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Tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) fruit, at five stages of development, have been analyzed for their carotenoid and chlorophyll (Chl) contents, in vitro activities of phytoene synthase, phytoene desaturase, and lycopene cyclase, as well as expression of the phytoene synthase (Psy) and phytoene desaturase (Pds) genes. During ripening, the total carotenoids increased with a concomitant decrease in Chl. Although the highest carotenoid content (consisting mainly of lycopene and [beta]-carotene) was found in ripe fruit, the greatest carotenogenic enzymic activities were found in green fruit. Phytoene synthase was located in the plastid stroma, whereas the metabolism of phytoene was associated with plastid membranes during all stages of fruit development. The in vitro products of phytoene desaturation altered from being predominantly phytofluence and [zeta]-carotene in chloroplasts to …