What's going on with these leaves?

[ReikoX]

If I had to guess, I would say N deficiency.

@Characterization of Nutrient Dissorders in Cannabis Sativa

Symptoms of nitrogen (N) deficiency developed 6 weeks after treatment. The plants first displayed slight stunting when compared to the control. Initially, the plants developed a slight overall yellowing or paling of the lower foliage. As the deficiency progressed, the yellowing became more intense and progressed from the bottom most leaves to the middle foliage. In advanced symptoms, the yellowing leaves became completely yellow and eventually turned necrotic and abscised.

If you don’t want to guess, get a soil test done. That should tell you what is missing or out of proportion.