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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.

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The biosynthesis of scoparone (6,7-dimethoxycoumarin) was associated with resistance of mature fruits of tangelo Nova, Citrus aurantium, and Citrus paradisi to infection by Phytophthora parasitica. Levels of scoparone increased by treating fruit with Brotomax, which also enhanced the resistance of fruit to the fungus. Lesion development was reduced by as much as 40%. Scoparone is therefore proposed as a possible phytoalexin in fruits of these cultivars. Keywords: Citrus; coumarin; phytoalexin; scoparone