Probably stained from nutes..at the water level.

I’m tweaking this very thing myself. Looks like you are in clay. Doesn’t hold allot of moisture to make plants do that. How many times a day are you feeding? Are you certain that the water is oxygenated properly, and how long are those roots sitting in solution? A you using Hydroguard? I think something along that lines is a must in this type of system. The roots do not look like an issue, so don’t freak yet. The oxygen levels are probably within range, you’d see that. The nutrient ppm range is a freaking joke on most of these threads. You need those plants to eat up, and burn off. I’d guess that you got lockout. With the human body they say to eat smaller meals more frequently to speed up the digestive system. You have to feed frequently in clay, so it would make sense to keep those ppms low. DWC guys are sitting in solution constantly. The veterans seem to be saying a PPM range of 300-700 on a 500 scale. Even when they are in the higher range, twist still top of with fresh water. PPMs come down, and it’s considered a proper flush. Plain water would shock them having O EC. When I got this issue, I hit them with Boomerang. That stuff flip them in 24 hours. Then come back on low end of that PPM range. 5.5 PH, and let it climb no higher than 6.2. Let it slowly rise, and right down measurements on your run-off. I couldn’t see flooding clay less than 4 times in flower. Now the flood levels, screw me on that one. Still dialing that in. I’d get a veteran that can help you determine that sweet spot on level/how long/how often. So questions:

Hydroguard?

What stage are you in, and what is your current PPMS?

How many times, and for how long are you flooding now?

What is your Humidity level? I bet it’s low right now.

Stick up a pic of the full plant, maybe to wet at the stem, if so, lower your water level, or raise your plant.