Comparison of ascorbate metabolism in fruits of two citrus species with obvious difference in ascorbate content in pulp

Citrus fruit is widely consumed and provides ascorbate for human health. The ascorbate content in pulp is generally higher in orange (Citrus sinensis Osb.) than in Satsuma mandarin (Citrus unshiu Marc.). However, what contributes to such difference is still unknown. In the present study, ascorbate accumulation, expression profiles of genes involved in l-galactose pathway and activity changes of enzymes related with l-ascorbic acid (AA) oxidation and recycling were investigated during fruit development and ripening in fruit pulp of Satsuma mandarin and orange. As fruit ripens, total ascorbate (T-ASC) or AA content increased in mandarin whereas fluctuated on a relatively high level in orange. Concentrations of T-ASC or AA in pulp of orange were over 1.5-fold higher than that in pulp of Satsuma mandarin during fruit ripening. Further analysis showed that each transcript of four genes (encoding GDP-d-mannose-3′,5′-epimerase, GDP-l-galactose-pyrophosphatase, l-galactose dehydrogenase and l-galactono-1,4-lactone dehydrogenase respectively) in orange was almost on a higher level and the activities of oxidation enzymes (ascorbate oxidase and ascorbate peroxidase) were lower during fruit ripening as compared with Satsuma mandarin. As ascorbate pool size is decided by the combination of biosynthesis, oxidation and recycling, therefore, higher expression of four genes along with lower activity of oxidation enzymes should contribute at least partially to the higher ASC accumulation in orange pulp.