http://www.ilovegrowingmarijuana.com/marijuana-nutrient-deficiency-phosphorus/ would be my guess my purple kush don’t get purple stems in veg
You don’t think it is a magnesium problem?
How can I fix it?
answer is included on the link but most common is to flush your soil with correctly ph’d water 3-4 times the volume of your pot rinse your soil slowly usually this reduces the salt build ups in your medium and lets you correct ph and stop lock out. Since I have 2 suspects for your issue both would be rectified with a simple flush. check your run off as you flush for both ph and ppm/tds you want to get back into target range ph 6.3-6.8 and tdds around or very close to clean ph’d water you are flushing with after last flush feed plant add a bit of cal-mag or epsom salt to hit 3 possible issues at once
Mag usually shows signs between veins on leaves paling leaves not spotty discolouration darkening
that’s mag here’s my other suspect
take your pick they have similar fixes but a flush medium and fresh balanced feed covers all bases