I can't say for sure. But i've read about and experienced purple stems several times...well..everytime actually.

What i've come to find, as far as the purple stems are concerned, is that light intensity/available light has a lot to do with it. I have 4' T8 fluorescents in my veg room and everything in there has purple stems, but otherwise healthy. As soon as i put them under my HPS in flowering the stems go green. Perhaps it's the new nute solution in the flowering Res, but i tend to think it's the lights that bring on the improvement.

who knows. all my grows so far start off great then crash about 4 weeks into flowering. Damned learning curve. i hope i was able to help. good luck.

Skeet.