The effects of variety, rootstock, and geographical location were studied as to their effects on secondary metabolite production in grapefruit and pummelo. The concentration of the flavanones narirutin, naringin, and neohesperidin and of the sesquiterpene nootkatone, which is principally responsible for the grapefruit's aroma, varies during h i t development. The highest flavanone levels are detected during the juvenile stages of fruit development, while nootkatone expression is associated with the processes of maturation and senescence. The possibility of increasing the levels of these metabolites by regulating the associated processes of growth and cell differentiation is discussed.

The distribution of nootkatone and valencene in grapefruit was determined by gas-liquid chromatography-mass spectrometry (GLC-MS). Changes in the concentration of valencene and nootkatone were observed during the development of fruits, increases in these compounds coinciding with the linear and the maturation steps, respectively. The possible participation of valencene and 2-hydroxyvalencene as precursors in the biosynthetic pathway for nootkatone in grapefruit is discussed.

The effect of ethylene and ethephon treatments on nootkatone accumulation in the rind of grapefruit was investigated. Considerable increases in the levels of this sesquiterpene were observed in the picked and unpicked grapefruits treated. The changes induced in the maturation-senescence stage of grapefruit by these treatments consisted of an accelerated carotenogenesis process in the rind, along with morphological changes in the exocarp and ultrastructural changes in the plastids. These results suggest that ethylene regulates nootkatone biosynthesis by accelerating the maturation-senescence processes in grapefruit rind.