nute lock-out??

This is a picture of my 4.5x4.5 grow tent with a 600 watt and also a close up of one of the fan leaves that is yellowing and dying. I was wondering if anyone could help me diagnose this problem. My thoughts were heat stress, light stress, or some sort of nutrient lock out? Does anyone have an opinion on this?

p.s. My room temp is right around 78 degrees F, so I was thinking it was probably more of a nute lock out. Any feed back would be appreciated.

You have a good temp, big area with not alot of plants so they can breathe. What week of flowering are you into? Is it pure indica strain? Everything else looks good is that the only leaf doing it?
Don't think it would be nute lockout on just one leaf. How close are your lights? Probably a heat prob combined with maybe the end of cannabis life cycle. The buds will start to pull nutrients from the leaves around them and cause them to go yellow in late stages of flowering.

Thanks for the quick response! I did have 4 other plants in the tent, but they were just harvested. Also I've had about 8 to 12 leaves from that plant turn yellow, get spotty, and start to die at the tips like the one in the picture. The strain is Qrazy Train and it's at the end of its 5th week of flowering; I switched to 12/12 on June 18th. I thought the leaves might be changing cause its nearing the end of its cycle, but it seems more intense then just that. Also My light is about 16" from the top of my plant and it seems to be the older leaves that are acting this way

Hey could you turn of your lights and take a picture...

Is it the plant closest to an intake, or an oscillating fan? A lot of times when there are leaves that are yellowing and dying like that, they are unable to transpire the proper amount of moisture and/or CO2 to and from the leaves, and they will die off like that.

Its either one of those, or the leaf itself was so hidden under the plant canopy that it could not take in enough PAR light to continue photosynthesis. Hope this helps! -Cheif

Thanks Chief, those are definitely some things to consider, because I do have an oscillating fan just out of the picture about 2 feet away, and also below the canopy is pretty dense so it may not be recieving enough PAR. Would you suggest thinning out the lower fan leaves to try and let more light through?