SisterMaryElephant wrote:It wasn't that long ago that people thought that plants don't use green light but now we know that they do use some of it. There probably isn't a perfect spectra...

Nate: We’re seeing a trend in the industry where LED grow lights come out with more and more “bands” of light. Initially most of the LED lights were only two bands (red and blue). Now companies are starting to claim their lights have 11 bands and more. What are your thoughts on this? Is this marketing hype or is there something to this movement?

George: When it comes to the spectrum and the bands, it’s important to understand the basics. Enough research has been done to date to paint a good picture of how most plants use light. For example the chlorophyll a and chlorophyll b peaks at 430/470 and 620/660 nm are very well documented. The other thing you want to keep in mind is there is plenty of process variation in the manufacturing of LEDs. This means that if you buy a lot of say 450nm LEDs you’ll get some that peak at 440nm, others that peak at 460nm. So as much as the marketing hype coming out of some LED grow light companies leaves customers feeling that more bands is better this is not necessarily true.
Our research has shown the following:

1 The high absorbance spectral region of plants is between 420nm and 480nm in blue and 610-680nm in the red.
2 It’s important to deliver some light in the 500-600nm range (green/yellow) targeting the carotenoids for good plant morphology
3 Anything above 700nm can have a negative impact on plant growth
4 Some UVB (280nm-315nm), can be helpful
5 Light intensity and proper red/blue balance has a much higher impact on plant growth than specific bands.

We use 5 different LED spectra in our lights, plus UVB in the SolarStorm. We include warm white (3100K) to fill-in the spectrum between 500 and 600nm. On the SolarStorm we also have a switch to optimize the blue/red ratio for vegetative growth vs. bloom. Given the process variation in LEDs, when you look at the Spectro-radiometric chart of our lights, you’ll notice high coverage in the range between 420 and 480nm and 610-680nm and some coverage between 500 and 600nm."

One of the most common misconceptions about photosynthesis is that leaves reflect all the green light and do not use green light in photosynthesis. The truth is that leaves typically absorb half or more of the green wavelengths, and green light is used fairly efficiently in photosynthesis. Most leaves do reflect more green light than other colors so leaves appear green to our eyes.

The misconception about green light use in photosynthesis arises because of the chlorophyll absorption spectrum printed in many botany and biology textbooks. The chlorophyll absorption spectrum is made using a spectrophotometer and chlorophyll extracted into a test tube of an organic solvent such as acetone. The chlorophyll solution does absorb relatively little green light compared to red and blue, however, chlorophyll solution in a test tube behaves differently than chlorophyll in a leaf. In plants, chlorophyll occurs in highly structured chloroplasts, which are in complex plant cells. In a test tube, the light passes right through the chlorophyll solution so the chlorophyll has one chance to absorb a green light particle or photon. In leaves, a chlorophyll molecule in a highly structured chloroplast has many chances to absorb a green photon because the unabsorbed photon can be reflected repeatedly from chloroplast to chloroplast many times increasing its chances of being absorbed. There are also accessary pigments, the carotenoids, which can absorb green photons and pass the energy to chlorophyll.

In terms of your experiment, it may be that the green filter you used reduced the amount of light much more than the red filter. The human eye is not a good judge of the exact color makeup of the light nor the total amount of light the plant is receiving. You need an instrument called a spectroradiometer to determine the exact color composition of the light and a quantum sensor to measure the amount of photosynthetically active radiation or PAR provided.

David Hershey

Reference:

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.