Yer lower leaves will turn yellow from Nit def in flowering first many times. The reason the Nit levels are so low in the flowering process, is due to the plants need to survive. The nitrogen is needed during veg so that the plant can GROW and THRIVE. Wen we put the plants into flower via 12/12 lighting, the plants know that the end of their lives are nearing, so the energy that was directed towards GROWING and THRIVING, is no directed to survival thru ensuring plant species. The plants know that they must reproduce in order to survive (survival fer them referring to spreading their seed), so they attempt to either pollinate (male), or BE pollinated (female). At this cycle in their lives, the higher amounts of Nit are NO longer needed