Cause of plant bleaching with led?

wietefras said:

I had bleaching under a 1000W Gavita DE.

It's just that normally HPS lights are kept well away from the canopy because the radiating heat from the lights is damaging already before the light intensity gets too high. So with HPS you know quite quickly that you are too close because the leaves start curling up. In this case I simply ran out of space. Although later I cut a hole in the tent ceiling and lifted the light right to the top of the tent to fix the issue.

The light was somewhere between 60cm to 70cm (24"to 28") above the canopy and then this happened:
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I believe that would actually be Infrared Cooking?
So if a DE HPS at approximately 36" from canopy is far enough to prevent the heat damage issue and grow with an intensity at the canopy that still exceeds most LED, It would lead me to believe that it is a spectrum other than IR that is the culprit. That was the intention of this post. Looks like most are agreeing it is in the blue side of the spectrum.
Any consensus on a nm range?

As positivity mentionned, it's more to do with leds being directional, and hence reaching higher intensites, quickly, as you get close to them.

I've had bleaching with 600W HPS, not often, but at least twice, that I can still remember ... Definitely easier to get it with LEDs. Again, as positivity said, back off the light

OneHitDone said:

I was referring to the "bleaching" that is always mentioned under too high of intensity in the diy posts

You don't ever here of bleaching even under DE1000W HPS

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EfficientWatt said:

As positivity mentionned, it's more to do with leds being directional, and hence reaching higher intensites, quickly, as you get close to them.

I've had bleaching with 600W HPS, not often, but at least twice, that I can still remember ... Definitely easier to get it with LEDs. Again, as positivity said, back off the light

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All of the LED par grids I have seen - backing off the light = a huge drop in par levels

The white tips are a strange phenomenon. Rives has a great example. It is living plant tissue and continues to grow and add trichomes. First time I ever saw it was after switching to white COB and running them at 38W ea. It only happens on certain ladies. I think this part of the plant can still create viable seed and viable cannabinoids but the plant chose not to use this area for photosynthesis for some reason. Maybe it is the stage just before tissue death.


The strange part, it concentrates on the tips. Why only the tip of the bud even though other buds are closer to the light?



Bleaching/burning is a different issue, the overexposed plant tissue dies. It can be caused by pure photon intensity and will happen to any part of the plant that is getting too much intensity.

@OneHitDone :Sure but if your plants are bleaching, they were too close in the first place, and you have too high par ... or/and very bad light distribution ...
In other words, bleaching === too high par, basically.

Over saturation of pigments. Chlorophyll a&b being the most represented in the plant and also targeted by led's. So things like only R/B lights that target only those peaks with high intensity get bleaching easier. Broadening the spectrum out allows plants to receive higher photon counts with less to no ill effects. And also how the sun can pump 2000µmols and plants of all kinds love it. Full spectrum and then some.

Greengenes707 said:

Over saturation of pigments. Chlorophyll a&b being the most represented in the plant and also targeted by led's. So things like only R/B lights that target only those peaks with high intensity get bleaching easier. Broadening the spectrum out allows plants to receive higher photon counts with less to no ill effects. And also how the sun can pump 2000µmols and plants of all kinds love it. Full spectrum and then some.

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Well said... Completely agree with that...

Great Thread guys!

Relating to this discussion -

"The McCree Curve, also known as the Photosynthesis Action Spectrum was developed in 1971 by Dr. Keith McCree to show how efficiently plants use each different wavelength (color) of light in the PAR region (and slightly outside of it). This study was tested by analyzing the levels and differentials of CO2 in a testing chamber using C3 and C4 plant leaf samples. The leaf samples were given monochromatic light ranging from 390nm to 760nm in intervals of 10nm and after a bunch of calculations the points were plotted in what we now know as the McCree Curve or the Photosynthesis Action Spectrum. By doing this study, it was determined that plants respond more efficiently under certain wavelength (colors) of light than others."

In the age of High Tech lighting why is everything still based on a study from 1971? Is it not possible that McCree was full of shit, or only partly correct? Would think in 2015 a new study could be done with much greater precision

Greengenes707 said:

Over saturation of pigments. Chlorophyll a&b being the most represented in the plant and also targeted by led's. So things like only R/B lights that target only those peaks with high intensity get bleaching easier. Broadening the spectrum out allows plants to receive higher photon counts with less to no ill effects. And also how the sun can pump 2000µmols and plants of all kinds love it. Full spectrum and then some.

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I agree,especially about the R&B panels but think it's more of a overload on the blue side. I haven't been able to bleach any of my strains under my COBs,they have gotten so close they fry rather than bleach.

The Mcree curve is great I think at isolating the more important wavelengths, when having to choose from available spectrums. I don't think Mcree would say his spectrum is better than the sun. Probably very accurate for its purposes.

I had a photo of a bleached top from cobs in my micro cab, looks like I erased it, it's definitely possible. But I wouldn't expect any different really...it was very close to a cob,not something that should normally happen.

Blue or red bleaching? They probably both have the ability with their narrow band. I'm more inclined to believe red is just as guilty after all my spectrum runs. White seems to counter it quite well, filling the gaps.

need more carotenoids (carotene, xanophyll etc.). Carotene and other carotenoids help with photosynthesis and protect the plant from damage that shows as bleaching. Doesn't happen until the photosynthetic compounds are overwhelmed by the amount of light available. Carotenoids allow the plant to dissipate excess light energy as heat and absorb the energy thus saving the surrounding tissues (such as chlorophyll) from damage. They also break down into chemicals that produce smell so they're great overall.