Nano-Seed Priming under Heavy Metal and Abiotic Stress: A Critical Assessment of Efficacy, Toxicity Thresholds, and Application Safety

Recently, it has been considered a prospective approach for seed germination and early seedling establishment under heavy metal and abiotic stress conditions. Though the number of studies examining nanoparticle-assisted priming is increasing rapidly, most of the existing reviews describe this approach broadly, with insufficient attention being paid to dose-dependence, toxicity thresholds, and material-specific safety at the seed stage. Four types of nanoparticles: metallic nanoparticles, metal oxide nanoparticles, silicon nanoparticles, carbonaceous nanoparticles, and biopolymer nanoparticles have been investigated for seed priming under the conditions of heavy metal and abiotic stress, which will be critically reviewed here, drawing on recent literature. The current evidence provides a strong dependence on NP type, concentration, and plant genotype, which shows that nano-seed priming can improve water uptake, activate antioxidant defense systems, and increase stress tolerance. Of the various materials compared, silicon-based nanoparticles almost always strike a more favorable compromise between effectiveness and low risk of phytotoxicity, while a number of metal and metal-oxide nanoparticles show limited safety windows where modest changes in concentration may change plant responses from stimulation to toxicity. This review highlights the necessity of distinct, material-specific toxicity thresholds and standardized protocols to guide application by combining comparative performance with safety. Conclusion: Overall, the review offers decision-making guidelines on choosing the nanoparticles that are safe and effective for seed priming and emphasizes a precision-based application as a prerequisite for reproducible agricultural benefits and environmental sustainability.