Investigation of chromoplast ultrastructure and tissue-specific accumulation of carotenoids in citrus flesh

Citrus flesh color due to the accumulation of carotenoids in chromoplasts is one of the most important traits for breeding. However, the relationship between carotenoids and chromoplast ultrastructure in citrus is largely unclear. Thus, in this study fourteen representative citrus cultivars including those with yellow, orange, and red flesh were comparatively analyzed by light microscopy, exhibiting three types of chromoplasts (i.e. globular, fusiform and crystalloid). Significant differences in carotenoid content and cell wall thickness were found in subdivided tissues of three grapefruits by high performance liquid chromatography and transmission electron microscopy. Based on our findings, it could be inferred that the accumulation of lycopene was closely related to the formation of crystalline chromoplasts. In the meantime, 'Flame' grapefruit with high content of carotenoids and cells with thin cell wall would have greater bio-accessibility, which indicated that 'Flame' grapefruit was superior to 'Star Ruby' grapefruit in the processing of citrus pulp juice. In conclusion, bio-accessibility influenced by chromoplasts and cell wall should be taken into consideration in citrus nutritional breeding, and differences in carotenoid accumulation and chromoplast morphology with tissue-specific characteristics would provide a new insight for further research.