Stem discoloration only

[epochz]

Hello all, just wanted to see if I could get some input on how my babies are doing; well at least one of my babies.

My setup
2x6x8 closet
1 T5 FL
2 35W CFL
Avg tmp 80
Avg RH 40%
FoxFarm Soil
PureBlend Grow (1 week on, 1 week off)

Currently 6 babies in 6 inch pots, 3 of them with 5 nodes (topped) and 3 are 4 weeks behind the others. All are looking pretty healthy to me except for some newbish nute burn that I have corrected since joining this great forum

So my oldest 3 are about 7-8 weeks old and one specifically has really darkened stems on the fan leaves. Now from all my reading this seemes that it could be the beggining of a PHosphorus deficiency but can also be the genetic makeup of the plant itself. Since I am not seeing any discoloration whatsoever on the leaves themselves I am led to believe this is the strain and not a deficiency. Unfortunetly I dont have a digital camera at the moment so you will have to go with my description.

My 2nd, 3rd, and 4th set of fan leaves have the most discoloration on the stems but still looks healthy. Since I topped my oldest 3 almost a week ago I am seeing massive side growth which is getting me really excited. I will try to borrow a cam to post some pics by this evening, otherwise any input is very much appreciated.