Bioactive metabolites upregulation in tomato seedlings (Solanum lycopersicum L.) through non-soil damaging foliar applications of multi-walled carbon nanotubes

• MWCNTs changed the metabolite profile in tomato seedlings in a concentration-dependent manner. • MWCNTs increased the synthesis of bioactive compounds in tomato seedlings in a concentration-dependent manner. • MWCNTs changed the amino acid and photosynthetic pigments in tomato seedlings according to their concentration. • MWCNTs did not significantly modify growth and nutrient uptake in tomato seedlings. • Foliar elicitation with MWCNTs did not significantly alter biological indicators of cultivated soil quality. The active compounds are metabolites synthesized by plants that can prevent and treat human degenerative diseases. Multi-walled carbon nanotubes (MWCNTs) are crystalline structures with the ability to interact cellularly and induce the synthesis of metabolites in plants. However, the effects of MWCNTs on ecosystems directly related to agriculture, such as soil, are not yet fully understood and require more extensive research to evaluate the environmental impact. Therefore, this study aimed to evaluate the effect of different concentrations of foliar applications of MWCNTs on the content of bioactive metabolites in tomato seedlings ( Solanum lycopersicum L.) and biological indicators of soil. The experiment was conducted in a greenhouse under a randomized complete block design; the MWCNTs were applied with distilled water using suspensions. The tomato seedlings and soil were evaluated twenty days after exposure. Non-targeted analyses demonstrate that MWCNTs changed the metabolite profile, the content of fifteen protein amino acids, and thirteen free polyphenols in tomato seedlings compared to the control in a concentration-dependent manner. Compared with the control treatment ( p < 0.05), 0.25 MW, 0.50 MW, and 1.0 MW improved seedlings' chlorophylls and β-carotene content. The 0.50 MW, 1.0 MW, and 2.0 MW treatments increased myricetin content. 1.0 MW improved iron absorption in plants, while the 2.0 MW treatment increased the glutamic acid content but decreased the absorption of calcium and manganese in the seedlings. Additionally, the MWCNTs did not obtain significant differences in the growth parameters of the seedlings and soil quality indicators compared to the control seedlings and soil. Therefore, the MWCNTs could be used as elicitors in producing bioactive compounds such as amino acids and phenols in seedlings of nutritional interest under normal conditions without compromising agricultural production and the quality of the cultivated soil.