Biochemical investigations on the tobacco plant during growth, maturation and senescence suggested that the key to flue-cured quality is the timing of the metabolic transition from nitrogen reduction to starch accumulation. In the ideal situation, this shift occurs coincident with flowering; such leaves fill out, ripen properly and cure easily; the cured tobaccos have good physical characteristics and are compositionally balanced (S/N = 6 to 9). In experiments involving N-rates and moisture managements, it has been demonstrated that underfertilization or/and leaching triggers this transition prematurely, resulting in tobaccos that are pale and chaffy and imbalanced chemically (S/N >= 10). Conversely, overfertilization or/and drought postpones this metabolic shift with the result that the leaves ripen poorly and are difficult to cure; such tobaccos are usually brown and are high in nicotine and low in sugars (S/N <= 4). Through appropriate combinations of N-fertilizations and moisture managements, the chemistries of tobaccos have been tailored to pre-specified levels.