University of Birjand Journal of Horticulture and Postharvest Research 2588-4883 9 Issue 3 2026 09 01 Effects of nitrogen levels on growth, photosynthesis, and nutrient interactions in sour orange seedlings grown in nutrient solution 427 444 4096 10.22077/jhpr.2026.9539.1527 EN Farnaz Kargar Department of Plant Production, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Iran Abbas Mirsoleimani Department of Plant Production, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Iran Mahdi Najafi-Ghiri Department of Soil Science, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Iran. Journal Article 2025 06 09 <strong>Purpose:</strong> Nitrogen (N) plays a crucial role in citrus growth, but its deficiency or excess can disrupt nutrient balance and physiological functions in plant. This study investigated how varying N levels (2, 4, 8, 16, and 32 mM) affect growth, photosynthesis, root morphology and nutrient interactions in sour orange (<em>Citrus aurantium</em> L.) seedlings. <strong>Research Method:</strong> Seedlings were grown in nutrient solutions with different N concentrations. Biomass, photosynthetic efficiency (Fv/Fm, PI), chlorophyll content, nutrient uptake, and nitrate reductase activity were analyzed. <strong>Findings: </strong>Maximum and significant plant dry weight (4.95 g), root length (51.5 cm), chlorophyll content (8.6 mg g⁻¹ FW), and photosynthetic efficiency (Fv/Fm = 0.73; PI = 4.2) occurred at 16 mM N compared to 2 mM N. In contrast, N deficiency (2–8 mM) reduced growth and photosynthetic performance, while N toxicity (32 mM) decreased plant biomass by 50%, impaired chlorophyll synthesis, and disrupted photosystem II efficiency (Fv/Fm = 0.55). Excessive N (32 mM) altered nutrient homeostasis, increasing root Ca and K concentration by 0.82% and 2.49% respectively but reducing their translocation to shoots, elevating the Ca/K ratio a key indicator of K deficiency risk in calcareous soils. Nitrate reductase activity declined under toxicity, reflecting suppressed N assimilation. Root N/Ca ratio decreased with increasing in N concentration and Ca/K ratio of root in N₁₆ and N₃₂ treatments was higher than that in other N treatments. Phenolic compounds accumulated in roots under high N, suggesting oxidative stress mitigation. <strong>Research limitations:</strong> No limitations were found. <strong>Originality/Value:</strong> Excessive N disrupts nutrient balance, photosynthetic inhibition, and growth suppression while<strong> </strong>16 mM N optimizes sour orange growth<strong>.</strong> https://jhpr.birjand.ac.ir/article_4096_009d2a84866524258533d4b07dfefe11.pdf