Variation in Floral Nectar Traits of Four Chestnut (Castanea spp.) Cultivars: Implications for Pollinators and Apiculture

Pollinator nutrition and honey production potential depend on nectar quantity, nectar availability across flowering phases, and sugar concentration. For chestnut (Castanea spp.), cultivar- and flowering phase-specific nectar data remain limited. This study analyzed nectar traits of four Castanea cultivars to evaluate their potential importance for pollinators and apiculture. A two-year field study (2023–2024) was conducted on four major South Korean cultivars (‘Daebo’, ‘Okkwang’, ‘Riheiguri’, ‘Tsukuba’) to quantify catkin floral traits, nectar volume, free sugar concentration (sucrose, glucose, fructose), and estimated nectar yields across four flowering phases. Standardized catkin-scale sampling and multivariate modeling revealed that flowering phase, rather than catkin size, determined nectar rewards in all cultivars. Nectar volume and sugar concentration per catkin peaked at mid anthesis (phase 3), while sugar concentration and hexose proportion increased in late anthesis (phase 4). ‘Daebo’ led in phase 3 nectar yields, ‘Okkwang’ was intermediate, and ‘Tsukuba’ and ‘Riheiguri’ provided more hexose-rich, concentrated nectar during phase 4. Notably, cultivar × flowering phase interactions determined both the amount and sugar profile of nectar resources. These findings indicate that phase 3 measurements are optimal for yield comparisons, while phase 4 profiles guide honey chemistry and handling. Mixed-cultivar plantings combining ‘Daebo’ (high honey yield) with late-phase hexose sources (‘Riheiguri’, ‘Tsukuba’) can help stabilize pollinator resources.