Abstract
This study explores the complex regulatory mechanisms of nitrogen (N) and phosphorus (P) supply interactions on the growth, root architecture, and nutrient uptake of Populus × euramericana 'Neva' seedlings. It shows that these responses depend on nutrient concentrations and exhibit organ-specific patterns. Low P (0 mM) and sufficient N (15-30 mM) enhances plant height and aboveground biomass by promoting P acquisition processes. At moderate N levels (5-15 mM), P supply is sufficient (0.5-1.5 mM) for root and stem growth. Nitrogen application prioritizes aboveground biomass, reducing the root-to-shoot ratio. Root architecture also responds organ-specifically: sufficient N under low P promotes fine root growth to increase P absorption; under moderate P (0.5 mM), balanced N optimizes branching; and under sufficient P (1.5 mM), N increases root thickness while reducing fine root investment. In terms of P metabolism, moderate N under low P increases P concentrations by upregulating phosphate transporter genes, while sufficient N maintains P use efficiency (PUE). For N metabolism, added P under low N (0 mM) maintains N use efficiency (NUE), while higher N levels (15-30 mM) reduce NUE due to interference in nitrogen transport and enzyme activity. This study highlights the importance of organ-specific resource allocation in adapting to N-P interactions and suggests optimizing fertilization strategies based on soil nutrient status to avoid physiological imbalance.