Which part of the light spectrum is used for photosynthesis?

Photosynthetically active radiation (PAR)

Plants use visible light for photosynthesis. Visible light ranges from low blue to far-red light and is described as the wavelengths between 380 nm and 750 nm. The region 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 for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD) in the units μmol/s.

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, giving rise to more compact plants. However, the photomorphogenesis effects of spectrum shifts in aquatic plants are quite different from terrestrial plants; things such as stem elongation are determined more by gaseous exchange (access to O2/CO2) than by blue shift in lighting. (Link to article) ​Similarly, flowering cycles for aquatic plants are triggered by access to surface air rather than red spectrum, for practical purposes.

The main impact of spectrum on aquatic plants is stronger pigmentation for certain species when stronger red/blue light is used. Having more red/blue spectrum also gives higher visual color contrast and saturation - which is why I highly recommend it.

WHAT WAVELENGTH REALLY MATTERS?

Absorption spectrum vs action spectrum curves

The ​Absorption Spectrum of chlorophyll defines the wavelengths that are absorbed by chlorophyll pigments. Many of such charts are constructed by experimenting with extracted chlorophyll pigments under lab conditions - which may not mirror what actually happens in a living leaf.

​The Action Spectrum defines the wavelengths that are most effective for photosynthesis - done by measuring oxygen output of actual leaves under different spectrum lighting. The two are quite different and it is the latter; action spectrum that is important in determining photosynthesis.

Absorption Spectrum

The chart below shows the absorption spectra of photosynthesis; charts like the one below are constructed from in-vitreo (in test-tube) lab work data- primarily by shining light through an extraction of chlorophyll and seeing what light spectrum makes it through. The pigment peaks can differ depending on the solvent used and the charts do not tell how much there is of a particular pigment in an actual leaf, neither does it mimic the full complexities of photosynthesis of a living leaf, where light absorption also depends on specific proteins bounded to chlorophyll pigments and overall orientation of the pigment in the leaf. Charts like these give the false impression that green/yellow light plays only a minor role in photosynthesis.

As the structure of the whole leaf is considered, we see more and more absorption in the green/yellow region. Therefore, plants absorption of green light is about 70%; and green light play a significant role in photosynthesis. ​​Interestingly, since then, many textbooks have 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 topic.

Action Spectrum

​Action spectrum for photosynthesis (vs absorption spectra charts), 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, while YPF weights photons in the range from 360 to 760 nm based on plant's photosynthetic response. i.e. In the McCree chart on the right, red is shown to be more efficient for photosynthesis than blue, which is more efficient than green. Some sources will also refer to this concept as PUR (Photosynthetically usable radiation).

From the chart below, it gives the impression that red light is 20-30% more efficient than blue/green light for the purposes of photosynthesis. The curve was developed from short-term measurements made on single leaves in low light. Some longer-term studies with whole plants in higher light indicate that light quality may have a smaller effect on plant growth rate than light quantity. Leaves absorb mostly red and blue light in the first layer of photosynthetic cells. Green light, however, penetrates deeper into the leaf interior and can drive photosynthesis more efficiently than red light at higher lighting levels.

Photosynthetically useful radiation ? (PUR)

The action spectra (YPF) does not function in a linear manner as light intensity increases, the idea of PUR (photosynthetically useful radiation) becomes vague as efficiency of different spectrums change with changes in overall light levels. As the article below describes, as red/blue pigmentation becomes light saturated, additional green gives a higher marginal gain to photosynthesis. This effect is also different across different plant species. Thus, commercial firms that market their aquarium lights as having more PUR  have no accuracy to their claims - its purely a marketing gimmick.  PUR is neither linearly measurable and is plant specific thus is it not possible to issue a general numerical value of it without examining the specific circumstances of which it is 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. Thus, measuring PAR is generally accepted by leading scientists currently as the best gauge for measuring how strongly a light will stimulate photosynthesis.

Reading through the popular literature on the Internet... there is much misunderstanding about which wavelengths plants use for photosynthesis. Absorption Spectrum is often confused with Action Spectrum. For hobbyists' purposes, aquatic plants use all of visible light for photosynthesis, including green, which is only partially reflected by green plants.

A good in-depth dive into light spectrum and photosynthesis by university researchers. Apogee instruments is industry standard for light measuring instruments with regards to plant farming.

Reading Spectrum Charts

Most serious light manufacturers will publish spectrum charts for their light units. Below shows the spectrum chart of a BML LED light unit and my tank under the light. The amount of each color light being produced is equivalent to the area under the curve. This particularly light unit has large spikes in blue and red, a smaller hump in green and produce little yellow and cyan light. This spectrum profile highlight reds and blues in the tank.

What is important is the relative area/size of peaks. To appear neutral white light, a light will have spikes 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 tone of the light by reading the spectrum chart. 

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

Head here to learn more about aquarium lighting for planted tanks

Head here to learn more about PAR values.

Head here to learn more about spectrum curves.