Exogenous Cytokinins and Auxins Affect Double Cropping in Vitis vinifera L. cv. ‘Ortrugo’ Grown in a Temperate Climate: Preliminary Results

The success of double cropping in Vitis vinifera L. cultivated in temperate climates relies on bud forcing efficiency, which requires the prompt unlocking of apical dormant buds with sufficient fruitfulness. Chemical dormancy-breaking strategies need to be tested to enhance dormant bud forcing in summer pruning, as hydrogen cyanamide, the most used agent, could damage green organs. This study tested whether foliar applications of cytokinins and auxins could modulate dormancy release, potentially affecting bud forcing dynamics and shoot fruitfulness. The forcing treatments involved trimming primary shoots at the eighth node, removing lateral shoots, and retaining the main leaves and inflorescences. Five treatments were investigated: unforced control, control + 6-Benzyladenine application, forcing (FR), forcing + 6-Benzyladenine application (FBA), and forcing + Naphthaleneacetic acid application (FNAA). Phenological evolution, vegetative and productive parameters, and physiological characteristics have been assessed. Results showed that among the forcing treatments, FBA showed the highest forced/primary shoots ratio (106%), followed by FR (94%) and FNAA (21%). Primary yields were similar across treatments (2.74 kg), but total yield was highest in FBA (4.78 kg, including 2.02 kg from forced grapes), followed by FR (3.62 kg, with 1.09 kg forced). FNAA yielded no forced crop. During forced grapes maturation, photosynthesis rates were higher in forced leaves (11.1 μmol m−2 s−1, as FR and FBA average) than primary leaves (−32%). Forced grapes ripened 47 days later and achieved higher sugar content (21.7 °Brix) and titratable acidity (10.6 g/L) than primary grapes. The findings suggest cytokinins application enhances bud forcing, supporting the feasibility of double cropping, while auxins limited it.