Accumulation and Variation Patterns of Terpenoid Metabolites in Citron Flavedo Based on Widely Targeted Metabolomics

Citron is a medicinal and edible plant. Terpenoids exhibit a wide range of biological activities. However, terpenoids in citron remain underexplored. In this study, a widely targeted metabolomics approach was employed to analyze the accumulation and variation patterns of terpenoid metabolites in citron flavedo at six developmental stages (50, 80, 110, 140, 170, and 210 d after flowering). The results showed that a total of 215 metabolites were detected, with monoterpenes (90) and triterpenes (51) being the major ones, accounting for 41.86% and 23.72% of the total, respectively. A total of 67 differential terpenoid metabolites were identified across different developmental stages. The number of up-regulated metabolites consistently exceeded that of down-regulated metabolites in each comparison group. Bitterness-related triterpenoid metabolites such as limonin, nomilin, obacunone, nomilinic acid, and obacunoic acid peaked at 170 d after flowering but exhibited an overall downward trend at 210 d. Most of the limonoid metabolites were significantly or extremely significantly positively correlated with one another; notably, nomilin displayed strong positive correlations with obacunone, deacetylnomilin, nomilinic acid, and obacunoic acid. This study delivers the first in-depth analysis of terpenoids in citron throughout development stages, laying a foundation for ongoing research and development of terpenoids in citron.