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Red Light vs Blue Light

Discussion in 'Talk to Tom Barr' started by VaughnH, Sep 5, 2007.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    In this article, Lighting for the Planted Tank, Karen Randall says "Red light encourages long, “leggy” growth, while blue light encourages compact, “bushy” growth." I have read where others say the same. The statement doesn't ring true to me, but I have no other basis for disbelieving it. Is it true? The whole subject of light spectrum vs. plant growth seems to be a myth-filled one to me, but I lack the experience to figure out where the myths begin and the facts end.
     
  2. Tom Barr

    Tom Barr Founder
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    I rarely disagree with Karen.
    I think what is being implied is that you need some of both, but red light alone does not promote leggy growth, red light is the primary driver of growth for aquatic and terrestrial plants.
    the type of red can promote leggy growth however, but red light is a fairly wide range of light and the devil is in the details.

    There are many potential things going here, both the molecular aspects that dictate leggy growth and this adaptation, as well as as the light quality itself. Another issue is the angel of incidence of the light and the spread.

    You must have the same PAR for both light types. Often, that is not the case.
    So comparison's are tough.

    As far as what is known to cause leggy growth:
    Plants can and do distingush between "shade" type.

    So if you have another plant shading a plant, it will become "leggy".
    If you have a piece of driftwood, or rock shading the same plant under the same conditions, it will not.

    Plants filter out the Red light leaving the Far Red light(longer than 700nm, which they cannot use for photosynthesis). Other objects do not filter out light selectively, they just reduce all light unless transparent to some degree etc.

    So a plant has a mechanism to "know" if they are being shaded by another plant(vs something else like a non living object) where they are growing versus a rock or something else that's not moving/growing /competing etc.

    What does this suggest?
    Good pruning does a tank good.
    Not allowing plants to shade eachother etc.

    It keeps the plants from competing against themselves.

    I see folks say a lot about algae competing with plants, or some light encourages algae.

    What I do not see much of is folks saying that plants competing with themselves/eachother or that some light types encourage some plants while hindering others.

    Some plants are far more tolerant of low/variable CO2 than others, same for PO4, NO3 and some other nutrients.

    Having a single point source of light such as MH's vs several FL's that span the entire top surface is also a good reason for leggy growth.

    You get fuller bushy growth if the lights move like the sun and is able to strike at many angles and sides rather than just one.

    I think a simple mixture, namely red.... with a little blue light is ideal for plant growth and our eyes as well.

    Most all bulbs are generally this anyway.
    I think folks get confused when they read about higher Kelvin temps and assume that it implies they will get bushier growth because of that. I do not see anything that suggest that personally and that's generally what many assume.

    I've read a lot about light and plants, but I've not met one single hobbyist on the web or in person that's even use a PAR light meter.

    and I do get around and talk about such things.........

    I've also yet to meet anyone that has measured the entire spectral intensity over the PAR range for their plants etc either. That is what is needed to show that plants become leggy under such conditions and under non limiting CO2/nutrients.

    You need to set up a test to show this and then measure it at the same time(purpose driven testing).
    Not just assume.

    Knowing how to set up a test that will answer the question also allows you to search for research on the matter also, a key in getting a good understanding about the background questions, pitfalls and other issues that always arise.









    Regards,
    Tom Barr













    Example:
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    Here I go again with intuition vs. experience: One thing that I think misleads people when discussing light spectra produced by the various bulbs we use, is misunderstanding what the spectra illustrations show. For example, here is a spectra of a bulb that was used in a comment on APC:
    [​IMG]

    The important thing to realize when looking at this is that it is the area under the curve that represents the energy given off by the bulb, not the height of the peaks. A casual look at that chart would lead you to believe that this is a bulb producing mostly green light. But, the areas under the various sections of the spectra are very close to the same for all colors. The green "band" may be have a bit more total energy than the other colors, but not substantially more. Those sharp narrow peaks are only significant if they are almost exactly aligned with the absorption peak for the chlorophyls in the plants. So, it is really the bottom part of that "curve" that shows more information of interest than the top part with the peaks, unless the peak coincides with an absorption band for the chlorophyl in the plant.

    Once you accept that, then the kelvin temperature rating for the bulb makes a lot more sense, and much of the argument about certain bulbs not being very good for plants goes away. This particular bulb, for example, produces a lot of energy in the red band, even though there is no high peak in that band, and it also produces a lot of energy in the blue band, with no high peak in that area either. The analysis of spectral charts for bulbs is just a lot more subtle than it appears to be.
     
  4. defdac

    defdac Lifetime Members
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    Calculating the PUR-efficiency is the best way regarding measuring the effective photosynthes-triggering amount of light in the spectral distribution. It does exactly what you are saying VaughnH - it calculates the amount of energy output by integrating the spectral distribution and weight compares it across the entire action spectrum.
    This was done years ago by Ivo Busko, and you know which article I'm talking about..
     
  5. Tom Barr

    Tom Barr Founder
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    Yep, you folks got it.
    Look at the blue spike there and the red, then the green/yellow.
    Not that much Blue really.

    I do not think we can generalize about a type of light producing bushier growth over another for all plants. If you want to test this for yourself: use atinic and a cool white which is fairly low in Blue.

    See which produces bushier growth, eg, more shoots per length of stem and shorter interodal distances.

    Many have already done the isolated blue and red light like test in the hobby.
    Now a nice mix of a little blue and a lot of red does produce decent growth, trimming, and other parameters, CO2, intensity, nutrients etc all play a role.

    I just do not think, believe nor see evidence that it's isolated into blue vs red light, .........."either or" business.

    Ivo did about the best estimate relative to the info that's usable. We talked a fair amount back several years ago about that. I was still not happy when making comparative test with plants, so I bought a LiCOR light meter and have not bothered with any of that since.

    I can test anywhere in the tank, any plant, any angle, any pattern etc.
    They do have a spectral analyzer over at the Engineering lab next door to us.

    But I'm not that interested in working with that just yet, someday perhaps.

    Regards,
    Tom Barr


    Regards,
    Tom Barr
     

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