Relationship Of Citrus Fruit Color And Carotenoid Accumulation

Carotenoids are the principal pigments of citrus fruits and the content and profile differ in different species and tissues. Most genes involving in the biosynthesis pathway have been cloned and study of gene expression patterns shows their function and relationship with citrus carotenoids biosynthesis. Lycopene-Accumulating Mutants are big group of citrus Mutants, the reason accounts for the accumulation of lycopene has remained unknown till now and further research is still working on. The effects of genetics, development, chemicals and environment on biosynthesis of citrus carotenoids were also reviewed.The relationship between the color of citrus fruits and accumulation were studied by research on the Citrus iyo and citrus red mutants.For Citrus iyo high content of chlorophylls in young fruit was responsible for its green color. Intensive increase in content of carotenoids and rapid loss of chlorophylls commenced at 8 and 4 weeks before harvest respectively, and both changes contributed to yellow color development in ripening fruit. 83.62% of total fruit carotenoids was found located in the peel, where carotenoid content was 16.62 times as high as that in the pulp. Composition of carotenoids in the peel within 12 weeks before fruit harvest was analyzed using high performance liquid chromatography - photodiode array detection (HPLC-PDAD) . 23 carotenoids were separated and 7 of them were identified. Among identified carotenoids, β-cryptoxanthin and zeaxanthin were relatively abundant, ranking 5th and 7th, and accounting for 8.00% and 6.78% of total carotenoids respectively;β-carotene was less than 1% of total while α-carotene and lycopene were undetectable. The increase in content of β-cryptoxanthin and zeaxanthin was accompanied by the decrease in content of lutein, α-carotene and β-carotene during late fruit development, indicating that carotenoid β pathway and β-ring hydroxylation activities were enhanced. Meanwhile, the appearance and accumulation of apo-carotenoids, characterized by short retention time and low maximum absorbance wavelength, suggested an increase in carotenoid cleavage activities as the fruit ripened.High performance liquid chromatography, coupled with photodiode array detection, was used to analyze the carotenoid composition of peel and juice vesicle tissues of ordinary and lycopene-accumulating mutants (referred to as red mutants in this article) of orange, pummelo and grapefruit. Thirty-six major carotenoids, including some cis-trans isomers, were separated on a C₃₀ reversed-phase column and 23 of them were identified on the basis of retention times and spectral characteristics with authentic standards. Carotenoid profiles varied with tissue types, citrus species and mutations. β-Citraurin occurred in the peel of oranges but not in juice vesicles, whereas the reverse was found for violaxanthin, 9-cis-violaxanthin and luteoxanthin. The diversity of carotenoids in peel and juice vesicle tissues and the fact that there was over 250 times higher content of total carotenoids in peel of Yuhuan pummelo than juice vesicles suggested that the biosynthesis of carotenoids in these two tissues was independent and exchange of carotenoids between the tissues was not likely. Lutein was observed in peel of pummelos and grapefruits and juice vesicles of ordinary pummelo but not in orange tissues. Accumulation of lycopene and β-carotene was observed in red mutant citrus, except for the peel of Cara Cara red orange. Additionally, phytoene accumulated in all tissues except for thepeel of Chuzhou Early Red pummelo. No obvious change in total content of xanthophylls was observed in the Cara Cara red orange. Ordinary grapefruit (Marsh) tissues and pummelo (Yuhuan) juice vesicles were almost devoid of carotenoids and in red mutants, the content of total carotenoids increased dramatically up to 790 fold. The different changes in carotenoid content and profiles in mutant (s) of different citrus species suggests that the underlying mechanisms for the mutations might be different.'Cara Cara navel' orange (citrus sinensis Osbeck) is a bud mutation of 'Washington Navel' Orange. The pigment which accounts for the red flesh of Cara Cara is lycopene and very high content of (3-carotene also exists in the juice vesicle comparing to the parental variety. Lycopene p cyclase genes get involved in the conversion from lycopene to |3-carotene in plants. In this research, we cloned two LCY-b genes in both yellow and red-fleshed genotypes. They shared 98.9% identities with each other. The sequencing results showed high identities with all citrus LCY-b genes in GenBank and functional analysis of both amino acid of ORFs showed that both of them were normal and active LCY-b enzymes. Considering the complicated relationship between citrus species they are maybe alleles. Real-time PCR results revealed similar levels of expression at ripen stage of fruits for both yellow and red-fleshed genotypes. From all the results above we speculate that the accumulation of lycopene in the flesh of Cara Cara does not connect to LCY-b gene.