Effects of ethephon on volatile metabolites in Shushanggan apricot fruit

【Objective】Shushanggan apricot is a main cultivar with Xinjiang characteristics. It is popular with consumers because of its suitable sugar-acid ratio, unique flavor and rich nutrition. As one of the key indicators to judge fruit quality, aroma has always been concerned. As the Shushanggan apricot enters the mature stage, its internal aroma components undergo a series of complex changes over time, which is directly related to its economic value. Ethephon is a synthetic plant growth regulator with high quality, high efficiency and low toxicity. Studies have shown that it can improve fruit flavor, promote aroma formation, and make fruits emit more intense aroma. This study focused on the effect of ethephon treatment on the aroma changes of Shushanggan apricot, and analyzed the relationship between ethephon treatment and the aroma components, so as to screen out the best ethephon treatment concentration to maintain the aroma of Shushanggan apricot fruit.【Methods】In this experiment, Shushanggan apricot was selected as the test material. In the late stage of the hard-core period, the ethephon solution with a concentration of 100 mg·L-1 (ST1), 200 mg·L-1 (ST2) and 300 mg·L-1 (ST3) was evenly sprayed on the fruit surface, and the spraying water was set as the control (SCK). When spraying, it was necessary to ensure that the spraying site was completely wet until the liquid dropped naturally. Apricot fruits were sampled when they reached maturity and stored in a refrigerator at -80 ℃. Headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS) were used to identify and analyze the volatile metabolites at maturity. The effects of ethephon treatment on volatile metabolites of Shushanggan apricot fruits at maturity were comprehensively analyzed by cluster analysis, principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA). 【Results】In the detection of volatile metabolites at the mature stage of Shushanggan apricot fruit, a total of 16 categories of volatile metabolites were identified, including 691 volatile metabolites, of which terpenoids, heterocyclic compounds and esters accounted for the highest proportion in the composition of volatile metabolites. Under four treatments, there were 651 common volatile metabolites in the samples, and 656, 679, 689 and 685 volatile metabolites were identified in SCK, ST1, ST2 and ST3, respectively. The relative contents of volatile metabolites were: 330.20 μg · g- 1 , 442.27 μg · g- 1 , 477.82 μg · g- 1 , 563.89 μg · g-1 , which showed that compared with the control group, the fruit treated with ethephon was not only more abundant in volatile metabolites, but also more abundant in relative content. Moreover, the content of volatile metabolites showed an increasing trend with the increase of ethephon treatment concentration. The relative odor activity value (rOAV) was calculated by combining the threshold value, and 124 components with rOAV greater than 1 were identified, which were identified as the key aroma components of Shushanggan apricot fruit. Among them, there were 104, 117, 117 and 119 substances with rOAV greater than 1 in SCK, ST1, ST2 and ST3, respectively. Among them, 5-Ethyl-3-hydroxy- 4-methyl-2(5H)- furanone, Benzenemethanethiol, 3-Mercapto-3-methylbutyl formate, Dodecanenitrile and 2,2,6-Trimethyl-cyclohexanone had the highest content, which contributed greatly to the overall aroma. Therefore, these aroma substances may be the key aroma components in the fruit of Shushanggan apricot, and the rOAV of ST1, ST2 and ST3 in the ethephon treatment group was higher than that of SCK. From high to low, it showed ST3＞ST2＞ST1＞SCK. Due to the differences in key aroma components, the aroma characteristics of the extracts under the four treatments were also different, each showing a unique flavor, providing consumers with a richer choice. Based on OPLS-DA analysis, it was found that there were robust separations between different ratio treatment groups, and 409 different aroma components were screened. SCK, ST1, ST2, and ST3 contained 391, 400, 409 and 409 species, respectively. With the increase of ethephon concentration, the contents of differential metabolites showed a gradual upward trend. Among them, the number of different components of heterocyclic compounds, terpenoids and esters metabolites was more, and there was no difference in halogenated hydrocarbons. Components and the contents of differential metabolites showed a gradual increase with the increase of ethephon concentration, among which (R)-3,7-Dimethyl-6-octen-1-ol, 5-Ethyl-2(5H)-furanone, 2,2,4- Trimethyl- 1,3- pentanediol diisobutyrate, E- 2- Octenal, (E, E)- 2,4- Undecadienal only existed in ST1, ST2 and ST3, and their rOAVs were all greater than 1. These were unique aroma components in Shushanggan apricot fruit after ethephon treatment.【Conclusion】In summary, as a plant growth regulator, ethephon can play an important role in the growth and development of Shushanggan apricot, and has a significant effect on the increase of volatile metabolite content, so that the aroma in the fruit is more intense, and the overall quality of the fruit is improved from the perspective of aroma quality. Among them, the promotion effect of 300 mg·L-1 ethephon treatment is the most ideal. The findings provide a scientific strategy for staggered harvesting, shelf-life extension, and flavor regulation. Ethephon likely activates enzymatic pathways (e.g., LOX, HPL or AAT) involved in aroma biosynthesis.