Biological effects of AgNPs on crop plants: environmental implications and agricultural applications

The varied use and disposal of silver nanoparticles (AgNPs) has allowed their presence in reclaimed water and sewage sludge, making them possibly available in agricultural soils. Furthermore, research advances have highlighted the agricultural application of AgNPs as alternative pesticides. As such, it is imperative to understand the phytotoxicity of AgNPs to evaluate their environmental implications. Although this is a subject of intense investigation, most of the reported results come from environmentally irrelevant concentrations. An analysis of the publications using relevant concentrations demonstrates that AgNPs may provide several benefits to crop plants. In addition, data show that one of the plant responses to AgNP exposure is the production of reactive oxygen species (ROS), which depending on the concentration may cause a variety of effects in plants. Excess ROS may damage tissues and interrupt biological functions, while low ROS may trigger beneficial responses. Data available are discussed and based on the evidence, it is hypothesized that the biological effects of AgNPs in plants may be attributed to ROS activation. In addition, the ROS generation mechanisms and molecular mechanisms of plant responses to AgNPs are discussed. Finally, the risk assessment of the application of AgNPs in agriculture, as well as the research questions to fill the knowledge gaps, is depicted in this review.