Tissue-Specific Browning in Postharvest Litchi Fruit: Comparative Analysis of the Pericarp, Aril, and Seeds

Although pericarp browning is a major postharvest challenge in litchi fruit, the browning behavior of the aril and seeds remains poorly understood. This study aimed to investigate tissue-specific browning and the underlying mechanisms. Mature litchi fruit were separated into pericarp, aril, and cut seed, stored at 25 °C. Browning occurred predominantly in the pericarp and seeds, while aril showed no visible browning. The pericarp exhibited greater increases in electrolyte leakage (REL) and malondialdehyde (MDA) content than the aril and seeds. The pericarp and seed were rich in total phenolics, flavonoids, and proanthocyanidins (PAs), all of which declined in both the pericarp and seed during browning, whereas PAs were nearly undetectable in aril. Notably, epicatechin (EC) content was higher in the pericarp than in the seed, while catechin (CT) was abundant in seed but absent in pericarp. Laccase (LAC) and peroxidase (POD) activities, along with transcript levels of LcLAC14-4/5, LcLAC7, and LcPOD3, were substantially higher in the pericarp than in the seed, and barely detectable in aril. The results indicate that tissue-specific browning in litchi fruit is associated with enzymatic oxidation of PA-associated phenolics in the pericarp and seeds, offering a mechanistic target for postharvest preservation. Further studies should explore the biosynthetic regulation of PAs in the pericarp and seeds.