The RED/Blue ratio

Discussion in 'Planted Tank Equipment' started by Greystoke, Mar 1, 2014.

  1. Greystoke
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    I’ve been looking for a simple indication that will give me a light source potential to support plant growth.

    I know that RED (600-700nm) and BLUE (400-500nm) light are the most important sectors for plant growth of the light spectrum, and the ratio between the two would be an important indicator.

    I have always thought that the solar R/B ratio (≈ 1.3) would be a good point of reference, after all, plants evolved under sunlight for millions of years, but recently I paid attention to what is known to be the Photosynthetic Usable Radiation (PUR), i.e. : the light that supports the photosynthesis process in plants.

    [​IMG]

    If you can measure the area underneath the curve (by integration) between 400 and 500nm and also between 600 and 700nm, you will get a measure of the amount of BLUE and RED covered in the graph.
    In doing so, and dividing the values into each other, we find a RED/BLUE ratio of ≈ 0.5. In other words: The photosynthesis process requires about twice as much BLUE compared to RED, when measured in PPF (PAR/m²).

    I find that level very surprising when compared to the solar R/B ratio of 1.3. I did not expect plants to be significantly different.

    Fortunately, I was not the only one.

    December 2008, Tom Barr said:
     
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  2. tyronegenade
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    Hello,

    Have a look at:
    [​IMG]from http://www.chm.bris.ac.uk/motm/chlorophyll/chlorophyll_h.htm

    I would use much narrow wavelength ranges to get some idea of the relative importance of the RED/BLUE light. My advice would be to use the nm range from 50% of peak height to 50% of peak height. So, for example, for Chlorophyll a I would use (approximately):400--440 and 660--680 nm; and then 440--490 and 620--650 nm. My rational for 50% is that all organisms need to maintain a degree of adaptability at the enzymatic level that does not require synthesizing new protein. At 50% the plant can adapt to more light but would still be sensitive to low light.

    Note how close the Chlorophyll a blue peaks are and where the maximum peak is in the PUR spectra (between the two chlorophylls)... Assuming this is no coincidence of evolution, it does imply a greater sensitivity to blue light for photosynthesis as well as the need to stimulate both chlorophyls. This could be an adaptation to clouds that will filter out red light. Photosynthesis is a quantum effect, so one blue photon is of equal photosynthetic value as one red photon. So, the more photons you have falling between these ranges the more photosynthesis happens.

    My problem with the above analysis (other than that I can only find a graph of PAR on wikipedia) is that when you rank the tubes by R/B then, yes, the Daylights, Skywhites etc.. rank close to 0.5 but the "best
    tubes have much higher R/B ratios. Perhaps, a useful analysis would be to take the PAR curve and Chlorophyll curves and determine at what wavelengths do you get the most Photosynthetic flux per quanta of light striking the chlorophyll and then determining which tubes deliver the most photosynthetically powerful photons?
     
  3. Greystoke
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    I agree with your comment.

    For the moment I'll use the 50% levels and find out what what happens.
    Perhaps it may be possible to determine an "effective" level based on the energy flux (?) at those wavelengths.
    That will keep me busy for a while ;D

    There a nice thread on the same subject going on @: The Planted Tank Forum, even Tom Barr is taking part.
     
  4. shihr
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    thanks for posting Cor :)
     
  5. Greystoke
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    As pointed out by Tyrone Genade, the absorption spectra of the two chlorophylls a & b are narrow and have different centers. This means that, even if you have a lot of Blue and Red light shining on your plants, but they don’t cover the effective bandwidth of the absorption peaks, those lights could serve little purpose.
    [​IMG]

    Compared to some typical LED spectra:

    [​IMG]

    There is definitely a mismatch here, which may well explain some unsuccessful attempts in our hobby.
    However, there are some small overlaps, and perhaps it would be a good idea to find out how much.
     
  6. Greystoke
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    Hi guys,
    I’m not through on this subject yet. I’m going to pursue it a bit further using the info I obtained. For the moment I worked on the problem of spectral mismatches between plants and light sources:

    [​IMG]
    This picture shows the mismatch between chlorophyll-a and the red and blue light sources.
    As shown, the chlorophyll only captures ≈10% of the blue light and ≈25% of the red light


    [​IMG]
    This picture shows a much better match for chlorophyll-b and the same light sources.
    The chlorophyll captures 80% of the blue light, and 90% of the red.

    Of course, this is the result using the available information. The chlorophyll spectra are accurate (I think), but I understand that the LEDs may be subject to production variables that move the central peaks around.
    That could spell disaster for chlorophyll-a.

    We need to find more info on this.
     
  7. Dirk B
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    Hi Cor,

    Just a quick comment because I am quite busy at the moment. Those absorption maxima at 400 and 700 nm are not that narrow. Chlorophyll a, chlorophyll b and the carotenoids all capture energy and funnel this energy to photosytem I and to photosystem II of the chloroplast for photosynthesis. So you CAN have a fairly broad spectrum of light and harvest the light energy for photosynthesis to occur.

    Kind regards,

    Dirk
     
  8. Greystoke
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    Thanks for your comment Dirk. It's appreciated

    So far I suspected that some of the light sources (fluorescent as well as LEDs) do not fully match the absorption spectra of our plants basically because these lights tend to generate distinct beams of very narrow bandwidth (≈20nm) which may or may not match (“near missing”) the demanding peaks.

    [​IMG]


    One member of The Planted Tank showed that plants have the surprising ability to shift their absorption peaks. However, from the example he presented the effect is marginal, and it doesn’t seem enough to bridge the gaps between the peaks of the various light sources that are available.

    The Chlorophyll absorption peaks also only have a 20nm bandwidth.

    It is true that a continuous spectrum (like the sun) would not cause this problem, but we only matched that with our inefficient incandescent lights. These days we have more efficient light, but they are a collection of individual colour peaks (fluorescent as well as LEDs).
     
  9. Dirk B
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    Hi Cor,

    I have now seen this reply and this comment, and I want to add that it does not work like this.

    In the so-called thylakoid membrane of the chloroplast, you have structures which are referred to "light harvesting complexes" or LHCs. There are two different LHCs, LHCI and LHCII. These complexes consist of protein molecules which hold a number of molecules of chlorophyll a, chlorophyll b and carotenoids in position relative to one another so that they can pass photons of light that they have absorbed from one to the other. Each one of the compounds can individually absorb light as you indicated at the wavelengths indicated by your spectra. However, photons that are absorbed by the carotenoids are passed on to chlorophyll b, and chlorophyll b can then pass on these photons to chlorophyll a and eventually chlorophyll a passes on all the collected photons to photosystem II or photosystem I for conversion of the light energy to chemical energy.

    So your original PUR curve is actually correct. Between the carotenoids, chlorophyll b and chlorophyll a all those wavelengths can be captured, and then the energy of the captured photons are passed on to the photosynthesis apparatus. The precise wavelength emitted by the lights is not critical as the photons are passed on from one pigment molecule to the next in any case. I also want to mention that this passing on of the photons from one pigment to another is associated with minor energy loss, but the photons eventually held by chlorophyll a does possess enough energy to drive the photosynthetic process. This is often referred to as a funnelling of the light energy by the LHC complexes to the so-called reaction centers which are the next set of chemical machinery which then perform the conversion of the light energy into chemical energy.

    So plants do not need to "shift their absorption peaks", they already absorb quite a broad spectrum of light in any case and can use this very efficiently for photosynthesis. I think "The Planted Tank" perhaps need a biochemist or plant physiologist to help them a little.

    Kind regards,

    Dirk
     
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  10. Greystoke
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    Hi Dirk,
    Just for my edification . . .

    Suppose we have a light source that generates all wavelength except 430nm, 453nm, 642nm and 662nm (the chlorophyll a and b absorption peaks)
    Question:Will there be ANY photosynthesis under this light?

    I'm asking this because I have a list of fluorescent lights where these wavelength are badly represented in their emission spectra, while some lights have them well represented.

    Example:
    The GIESEMANN POWERCHROME Aquaflora T5HO emits ±190mW in those wavelengths alone per Watt of electrical input power, as opposed to the BOYU Sun Light T8's 43mW.
    Obviously, the BOYU is NOT regarded as a plant growing bulb.
     
  11. Dirk B
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    Cor do I understand you correctly in that you are meaning not 430 nm, but 429 nm and below and 431 nm and above for example?

    If it is precisely those wavelengths that you have excluded, I can confirm that you will have photosynthesis and it will not be a problem!

    Obviously, if you have a range of wavelengths missing, say 400 to 450 nm, then you will have a problem, but a single wavelength with the immediate wavelengths present, will not present a problem.

    As you also say, it is the total power of the light emission cumulatively over that spectral area which will decide how well photosynthesis will work.

    Kind regards,

    Dirk
     
  12. Greystoke
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    I should have been a bit more specific ;D
    I did not mean a single missing wavelength, but a "gap" with bandwidth of say 30 nm (±15nm) around the indicated wavelengths

    However, you mentioned that: 400 to 450nm or a ≈50nm bandwidth around 430nm central might create a problem.

    I do understand that the occurrence of ANY wavelength within a critical band will support photosynthesis, although the process through which this happens is a bit complicated for me.

    I've learned enough about this subject to get confused, but I'm doing my best to - at least - understand this particular aspect.
    If so, then we may be able to better judge the "photosynthetic potential" of a light source by determining whether it's spectrum contains enough wavelengths within a critical band to support photosynthesis.
    My evidence suggests that the inclusion of the chlorophyll absorption wavelengths in the light spectrum is vital for the stimulation of our plants.
    Conversely . . .
    If these wavelengths are not or badly represented in the spectrum then the plant will struggle to grow under it.
     

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