We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we’ll assume that you are happy to receive all cookies from this website. If you would like to change your preferences you may do so by following these instructions.


LED Light Spectrum 101: The Action Spectra

By using different color lights and controls to tune color ratios, LED grow lights can activate specific areas of the spectrum in photosynthesis. The spectrum chosen can have a huge impact on your plants, both in terms of quality and yield.

Before christmas we published the blog post “Spectrum 101: Absorption Spectra”. Here we wanted to increase the understanding of the wavelengths plants use for photosynthesis by clarifying the difference between absorption spectrum versus the action spectrum.

There we defined an absorption spectrum as the spectrum of electromagnetic radiation, or light, plants absorb. This is dependent on the cellular and molecular build-up of the plant and therefore differ depending on species. Whereas the action spectrum defines the spectrum of light most effective for photosynthesis. In other words, it is the part of the light spectrum that does the work.

In the last post we also explained the concept of absorption spectra more closely. In this new blog post we will continue to explore the action spectrum…

The Action Spectrum

An action spectrum describes the efficiency with which specific wavelengths produce a photochemical reaction. Photosynthesis involves the harvesting of light (absorption spectrum) and the subsequent photochemical and biochemical reactions. Meaning, an action spectrum describes the wavelengths that actually drive photosynthesis.

In 1972, KJ McCree published his influential paper describing the action spectra for 22 plant species. This work was originally done in order to provide an accurate definition of PAR(Photosynthetic Active Radiation), which had not been previously described empirically. The action spectra described by McCree plots the efficiency or quantum yield of CO2 assimilation as a function of wavelength. Interestingly, similar action spectra were observed for the 22 plant species.


Action spectra for 22 plant species grown in the field (top plate) and a growth¨chamber (bottom plate). (McCree 1972).

However, there were slight variations between species in the blue end of the spectrum. The results from this work indicated that PAR was between 400 nm and 700 nm and that all wavelengths within this region were used in photosynthesis.

PAR and action spectra

The areas of the spectrum that drive photosynthesis are highest in the red end (600-700 nm), followed by the blue region (400-500 nm) and lastly, the green region (500-600 nm). These data show that between 50 and 75% of the green light is used in photosynthesis. Thus green light is necessary for photosynthesis.

Action Spectra image

The action spectra for higher plants and a green alga (Ulva) http://photobiology.info/Gorton.html. Figure reprinted with permission from Dr. Holly Gorton. ( Photosynthetic action spectra for the green alga Ulva (two cell layers) (Haxo & Blinks, 1950) and higher plants (multiple cell layers). The curve for higher plants represents the average of action spectra obtained for 22 crop plants (McCree, 1971/1972) recalculated on a photon basis.)

The action spectrum for higher plants presented here (b) is an average of the data presented in the McCree (1972) paper. On average, over 70% of the green light was used in photosynthesis.

Crop plants have been bred for uniformity and thus have similar action spectra. Algae and other photoautotrophic organisms (organisms deriving their energy for food synthesis from light and are capable of using CO2 as their principal source of carbon) have evolved differently.

LED Grow Lights - targeting the action spectrum

The benefit of an LED grow light is that it produces light at the specific wavelengths chosen to match needs of plants. So instead of converting electricity into a broad-spectrum light source such as an HPS lamp, LED grow lights target the photosynthetic action spectrum. In other words, they put the energy into the wavelengths that matter most for driving photosynthesis. This leads to healthier plants, less waste and can improve yield for the grower. A targeted light spectrum also allows growers to affect the appearance of the plant, for example enhance branching, increase leaf thickness, and obtain overall better plant vigor.  It can as well be used to influence the growth process by slowing down vegetative plant growth and speeding up flowering.

Next up: The importance of green light…


Would you like to find out how LED Lights can improve your growth facility?  Contact Us Today!