It's not even a nitrogen def. That would affect more than just the one leaf. Your plant is beautiful. It has just decided fir whatever reason it doesn't need that leaf, and is shedding it. Google abscission. Thats the process by which a plant sheds the leaf petiole at the stem. A lot of strains have a genetic disposition for red leaf stems, so barring any other obvious signs of N deficiency, I'd say that "You look MAHVELOUS"