What's the pH of your soil, and what nutrients are you currently feeding it? Make sure before you start treating for any deficiency that salt-buildup or an extreme pH aren't causing the problems/deficiencies.

Purple stems are also sometimes caused by genetics and many times by temperature. Since you are unsure of the genetics, we'll just leave that alone. I can't tell a whole lot by looking at your picture, but I can recommend again that you don't start treating for a P deficiency that you might not have. Check everything else first, then treat for a deficiency if all else checks out alright.

I sometimes see purple stems in seedlings, even with quality genetics, but generally attribute it to environmental factors and not the strain itself. Typically, a P deficiency is the problem when it happens in flowering.