Blue LED Light Promoted the Formation of Anthocyanin-Rich Red Halo During Penicillium expansum Infection of Apple

Blue light-emitting diode (LED) light can suppress the postharvest pathogen Penicillium expansum (P. expansum), but its effects on the phenolic response of infected apple skin remain poorly understood. In this study, ‘Idared’ apples were inoculated with P. expansum and incubated under continuous blue LED light irradiation or in darkness. Perilesional skin tissue was sampled at 2 days post-inoculation (dpi), while at 7 dpi, both perilesional tissue and an adjacent outer ring were sampled. Twenty phenolic compounds were quantified by UPLC-MS/MS and UPLC-PDA-QDa. A distinct red halo developed around inoculation sites only under blue LED light. At 2 dpi, the total concentration of quantified phenolics in apple skin was 22.6% higher under blue LED light than in darkness. Cyanidin 3-O-galactoside was 10.3-fold higher. Three minor cyanidin glycosides were detected only in irradiated tissue. At 7 dpi, cyanidin 3-O-galactoside was 5.4-fold higher in irradiated than in dark-incubated perilesional tissue. Chlorogenic acid was strongly concentrated in perilesional tissue relative to adjacent outer tissue, with concentrations 3.7-fold higher under blue LED light and 2.5-fold higher in darkness. In contrast, several flavan-3-ols, procyanidins, phloridzin, and naringenin 7-O-glucoside were more abundant in the adjacent outer tissue. Thus, total phenolic concentrations did not capture these contrasting spatial patterns. The red halo was associated with a localized anthocyanin signature within a broader, compound-specific phenolic response to P. expansum infection under blue LED light.