Integrating Transcriptomics and Metabolomics Analyses to Reveal the Potential Molecular Mechanism of Citrus junos Aroma Enhancement by Protected Cultivation

Protected cultivation is a cultivation practice that plays an important role in improving crop quality. Aroma is an important flavour that assesses the quality of yuzu. In this study, C. junos cv. ‘Kitou’ grown in open fields (CJKTF) and plastic greenhouses (CJKTP) were selected as the study material. Significant differences in aroma performance between CJKTF and CJKTP were found by the olfactory senses of the members of this research group and an electronic nose, with CJKTP having a stronger aroma. Regarding VOCs, GC-MS analyses revealed 13 VOCs up-regulated and 28 VOCs down-regulated in CJKTP compared to CJKTF. Transcriptome analysis revealed that 515 genes were up-regulated and 720 genes were down-regulated in CJKTP compared to CJKTF. The differential VOCs nerolidol and γ-cadinene, and the differential genes nerolidol synthase 1 (NES1), nerolidol synthase 1-like (NES1-like), and cadinene synthase (DCS), were in the sesquiterpene synthesis pathway and showed significant correlation. NES1, NES1-like, and DCS encode terpene synthases, which may be involved in the biosynthetic pathway of nerolidol and γ-cadinene. In conclusion, the use of plastic greenhouses for cultivation may improve the quality and aroma intensity of yuzu, as well as alter the expression of related genes, compared to field cultivation. These results suggest that protected cultivation is a suitable cultivation practice to enhance the aroma of yuzu.