Temporal changes in the proanthocyanidins to anthocyanins ratio during dormancy associate with bloom time variations in peach

Our previous research demonstrated that fall applications of ethephon, an ethylene-releasing plant growth regulator, delay bloom in peach, accompanied by changes in endogenous hormones, ROS, sugar metabolism, and transcriptomic profiles during bud dormancy phases (endodormancy and ecodormancy). In this study, floral bud tissues were collected from ethephon-treated and untreated trees at three time points (200, 600, and 1000 chilling hours, CH) during endodormancy and two points (1000 and 3000 growing degree hours, GDH) during ecodormancy. Using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOF/MS), we aimed to unravel the untargeted metabolic changes explaining ethephon-mediated bloom delay. Metabolite set-enrichment analysis (MSEA) revealed significant chemical group variations between dormancy phases, with a threefold increase in flavonoids during endodormancy and a doubling of organic and amino acids during ecodormancy. Further analysis of genes associated with the biosynthesis and transcriptional regulation of the flavonoid pathway showed that ethephon treatment upregulated genes associated with proanthocyanidin (PA) biosynthesis and downregulated genes related to anthocyanins (ACNs). We quantified PA and ACN contents in 12 peach cultivars with contrasting bloom times and chilling requirements (e.g. 727–1308 CH). Late-bloom cultivars had higher PA levels during endodormancy, while early-bloom cultivars had higher ACN levels during ecodormancy. Staining buds with 4-dimethylaminocinnamaldehyde (DMAC) dye revealed a decline in the PA/ACN ratio at later ecodormancy stages, correlating with bloom time. Integrated analysis of metabolite content and gene expression in late-bloom 'KV021779' and early-bloom 'John Boy' cultivars validated that late-blooming cultivars have higher PA levels during endodormancy, extending dormancy-release periods and resulting in later blooms.