they turn yellow as the plant uses up the chlorophyll and nitrogen in the lower parts of the plant ... normally, nitrogen isn't added in flowering (it is said to actually impede flowering) ... most flowering-type nutrients have at least some nitrogen in them, anyway ... I wouldn't worry about it, unless the plant takes an obvious turn for the worse ... with a sativa, you could still have a long way to go (my Willie Nelsons went 14 weeks, and could have used a couple more) ... :smokin: