Physiological and Biochemical Responses of Pearl Millet ( L.) Seedlings Exposed to Silver Nitrate (AgNO) and Silver Nanoparticles (AgNPs).

A rapid and continuous growth of silver nanoparticles (AgNPs) via their precursor "silver nitrate" (AgNO) has increased their environmental risk because of their unsafe discharge into the surrounding environment. Both have damaging effects on plants and induce oxidative stress. In the present study, differential responses in the morpho-physiological and biochemical profiles of (L.) seedlings exposed to various doses of AgNPs and AgNO were studied. Both have forms of Ag accelerated the reactive oxygen species (ROS) production, which adversely affected the membrane stability as a result of their enhanced accumulation, and resulted in a significant reduction in growth, that is, root length, shoot length, fresh and dry biomass, and relative water content. AgNO possessed a higher degree of toxicity owing to its higher accumulation than AgNPs, and induced changes in the antioxidants' enzyme activity: superoxide dismutase (SOD), peroxidase (POD), catalases (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), and glutathione reductase (GR) activity, as well as proline content, total phenolic, and total flavonoids contents (TFCs) under all tested treatments (mM). A decline in photosynthetic pigments such as total chlorophyll content and carotenoid content and alterations in quantum yield (Fv/Fm), photochemical (qP), and non-photochemical quenching (NPQ) indicated the blockage of the electron transport chain (ETC), which led to a significant inhibition of photosynthesis. Interestingly, seedlings exposed to AgNPs showed less damaging effects on (L.) seedlings, resulting in relatively lower oxidative stress in contrast to AgNO. Our results revealed that AgNO and AgNPs possessed differential phytotoxic effects on (L.) seedlings, including their mechanism of uptake, translocation, and action. The present findings may be useful in phytotoxic research to design strategies that minimize the adverse effects of AgNPs and AgNO on crops, especially in the agriculture sector.