If the plant were a deep green color, I could say it's excess nitrogen, but it isn't. It's likely a primary symptom of overwatering in the plant. If it progresses, the leaves will continue to curl downwards, and the new buds will look distorted like "crazy" leaves; that would clearly indicate excess water in the plant's cells. But I see you have a hydroponic system; I would like to know your watering intervals and the type of nutrient solution.
Cheers!