomatal opening → regulation of gas exchange (CO₂ uptake)
- • Chlorophyll synthesis and chloroplast development
- • Phototropism (leaf orientation towards light)
In production: 15–25% of total spectrum in blue is recommended for the vegetative phase.
Green — 500 to 600 nm
Foliar penetrationContrary to common belief, green is not useless for plants. Although less well absorbed by chlorophylls (hence the green colour of leaves through reflection), it has a unique property: its ability to penetrate into the lower layers of foliage that blue and red cannot reach.
- • Reaches deep mesophyll cells
- • Contributes to photosynthesis in dense plant canopies
- • Improves thermal balance (less heating than red)
Red — 600 to 700 nm
Maximum photosynthesisRed is the most photosynthetically efficient wavelength. Chlorophyll A absorbs strongly at 662 nm and chlorophyll B at 642 nm. Red:
- • Maximises photosynthesis rate (most efficient photon per joule)
- • Promotes elongation and biomass growth
- • Stimulates fruiting and sugar accumulation
660 nm red LED = most photosynthetically efficient LED. But alone, without blue, it produces etiolated and fragile plants.
Far-Red — 700 to 780 nm
Phytochrome & floweringFar-red is not absorbed by chlorophylls and technically falls outside the standard PAR range. However, it exerts considerable influence via the phytochrome system:
- • The active Pfr form is produced under red (660 nm), Pr under far-red (730 nm)
- • Low R:FR ratio → triggers flowering in short-day plants
- • Emerson effect: FR combined with red increases photosynthetic yield beyond predictions
- • Accelerates stem and petiole elongation
Application: adding 730 nm LEDs at the end of the photoperiod to accelerate flowering or increase biomass.
UV — 280 to 400 nm
Secondary metabolitesUV does not directly participate in photosynthesis but plays an important role in product quality:
- • UV-A (315–400 nm): synthesis of anthocyanins (red/purple leaf colour), flavonoids, terpenes
- • Improvement of aromas in aromatic herbs (basil, mint, thyme)
- • Induction of defence mechanisms against pathogens
- • UV-B (280–315 nm): low doses = beneficial stress; high doses = DNA damage
The McCree curve: what science actually recommends
In 1972, plant physiologist K.J. McCree published the relative action spectrum of photosynthesis — an undisputed scientific reference showing the relative efficiency of each wavelength for photosynthesis.
His findings are counter-intuitive: green (550–600 nm) is more efficient than deep blue (< 430 nm) for photosynthesis. The curve shows two main peaks around 450 nm and 670 nm, but remains significantly positive across the entire visible spectrum.
Relative photosynthetic efficiency by spectral range (after McCree, 1972)
Source: McCree, K.J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191-216.
Practical implication:
Do not discard green. A "white" LED (broad spectrum including green) can be more efficient than a purely red/blue spectrum because green contributes to photosynthesis in the lower foliage layers — where red and blue have already been absorbed by the upper leaves.
Full Spectrum vs targeted spectrum: the real professional debate
| Criterion | Targeted spectrum (R+B) | Full Spectrum (white) |
|---|---|---|
In practice, modern professional installations adopt a red-enriched white spectrum (white + deep red 660 nm ± FR 730 nm). This compromise offers high photon efficiency, acceptable staff comfort, and the spectral flexibility required for the different growth phases.
Adapting the spectrum to growth phase
🌱 Germination / Seedlings
- • Blue dominant: 30–40%
- • Red: 50–60%
- • Little or no FR
- • Objective: compactness, no etiolation
🌿 Vegetative phase
- • Blue: 20–30%
- • Red: 60–70%
- • FR optional: 5–10%
- • Objective: maximum biomass
🍅 Flowering / Fruiting
- • Blue: 15–20%
- • Red: 65–75%
- • FR: 10–15% (floral induction)
- • Objective: yield, calibre, Brix
Spectrum impact on organoleptic quality
The light spectrum influences not only the quantity of biomass produced, but also its intrinsic quality. Recent studies show clear correlations:
↑ Blue light + UV-A
- + Anthocyanins (red/purple leaf colour)
- + Antioxidant flavonoids
- + Aromas (terpenes, phenylpropanoids)
- + Vitamin C
↑ Red light (660 nm)
- + Sugar content (Brix)
- + Fruit size and calibre
- + Carotenoid content (tomatoes)
- + Total biomass
Real case: spectral optimisation for aromatic basil
Organic basil grower — 400 m² greenhouse
Switch from 4000K white spectrum → 3000K white spectrum + 660 nm red + UV-A 380 nm (10%)
+38%
Essential oil content
+22%
Total biomass (same consumption)
+15%
Average selling price (premium quality)
* Results over 3 growing cycles, compared to the same 4000K white spectrum used previously. Without modifying PPFD or lighting duration.
The key to this result: the addition of UV-A at 10% of total spectrum activated the synthesis of essential oils (linalool, eugenol) without reducing growth. The enrichment in 660 nm red compensated for the slight growth reduction linked to UV. The basil produced was visually greener, more fragrant, and sold at a premium price.