Yellow Leaves during flowering.

I am 3 weeks into flowering (maybe 4) and my plant is yellowing on many of the leaves. I have it in recirculating program, 600 watt MH bulb 4x4x6 grow room. here is a pic, i have my ph at about 5.8 and my ppm is general hydroponics, about 350 steady ppm. Any tips, my last couple of grows did not yellow so much. As a bit of history, a couple of weeks ago i left town, and my plants ran out of water. they wilted but not to badly, and seemed to recover well from the incident.

Depending on the strain the yellowing of the leaves is quite normal because in the flowering state the amount of nitrogen has significantly dropped and your introduction of phosphorus has significantly increased. You are basically bleeding out the nitrogen. Your buds look phenomenal @ week three of flowering btw.

No kidding. +rep

your plants look further on than 3 to 4 weeks.
but this yellowing looks normal.
what strain are they.

after having another look.
they look a little over fed, i would cut down to half for a little while.
what are you feeding and your feeding sced?.