Plant Problems :(

Hi guys im currently in my flowering stage with my auto's and sum older leaves are starting to die off and the ones that are affected have purple stems the ones that arent are nice and green. What u guys think is the problem?

yea i kno i read them all, but it doesnt say anything about purple stem, if you look at my grow journal and see the last pics i posted mostly all the stems to the leaves are green but sum older leaves and newer ones up top have a purple stem and are die'in off, im not sure if its just getting rid of them cause its putting mostly all its energy into budding. This is where im not sure if its a difieciency?

Purple stems could be from low temps.. Maybe when the lights are off.. But some pics would help bro..

Sometimes during flowering your plant deprives your leaves of nutes because they are all going to the buds. You will see lower leaves die off as part of the plants normal life cycle. I have seen purple stems before but not exactly sure what causes that, possibly and abundance of Nitrogen as it causes darkening in the plants color. Again, not positive on that.