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.

Water deficit can alter plant growth and stress-related biochemical and molecular responses. Four fungal isolates obtained from Pinus cembroides roots—Cadophora sp., Phialophora livistonae, Pochonia sp., and Pochonia chlamydosporia—were evaluated in vitro under increasing polyethylene glycol (PEG-6000) concentrations. Pochonia sp. and P. livistonae were subsequently selected for inoculation of tomato seedlings grown at 100, 75, 50, or 25% of substrate water-holding capacity. Seedling growth, root and leaf biochemical traits, antioxidant enzyme activities, and expression of five water-deficit-associated genes were evaluated using factorial analyses. Pochonia sp. maintained the greatest colony diameter across the PEG-6000 treatments, while Pochonia sp. and P. livistonae produced the highest fresh fungal biomass at 20% PEG-6000. Irrigation level significantly influenced most seedling growth traits. Fungal inoculation significantly affected root fresh and dry weights overall, but within level comparisons identified a higher root fresh weight with P. livistonae than in uninoculated seedlings only under mild water stress (75%). Under the same conditions, P. livistonae reduced root proline content relative to the uninoculated control and Pochonia sp. Root catalase activity showed a significant inoculation × irrigation inter-action, whereas total phenolic content and leaf antioxidant enzyme activities showed no significant treatment effects. Irrigation significantly affected Fe-SOD gene expression, but fungal inoculation had no significant effect on any evaluated gene; none of the planned inoculation contrasts remained significant after Holm adjustment. These findings indicate selective, mainly root-associated responses to fungal inoculation rather than consistent enhancement of tomato growth or water-deficit tolerance. Further work should verify fungal colonization and assess plant water status and performance under production conditions.