Influence of nitrogen fertilization rate and application frequency on physical and phytochemical quality of ‘Owari’ Satsuma Mandarin across fruit developmental stages in North Florida

Citrus trees require substantial amounts of nitrogen (N), making it essential to balance N inputs with tree demand to optimize yield and fruit quality. Because N requirements vary considerably over the growing season, identifying the appropriate application rate and timing is critical to align N supply with tree physiological needs. 'Satsuma' mandarin (Citrus unshiu) is a promising cultivar for north Florida due to its tolerance to cold temperatures; however, guidelines on how N management affect fruit quality during the fruit developmental stages remain limited. The present study evaluated the effects of varying N rates and application timings on fruit quality parameters of 'Owari' Satsuma mandarins and tracked their changes at different stages of fruit development. Treatments consisted of four N rates (112-280 kg ha⁻¹) and three applications frequencies (3, 5, and 7 split applications) were arranged in a randomized complete block design (RCBD) with four replications. Fruit quality was monitored at different stages of fruit development from early September through mid-December. Results indicated that N rate had minimal influence on physical fruit quality parameters, whereas split frequency of N application significantly affected fruit weight, peel weight, and fruit diameter, with the highest values observed with 7 split applications. Conversely, N rate significantly affected the juice percentage, particularly during the peak fruit development period (mid-November), with the highest juice content observed at the highest N rate (280 kg ha^{- 1}). Biochemical traits, including total phenolic content (TPC), total flavonoid content (TFC), and total antioxidant activity (TAA), peaked at late maturity, showing strong positive responses to N rate but little response to split frequency. Significant interaction among N rate and split frequency was recorded for total flavanols content (TFl) with differential response under various sampling dates. Overall, a moderate N input (112-168 kg ha⁻¹) applied in seven splits may represent an ideal strategy for improving fruit quality in 'Owari' Satsuma mandarin under the growing conditions of north Florida. Furthermore, most parameters in our study reached their optimum levels between late October and mid-November, suggesting that delaying harvest beyond this period may compromise fruit quality.