Silver nanoparticles as potential fungicide against rice brown spot: physiological and biochemical responses in plants

The current study aimed to assess the impact of chemically synthesized silver nanoparticles (AgNPs) on brown spot (BS), which is caused by the fungus Bipolaris oryzae . After the chemical synthesis of AgNPs, we conducted in vitro tests to examine the growth of B. oryzae in response to increasing concentrations of AgNPs. Subsequently, an in vivo experiment was installed under greenhouse conditions to evaluate the effects of rice plants previously treated with AgNPs and infected with B. oryzae . Epidemiological parameters (number and size of lesions, severity, and area under the disease progress curve (AUDPC), physiological parameters (chlorophyll [Chl] a fluorescence and concentration of photosynthetic pigments), biochemical parameters (activities of enzymes of the antioxidative system (ascorbate peroxidase [APX], catalase [CAT], peroxidase [POX], superoxide dismutase [SOD]), and defense system (phenylalanine ammonia-lyase [PAL]) were evaluated. The nanoparticles exhibited low polydispersity and had a diameter of approximately 20 nm. The velocity of mycelial growth, the diameter of the mycelia, and the germination rate of B. oryzae conidia showed a decrease with the escalating doses of AgNPs. The application of AgNPs in the plants significantly reduced the intensity of BS in rice. Leaves of plants treated with AgNPs, due to their lower BS severity, showed higher concentrations of Chl a and Chl b , in addition to greater quantum efficiency of photosystem II. Additionally, the lower cell damage due to the low level of symptoms in leaves of plants treated with AgNPs resulted in lower SOD, CAT, POX, and APX activities. PAL activity was significantly higher in plants treated with AgNPs compared to control. We conclude that the photosynthetic capacity was preserved in plants treated with AgNPs. There was a lower activity of reactive oxygen species sequestering enzymes in plants with AgNPs due to the lower level of cellular infection. Furthermore, the greater PAL activity shows a greater ability to activate defense routes in rice plants previously treated with AgNPs.