Inorganic nanoparticles (NPs) have been proposed as alternative fertilizers to suppress plant disease and increase crop yield. However, phytotoxicity of NPs remains a key factor for their massive employment in agricultural applications. In order to investigate new effective, nonphytotoxic, and inexpensive fungicides, in the present study CuZn bimetallic nanoparticles (BNPs) have been synthesized as antifungals, while assessment of photosystem II (PSII) efficiency by chlorophyll fluorescence imaging analysis is utilized as an effective and noninvasive phytotoxicity evaluation method. Thus, biocompatible coated, nonoxide contaminated CuZn BNPs of 20 nm crystallite size and 250 nm hydrodynamic diameter have been prepared by a microwave-assisted synthesis. BNPs' antifungal activity against Saccharomyces cerevisiae was found to be enhanced compared to monometallic Cu NPs. Reactive oxygen species (ROS) formation and photosystem II (PSII) functionality at low light (LL) and high light (HL) intensity were determined on tomato plants sprayed with 15 and 30 mg L-1 of BNPs for the evaluation of their phytotoxicity. Tomato leaves sprayed with 15 mg L-1 of BNPs displayed no significant difference in PSII functionality at LL, while exposure to 30 mg L-1 of BNPs for up to 90 min resulted in a reduced plastoquinone (PQ) pool that gave rise to H2O2 accumulation, initiating signaling networks and regulating acclimation responses. After 3 h of exposure to 30 mg L-1 of BNPs, PSII functionality at LL was similar to control, indicating nonphytotoxic effects. Meanwhile, exposure of tomato leaves either enhanced (15 mg L-1) or did not have any significant effect (30 mg L-1) on PSII functionality at HL, attributed to the absence of semiconducting oxide phases and photochemical toxicity-reducing modifications. The use of chlorophyll fluorescence imaging analysis is recommended as a tool to monitor NPs behavior on plants.
Keywords: Lycopersicon esculentum; Saccharomyces cerevisiae; bimetallic nanoparticles; hydrogen peroxide; nonphotochemical quenching; photoprotection; reactive oxygen species; redox state.