Grundle
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This looks like you are having VPD (Vapor Pressure Deficit) issues caused by less than ideal humidity (if I had to guess). This same situation happened to me and took me a long time to figure out. When this happens the plant uptakes more water along with nutrients because the lack of vapor pressure (humidity) in the air is causing increased transpiration. This leads to nutrient lockout and you end up seeing purple stems, slow growth and yellowing between the veins of the leaf. You may also see the leaves getting rough and have a puffy appearance between the veins. I copied a link below that has more information and VPD charts but there are a lot of online resources for this issue. What I did to solve it was to dim the lights a bit and increase the humidity to 65% until I saw normal growth again, Based on my own experience, it was not worth the effort because it took many weeks to resolve and when I put the plants into flower, they still struggled a bit. If I were to run into this issue again, I might be tempted to trash the grow and start over. I could be wrong but this is what I think is happening here.
Vapor Pressure Deficit - The Ultimate Guide - Dimlux Lighting - The Best Grow Lights
Simply put, Vapor Pressure Deficit measures the difference between the amount of drying power the atmosphere is exerting upon your plants. VPD is the key...
www.dimluxlighting.com