Identification of volatile compounds and analysis of aroma characteristics in litchi fruits of 6 special-early maturing and early maturing germplasms

Abstract: 【Objective】Litchi germplasm resources are very important for litchi breeding, scientific research, and production. Compared with mid and late maturing germplasms, special-early maturing and early maturing lychee germplasm have smaller proportion of planting area and as maller less number ofvarieties due to their sour and astringent taste. However, due to the early maturity trait, they still have a significant place in the market. Volatile substances affect the flavor quality of fruits. Terpenes are extremely important volatile substances, among them monoterpenes and sesquiterpene have special smell and widely exist in plants. The volatile components are important sources of litchi flavor and resistance. However, there have been a few reports on special-early maturing, early maturing germplasm and wild germplasm of litchi. The purpose of this experiment was to explore the characteristics and differences of volatile components in the fruits of special-early maturing and early maturing litchi, and to provide a reference basis for litchi cultivation, breeding, and processing.【Methods】6 accessions of litchi germplasm were selected and divided into special-early maturing group and early maturing group, namely Xiangmizao, Sanyuehong and Daxinli were in the special- early maturing group, and Feizixiao, Shuidong and Baitangyin in the early maturing group. The volatile compounds in mature fruits were systematically analyzed using headspace solid-phase microextraction (HS-SPEM) combined with gas chromatography- mass spectrometry (GC- MS). Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were used to predict the stability and reliability of the model. The multivariate statistical analysis was used to screen differential metabolites; Hierarchical clustering analysis was used to analyze the metabolites of each group, and the difference of volatile components between the special- early maturing and early maturing germplasm was compared.【Results】A total of 103 volatile components were identified, of them the monoterpenes, alcohols, aldehyde ketones and sesquiterpene were the main components. The alcohols, aldehyde ketones and lipids were the main volatile components of the special-early germplasm, while the terpenoids (monoterpenes and sesquiterpene) and alcohols were the main volatile components of the early germplasm; Feizixiao and Shuidong had the highest cumulative content of volatile compounds, while Baitangyin, Sanyuehong, Xiangmizao, and Daxinlihad had lower content. It was discovered that 10 volatile components were common compounds (ethanol, 1-octen-3-ol, acetaldehyde, hexanal, ethanol, 1-octen-3-ol, acetaldehyde, hexanal, trans-2-Hexen, 1-nonanal, linalool, citronellol, geraniol, (-)-rose oxide) for all the germplasms and 34 compounds were unique compounds to each of the germplasms. Using the OPLS-DA method, 39 differential metabolites were obtained between the special-early maturing group and the early maturing group, among them the downregulated compounds included 3-Methyl-1-butanol, 1-Octanol, Isovaleraldehyde, (E)-2-Octenal, Ethyl Acetate, 3-methylbut-3-enyl acetate, Benzyl acetate, Benzyl valerate, Isoamyl benzoate, α-Cubebene, α-Copaene, Isovaleric acid, Tetradecane, the upregulated compounds included α-Pinene, Myrcene, α-Terpinene, o-Cymene, (+)-Dipentene, Ocimene, β-trans-Ocimene, γ-Terpinen, Terpinolene, Allocimene B, p-Mentha-1, 5, 8-triene, Linalool, β-Citronellol methyl ether, 2- (4- Methylphenyl) propan-2-ol, Pinane-10-ol, α-Terpineol, Nerol methyl ether, Citronellol, iso-Geraniol, β- Citral, Geraniol, Geranyl acetate, (E)- beta- Farnesene, ar- Curcumen, α- curcumene, α- Zingiberene. These differential metabolites combined with specific compounds of different germplasms could be employed as characteristic volatile compounds to distinguish the special-early maturing and early maturing germplasm. Based on the aroma activity value (OAV), 35 compounds contributing to the aroma of 6 litchi germplasm were screened, including 5 alcohols, 9 aldehydes, 3 lipids, 6 monoterpenes, 10 monoterpene oxides, and 2 sulfides. The main compounds were monoterpenes and aldehydes with geraniol having the highest OAV value; 18 key aroma components also obtained: geraniol, dimethyl trisulfide, myrcene, 3-methyl-1-butanol, iso-Geraniol, 1-octen-3-ol, (-)-rose oxide, ethyl acetate, hexanal, 1-hexanol, citronellol, 1- nonanal, isovaleraldehyde, o-cymene, acetaldehyde, phenethyl alcohol, dimethyl disulfide, and linalool, but there were significant differences among different germplasms. 1-octen-3-ol,acetaldehyde, hexanal, 1-Nonanal, (-)-rose oxide, iso-Geraniol and trans-2-hexen were the common aroma compounds in the 6 lychee germplasms, while (-)-rose oxid, 3-Methyl-1-butanol, ethyl acetate, 1- nonanal, and 1-octen-3-ol were key aroma components common in the 3 special-early maturing lychee germplasms, and myrcene, geraniol, 1- nonanal, o- cymene and 1- octen-3- ol were key aroma components common in the 3 early maturing lychee germplasms. Through analysis of aroma characteristics, it was found that six litchi germplasms mainly contained 10 aroma types: floral, fruity, green plant, citrus, woody, and green fruit. In addition, some germplasms also had a small amount of vanilla, cream, nut, and sulfur-containing vegetable flavors; The aroma of the special-early maturing germplasm was mainly fruity, supplemented by floral and green plant aromas. The aroma of the early maturing germplasms was mainly floral type, supplemented by fruity and green plant aromas.【Conclusion】In summary, the volatile metabolites of the special-early maturing and early maturing litchi germplasms had rich diversity. There were significant differences in the volatile and aroma components in litchi fruits with different maturity, and volatile compounds could be used as indicators for identifying different litchi germplasms.