Biostimulants are materials that when applied in small amounts are capable of promoting plant growth. Nanoparticles (NPs) and nanomaterials (NMs) can be considered as biostimulants since, in specific ranges of concentration, generally in small levels, they increase plant growth. Pristine NPs and NMs have a high density of surface charges capable of unspecific interactions with the surface charges of the cell walls and membranes of plant cells. In the same way, functionalized NPs and NMs, and the NPs and NMs with a corona formed after the exposition to natural fluids such as water, soil solution, or the interior of organisms, present a high density of surface charges that interact with specific charged groups in cell surfaces. The magnitude of the interaction will depend on the materials adhered to the corona, but high-density charges located in a small volume cause an intense interaction capable of disturbing the density of surface charges of cell walls and membranes. The electrostatic disturbance can have an impact on the electrical potentials of the outer and inner surfaces, as well as on the transmembrane electrical potential, modifying the activity of the integral proteins of the membranes. The extension of the cellular response can range from biostimulation to cell death and will depend on the concentration, size, and the characteristics of the corona.

In recent years, interest in nutrient and nanoscale biostimulant use in agriculture to improve seed germination and crop productivity has increased. Nanoparticle seed priming has improved growth and quality in crops of agricultural value. The following study shows the effect of seed priming on bell pepper RZ F1 (35-71) with zinc oxide nanoparticles (ZnO NPs) with different morphology: spherical and hexagonal. Bell pepper seeds were primed with ZnO NPs at different doses, 50 and 100 mg L-1. The study was carried out in two phases. The first phase consisted of an in-vitro study in a germination chamber (28°C), where early growth variables were assessed: germination percentage, radicle, plumule and hypocotyl length; while the second phase was carried out under greenhouse conditions, where variables such as plant height, stem diameter, dry weight, leaf area, total chlorophyll and phenols were assessed, 45 days after sowing. Seed priming with different morphology ZnO NPs showed a positive influence, 100 mg L-1 dose giving the best results for early growth parameters, as well as for plant height, stem diameter, leaf area, total chlorophyll and phenolic content. These results suggest that ZnO NPs can be considered a promising seed preparation agent to improve germination, early growth parameters, and chlorophyll and phenolic contents.