No I gotta be honest that doesn't look like any deficiency it is definitely stress, if it's heat stress it's weird that a lower leaf would experience it first, unless your using cfls, what are you using for lights, if your using hps and it's within 2 feet of the plants it's definitely not heat stress, you would see it on the top leaves first. If it's not heat stress I would guess water stress. Do you have sufficient drainage?