## 9 Biosafety of Nanomaterials in Sustainable Agriculture
Previous studies have shown that there are decreased macro- and micronutrients in plants when exposed to high concentrations of ZnO NPs, due to blockage of the roots by the nanoparticles, which has not been observed using zinc ions [157; 158; 159]. They block the uptake of other nutrients while becoming concentrated on the roots. [PERSON] et al. showed higher Zn levels in the roots of _Schoenoplectus taberaemontani_ than in the shoots, which reflects poor mineral translocation potential to other parts of the plants as a result of excess ZnO nanoparticles treatment of the soils [160]. They carried out their experiment out using zinc ions, and it observed that the roots accumulate more zinc in the nanoparticulate form than in the zinc ions treatment. Further reasons are that ZnO NPs achieves higher sorption into the plant organs with lower mobility than the Zn\({}^{2+}\) ions [129]. Zinc in any form, however, should not be used in excess in the soil to avoid toxicity due to chlorosis and root morphology impairment [157; 161]. In general, metal and metal oxide nanoparticles are more detrimental to soil microorganisms than organic nanomaterials. ZnO-NPs has been reported to affect the soil microbial metabolism and reduce the numbers of the nutrient fixing bacteria such as the azotobacter, and P- and K-solubilizing bacteria [130; 162]. CuO NPs are also reported to inhibit the root growth of wheat plants [163; 164]. Copper nanoparticles can affect the photosynthetic, respiratory and growth processes of the plant at higher concentration and would pose dangers on humans when such crops are consumed. [PERSON] et al. [165], used SiO\({}_{2}\) and TiO\({}_{2}\) NPs to enhance the activities of nitrifying bacteria and the uptake of fertilizer and water in soybean. In spinach, TiO\({}_{2}\) NPs was used to improve the chlorophyll concentration, enhancing the plant photosynthesis and dry weight [88; 141]. They did not constitute any toxic effects, rather they caused obstruction to the apoplastic flow through the cell walls. AgNPs are reported to be toxic at higher concentrations of \(\geq\)60-100 \(\upmu\)g/mL. At low concentrations of about 30 \(\upmu\)g/mL, they cannot penetrate the roots of plants and do not cause any harm [126]. In a study conducted by [PERSON] et al. (2012) on _Bacopa monnieri_ plant, the biologically synthesized AgNPs offered less or no toxicity and positively affected protein and carbohydrate synthesis with lower phenol contents [166]. These reports generally imply that nanoparticles affect the crop nutrient quality in diverse ways. In cucumber, ZnO added to the soil has improved the starch and protein content of the plant, while reducing the copper and molybdenum micronutrient content [167; 168]. However, in CeO\({}_{2}\) modulated soil, the nutritional content of the wheat grains used in the study was not detrimentally affected by the nanoparticles. In other crops such as rice, soybean, tomatoes and cucumber, there was high accumulation of cerium, which shows different transport mechanism in plants and root storage of engineered nanomaterials [168; 169]. The use of nanomaterials to boost plants nutrient and agricultural production is obviously gaining attention, but there should be more attention on the safety of these materials, as there is a thin line between deficiency and toxicity [115]. Although there are successful developments in the use of nanotechnologies in the field of agriculture, we cannot entirely rule out the dangers inherent in their usage as well. Modern research has adopted this technology as a way towards unparalleled development and, therefore, should also invest more in evaluating the safety of the materials to improve on their processing, characterization and overall application standards.