YELLOWING LEAVES

Looks like burn to me mate. Either your feed or soil is / was a bit hot. Looks like they have already started recovering though 👍

If it´s light burn, why aren´t the upper leaves affected? At this early stage you don´t feed and water too much i hope? Did you check your water in general? Maybe it lacks cal? I have too add too... I would also dimm the lamp or raise the lamp/plant distance to 50-60 cm. Just my opinion, good luck,.

Seems to me like either a Magnesium (Mg) or Iron (Fe) deficiencies! From the looks of it your plants suffer it from a few reasons, first is the plant size is too big for those small solo cups (unless you are pruning the roots which I highly doubt) this causes the roots to grow around themselves with nowhere else to grow and then locking themselves from nutrients - this is a problem you need to tend to ASAP. *Another reason for the deficiencies may be either pH fluctuation and getting out of the 5.5-6.5 range or Calcium (Ca) toxicity which inhibits Potassium (K), Iron (Fe), Magnesium (Mg), and Manganese (Mn) uptake. I believe the only thing you need to worry about is transplanting to a bigger pot, give them a good nutrient rich irrigation with at least 20-30% runoff, plants should show good results in about 2-3 days. Hope this helps! Good luck and happy growing :)

i can think about two factors: 1st is the ph fluctuation. 2nd is the magnesium deficiency let me know if you can find a solution

You have nute burn. The tips turn brown and curl up with nute burn. With sun stress they turn yellow like they are and progresses in on leaf. Might have a touch of both. Most definitely nute burn. Make sure you are getting 20 percent run off each feed to flush out salts from prior feeds. Plus helps maintain proper ph. I just went through this. Good luck and happy growing.

They need to be moved from the solo cups to a bigger pot. Try transplanting your girls into a new home. 🤘😤