Foliar-Applied LW-CNs Alleviate Cadmium Toxicity in Soybean by Regulating Cd Accumulation, Photosynthesis and Nitrogen Metabolism

Cadmium (Cd) pollution seriously threatens food safety. Plant-derived nanomaterials, due to their environmental friendliness, have emerged as a promising approach for crop stress tolerance regulation; however, their mechanisms under Cd stress remain unclear. In this study, licorice–wolfberry-derived nanocomposites (LW-CNs) were synthesized via a one-step hydrothermal reaction following an established protocol, and their physicochemical properties have been characterized in our previous studies. These were foliarly applied at 200 mg/L to soybean seedlings under 300 μmol/L Cd stress, and the physiological mechanisms underlying their alleviation of Cd toxicity were systematically analyzed. The results showed that Cd stress significantly inhibited soybean seedling growth, reducing net photosynthetic rate from 6.42 to 0.29 μmol·m−2·s−1, and induced substantial accumulation of H2O2, O2·−, and MDA in leaves. After LW-CN application, plant height, shoot fresh weight, and root fresh weight increased by 5.8%, 29.0%, and 28.2%, respectively, while Pn recovered to 0.94 μmol·m−2·s−1. Mechanistic analysis revealed that the ameliorative effects of LW-CNs resulted from four synergistic actions: (i) maintaining chloroplast ultrastructural integrity, with Pn recovery accompanied by stable intercellular CO2 concentration, suggesting that the relief of photosynthetic limitation was unlikely to be attributable to stomatal factors alone; (ii) systemically upregulating SOD, POD, CAT, APX, DHAR, and GR activities, modulating the redox status of the AsA-GSH cycle, and significantly reducing H2O2 (2.77 to 2.14 μmol/g), O2− (284.73 to 247.40 nmol/g), and MDA (79.29 to 58.40 nmol/g) contents, thereby maintaining ROS homeostasis; (iii) decreasing Cd concentrations in roots, stems, and leaves, and reducing chloroplast Cd co-localization signals (0.774 to 0.171), thus mitigating direct damage to photosynthetic machinery at the source; and (iv) organ-specifically reconfiguring nitrogen metabolism, enhancing NR, GOGAT, and GDH activities and glutamate accumulation in roots, and promoting nitrogen redistribution among roots, stems, and leaves. These findings provide a multidimensional physiological framework—covering Cd-accumulation reduction, photosynthesis protection, redox stabilization, and nitrogen redistribution—for understanding how a plant-derived nanomaterial enhances soybean Cd tolerance, and offer a reference for developing plant-based strategies to sustain soybean production in Cd-contaminated farmlands.