It looks like a bad atmosphere.Fress air,heat,lack of heat,humidity.Its not the only one in the tent like it.The others are going the same way.Are you extracing the air?Make sure everything else is how it should be.Get everything dialed in then foliar feed your plants at half strength.Its by far the best way to get nutrition into your leaves,and it gets there fast.They say the plant only take up 2% of the root drench.I guess your lights are high?If they're strong lights then turn them down 20-30%,and give it a few days.