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In this study, grafted and own-rooted young hazelnut plants of three high-quality cultivars were cultivated in Central Italy to investigate possible differences in growth, fruit and flower production, and physiological processes encompassing water uptake, photosynthetic variables and non-structural carbohydrate allocation. Stable isotopes and photosynthetic measurements were used to study carbon and water fluxes in plants. For the first time, an ecophysiological study was carried out to understand the seasonal growth dynamics of grafted plants in comparison with own-rooted plants. The own-rooted hazelnuts showed rapid above-ground development with large canopy volume, high amount of sprouts and earlier yield. The grafted plants showed greater below-ground development with lower canopy volumes and lower yield. However, later, the higher growth rates of the canopy led these plants to achieve the same size as that of the own-rooted hazelnuts and to enter the fruit production phase. Different seasonal behaviour in root water uptake and leaf photosynthesis-related variables was detected between the two types of plants. The grafted plants showed root development that allowed deeper water uptake than that of the own-rooted hazelnuts. Moreover, the grafted plants were characterized by a higher accumulation of carbohydrate reserves in their root tissues and by higher stomatal reactivity, determining significant plasticity in response to seasonal thermal variations. [ABSTRACT FROM AUTHOR]