Which part of the light spectrum is used for photosynthesis?

Photosynthetically active radiation (PAR)

Plants use visible light to photosynthesise. Visible light ranges from deep blue to far red light and is described as wavelengths between 380 nm and 750 nm. The range between 400 nm and 700 nm is what plants primarily use to drive photosynthesis and is typically referred to as Photosynthetically Active Radiation (PAR). Plant biologists quantify PAR using the number of photons in the 400-700 nm range received by a surface over a given period of time, or the Photosynthetic Photon Flux Density (PPFD) in units of μmol/sec.

Photomorphogenesis

Different light wavelengths (including portions of the UV spectrum outside of PAR) stimulate different hormonal changes in plants. This phenomenon is known as photomorphogenesis, which is light-regulated changes in development, morphology, biochemistry and cell structure and function.

 In terrestrial plants, red light stimulates flowering cycles and blue light suppresses stem elongation, resulting in more compact plants. However, the photomorphogenetic effects of spectrum shifts in aquatic plants are quite different from those in terrestrial plants; things like stem elongation are determined more by gas exchange (access to O2/CO2) than by the blue shift of the light. (Link to article) ​Similarly, for practical purposes, the flowering cycles of aquatic plants are triggered by access to surface air rather than the red spectrum.

The main effect of spectrum on aquatic plants is greater pigmentation in certain species when more red/blue light is used. More red/blue spectrum also gives greater visual colour contrast and saturation - so it is highly recommended.

What wavelengths of light actually matter?

Absorption spectrum vs action spectrum curves

The absorption spectrum of chlorophyll defines the wavelengths absorbed by chlorophyll pigments. Many of these tables are constructed by experimenting with extracted chlorophyll pigments under laboratory conditions - which may not reflect what actually happens in a living leaf.

The Action Spectrum defines the wavelengths that are most effective for photosynthesis - this is done by measuring the oxygen output of real leaves under different spectral illumination. The two are quite different and it is the latter, the action spectrum, that is important in determining photosynthesis.

Absorption Spectrum

The diagram below shows the absorption spectra of photosynthesis; diagrams such as the one below are constructed from in vitro (in the test tube) laboratory data - mainly by shining light through an extraction of chlorophyll and seeing what spectrum of light passes through. Pigment peaks can vary depending on the solvent used, and the graphs do not tell you how much of a particular pigment is present in an actual leaf, nor do they mimic the full complexity of photosynthesis in a living leaf, where light absorption also depends on specific proteins bound to chlorophyll pigments and the overall orientation of the pigment in the leaf. Graphs such as these give the false impression that green/yellow light plays only a minor role in photosynthesis.

Looking at the structure of the whole leaf, we see more and more absorption in the green/yellow region. Therefore, the absorption of green light in plants is about 70%, and green light plays an important role in photosynthesis. Interestingly, many textbooks have since found it necessary to update their texts to reflect the new information we know today. However, many older textbooks still contain incorrect information on this subject.

Action Spectrum

The action spectrum for photosynthesis (vs absorption spectra diagrams) describes the efficiency with which specific wavelengths produce a photochemical reaction. The curve is also known as the Yield Photon Flux (YPF). PAR values all photons from 400 to 700 nm equally, whereas YPF values photons from 360 to 760 nm based on the photosynthetic response of the plant, i.e. in the McCree chart on the right, red is more efficient for photosynthesis than blue, which is more efficient than green. Some sources also refer to this concept as PUR (photosynthetically usable radiation).

The graph below gives the impression that red light is 20-30% more efficient for photosynthesis than blue/green light. The curve was developed from short-term measurements on single leaves in low light. Some longer term studies with whole plants in higher light suggest that light quality may have less of an effect on plant growth rate than light quantity. Leaves absorb mainly red and blue light in the first layer of photosynthetic cells. However, green light penetrates deeper into the leaf and can drive photosynthesis more efficiently than red light at higher light levels.

Photosynthetically useful radiation ? (PUR)

The action spectra (YPF) do not work in a linear manner.  As light intensity increases, the idea of PUR (photosynthetically useful radiation) becomes vague as the efficiency of different spectrums changes with changes in total light levels. As the article below describes, as red/blue pigmentation becomes light saturated, the addition of green gives a higher marginal gain to photosynthesis. This effect also varies between plant species. Therefore, commercial companies that market their aquarium lamps as having more PUR have no accuracy to their claims - its purely a marketing gimmick. PUR is neither linearly measurable nor plant specific so it is not possible to give a general numerical value of it without examining the specific circumstances in which it is being measured.

For further reading, read the paper "Green light drives photosynthesis more strongly than red light in strong white light: Revisiting the question of why leaves are green" by Ichiro Terashima/Takashi Fujita/Riichi Oguchi.

PAR is still the best way to quantify how effectively a light will stimulate photosynthesis

There is a strong correlation between higher PAR levels and increased photosynthesis. As a result, the measurement of PAR is currently accepted by leading scientists as the best measure of how much light stimulates photosynthesis.

Reading the popular literature on the internet... there is much misunderstanding about which wavelengths plants use for photosynthesis. The absorption spectrum is often confused with the action spectrum. For hobbyists, aquatic plants use all visible light for photosynthesis, including green light, which is only partially reflected by green plants.

A good in-depth dive into the light spectrum and photosynthesis by university researchers. Apogee Instruments is the industry standard for light meters used in plant production.

Reading Spectrum Charts

Most reputable lamp manufacturers will publish spectral charts for their lamps. Below is the spectrum chart of a BML LED light unit and my tank under the light. The amount of each colour of light produced is equal to the area under the curve. This particular light unit has large peaks in blue and red, a smaller bump in green and produces little yellow and cyan light. This spectral profile highlights the reds and blues in the tank.

What is important is the relative area/size of the peaks. To appear neutral white, a lamp will have peaks in blue, green and red. A light that is all blue and red, with very little green, will appear pink/purple and cast a reddish hue over the tank. In this way we can roughly gauge the overall colour rendering of the light by reading the spectrum chart.

What does the K rating such as 6500K really mean? Head over to this article

Click here to learn more about aquarium lighting for planted tanks

Click here to learn more about PAR values.

Click here to learn more about spectrum curves.