Silicon Application Alleviates Arsenic Toxicity in Isatis (Isatis cappadocica Desv.) by Modulating Key Biochemical Attributes and Antioxidant Defense Systems

Arsenic (As) toxicity hinders plant growth and productivity and poses human health concerns. However, silicon (Si) nutrition can help improve plant tolerance to abiotic stresses. This study evaluated the influence of Si application on plant growth and As accumulation in an As hyperaccumulator plant, Isatis (Isatis cappadocica Desv.) under As stress. Isatis seedlings were exposed to 0, 5, 25, 125, and 625 μM Na₂HAsO_4.7H₂O (AsV) and NaAsO₂ (AsIII) and then Si was applied as Na₂SiO₃ (1 and 2 mM). Plant exposure to As(V: 650 μM) and As(III: 125 µM) caused significant reduction in plant fresh weight by 31 and 42%, and photosynthetic pigments by 47 and 62%, respectively. The same treatments caused 37- and 4.2-fold increase in levels of reactive oxygen species. However, exogenous application of Si effectively mitigated As stress, and caused a decrease in shoot As concentration up to 15% and 21% in As(V) and As(III) treated plants, respectively. In this regard, Si application at 1 mM level was more effective. The Si-induced tolerance against As in Isatis was attributed to the activation of antioxidant defense system (increase in the activities of gluthatione-S-transferase and guaiacol peroxidase, carotenoids, proline and anthocyanin contents). In conclusion, Si application improved the As tolerance in Isatis plants by modulating plant growth, decreasing As accumulation and activating the antioxidant defense system.