Welcome to ICMag. Looking forward to your side-by-side. Make sure you get permission from the mods in that sub forum
BigPerm's stem coloration seems to be indeed a light response in this particular instance.Great purple stems video - part two was great. I like when people approach things from a scientific perspective rather than hysteria as seems to be so common these days. People assume way too much rather than being logical as you have shown.
It's hard to believe we have people who think that purple stems are a BIG issue. So much that they raise lights to a point where the lack of intensity causes plants to once again turn green (as you have shown green is a shade/low light response). They're purposely causing the plants to grow slower and stretch more just to achieve green stems. And yet this same person is considered by many an "expert".
Thanks for taking the time to clear this little question up.
BigPerm's stem coloration seems to be indeed a light response in this particular instance.
That is not the same as what many have seen and are battling. I personally have seen it even under HID light. When the plant just isn't happy / healthy it will get more of a red coloration to the petiole and in bad cases it will move into the stem. These are genetics that I have watched with my own eyes grow out of the red petiole as they regain health and as light levels are increased the color does not return.
I received the (4) 84x's on May 1st (a day early). I put them in my Sky Lotus tent and am going to run them through harvest which is in roughly 3 weeks.In the 5-6 years I grew commercially, never once were purple petioles an issue in my garden. They certainly happened, but never lead to any issues even if the stems turned purple or had purple veins. It happened whether I was running CFL, HPS, MH or LED, and typically when intensity was in the optimal zone (45-65 DLI). If DLI was low, they stayed green (20-40 DLI).
I think what we need is a grower who will do a 3-4 way side-by-side with 1 plant in each 2x2 area under a single light. Run one at 500 umol, another at 800, another at 1000 and another at 1200. Watch how they all grow, watch the coloration, and at the end draw some conclusions. This will also help determine optimal DLI/LSP under our spectrum compared to sunlight. But finding someone with a scientific curiosity who would run a test of this nature isn't the easiest lol.
To me the purpling for cannabis is the equivalent of a sun tan for humans. It's not a cause for concern, rather the plant telling you "thanks for giving me proper lighting". Too much intensity will lead to a "sunburn" which is displayed by the leaves rather than stems/petioles. If you see conditions like the image below, that's when your lights are too close.
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I can't comment on what you or others might have experienced, but I can tell you that most people are hysterical in their responses. They see purple and immediately assume they have a problem, then they add in a bunch of cal-mag which can actually cause issues when none actually existed. Or they raise the lights which light-starves the plants. If the plant is receiving optimal DLI and is expressing purple, it's certainly not due to excess light intensity. Now if the plants are receiving 80 DLI or higher there could be a cause for concern, but that would require a completely even spread of 1850 umol over 12 hours. The way our lights are designed you'll never reach these levels if using as directed. Our lights peak out between 1300-1500 umol at 12" where we recommend them for bloom (depends on model).