| In recent years, the investigation on aroma of citrus fruits is mainly concentrated on discovering novel volatiles and establishing new qualitative or quantitative determination methods. In spite of that more than 200 kinds of volatiles have been identified in citrus, the biosynthetic pathway and its regulatory mechanism have rarely been studied.In this study, we attempted to explore the correlation between the biosynthesis of citrus aroma compounds and its related key genes. Cara Cara navel orange(Citrus sinensis Osbeck cv. Cara Cara) and Washington navel orange(Gitrus sinensis Osbeck cv. Washington) were used as experimental materials. Aroma compounds of these fruits were extracted by head-space solid-phase microextraction and determined by Gas Chromatogram/Mass Spectrum. Three key genes (Csgpps. Csdlims and Cslinas) in the biosynthetic pathway have been cloned. Furthermore, the expression of these genes during citrus fruits maturation was analyzed by real-time quantity PCR. The main results show as follows:1. The content of d-limonene from citrus flavedo and pulp was determined by SPME-GC-MS, which used cyclohexanone as internal standard as well as d-limonene as external standard. The content of d-limonene from Cara Cara navel orange flavedo displayed a tendency that decreased at the early mature stage and raised gradually at the middle mature stage, then dropped again at the last stage. While the content of d-limonene from its pulp maintained a stable level at the early mature stage, and then soared at the last stage. As for the Washington navel orange, the content of d-limonene from its flavedo and pulp shared an almost identical trend, which increased at the early mature stage and then decreased. This indicated that the aroma in different citrus varieties presented diverse variation patten. While the aroma in different tissues from the same variety shared the similar or distinct variation patten. 2. Contents of the main volatiles from Cara Cara and Washington navel orange flavedo and pulp during different stages of maturity were determined using semi-quantitative method by SPME-GC-MS, which utilized cyclohexanone as the internal standard. The main volatiles a-pinene,β-myrcene, r-terpinene from Cara Cara navel orange flavedo had the similar variation patten during fruits maturation, and all of them reached a maximum content at 210 days after flowering. However, the variation patten of B-linalool were contrary to the above three kinds of volatiles. a-pinene,β-myrcene, r-terpinene and valencene from Cara Cara navel orange pulp also showed the similar trends, which decreased rapidly to zero at 220 days after flowering.α-pinene, B-myrcene, and r-terpinene from Washington navel orange flavedo presented a linear increase during fruits maturation, whileβ-linalool declined slightly during 200 days to 210 days after flowering.α-pinene, B-myrcene and r-terpinene from Washington navel orange pulp decreased suddenly during 190 days to 200 days after flowering and then remained unchanged, while valencene showed a different variation patten from the above three kinds of aromas. In addition,β-linalool was not detected in neither Cara Cara nor Washington navel orange pulp.3. Expression of key genes for volatile biosyntheis in citrus was analyzed by real-time quantitative PCR. The results declared that the expression variation pattern of Csdlims gene was in accordance with the variation patten of its product during fruit maturation both in Cara Cara and Washington navel orange pulp. The expression of Cslinas gene was very low in the above two citrus varieties, which could be regarded as nearly no expression. Similarly, the product of Cslinas gene was not determined in both of the above citrus. This implied that the content of main volatiles in citrus was closely connected with the expression of its key genes. |
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