Whats going on with my plants?

Could be nute burn, or Ph lockout, you will have try use a process of elimination to figure it out. Have you checked your PH and is it within normal range for your setup? What are the PPMs of your nute solution, and are they within normal limits? Probably not but have you ruled out pests?

If you're sure it isn't pests then the remedy is to flush the plants with clean PH'd water. Adjust the PH of your medium, and then slowly add nutes till you get back up to max.

You can tell by the red leaf stems that your plant is very stressed. Find the cause of that stress and your plants will reward you in kind.

 

not burn, surely not pests

you have nute def. and it looks like complete lockout, check the PH soil runoff first

 

thats good that eliminates PH lockout, I,d like to see a pic of the whole plant

they are nute deficient, so either fert with an even NPK value fert like 10. 10. 10.

or if you have been fertilizing recently, something is locking out the nutes, (you may need to try a flush)

give us the rest of the grow details to ferret out this problem

 

Ganj, it looks a little like nute burn. We do need him to answer what when and how much he feeds it and does the yellowing start at the bottom and moves up the plant or vice versa?

 

red stems is a sign of N P K and MG def's

 

The stems can also turn purple during over fert or it can be a strain characteristic. I would kick back on the nutes for a bit and see what happens.

 

purple can be in the plant genitics, however if you saw a fairly rapid change in color to purple, that is either phosphorus not being pumped up the plant or cold temps can also induce this, it still is phosphorus not being pumped up but its the cold causing it.

as I said I see nute lockout, and now the red stems are congruent with my diagnosis.
If you feel you over ferted your plants then I,d suggest you flush or transplant to new soil.