Physiological and Yield Productivity Responses of Hazelnut (Corylus avellana L.) to Exogenous Cytokinin and Girdling Treatments

Hazelnut (Corylus avellana L.) productivity may be constrained by source–sink imbalances. However, field-based evidence under commercial orchard conditions on the use of branch girdling and cytokinin sprays in hazelnut remains limited. This two-season study conducted in a commercial orchard evaluated the effects of branch girdling (30 mm in October; 3 mm in November) and foliar 6-benzyladenine (6-BA; 30 or 60 mg L−1) applications on the physiology, yield, and nut quality of ‘Tonda di Giffoni’ under Mediterranean conditions. Treatments were evaluated in a randomized complete block design (eight trees per treatment) using linear mixed models. Neither girdling nor 6-BA significantly improved fruit set or estimated yield (p > 0.18) and branch productivity was primarily determined by the initial floral load. However, intense October girdling markedly reduced return bloom (p < 0.001) and impaired gas exchange. In contrast, late-season or split 6-BA applications (T7–T9) consistently increased kernel yield (%), although sometimes at the expense of fruit size and weight. These findings suggest that while the total yield remained unchanged, specific treatments modulated physiological and quality traits, with late 6-BA enhancing kernel fill and early girdling posing risks to subsequent reproductive performance.