Foliage starting to yellow and age on most leaf crops is a clear signal that mature leaves are no longer contributing through photosynthesis to biomass accumulation and are past their prime, both physiologically and commercially. Tobacco leaves are different because colouration and senescence (ageing) is the signal for harvest time and subsequent post-harvest proessing (drying, curing and perhaps fermentation) to produce marketable leaf products. Chemical changes occurring during this period will ultimately determine the quality of tobacco leaf and smoking experience.