Impact of Flowering Period on Fruit Quality and Antioxidant Systems in ‘Femminello Zagara Bianca’ Lemons Under Different Irrigation Regimes and Rootstock Combinations

‘Femminello Zagara Bianca’ is one of the most important lemon cultivars in Sicilian citriculture. This study evaluated how flowering period (early-season ‘Primofiore’ and late-season Verdello, obtained by forcing technique), rootstock genotype (Citrus volkameriana [VK] and Citrange Carrizo [CC]), and irrigation regime (full irrigation [FI] and summer forcing) affected fruit quality and antioxidant systems of ‘Femminello Zagara Bianca’ lemons. ‘Primofiore’ lemons achieved higher fruit weight, yellow colour, and antioxidant enzyme activities (ascorbate peroxidase, catalase, and peroxidase) than Verdello lemon fruit. The forcing technique enhanced total soluble solids, titratable acidity, individual sugars, organic acids, and flavedo flavonoids (hesperidin and eriocitrin). The rootstock genotype VK increased fruit weight, while CC combined with forcing showed the highest accumulation of chemical and bioactive compounds. Principal component analysis confirmed that the quality of lemon fruit was determined by the management of the flowering period and irrigation in a rootstock-dependent manner. Therefore, these findings provide growers with a strategy for utilising specific scion–rootstock–irrigation combinations to ensure high-value fruit throughout the harvest season.