Integrated metabolomic and structural analyses reveal ripening-driven metabolic landscapes in passion fruit peel: a promising by-product resource

Passion fruit peels have attracted considerable interest due to their edibility and abundant bioactive potential. By employing integrated methodologies, this study systematically investigated the tissue structure and metabolic profiles of passion fruit peel across various ripening stages. Microstructurally, the exocarp underwent progressive lignification, while sclerenchymatous cells exhibited cell wall wrinkling and chloroplast disassembly initiated from the T3 ripening stage. During pre-harvest ripening, a total of 153 differentially accumulated metabolites (DAMs) were identified, with 10 flavonoids, 9 fatty acids, 7 organic acids and 5 amino acids as the key components. These DAMs collectively contribute to the growth promotion and stress resistance of passion fruit peel. In contrast, 176 DAMs with higher diversity were detected during the post-harvest period, among which 17 amino acids, 11 fatty acids, 8 phenols and 7 carbohydrates were the core metabolites. These DAMs were primarily enriched in metabolism associated with color development, flavor formation, and secondary metabolite biosynthesis of passion fruit. Pyruvate and L-glutamate were identified as the core hub metabolites during passion fruit peel ripening, whose levels decreased significantly throughout the post-harvest ripening progression. Targeted quantitative analysis revealed that noradrenaline and malic acid in post-harvest fruit peel were approximately 2-fold and over 20-fold higher than their initial levels in pre-harvest T3 peel samples, which was consistent with the untargeted metabolomic trends. The findings of this study deepen our understanding of the peel's dynamic metabolic changes during ripening and provide a theoretical basis for the targeted utilization of passion fruit peels, as well as for optimizing harvest timing and postharvest preservation strategies.