My plants are yellowing from the bottom upwards and each stem and vein is a reddish colour, the plants are bout 5 weeks into veg and have been growing very nicely untill i upped to a 600 w (im growin in my shed so temps are very low wen lights out) Could this be the problem

cold could be your problem - though like everything it could be something else as well

i was told at last years expo that a purple/red stem is an indicator the plant is/has been cold

they suggested a way to overcome it is to keep the nutrient/roots warm 24hrs, then the above ground part of the plant can handle lower temperatures

i don't know this to be fact, but i have quite a cold nighttime during the winter and have got a 200w aquarium heater in a 100litre tank keeping it at 24 deg

yield is pretty constant throughout the year, or not noticeably different

the yellow leaves sounds more like a nutrient thing, how are you growing them?

Their in 11l pots under 600 w in a 1.2 x 1.2 x 2 room in my shed with rvk extract and passive for now temps hittin 8-9 and sod all humidity -- ive given them a water with nutes at half strength for the 1st time 2 days ago with alittle epsom and watered with just water tonight, their drooping at the mo and look rather pathetic.

Warm them roots, underfloor heating kit, ebay £70 or so, use "underwood" type.

Some strains are ok, my Him Golds seem to droop @ 15C or less, give them 15.5-16C they perk up!

I am keeping my roots @ 23-25C by heating them.

"haze" from sativa seedback will grow/flower in near freezing conditons

8-9C ouch...Also Blue Cheese can take FREEZING weather.

If your stain is not a cold lover they will just stunt, be REALLY slow and yeild peanuts.