The present study explores the potential impact of silicon nanoparticles (Si NPs), in comparison with their bulk counterpart (silicate), on post-stress recovery performance of barley (Hordeum vulgare) seedlings under different drought stress intensities during vegetative growth. Barley plants were grown under 100% field capacity (FC), or mild (75% FC), moderate (50% FC) and severe (25% FC) drought stress levels, and were subsequently recovered by different treatments including soil application of 150 mL of Si NPs and silicate (at 125 and 250 mg Si L⁻¹), and water. Si NPs application at 250 mg L⁻¹ led to formation of Si NP aggregates in plant tissues, large pores in roots, and also rapid stomata closure in leaves. However, the lower Si NPs dose (125 mg Si L⁻¹) was accompanied by a wider distribution of Si NPs in cells, and formation of a regular porosity pattern in roots i.e. more frequent pores of a smaller size. Upon recovery from all the drought stress levels, shoot biomass increased significantly in recovered plants compared to the respective non-recovered controls, and the maximum shoot biomass increase (27.3%) belonged to the moderate-stressed plants treated with Si NPs at 125 mg L⁻¹. Exposure to Si NPs and silicate (at both doses) after all drought stress intensities caused a significant increase in total chlorophyll (up to 17.1%) and carotenoid (up to 24.1%) content of leaves except for the carotenoid content under severe drought stress. Post-drought recovery with Si NPs and silicate was linked to alterations in the plant osmolyte and metabolite profile, cellular injury and membrane stability indices, and the activity of antioxidant enzymes. Soil application of Si NPs (at a low dose of 125 mg Si L⁻¹) showed a promising potential for post-drought recovery of barley plants via modifying plant morpho-physiological and antioxidative attributes and synthesis of specific metabolites.

Soil microorganisms can be exposed to, and affected by, nanoparticles (NPs) that are either purposely released into the environment (e.g., nanoagrochemicals and NP-containing amendments) or reach soil as nanomaterial contaminants. It is crucial to evaluate the potential impact of NPs on key plant-microbe symbioses such as mycorrhizas and rhizobia, which are vital for health, functioning and sustainability of both natural and agricultural ecosystems. Our critical review of the literature indicates that NPs may have neutral, negative, or positive effects on development of mycorrhizal and rhizobial symbioses. The net effect of NPs on mycorrhizal development is driven by various factors including NPs type, speciation, size, concentration, fungal species, and soil physicochemical properties. As expected for potentially toxic substances, NPs concentration was found to be the most critical factor determining the toxicity of NPs against mycorrhizas, as even less toxic NPs such as ZnO NPs can be inhibitory at high concentrations, and highly toxic NPs such as Ag NPs can be stimulatory at low concentrations. Likewise, rhizobia show differential responses to NPs depending on the NPs concentration and the properties of NPs, rhizobia, and growth substrate, however, most rhizobial studies have been conducted in soil-less media, and the documented effects cannot be simply interpreted within soil systems in which complex interactions occur. Overall, most studies indicating adverse effects of NPs on mycorrhizas and rhizobia have been performed using either unrealistically high NP concentrations that are unlikely to occur in soil, or simple soil-less media (e.g., hydroponic cultures) that provide limited information about the processes occurring in the real environment/agrosystems. To safeguard these ecologically paramount associations, along with other ecotoxicological considerations, large-scale application of NPs in farming systems should be preceded by long-term field trials and requires an appropriate application rate and comprehensive (preferably case-specific) assessment of the context parameters i.e., the properties of NPs, microbial symbionts, and soil. Directions and priorities for future research are proposed based on the gaps and experimental restrictions identified.