Could be a lot of things, but based on what's visible of the other plant it looks like what's wrong isn't exclusive to this single plant....it looks to me like a lot of leaf drop (stress)...when I grew in those black pots with soil, the roots were always in trouble, can they breathe? What's the medium?
My initial impression is ph/overfert and a lot of root zone stress. If your sun is intense those black pots get really hot.
Without more details it's hard to say....maybe a foliar feed that was way strong?