Ascorbate Accumulation And Activity Change Of Related Enzymes During Citrus Fruit Development

The contents of ascorbate including total AsA (T-AsA), L-ascorbic acid (AsA, The reduced form) and dehydroascorbate (DHA, the oxidized form), along with the activities of L-galactono-1,4-lactone dehydrogenase (GalLDH), ascorbic acid oxidase (AAO), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) were assayed during the fruit development in ’Guoqing No.1’ (Citrus unshiu Marc. cv. Guoqing No.1) and ’Newhall’ (C. sinensis Osbeck cv. Newhall). The results showed as follows:1. Ascorbate accumulation during fruit development varied in two citrus species and in different fruit tissues as well. On the other hand, the AsA content in navel oranges was higher than that in the mandarin fruits; AsA contents in peel were higher than those in fruit pulp either in navel oranges or in mandarin fruits.2. In the peel, the changes of GalLDH activity and AsA content showed a similar trend, while in fruit pulp, GalLDH activity almost kept constant during fruit development. Whether in the peel or pulp, the GalLDH activities of ’Newhall’ were significantly higher than those in ’Guoqing No.1’3. Activities of AAO and APX in the peel of’Guoqing No.1’ were significantly higher than those of Newhall, while in the pulp, activities of AAO and APX of’Guoqing no.1’ were significantly lower than those of ’Newhall’. Additionally, no significant difference was found about AAO activity between pulp and peel of’Guoqing No.1’. However in ’Newhall’, activities of AAO and APX in peel were higher than those in fruit pulp.4. In the pulp and peel of each cultivar, the DHAR activity was higher than the MDHAR activity. At each sampling point, activities of DHAR and MDHAR in fruit peel were higher than those in fruit pulp. Comparision between the two species was found that activities of DHAR and MDHAR in peel of ’Guoqing No.1’ were similar as in peel of ’Newhall’; some difference without special regularity were found about MDHAR activity between pulp of both cultivars. Additionally, the activity of DHAR in the pulp of Newhall was higher than that of ’Guoqing no.1’ during the fruit development.5. Taken together, GalLDH might play an important role in the process of AsA accumulation in the peel rather than in the pulp. The differences of AsA oxidase activity were more likely to affect the proportion of AsA and DHA instead of ascorbate total level. On the other hand, higher AsA content in ’Newhall’ than in Satsuma mandarin may be due to its high synthesis ability. However, higher AsA content in fruit peel than in fruit pulp may be related with other factors except for its higher synthesis ability. For example, APX was the major enzyme to oxidate the AsA to DHA in the pulp and may be an important factor to affect AsA content therein.