Determination of chilling requirements for anthocyanin accumulation in blood orange ‘Tarocco Ippolito’

Cold exposure is a key factor in anthocyanin accumulation during the postharvest period of blood oranges; however, its role in the accumulation of these compounds while the fruit is still on the tree has not been investigated. Herein, the influence of temperature, thermal amplitude, and solar radiation on the onset of anthocyanin biosynthesis in ‘Tarocco Ippolito’ blood oranges was investigated. Over three consecutive seasons, environmental factors and major physicochemical changes during fruit maturation were monitored in parallel. Our findings confirm that anthocyanin biosynthesis in the pulp and peel occurs as independent processes. While thermal amplitude and solar radiation did not explain the onset of anthocyanin accumulation, temperature emerged as the key environmental factor triggering this process. For the first time, the main chill accumulation models developed to explain endodormancy release, namely the Chilling Hours, Utah, Positive Utah, and Dynamic models, were applied in a new context to determine the chilling requirements for the onset of anthocyanin accumulation. In the flesh, anthocyanin biosynthesis began after a total cold accumulation of 4–5 chill portions (Dynamic model) or approximately 128 chilling P-units (Positive Utah model). A higher cold requirement was observed for the peel, where red colouration appeared 8–15 days later, after accumulating of approximately 10 chilling portions (Dynamic model) or around 240 chilling P-units (Positive Utah model). The Chilling Hours and Utah models were unsuitable for this purpose. This information represents a significant advancement in the understanding of fruit physiology and may be key to the success of new plantations. • Chilling models evaluated for first time to explain anthocyanin buildup in fruit. • Peel anthocyanin accumulation requires more chilling than flesh. • Flesh pigmentation starts after 4–5 chill portions or 128 chilling P-units. • Peel pigmentation requires 10 chill portions or 240 chilling P-units. • Thermal amplitude and solar radiation were not linked to anthocyanin accumulation.