Seed Nanopriming with Spirulina-Derived Carbon Dots Enhances Rice (Oryza sativa L.) Germination, Crop Establishment, and Seedling Metabolic Performance

Biogenic carbon dots (CDs) are emerging as promising plant biostimulants, yet their effects during early crop establishment remain underexplored. Here, we synthesized and characterized Spirulina-derived CDs and evaluated their efficacy as seed nanopriming agents in rice (Oryza sativa L.). CDs exhibited nanoscale size, abundant surface functionalities, and a highly negative ζ-potential, indicative of stable aqueous dispersions. Spectroscopic characterization (Raman and FTIR) confirmed a graphitic–amorphous carbon structure. Near-infrared spectroscopy coupled to principal component analysis revealed time-dependent metabolic changes during imbibition, identifying 8–12 h as the optimal priming window. Nanopriming with Spirulina CDs (0.2 mg mL−1 for 12 h) increased the seed germination rate (25%), the germination speed index (17%), vigor index I (22%), and root length (37%) compared to hydropriming. Biochemically, the nanoprimed seedlings accumulated higher levels of starch (24%), total carbohydrates (8%), and total phenolics (20%), without evidence of oxidative imbalance, based on antioxidant capacity measurements and proteomic profiling. Proteomic analysis revealed coordinated metabolic reprogramming, characterized by increased abundance of proteins involved in translation, energy metabolism, and ion/nutrient homeostasis, alongside reduced abundance of proteins associated with defense and catabolic processes. This shift from stress-preparation to growth-oriented metabolism supports improved seedling establishment. Overall, Spirulina-derived CDs function as effective nanobiostimulants that promote early metabolic activation and resource mobilization, offering a sustainable strategy to enhance rice seedling establishment.