Effect Of Grafting On Biosynthesis Of Esters Through Amino Acid Pathway In Oriental Sweet Melon

The oriental sweet melon (Cucumis melo var. makuwa Makino)'Yumeiren'as scion, and white seed pumpkin'Shengzhen No.l'as rootstock, both extensivelly cultivated in northeast of China, tongue grafting approach was adopted. The fruits of own-root plant were used as control. This experiment was focus on effects of grafting on aromatic compositions and contents, amino acids contents and activities of aroma-related enzymes, including alcohol dehydrogenase (ADH) and alcohol acyl transferase (AAT) at different development stages in peel and flesh tissues of oriental sweet melons. Effects of grafting on various aspects of branched-chain ester (i.e.2-methyl-l-butyl acetate) biosynthesis were investigated in peel and flesh tissues of melons using fruits incubated in vivo bioconversion precursors synthesized in different stages, L-isoleucine,2-methylbutanal and 2-methylbutanol. Then through immersion of exogenous L-isoleucine in fruits, effects of exogenous incubated amino acid on contents of related ester were studied. With a view to further control the grafting melon flavor quality and provide a theoretical basis. The main results showed that:1. Aroma compounds in peel and flesh tissues of'Yumeiren'fruits at different development periods were detected and quantitatively analysis using SPME-GC-MS after grafted. Varieties and total content of esters increased gradually with the maturation of fruits, while alcohols and aldehydes followed opposite trend in peel and flesh and no major variation in acids and hydrocarbons were not found. Acetic esters predominated among esters, accounting for 64.67-85.85%of the total content of esters. Effects of grafting on varieties and content of esters were different in peel and flesh tissues during fruit development. For example, acetic esters and total content of esters decreased by grafting in peel and flesh tissues. But content of butyrates and caproates increased in peel, while no significant variation obtained in flesh. The varieties and contents of the esters were higher in peel than in flesh. But varieties of alcohols, acids and hydrocarbons followed the opposite trend.2. After detected and analyzed,7 kinds of characteristic aroma volatiles including ethyl acetate, hexyl acetate, phenylmethyl acetate, phenylethyl acetate,2-methyl-l-butyl acetate, ethyl 2-methylbutanoate and 2-methyl-l-butanol obtained in peel and flesh at maturity period (36 days after anthesis). These characteristic aroma volatiles make a great contribute to the flavor of melons. But we found three kinds of compounds vanished in peel after grafting, and there were no significant variation in flesh. The odor values also reduced in peel and flesh tissues of melons after grafting.3. Four characteristic esters (i.e. ethyl acetate,1-butanol,2-methyl acetate, hexyl acetate and phenylmethyl acetate) were detection and quantitative analysis. The results found that these four kinds of esters content in the peel and flesh tissues were changed and influence differently by grafting. Grafting increased ethyl acetate content in peel and flesh tissues and decreased hexyl acetate and phenylmethyl acetate content.2-methyl-l-butyl acetate content in peel tissue increased, while that declined in flesh tissue in grafted melons. In addition, the esters contents in peel tissue were higher than in flesh at maturity.4. Sixteen kinds of free amino acids were detected by antomatic acid analyzer in peel and flesh tissues of melon. Amino acids content gradually increased with fruits development, and the content in grafted melons was higher than own-rooted fruits at early stages; however, which lower than own-rooted fruits at middle and later stages. The biosynthesis relationship between valine, leucine and isoleucine and 2-methyl-1-butyl acetate were validated.5. The ADH activity decreased, while the AAT activity increased gradually in peel and flesh tissues with fruits development. The enzymes activities in peel (P<0.01) and flesh tissue (P<0.05) of ADH and AAT singnificantly decreased in grafted melons. In addition, the enzymatic activities in peel tissue were higher than in flesh during fruits development.6. The different stages of 2-methyl-1-butyl acetate synthetic precursors were added to peel and flesh tissues incubated at 30 days after anthesis respectively. The results showed that esterification of 2-methylbutanol and conversion of L-isoleucine to 2-methylbutanal and 2-methylbutanol in peel tissue were reduced by grafting, while the transformation from 2-methylbutanal into ester were uninfluenced. In flesh tissue, conversion of aldehyde, alcohol and L-Isoleucine to 2-methyl-1-butyl acetate were all uninfluenced. It showed effects on grafting of biosynthesis mechanism were different between oriental sweet melon peel and flesh tissues. These findings also indicated grafting through conversion of amino acid to its corresponding branched-chain alcohol and branched-chain aldehyde to its corresponding ester, were differentially affected 2-methyl-1-butyl acetate biosynthesis by grafting in oriental sweet melon.7. Own-root and grafting oriental sweet melons (28 days after anthesis) were immersed with exogenous L-isoleucine, and then variations for content of aromatic compounds were detected in peel and flesh. The results suggested that content of butyrates and butyl esters with branched-chain after treatment increased markedly, especially for butyl 2-methyl acetate, while, little effect on quantity of acetate and propionate esters. These findings indicated L-isoleucine could enhance content of branched-chain butyrates and butyl esters through conversion of amino acids, while little effect on other esters. Moreover, content of esters were higher in own-root than in grafting. The content of butyrates and butyl esters in peel were higher than in flesh. AAT activity increased significantly after exogenous L-isoleucine- soaked. This indicated L-isoleucine could induce this enzyme activity and promote the biosynthesis of branched-chain butyrates and butyl esters.