Developmental Dynamics of Anthocyanin and Carotenoid Accumulation and Associated Gene Expression in Two Red-Fleshed Apple Cultivars

Fruit color, a key quality trait in apples, is primarily determined by anthocyanin and carotenoid accumulation. Although the regulation of pigmentation in red-skinned apples has been extensively investigated, comparative information regarding pigment dynamics in red-fleshed cultivars during fruit development remains limited. In this study, two red-fleshed apple cultivars, ‘Okanagan’ and ‘Pink Wood,’ were examined at five developmental stages, 30, 60, 90, and 120 days after full bloom (DAFB), and at harvest (135 DAFB), to evaluate changes in peel and flesh coloration, pigment accumulation, and expression of genes associated with anthocyanin and carotenoid biosynthesis. Both cultivars exhibited peak peel redness during early fruit development. The peel anthocyanin concentration was highest at 30 DAFB. Anthocyanin accumulation in flesh tissues was comparatively low but increased slightly during later stages. Transcripts of anthocyanin pathway genes and the regulatory transcription factor MdMYB10 were abundant in peel tissues during the early stages. Strong positive correlations were observed between anthocyanin-associated gene expression, peel and flesh redness (a*), and anthocyanin concentration. ‘Pink Wood’ exhibited stronger red pigmentation in flesh tissues at harvest. These findings provide comparative insights into the cultivar-dependent mechanisms regulating fruit coloration in red-fleshed apples, and may support breeding strategies targeting enhanced visual and nutritional quality.