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based on my experience growing AK48's they seem to shed thier lower fan leaves (and ones that dont get much light) as they get closer to harvest, as you can see in this pic of my AK48's most of the lower fan leaves are gone (pic = 32days from the 12/12 flip), and I assure you that they are not low on Nitrogen as I feed on ever watering and my tops are still dark green. If the tops look good and green and the buds are forming nicely I wouldnt worry about it, I have also had other strains that tend to lose the lower fan leave after a month of flowering. You do need to get a better fertilizer, something higher in "P" and lower in "N" , i use Fox Farms Big Bloom and Tiger Bloom
i was goin to say tho i never grew AK wouldnt he be close to half way done being his nute regiment isnt strong thus plant could be takin what ever nutrients it can from lower leafs ???