Green or orange - towards understanding the molecular basis of citrus fruit peel color development.

Peel color development is one of the most important factors to determine commercial quality of citrus fruits. During ripening, fruits of late season varieties such as Valencia orange show in their peel color first degreening, then later sometimes an apparent revert from orange to green. This “regreening” phenomenon appears to correlate with increasing temperature in late spring to summer when fruits remain on the tree, and can cause a significant marketing problem. This color change is known to associate with the conversion of plastids, i.e., from non-photosynthetic chromoplasts to photosynthetic chloroplasts. As an initial step to develop efficient strategies to control citrus fruit peel color development, we analyzed the accumulation pattern of chlorophylls and carotenoids, as well as that of transcripts involved in i) the biosynthesis of the green and orange pigments, and ii) plastid development, in peels from different developmental stages of Valencia orange fruits. First, high performance liquid chromatography (HPLC) analyses revealed that the regreened peel of ripe fruits accumulates chlorophylls a and b, as well as the three major photosynthetic carotenoids, violaxanthin, lutein, and β-carotene, in a roughly similar ratio as does the unripe green peel (compare elution patterns of pigments in RG and MG in Fig. 1).