Plant Morphology and Spectrum: How Plants Respond to Different Wavelengths

Plants photosynthesize using light in the ePAR range of 400 to 750 nanometres, which extends the traditional PAR range of 400 to 700 nanometres to include far-red. The individual wavelengths inside that range do more than feed the crop. They tell it how to grow. Red and far-red light drive flowering and stem extension, blue light keeps plants compact, green light travels deeper into the canopy, and UV can trigger defence and quality responses. Knowing the impact of each wavelength on plant growth and what it does to the plant is the difference between a light source that grows a crop and one that shapes it.

Light is one of the most important factors for optimizing plant growth. Plants collect energy from light to make sugars and these sugars are used for the growth of roots, leaves, stems, fruit and flowers. Plants use light for both photosynthesis and to respond to the environment to optimize their growth. Light can be described as the visible portion of the electromagnetic spectrum which extends from about 380 to about 770 nanometres.

Plants primarily respond to wavelengths from 400-700 nanometres (nm) for photosynthesis; light within these wavelengths is referred to as photosynthetically active radiation (PAR). How those wavelengths are balanced is a separate decision from how much light you deliver, and it is the reason full spectrum and red-blue LED lighting produce different results in the same greenhouse.

Light Spectrum and Plant Response at a Glance

Band boundaries follow the conventions used in horticultural lighting research and vary slightly between sources. Individual cultivars and varieties can respond differently to the same spectrum, so treat this as a starting point rather than a definitive recipe.

What Chlorophyll Absorbs, and Why Leaves Look Green

Chlorophyll does not absorb the spectrum evenly. Extracted into solvent and measured in a spectrophotometer, chlorophyll a peaks at roughly 430nm in the blue and 662nm in the red, while chlorophyll b peaks at roughly 453nm and 642nm. The exact positions shift depending on the solvent used. Between those peaks, through the green band from about 500 to 600nm, absorption falls away sharply.

That gap is why leaves look green. Green photons are the ones a leaf is least likely to absorb on first contact, so proportionally more of them are reflected back out or transmitted through the leaf, and green is what reaches your eye.

This two-peak curve is the absorption spectrum of chlorophyll, and for years it was read as a specification for how to build a grow light. If the pigment absorbs blue and red and mostly ignores green, the argument went, then every green photon a fixture emits is wasted energy. That reading, together with the early availability and efficacy of red and blue LEDs, is what produced the purple fixtures common in early horticultural LED lighting.

The reasoning does not hold, and understanding why matters for anyone specifying a fixture today.

Absorption Spectrum Versus Action Spectrum

An absorption spectrum shows which wavelengths a pigment absorbs. An action spectrum shows which wavelengths actually drive a process, measured from the process itself rather than from pigment absorbance. For photosynthesis that means a direct measure of photosynthetic activity, such as carbon dioxide uptake or oxygen evolution. The two are not the same curve, and the difference between them is where the practical answer sits.

K.J. McCree measured the action spectrum, absorptance and spectral quantum yield of photosynthesis across 22 crop species, over 350 to 750nm. The result was far flatter than the chlorophyll absorption curve predicts. The quantum yield curve showed two broad maxima centred near 620nm and 440nm with a shoulder at 670nm, and the blue peak averaged only about 70 per cent of the height of the red peak. Green wavelengths returned real quantum yield rather than the near-zero that the test-tube curve implies. McCree went on to test the competing definitions of photosynthetically active radiation against leaf photosynthesis data, and that body of work helped motivate the convention of counting PAR as an unweighted photon flux from 400 to 700nm. That even weighting is a practical measurement convention rather than a finding that every wavelength in the band is equally effective, because McCree’s own curves show that it is not.

Three things explain the gap between the two spectra:

• A leaf is not a solution. Chlorophyll in a living leaf sits inside pigment-protein complexes, which broadens and shifts its absorption compared with pigment extracted into solvent.
• Leaves scatter light internally. A green photon that passes the first chloroplast is not necessarily lost. It scatters between cell walls and air spaces, which lengthens its path through the tissue and raises the chance it is absorbed further in.
• Leaves are layered. Red and especially blue photons are absorbed steeply by the upper cell layers, so a smaller share of them reaches the lower mesophyll. Green attenuates less steeply and delivers proportionally more of its photons to chloroplasts deeper in the leaf.

That last point has a counter-intuitive consequence. Terashima and colleagues found that under strong white background light, adding green light increased leaf photosynthesis more efficiently than adding red light, because the upper chloroplasts absorbing the red were already close to saturation while the deeper ones were still light-limited. This finding is specific to high background light levels and is not a general claim that green outperforms red.

For fixture selection the practical consequence is that a whole leaf makes useful work of the full 400 to 700nm range, so a broad spectrum white source is a sound basis for many production fixtures, and green is not the wasted output that the pigment absorption curve alone appears to suggest. How much green contributes still depends on canopy density, leaf thickness and background light level, so it is not universally interchangeable with red or blue.

How Plants Sense Light Quality

The various wavelengths of light in the spectrum can trigger morphological responses. Light spectrum in terms of plant growth and morphology is often referred to as light quality, and collectively these responses to light are called Photomorphogenesis. Plants have developed sophisticated photoreceptors that enable them to respond to light quality. In this article we will outline some of these photoreceptors and their responses in relation to light quality. Definitions for PAR, photoperiod and other lighting terms used below are collected in our horticultural lighting glossary.

Red and Far-Red Light

Red and far-red light control how tall a plant grows and how strongly it responds to the day length that triggers flowering and fruiting.

Light from the red (600-700nm) and far-red (700-750nm) wavelengths is responsible for enabling the photoreceptor Phytochrome. The pigment phytochrome allows plants to detect light and regulate morphological processes such as flowering, fruiting, vegetative growth and set the plant’s circadian rhythm. The pigment exists in two forms, one that absorbs red light and one that absorbs far-red light. Red light causes a response in the phytochrome pigment that puts it into an active form and triggers processes such as regulating photoperiod in greenhouse crops. Since photoperiodic plants measure the length of the uninterrupted dark period, flowering or fruiting is controlled by day length rather than by spectrum. Growers create short days with blackout curtains, and long days with day-extension or night-interruption lighting. Spectrum still matters within that schedule, because the balance of red and far-red light sets how strongly the plant reads the signal.

Plants also use phytochrome to move towards light and away from shade, and this is called the Shade avoidance response. Since more blue and red light is absorbed by the upper canopy compared to green and far-red, this distributes more green and far-red light to lower in the canopy. An environment high in far-red light can trigger stem extension, leaf extension, petiole elongation and apical dominance in the more shaded areas of the canopy (Lopez & Runkle, 2017). This can result in an elongated plant with increased leaf area and overall growth.

Certain seeds also require red light to initiate germination. This is usually true for smaller seeds such as snapdragons or coleus, which is one of the reasons spectrum choices matter so early in the crop cycle. Our lighting recommendations for plant propagation cover that stage in more detail.

Blue Light

Blue light is the band growers reach for when they need shorter, more compact plants and more vibrant colour.

Blue light (400-500nm) is known to trigger morphological responses associated with the photoreceptors Cryptochrome and phototropin.

This blue light can act as a growth regulator, and generally suppresses growth extension, which can be advantageous for when height requirements are a factor or when plants need to be more compact in size, though in some crops extension can be promoted instead. The blue light receptors may also play a role in the production of carotenoid and anthocyanin development. This can be seen in leaf colouration, such as enhancing the colour of red lettuce varieties or increased pigment and nutrition in peppers (Naznin et al., 2019). The same principle is at work when growers use LED spectra for enhancing the colour of kale and cabbage.

Blue light responses in relation to phototropin include phototropism (Jones, 2018), which is the orientation of the plant in relation to light, where the plant’s growth will be directed toward where the light source is. This is often seen in indoor plants whose only light source is that of a window.

Chlorophyll accumulation, leaf expansion and positioning can also be triggered by blue light. An increase or decrease in blue light can control the opening and closing of stomata, which affects photosynthesis and transpiration. Blue light is necessary, even in low intensities, for healthy plants. Plants require blue light for full functioning photosynthesis and a lack of blue light may lead to further developmental problems such as blistering on leaves and stems.

Green Light

Green light is the band that reaches the leaves the others cannot, and it is the band most often misjudged from the chlorophyll absorption curve alone.

Green light (500-600nm) responses appear to be triggered under low light intensities. It has been studied that high far-red light with high green light can cause an increase in the shade avoidance response (Wang & Folta, 2013). Green light is directly useful for photosynthesis. Far-red sits outside the 400-700nm PAR range and drives little photosynthesis on its own, although research since 2020 has shown that far-red photons do contribute to canopy photosynthesis when they are delivered alongside PAR. And one potential advantage to green light is that, since it can penetrate the lower canopy better than red or blue light, the lower leaves can continue to photosynthesize. This is because red and blue light are more readily absorbed higher in the canopy and green penetrates further down. Adding green light to a spectrum also makes the work environment more pleasant for employees, as the combination of red, blue and green light creates white light, which means workers can more easily identify pests, disease and other issues.

UV Light

UV sits outside the photosynthesis range, so it does not feed the crop, but it does trigger defence, quality and developmental responses.

While UV (280-400nm) is not used for photosynthesis, it does influence how a plant grows. Low doses, particularly UV-B in the 280-315nm range, help activate a plant’s defence mechanisms and drive production of protective compounds such as flavonoids. Reported effects on plant form vary by species and dose, so UV should not be assumed to produce more compact growth. Although damaging in large quantities, UV light can have important benefits such as producing different defence proteins that give them protection against pests and disease. Plants are also able to increase antioxidant compounds to protect themselves against UV light damage, and many of these also add to the nutritional value of the plant. The same UV-B response also drives the development of phenolic acids (Escobar-Bravo et al., 2019).

What This Means for Your Crop

So, what does all this mean for your growing needs? Depending on what type of crop you are growing and desired goal, light spectrum can play a significant role in shaping your plants’ development. It should be noted though, that different plant species and varieties will respond differently to light quality and the percentages of various light wavelengths. More research is needed to effectively use these wavelengths and spectrums to create light recipes for enhanced plant development. For example, Naznin, et al., (2019), found that carotenoid accumulation, triggered by blue light, increases in lettuce and pepper under a 91% red and 9% blue treatment, but was different for kale and basil where carotenoid accumulation was higher under 83% red light and 17% blue light. This demonstrates that to reach your desired goal when growing using customized light spectrums, you must take each individual plant and cultivar into account.

Spectrum is only one of the levers. Light intensity and its effect on yield works alongside light quality, and dynamic LED grow lighting allows the spectrum to be adjusted as the crop moves through its stages. If you would like a spectrum matched to your crop, your facility and your production targets, you can request a light plan and our team will build one for your site.

Common Questions About Light Spectrum and Plant Growth

What Wavelength of Light Is Best for Plant Growth?

No single wavelength is best on its own. Plants use the whole 400-700nm PAR range for photosynthesis, with red (600-700nm) and blue (400-500nm) driving the strongest responses. Red light supports flowering, fruiting and production, while blue light keeps plants compact. Most commercial crops perform best under a balanced spectrum rather than one colour.

What Light Spectrum Is Best for Photosynthesis?

Photosynthesis is driven by photosynthetically active radiation, which covers 400 to 700 nanometres. Within that range, red and blue wavelengths are absorbed most readily by the upper canopy, while green light penetrates deeper and lets lower leaves keep photosynthesizing. Far-red light above 700nm sits outside the 400-700nm PAR range and drives little photosynthesis on its own, but far-red photons do add to canopy photosynthesis when they are supplied together with PAR.

How Do Plants Respond to Light?

Plants respond to light in two ways. They use it as an energy source for photosynthesis, and they read it as information about their environment through photoreceptors such as phytochrome and cryptochrome. These collective responses to light quality are called photomorphogenesis, and they govern flowering, fruiting, stem extension, leaf shape and pigment production.

Do Plants Use Green Light?

Yes. Green light between 500 and 600nm is photosynthetically useful, and because red and blue light are absorbed more readily by the upper canopy, green light penetrates further down so lower leaves can continue to photosynthesize. Green light also combines with red and blue to create white light, which makes crop inspection easier for staff.

Does Blue Light Make Plants More Compact?

Generally yes. Blue light acts as a growth regulator that suppresses growth extension, which produces shorter, more compact plants. This is useful where height is a constraint. The response is not universal, and in some crops extension can be promoted instead, so the effect should be confirmed for your specific species and variety.

What Colours of Light Does Chlorophyll Absorb?

Chlorophyll absorbs mainly blue and red light. Measured in solvent, chlorophyll a peaks near 430nm and 662nm, and chlorophyll b peaks near 453nm and 642nm. Absorption is weakest through the green band from about 500 to 600nm. Inside a living leaf the picture is broader, because chlorophyll is bound into pigment-protein complexes and because internal scattering lengthens the path light travels through the tissue and raises the chance it is absorbed.

Why Do Plants Reflect Green Light?

Green is the band chlorophyll absorbs least strongly, so proportionally more green light is reflected or transmitted rather than absorbed, and that reflected light is what reaches your eye. It does not mean green light goes unused. Whole leaves still absorb a large share of the green light falling on them, and green penetrates deeper into both the leaf and the canopy than red or blue.

What Is the Difference Between an Absorption Spectrum and an Action Spectrum?

An absorption spectrum records which wavelengths a pigment absorbs. An action spectrum records which wavelengths actually drive a process, measured from the process itself, such as carbon dioxide uptake or oxygen evolution for photosynthesis. In plants the action spectrum is considerably flatter than the chlorophyll absorption spectrum, and that helped motivate the convention of counting photosynthetically active radiation evenly across 400 to 700nm as a practical measurement standard.

References

Escobar-Bravo, R., G. Chen, H.K. Kim, K. Grosser, N. Van Dam, K.A. Leiss, & P.G.L. Klinkhamer. Ultraviolet radiation exposure time and intensity modulate tomato resistance to herbivory through activation of jasmonic acid signaling. 2019. Journal of experimental botany 70:1: 315-327.

Jones, M.A. 2018. Using light to improve commercial value. Horticulture Research 5: Article 47. doi:10.1038/s41438-018-0049-7.

Lopez, R. & E.S. Runkle (eds.). 2017. Light Management in Controlled Environments. Meister Media Worldwide, Willoughby, OH, USA.

McCree, K.J. 1971. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology 9: 191-216. doi:10.1016/0002-1571(71)90022-7.

McCree, K.J. 1972. Test of current definitions of photosynthetically active radiation against leaf photosynthesis data. Agricultural Meteorology 10: 443-453. doi:10.1016/0002-1571(72)90045-3.

Naznin, M.T., M. Lefsrud, V. Gravel & M.O.K. Azad. 2019. Blue light added with Red LEDs Enhance growth characteristics, pigment content and antioxidant capacity in lettuce, spinach, kale, basil and sweet pepper in a controlled environment. Plants 8(4):93. doi:10.3390/plants8040093.

Terashima, I., T. Fujita, T. Inoue, W.S. Chow & R. Oguchi. 2009. Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting the enigmatic question of why leaves are green. Plant and Cell Physiology 50(4): 684-697. doi:10.1093/pcp/pcp034.

Wang, Y., & K.M. Folta. 2013. Contributions of green light to plant growth and development. American Journal of Botany 100(1): 70-78. doi:10.3732/ajb.1200354.

To learn more about the photoreceptor phytochrome and the role of red and far-red light, see Kusuma and Bugbee’s 2021 work on phytochrome photoequilibrium from the Utah State University Crop Physiology Lab.