| China is the largest country of sweet melon (Cucumis melo L.) fruit production that accounted for more than 40 percent of total growing acreage and yield in the world. However, the poor quality of sweet melon fruits give rise to a great reduction in the economic benefit. With the increasing living standard and abundance of various fruitage supply, there is a large demand for higher quality sweet melon fruits . The quality of sweet melon is mainly determined by sugar contents because it affects the tastes and color of fruit and serve as a precursor as well for synthesis of organic acid, vitamin and carotenoids. Therefore, it is very imperative to study the mechanism of sugar accumulation for the ultimate goal of improving the quality of sweet melon.Four genotypic sweet melon varieties were used as research materials. The experiments were carried out in 2001 and 2002 at Department of Horticultural Science, Zhejiang University. The genotypic difference in sugar-accumulating patterns and its related sucrose metabolizing enzymes were systemically analyzed for elucidating the physiological aspects that are possibly responsible for the difference. The main results were as follows:By comparing two netted melon genotypes, the result showed that 'Crystal' has the higher sweetness than 'Nayiduo' at maturity. Using the mesocarp as example, the sweetness index (Fructose 200+Glucose 100+Sucrose 145) of 'Crystal' was 11306.45 and the sweetness index of 'Nayiduo' was 10734.25. Meanwhile it was 8339.05 and 4362.75 in 'Huangjingua' and 'Yuegua' respectively. The contribution ration of sucrose to sweetness index were 57% and 35% in 'Crystal' and 'Nayiduo' mesocarps respectively. Meanwhile it was 32.5% and 0.5% in 'Huangjingua' and 'Yuegua'. These findings suggested that the sucrose level rather than the hexose content, in our studies, was the main determinant of sweetness in different genotypic sweet melon.After hand-pollination, the contents of glucose and fructose increased stably and were the predominant soluble sugars detected in flesh of melon before sucrose accumulation. They had no apparent differences in contents and had very similaraccumulating patterns. The accumulation of sucrose occurred in mid-stages and latter stages of fruit development. The genotype with high sucrose accumulation had much intensive and faster sucrose accumulation with much higher sucrose content at maturity. There was a gradient in sucrose content from umbilicus, mesocarps to pedicel parts in sucrose accumulation genotypes. In addition, the pedicel part had much higher sucrose content than any other parts, implying that the rate of sucrose accumulation for each part was varied, which maybe due to have different changes of enzymes related to sucrose metabolism in different tissue parts.In early stage, the sucrose cleavage enzymes behaved much higher activities than sucrose synthetic enzymes. The net activities between synthesis and cleavage had higher negative value. The photosynthetic substances translocated were decomposed, consumed and utilized, and the rate for sucrose accumulation was slower. In late developmental stage, the sucrose was quickly accumulated as a result of the enzymes of sucrose cleavage activities were very low and the enzymes of sucrose synthesis activities were high at this stage. In the present investigation, there was no or very low sucrose accumulation when the net activities was negative. After net activities was changed to positive, sucrose accumulation was enhanced.The sucrose phosphate synthase activity coincided with sucrose accumulation. It increased earlier in activity and was significantly enhanced during late developmental stage in genotype "Crystal" with higher sucrose-accumulation than "Nayiduo" with lower accumulation. In oriental melons, the SPS activity in sucrose genotype increased earlier and reached maximum level at maturity. But in minor-sucrose genotype, its activity was reduced to very low level after anthesis and retained at low level. The alteration of SPS activity had a paral...
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