| The melon production plays an important role in the horticultural industry of the world, and China is one of the great melon-producing countries. The fruit quality became more and more cruial to consumers with the improvement of living standards of people. To improve the fruit inner quality is the key to raise melon quality, and the compositions and contents of sugars and acids impact the melon inner quality significantly because they reflect an important indicator of fruit flavor. Cucumis melo L. Flavor Melon series were a kind of novel melon and bred out by Xinjiang Academy of Agricultural Science. These melon fruits cause both high sugar and high acid accumulation. In this reserach, the leaves and fruit flesh of Flavor Melon series ('Flavor No.2','Flavor No.3', and'Flavor No.4'), some parents of Flavor Melon ['Suangua'(high organic acid accumulation-type) and'Shouxing'(low organic acid accumulation-type)] and some traditional melon cultivars ('Xueli'and'Xuelihong') as the experiment materials, in order to conduct a combined physiology, biochemical and gene expression study to understand the differential accumulation of sugar and acid in melon fruits and to explore the biological basis of the formation of high sugar and high acid contents. The main results were as follows:1. The observation of fruit characteristics of Flavor Melon revealed that the fruit of 'Flavor No.2'obtained the best quality during 30-35 DAP, involving fresh weight, size, pulp thickness, contents of sugar and acid, and some other index. The sour tasted fruit was the novelty of'Flavor No.2', and this was resulted from the accumulation of citric acid.2. The observation of photosynthetic characteristics of Flavor Melon and control cultivars revealed that there were some differences in different melon cultivars. The net photosynthetic rate of'Flavor No.3'and'Xueli'in the initial stage of fruit development were higher than those in vegetative growth stage, instead'Xuelihong'was lower. In the middle stage of fruit development, the net photosynthetic rate of three cultivars were all lower than those in the initial stage, additionally the decline in'Flavor No.3'was the least. The SPAD value of 'Flavor No.3'was higher than those of control melons, and this was an obvious appearance suggesting its higher photosynthetic rate. The declines of photosynthetic rate in three cultivars were because of the stomatal limitation in different degree. In the middle stage of fruit development, the intercellular CO2 concentration of 'Flavor No.3'was significant lower than the control melon, this indicated that the carboxylation efficiency of Flavor No.3'was higher, and more CO2 was fixed, so more photosynthate were supplied for the fruit development.3. GC and real-time PCR techniques were used to determine the main soluble sugar content, sugar metabolic pathway key enzyme genes expression and activity in Flavor Melon and control cultivars at different developmental stages. The results showed that there was a large number of sucrose accumulation when 15-20 d before melon fruit got mature. And sucrose is the major soluble sugar in mature fruit. In'Flavor No.4'and 'Suangua', SPS gene expression and enzyme activity consistent with the accumulation of sucrose, while the'Shouxing' performed a very low activity and high sucrose content of AI during the entire development. Therefore, it is likely that there are two mechanisms for the accumulation of sucrose, one is the accumulation of sucrose is along with the increasing SPS activity, the other is the accumulation of sucrose occurs when AI activity was below a certain threshold.4. GC and real-time PCR techniques were used to determine the dominant orgaic acids content, acid metabolic pathway key enzyme genes expression and activity in Flavor Melon and control cultivars at different developmental stages. The results revealed that citric acid is the major organic acid in mature fruit. During 5-25 DAP, the ACO and ME activities were correlated to the citric acid content, and the activities in 'Flavor No.3' were hgher than those in 'Xuelihong', so the citric acid content was higher in the former. The activities of citric acid metabolic pathway key enzymes in'Flavor No.4'and 'Suangua' were mostly higher than those in'Shouxing', as a result, citric acid increased. However, the sucrose content in'Shouxing'was the highest, and followed by'Flavor No.4'and'Suangua', and the acid content was inverse。So it suggested that the Krebs cycle was accelerated and more photosynthate were subjected to the partitioning in the citric acid metabolism in acid-accumulation melons. 5. The volatile substances in mature Flavor Melon and control cultivars fruits was measured by GC-MS. The results showed that, the representative volatile substances in mature'Flavor No.2'fruit were diethyl phthalate, dimethyl phthalate, and (E, E)-2,4-decadienal; and the volatile substance in'Flavor No.3' and 'Xueli'fruit were mainly esters and ketones, which accounted for more than 70% of the total content of volatile substances, and the rest were alcohols, aldehydes, sulfur-containing compounds and so on. The ester content was 43.03% in 'Flavor No.3', and 65.61% in 'Xueli', while the ketone content was 30.08% in 'Flavor No.3', and 10.07% in 'Xueli'.4-Hydroxy-2-butanone and 1,2-benzenedicarboxylic acid butyl 2-methylpropyl ester were the main volatile substances is'Flavor No.3'.Taken together, the quality of Flavor Melon fruits are dramatically different from the traditional cultivars and its parents, and the formation mechanism is speculated in two aspects as follow:on the one hand, the photosynthesis of leaves are stronger, so they could supply more photosynthate for fruit development; on the other hand, as there are more photosynthate in fruit, some are metabolized to be sugars and accumulation, at the same time, some assimilates are subjected to the partitioning in the citric acid metabolism, and as a result citric acid increased in amount as an intermediate product in the cycle.
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