Anatomical and Physiological Responses of Maize Nodal Roots to Shading Stress and Nitrogen Supply

Abstract

Although the upper nodal roots are vital for resource uptake in late-stage maize, their anatomical and physiological responses to varying nitrogen and light regimes remain unclear. In a field experiment, maize was grown under three nitrogen levels (0, 192, and 240 kg·ha−1) and two light regimes (normal light and 50% shading). At flowering (R1), we selected the number, diameter, anatomy of nodal roots, root-bleeding sap composition, and grain yield. Sample sizes ranged from three to twelve replicates per treatment, depending on the trait measured. Data were analyzed using ANOVA and Duncan’s test (p < 0.05). Under normal light, N192 and N240 significantly enhanced stele and vessel diameters in the sixth and seventh nodal root whorls, vessel number and cortical cell layers in the fifth and seventh whorls, root-bleeding intensity, exudation rates of sucrose, abscisic acid, key free amino acids (Asn, Asp, Glu), and grain yield, compared to N0. Shading markedly suppressed the nodal root anatomical structure, reducing root-bleeding intensity by 18.2–26.6% and yield by 30.6–40.8%; especially under SS-N0, which also notably increased the exudation of stress-related amino acids (particularly Asp and Glu). Correlation analysis revealed positive relationships of root-bleeding intensity with vessel area and grain yield, indicating that impaired root anatomy restricts resource transport under shading. Sufficient nitrogen partially alleviated these adverse effects. This study demonstrates that light and nitrogen synergistically regulate the upper nodal root anatomy, thereby modulating root-bleeding sap and ultimately influencing grain yield. These results provide a theoretical basis for high-yield maize cultivation and precision nitrogen management under low-light stress.