Weilin Li

Cytokinin and nitrogen synergistically regulate plant physiological metabolism and developmental processes; however, their combined application on the growth, yield and fruit quality of blackberry remains unreported. This study aimed to investigate the effects of different ammonium-to-nitrate (\({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\)) ratios combined with 6-benzylaminopurine (6-BA) on the physiological characteristics, fruit quality and yield of blackberry to screening of the optimal treatment combination. Three-year-old potted 'Ningzhi 4' blackberry plants were subjected to four treatments: nutrient solutions with \({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\) ratios of 100 : 0 and 75 : 25, either alone or combined with a foliar application of 40 mg L^–1 6-BA, administered continuously for 90 days. Physiological parameters of leaves, including photosynthetic pigments, sugar accumulation and antioxidant enzyme activities, alongside fruit morphological indices, yield and bioactive compound contents, were evaluated. The \({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\) (75 : 25) + 6-BA treatment yielded the highest leaf chlorophyll and nitrogen contents, significantly elevated peroxidase (POD) and catalase (CAT) activities and minimized the accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA). Furthermore, this specific treatment achieved the highest theoretical yield per plant and significantly increased the contents of bioactive compounds such as total phenols, anthocyanins and ellagic acid in the fruits. In contrast, the \({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\) (100 : 0) + 6-BA treatment was most effective in increasing single fruit weight, transverse and longitudinal diameters, and fruit firmness. Additionally, the application of 6-BA decreased the soluble solids content and the solidity-acid ratio while increasing titratable acidity in the fruits. Different \({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\) ratios combined with 6-BA exert differential regulatory effects on blackberry growth and metabolism. Specifically, the 75 : 25 \({\text{NH}}_{4}^{ + }\) : \({\text{NO}}_{3}^{ - }\) ratio combined with 6-BA maximizes the plant’s antioxidant defense capacity, boosts yield, and promotes the accumulation of bioactive compounds, whereas the pure ammonium (100 : 0) treatment combined with 6‑BA is more conducive to enhancing fruit size and firmness.