Effects of summer nitrogen application rate on citrus shoot-root-soil interactions

• Identified a critical pure nitrogen threshold of 3–4 g plant −1 for regulating summer shoot growth and maintaining physiological health in potted 'Newhall' navel orange seedlings. • Elucidated the complex regulatory mechanism of summer shoot growth, demonstrating joint mediation by root-zone nitrogen availability, root system architectural plasticity, and the intricate ABA-Pro-Chl metabolic balance. • Developed a robust multiple regression predictive model integrating soil available nitrogen (NO 3 − -N and NH 4 + -N) and root morphological parameters to effectively forecast summer shoot growth dynamics (R 2 = 0.53–0.72). Nitrogen (N) is the primary nutrient that initiates the summer-flush (SF) in citrus trees. However, the integrated "soil-root-shoot" feedback mechanism regulating SF intensity remains poorly understood in fertigation-based systems. To quantify dose-dependent effects on SF morphogenesis, nutrient allocation, root-tissue, and soil N concentrations, we conducted a greenhouse pot experiment with two-year-old 'Newhall' navel orange grafted onto Poncirus trifoliata rootstocks. The plants were subjected to six summer N rates (pure N 1, 2, 3, 4, 5, and 7 g plant −1 ). The results showed that: (1) Reducing N supply progressively delayed SF emergence and advanced maturation, thereby shortening the flush period; (2) Compared to high N treatment (5–7 g plant −1 ), moderate N (3–4 g plant −1 ) application treatment significantly suppressed SF growth, reducing shoot number, length, basal diameter, leaf number, emergence rate and biomass by 42.86 %, 8.99 %, 9.76 %, 16.74 %, 29.37 %, and 47.59 %, respectively. Conversely, this optimal N range maximized the net increments in plant height (1.96 cm) and main stem diameter (1.52 cm); (3) Within this range, the summer shoots maintained medium-to-high levels of nutrient content (N, phosphorus, potassium) and physiologically active substances (abscisic acid, chlorophyll, proline), while root growth was moderated to a medium-low level. Meanwhile, the nutrient status of the root zone soil and roots remained in the medium-to-high range; (4) Root zone soil ammonium nitrogen and nitrate nitrogen concentrations and root length density were strongly correlated with SF architectural traits. Multiple regression models incorporating these variables explained >50 % of the variance in SF phenotype. This regulation was attributed to an N-driven "soil-root-shoot" feedback mechanism, where the root zone inorganic N pool, root morphology, and the balance of key shoot physiologically active substances collectively defined summer-flush growth patterns. Therefore, it is advisable to apply 3–4 g N plant −1 as the ideal summer rate for potted citrus seedlings in controlled environments. This effectively maintains a balance between vegetative growth and nutrient homeostasis through the identified "soil-root-shoot" feedback loop. However, these conclusions need to be verified in orchard-grown, fruit-bearing trees to substantiate their relevance in commercial cultivation systems before broader implementation.