I found this issue on the seedling. I have never seen this. I usually have issues with edges, not the center.
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Sulfur deficiency?
What is it planted in?
Cheers
G
It is in the coot soil I mix. This is what I always grow in. The only thing different from others is R/O water. What contains Sulfur? Maybe I forgot something in the mix.
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Good ro water is stripped of everything from what I understand
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IMO, looks like “Interveinal Chlorosis, a symptom of Iron, Zinc and Manganese deficiencies, evident in yellow PARTS of leaf” - Extension Arizona.edu NOTE: Ref @Hydro921, what RO water strips out, YOU must replace. Indeed, if that’s the ONLY change, you can safely narrow it down. Good luck, may be an easy fix. SS/BW…mister 
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I see it venial, not intervenial 
, I would think about some root problem … 
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it looks like its using up the leaf energy because it cant get it from elsewhere.
kinda like what clones experience when they are making roots
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One node above the cotyledons and a single blade, means it’s the plant’s first true leaf.
First true leaves on a seed plant like that often yellow and start to fade as the plant gets taller, stem thickens, and builds more leaves higher up. Notice how the cotyledons below have almost completely fallen off.
Unless the rest of the plant looks like that don’t sweat it.
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If you’re new to using RO, you’ll find better results if you add a Cal-Mag type product to it. For the last 13+ years, I’ve been using Botanicare’s Cal-Mag Plus, which adds Cal, Mag and micronutrients. Use less for young seedlings or clones, more for established plants, and even more for needy plants. For established plants, 1 tsp/gallon to 1.5 tsp/gallon is normal, so for young seedlings, use between 1/2 to 3/4 tsp/gallon.
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Gypsum (Calcium Sulfate) but coots soil mix has that in it.
I think Vernal has it right, it should green up in a few days.
Cheers
G
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I never worry too much about seedlings. They usually recover on their own.
Epsom salts! I use 1 tsp/gal every water/feeding, through first 2 weeks of flowering. Adds needed Mg too! Just my .02. 
That looks like Variegation to me. Photos of the entire plant would help.
You say you are coots mix, but is it the first run in the container or ???
I would advise moving forward you ditch the RO. A lot of problems in living soil come from the use of RO water. You want carbon and sediment filtration. We are trying to remove chlorine and chloramine with the carbon. You want to retain the minerals in your water. You can pull the ro membrane out and just run 2 stage if you have a 3 stage system.
And whatever you do, do NOT add cal mag.
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Variegation or deficiencies occur normally in the leaf tissues, what’s strange here is that the veins are yellow , I agree that a photo of the whole plant would help … 
I think I see something…
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Are you kidding? 
i have to zoom in more for you to see?
he lives on!
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Man, do you know the difference between a vein and a leaf tissue? Are you too stoned to see it? Do you see intervenial chlorosis or yellowing tissues? Please stick to facts, keep your jokes for those who like them …
Alrighty then, Buzz Killington
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