| China is the world's largest melon producer.Soluble sugar,organic acid composition and content determine the quality and flavor of fruits.In general,the flavor of melon ismostly sweetbut lack sour.Recently,the melon with the sour flavours is getting more popular for the consumers.In order to investigate the variation of organic acid accumulation in different melon fruits,40 melon germplasms were used as test materials.The contents of soluble sugars in fruits were determined by High Performance Liquid Chromatography(HPLC),and the contents of organic acids were determined by Ion Chromatography(IC).Base on the differentcomponents and contents of the sugars and acids in fruits,8melon germplasms of different flavor fruits were selected,including High-sugar with high-acid(201,892),high-sugar with low-acid(43,677),low sugar with high acid(743,965),low-sugar with low-acid(271,979).For these 8 selected materials,their organic acid components,contents,and the metabolic-related enzyme activities were investigated during the fruits development stage,and the correlation between organic acid components and metabolic enzyme activities in different germplasm was analyzed,in order to understand the mechanism of the accumulation and metabolism of organic acids in fruits,it may provide a theoretical basis for melon breeding in flavor quality.The main results were as follows:1.The organic acid components of melon is mainly composed of citric acid,which accounting for about 75% of the total acid content;followed by malic acid,which about 15% of the total acid content;the rest also contains small amounts of oxalic acid and succinic acid.The soluble sugar components of melon fruit were mainly composed of sucrose,fructose and glucosamine.The contents of sucrose in melon picking stage was the key factors for the good quality.2.In the 8 germaplasms,the citric acid contents was generally low in the early development stage;then it rose rapidly in the middle period,and peaked at 25?30DAP;later,it slightly declined during the mature period.The accumulation of citric acid in the 4 high-acid flavor melons(201,892,743,965)was significantly higher than that in the low-acid melons(43,677,271,979).During the development stage,the contents of the total acids and citric acid in 8 melon germplasms were basically consistent.The contents of malic acid,oxalic acid and succinic acid is getting smaller during the melon development stage.showed a decreasing trend overall.There were no significant difference in the contents of the organic acids during picking stage.It showed that the accumulation of citric acid in the lately development stage was the key factor for the contents of total acids in melon In the 8 germaplasms,7 germplasms were significantly correlated between the citric acid and the total acid content(r=0.790*?0.993**),while the malic acid in 979 was significantly correlated with the total acid(r=0.990**).3.The colorimetric method was used to determine the changes in the activities of enzymes related to organic acid metabolism during the melon development stage,and the correlations between acid-metabolizing enzymes,citric acid content,and the total acid content were analyzed.(1)During the development stage,the activity of Citrate Synthase(CS)in the melon germplasms was continuously increased,and the activity of the high-acid flavor melons CS enzyme was about twice of the low-acid taste in the lately development stage.The change of CS enzyme activity was similar with the citric acid accumulation and the total acid changes.The correlation analysis showed that there was a significant strong correlation relationship between CS enzyme and citric acid content,r=0.532?0.991**(6 germplasms).This indicated that the CS enzyme was the key enzyme in the citrate synthesis of melon fruits(2)The activity of the Cytosolic Aconitase(Cyt-ACO)in low-acid flavor melons increased first and then decreased,while the activity of enzymes in high-acid flavor melons tended to be stable;the activity of Mitochondrial Aconitase(Mit-ACO)in the fruit development stage was getting lower and lower during the melon development stage.The activity of Mit-ACO in low-acid flavor melons was higher than that in high-acid flavor melons at the early fruit development stage and mature stage.Correlation analysis showed that the Cyt-ACO enzymes were stongly correlated with total acid and citric acid,respectively(r=0.911**,r=0.794);there was no significant correlation between Mit-ACO enzymeand citric acid and total acid contents.The acitivities of Isocitrate Dehydrogenase(NAD-IDH)in low-acid melons were significantly higher than that of in high-acid melons at mature stage.There is no significant correlation between NAD-IDH enzyme and citric acid content(r=-0.673 ?-0.722),.This indicated that Cyt-ACO enzyme was closely related to the accumulation of organic acids in melon fruits;Mit-ACO may be involved in organic acid metabolism during the early development of melons.The NAD-IDH enzyme has little effect on the accumulation of organic acids.(3)The activity of Phosphoenolpyruvate Hydroxylase(PEPC)has fluctuated during the fruit development stage.The activity of PEPC in the high-acid flavor melons was higher than that of low-acid flavor melons in the early stage of development;There was no significant difference in the 8germaplasms;Malate Dehydrogenase(NAD-MDH)showed a slowly rising trend in development stage.The activity of enzymes in low-acid flavor melons was significantly higher than that of high-acid flavor melons.There was no significant correlation between PEPC enzyme and citric acid and malic acid levels(r=-0.386?0.708,r=-0.337?0.374).There was a negative correlation between NAD-MDH enzyme and malate content during fruit development(r=-0.803?-0.246).It was suggested that the NAD-MDH enzyme involved in the decomposition of malic acid was closely related to the accumulation of malic acid in fruit.The organic acid component of melon fruit was mainly citric acid at mature stage;the accumulation of sucrose and citric acid was the key factors for the contents of total soluble sugar and total acids in the later developmental period;The CS and Cyt-ACO may affect the accumulation of the citric acid.Meanwhile,the activity of NAD-MDH is related with the decomposition of malic acid. |
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