Pretreatment with salicylic acid and nitric oxide alleviates silver nanoparticles toxicity in medicinal Hypericum perforatum plants

Since the use of seed priming agents to reduce the potential toxicity of AgNPs has not been explored, we evaluated the combined effects of 0.1 mM salicylic acid (SA) and 0.1 mM nitric oxide (NO) on responses of Hypericum perforatum plants to Ag-induced stress. The results of growth parameters together with measurement of malondialdehyde (MDA) revealed that exposure to AgNPs or AgNO3 caused more toxicity, which was closely associated with the over–accumulation of ROS. However, SNP (NO) and SA addition successfully mitigated adverse impact of AgNPs on H. perforatum seedlings, which was associated with the higher synthesis of chlorophyll b pigments and proline. In addition, a further increase in antioxidants (phenolic compounds) was observed in NO-primed plants under AgNPs-induced stress, which was concomitant with the high level of SOD activity. We concluded that NO minimized the inhibitory effects of AgNPs stress by increasing the phenolic compounds as well as by regulating accumulation of Ag, ROS and antioxidants. To sum up, exogenous NO plays a key role in enhancing its resistance to Ag stress in physiological base, thereby improving the growth and phenolic compounds production (chlorogenic acid, quercetin and rutin) of H. perforatum plant.