Impact evaluation of bare and chitosan complexed silver nanoparticles on the growth of Sorghum bicolor (L.) Moench and soil bacterial diversity

Silver nanoparticles (Ag NPs) are utilised mainly as bio-stimulants to elevate plant growth; however, they may be toxic due to their oxidative behaviour. Nanocomposites (NCPs) of Ag with chitosan (CS/Ag NCPs), which combine the beneficial properties of both, can considerably alleviate this issue. In the present study, the effect of unmodified (bare Ag NPs: size 4-8 nm) and chitosan-modified Ag NPs (CS/Ag NCPs: size 7-14 nm) in supporting the growth of Sorghum bicolor (L.) was tested on 30 days old plants by growing them in soil spiked with two different concentrations (100 and 200 ppm) of nanomaterials. Both Ag NPs and CS/Ag NCPs positively influence the growth of S. bicolor; however, higher growth and root protein contents, increased amounts of nitrogen and phosphorous and lower malondialdehyde (MDA) contents in presence of 200 ppm CS/Ag NCPs suggested their superiority over control and Ag NPs treated plants. In contrast to CS/Ag NCPs treated plants, Ag NPs treated plants accumulated more Ag, which may be correlated with higher MDA levels in their shoots. However, plants exposed to 200 ppm of Ag NPs also had the highest levels of starch, cellulose, magnesium and potassium. Both nanomaterials improved the number of phosphate-solubilising bacteria, Azotobacter species and Azospirillum species, which improved the uptake of nutrients by the treated plants. The results demonstrated the safety of both nanomaterials and the superiority of CS/Ag NCPs over Ag NPs for their application as growth-promoting agents for S. bicolor and future investigation into their potential for use in nanobiotechnological applications.