Studies On Volatile Components And Metabolism Of Muskmelon (Cucumis Melo L) During Fruit Development

Volatile component is one of the most important factors determining muskmelon fruit quality. Cv.'Shannong Golden 1', cv.'Shannong Cuibai 1', S17, cv.'Cantaloupe Iroquois', cv.'Cantaloupe Imperial 45', cv.'Sweet Delight', cv.'Amusu', cv.'Honoka'and cv.'Takami'muskmelon varieties were grown in a solar greenhouse in the Horticulture Experimental Station of Shandong Agricultural University from 2006 to 2007. Volatile compositions in mature fruits and during fruit development of different varieties, ethylene production effects on fruit volatile composition during different muskmelon fruit development, and volatile metabolic precursors (fatty acids, amino acids, sugar) and lipoxygenase (LOX), alcohol dehydrogenase (ADH), alcohol acyltransferase (AAT) and other related metabolic activities in the volatile formation in the experiment were studied. The main results were as follows:1. Different muskmelon cultivars fruits present different flavors owing to different aroma components and contents in their fruits. 204 volatile components were separated from all of the nine muskmelon cultivars. Esters were the major aroma substances in cv.'Shannong Golden 1', cv.'Shannong Cuibai 1', S17, cv.'Cantaloupe Iroquois', cv.'Cantaloupe Imperial 45'and cv.'Honoka'fruits. Aldehydes and some alcohols were the major aroma substances in cv.'Sweet Delight'ripe fruit. Hexanal, acetic acid phenylmethyl ester volatilizing the green note were the major aroma in cv.'Amusu'and cv.'Takami'Japanese muskmelons and their fruits smelled non-aromatic.2. Volatile component and content is different significantly in the course of muskmelon fruit development. C6, C9 aldehydes were the main volatile components in cv.'Shannong Golden 1'immature fruit. Whereas, esters, including methyl acetate, acetic acid methyl benzene, were the main volatiles in cv.'Shannong Golden 1'mature fruit. On the contrast to cv.'Shannong Golden 1', C6, C9 aldehydes and alcohols were the main volatiles in cv.'Sweet Delight'30 DAP and 45 DAP fruit, and aldehydes were the main volatiles in the ripe fruit. Alcohols and aldehydes were the main volatiles in cv.'Takami'30 DAP fruit. And aldehydes and esters were the main volatiles in the ripe fruit.Esters including methyl acetate and butyl acetate, acetic acid-3-methyl-1 -butyl acetate, methyl benzene acetic acid were the main aroma components in cv.'Shannong Golden 1'fruit. Acetate methyl benzene was the main aroma component in cv.'Sweet Delight'muskmelon fruit. Esters including acid-2 -methyl propyl acetate-3-methyl-1-butyl acetate methyl benzene, acetic acid-2 -ethyl benzene were the main aroma components in cv.'Takami'fruit. In addition, the detection of sulfur- compounds such as the 2-(methylthio)-methyl acetate were the main aroma components.3. Ethylene production plays especially important role in fruit ripening. In our work, we observed the level of ethylene production increased significantly with fruit ripening in cv.'Shannong Golden1'and the level of esters increased significantly with the ethylene increase. On contrast, ethylene production was very low and keeping almost still during the step of cv.'Sweet Delight'fruit ripening and the levels of esters, alcohols and aldehydes changed slightly. In addition, the ethylene production kept intermediate in cv.'Takami'with fruit ripening compared to the above two varieties, and some esters formed and increased with the fruit ripening and some esters and aldehydes were the major volatile when the fruit ripening. The results suggest that the faster ripening rate that was associated with a sharper peak of ethylene production and respiration climacteric, the more concentration of volatile esters in muskmelons.4. The trend that fatty acid content appeared lower - increase - to reduce in cv.'Shannong Golden 1'fruit is a complex physiological and biochemical metabolic process. And the effect of fatty acids on the aroma formation was appeared in the process of fruit ripening. Oleic acid, linoleic acid and linolenic acid levels increased significantly in the 25 DAP fruit, and they were generated to alcohols, aldehydes esters on the action of LOX, ADH, AAT and acyl-CoA. The levels of oleic acid, linoleic acid and linolenic acid were lower and the straight-chain esters were absent during cv.'Takami'fruit development. Fatty acids contents decreased and LOX activity increased during cv.'Sweet Delight'fruit ripening, but the esters were in a very small amount because of the low activity of AAT.5. The levels of some amino acids participating in volatile formation, such as Isoleucine, Leucine, Phenylalanine, were high in cv.'Shannong Golden1'15 DAP fruit, but then decreased significantly in 20 DAP fruit except Tyrosine and kept stability in 25 DAP fruit. And then the levels decreased insignificantly except Cystine until ripe fruit. In contrast, the levels of some amino acids participating in volatile formation were low in cv.'Takami'15 DAP fruit, and kept stability in 20 DAP fruit except Valine and Leucine had higher levels. But the levels decreased until ripe fruit. For cv.'Sweet Delight'muskmelon, the level of the amino acids relating to volatile formation changed different from the kinds of amino acids during fruit development. The levels of all kinds of amino acids were higher in the 15 and 30 DAP fruit. The levels decreased until fruit ripe except the levels of Isoleucine and Phenylalanine keeping stability in the 45 DAP fruit.6. Sugar content was higher in cv.'Shannong Golden 1'25 ~ 30 DAP fruit, at this time lots of esters were generated by alcohols and acyl-coa derived from fatty acid, amino acid and sugar metabolism, in a reaction catalysed by the enzyme alcohol o-acyltransferase . The sugar was increased significantly in cv.'Sweet Delight'45 ~ 55 DAP fruit, but the esters were absent because of the lower levels of ADH and AAT activities, and the lower level of ADH caused the lower levels of esters in cv.'Takami'fruit.