Nitrogen addition promotes biomass recovery and cadmium immobilization in Cosmos bipinnatus through physiological and molecular regulation

Nitrogen (N) addition enhances cadmium (Cd) tolerance and biomass performance in Cosmos bipinnatus , which was investigated using integrated morphological, physiological, microscopic and root transcriptome–metabolome analyses. Notably, under Cd stress, N supplementation led to significant improvements in the growth and biomass production of C. bipinnatus , while also enhancing the levels of nutrient elements in both its shoot and root tissues. This response is consistent with N-driven shifts in root traits and improved Cd handling capacity, indicating that N-induced physiological and structural homeostasis is closely linked to biomass recovery and that N addition enhances the potential of C. bipinnatus for industrial soil remediation in Cd-contaminated environments. Moreover, N supplementation restored growth and photosynthetic function, suppressed lipid peroxidation, and strengthened antioxidant and osmotic defense capacity. Microscopy showed recovery of root tips with thick, compact walls; fractionation confirmed that Cd was predominantly immobilized in the cell wall, with about 83% retained in low-mobility fractions, restricting root-to-shoot transfer. Transcriptome and metabolome analyses indicated coordinated activation of peroxisomal functions and the ascorbate–glutathione cycle, together with reinforcement of phenylpropanoid metabolism and cell-wall remodeling. The metal-transport network was reorganized across the ZIP, NRAMP, HMA, and YSL families, with tighter Fe and K homeostasis.These adjustments stabilized Cd within roots, reduced its mobility and reactivity, protected cellular structures, and sustained biomass accumulation and recovery. The findings provide a physiological and molecular rationale for N management that couples growth rescue with in situ stabilization and informs remediation strategies in C. bipinnatus and related ornamentals.