Model-based analysis of chill accumulation in European hazelnut catkins highlights site-specific climatic sensitivity across Italian locations

Rising global demand for hazelnuts requires improved tools to support cultivar adaptation without compromising quality. This study evaluated male flowering phenology and chilling accumulation over three years in four European hazelnut cultivars and one wild type accession grown in two contrasting environments: Viterbo (central Italy, sub-coastal temperate climate) and Chieri (northern Italy, sub-continental temperate climate). Pollen dispersal was monitored, and chilling accumulation was calculated from November 1st to dormancy release using chilling hours (CH, 0-7 degrees C), Utah (chilling units, CU), and Dynamic (chilling portions, CP) models. Results revealed clear genotype-specific differences in flowering timing and duration. Early-flowering cultivars reached anthesis two to four weeks earlier in Viterbo than in Chieri, reflecting adaptation to local mesoclimatic conditions. Chilling accumulation differed significantly between sites, with consistently lower values in the warmer environment. Among models, CH showed the most stable and consistent response to environmental differences and best agreement with observed phenology, while CU provided comparable but slightly less robust estimates. CP captured the influence of temperature fluctuations and warm spells, but showed less consistency in cross-site comparisons. These findings highlight the strong interaction between genotype and environment in hazelnut phenology and emphasize the importance of selecting appropriate chilling models to support cultivar choice and orchard management under variable climatic conditions.