Engineered polysaccharide-based nanoparticles as hybrid fertilizer for efficient plant nutrition and stress tolerance.

Abstract

According to the Food and Agriculture Organization of the United Nations report 2022, nearly 828 million people were reported to suffer from chronic undernourishment in 2021-2022, with over 250 million affected in Africa. These figures demonstrate that achieving the United Nations Sustainable Development Goal 2 remains an elusive utopia unless it has happened in Africa. According to the global agriculture leaders in the 2023 Dakar 2 Summit, the continent should draw on its vast potential to feed on itself. However, the agricultural sector in the most populated continent, after Asia, is also increasingly facing the negative impacts of changing climates due to various stress conditions. Low-risk practices such as decreasing fertilizer inputs and seed priming have been promoted to curb this scenario. Breakthroughs in agricultural research have led to the development of nanofertilizers as priming agents that could contribute less to environmental pollution while promoting the early growth stages of plants, even under stress.In this study, nano-metal oxides (ZnO and Fe3O4) were developed following the phyto-assisted procedure. Aqueous extracts of the South Africa's sacred herb Helichrysum odoratissimum (Impepho) were used, at different concentrations. The chemical profiles of these extracts were obtained using the colorimetric assessments complemented with the high-performance thin layer chromatography (HPTLC) analysis. The synthesized nano-metal oxides were characterized microscopically; scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and spectroscopically; ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) techniques. Results revealed spherical ZnO and Fe3O4 nanoparticles from microscopic techniques. For UV-Vis, the absorption bands were observed at 370 nm (ZnO) and 416 nm (Fe3O4). The FTIR analysis confirmed the formation of nano-metal oxides and XRD patterns depicted crystalline structures for the nano-oxides. Furthermore, the electrochemical measurements established that the tested phytoconstituents (flavonoids, phenolic compounds, alkaloids, and terpenoids) acted as reducer and oxygen providers to convert the di-cationic Zn(II) and Fe(II) species into ZnO and Fe3O4 nanoparticles. Dynamic light scattering (DLS) technique revealed negatively charged and relatively stable particles (surface charge < -26 mV) with average hydrodynamic sizes in the range of 144.6 ± 2.6 - 321.2 ± 6.2 nm. The developed metal oxides and their metal ionic precursors were integrated into tripolyphosphate (TPP)-crosslinked chitosan (CS/TPP) biopolymeric nanostructures by ionic gelation. The physicochemical characterization techniques SEM-EDX, TEM, DLS, FTIR, TGA, and XPS were exploited to report the integrated nanomaterials' morphology, hydrodynamic size, surface charge, and structural organization. These revealed positively charged particles with average hydrodynamic sizes of 149.4 ± 22.3 - 498.8 ± 12.3 nm. The nutrient release behaviour of the developed polysaccharide-based nanoparticles was investigated by ICP-MS technique. The ZnO[15S], ZnO[15s]-loaded CS/TPP, and Zn(II)-laden CS/TPP nanoparticles were used as models. A slow change in zinc concentration of ZnO[15s]-loaded CS/TPP in water was observed as compared to other specimens during the study. It has been argued that priming seeds with ZnO nanoparticles and TPP-crosslinked CS nanoparticles enhanced seed germination and seedling growth. Therefore, in this study, the developed ZnO nanoparticles and zinc integrated CS/TPP nanoparticles were used as nanopriming agents to stimulate the germination and early seedling growth of maize seeds. The seeds were soaked in the prepared treatments, arranged in triplicates inside the growth chamber in a complete randomized design (CRD). Ten days after priming Zea mays seeds with the ZnO nanoparticles at 0.04%, more than 3-fold increase in radicle elongation and number of lateral roots was recorded relative to the control treatment. Under salinity, ZnO nanoparticles (0.01%) improved the radicle length by 78% and 84% compared to the control and NaCl treatment, respectively. Moreover, the CS/TPP, Zn(II)-laden CS/TPP, and ZnOloaded CS/TPP nanoparticles at 0.01%, significantly promoted the early seedling development under salinity stress. Nanotechnology, by virtue of the important properties of nanoscale particles, has potential applications for the development of alternative pesticides. Iron oxide nanoparticles have demonstrated higher value in preventing and treating fungal infections in both plants and humans. Therefore, the antifungal properties of the prepared iron oxide nanoparticles and iron integrated CS/TPP nanoparticles were evaluated against selected plant and human pathogens. The Fe3O4-loaded CS/TPP nanoparticles exhibited noteworthy activities with minimum inhibitory concentrations lower than 1.0 mg/mL against four phytopathogenic fungal strains, namely, Aspergillus niger, Fusarium solani, Alternaria solani, and Aspergillus flavus. The Fe(II)-laden CS/TPP nanoparticles were also found to show a better inhibitory effect against F. solani (MIC 0.3125 mg/mL) and A. flavas (MIC 0.625 mg/mL). Furthermore, the prepared Fe3O4-loaded CS/TPP nanoparticles showed the potential as therapeutic agents against Candida species (Candida tropicallis and Candida glabrata). This study envisaged promotion of the usage of phytoassisted procedures for the preparation of non-toxic nano-metal oxides for sustainable agriculture and food security.