After creating dark tobaccos in which the alkaloidogenesis was dependent on recessive factors, the authors have studied their behavior in relation to topping and three different harvest dates. The effect of topping observed on upper and middle leaves was evidenced by a better expression of the hereditary factors of the plant. With these very low alkaloid varieties, the variations in tar content of the smoke were sometimes inverse to those in nicotine. The chemical differences between leaf stalk positions were due to physiological factors, the effects of which depended on the nature of the genes responsible for the production of alkaloids. However, this relation was not linked only to the plant's biosynthetic activity.