Looks like over transpiration to me, I had a similar problem, I thought it was nute burn. Turned out my plants were losing water faster than they could take it in. Problem was poor ventilation, grow room gor too hot.
my room is like 72F max now, i did raise the lights
if you look at the small leaf in the bottom right corner its got a brown tip and is curling downward ... quite a few leaves are doing that too