Insecticidal and antifungal activities of Rheum palmatum L. anthraquinones and structurally related compounds

As a well-known herb worldwide, rhubarb presents a wide range of pharmacological activities. In our studies, Rheum palmatum L. extracts and five isolated anthraquinones possessed insecticidal activity. The acetone extract (yield, 8.84%), which contained the highest amount of compounds (82.87 mg/g), showed stronger insecticidal activity than did the aqueous extract (yield, 37.17%) and the ethanol extract (yield, 36.92%). Notable amounts of compounds were also found in water extract residues (18.2 mg/g). Among the isolated quinones and structurally related compounds, emodin exhibited the highest insecticidal activity (LC 50  = 84.30 μg/mL) against Nilarparvata lugens ; and very moderate activity against Mythimna separata (LC 50  = 548.74 μg/mL). The LC 50 values of positive agent toosendanin against two insects were 89.34 and 418.24 μg/mL, respectively. Moreover, emodin exhibited significant AChE and GST inhibitory activities (with respective IC 50 values of 11.36 and 4.18 μg/mL). Plumbagin exhibited stronger and broader antifungal activity against eight phytopathogenic fungi (with EC 50 values ranging from 2.84 to 10.53 μg/mL) compared to a commercial fungicide, azoxystrobin (EC 50 ranging from 20.40 to >100.00 μg/mL). Plumbagin significantly damaged the cell and mitochondrial membrane systems, changed the morphology, triggered the release of cellular contents and generate reactive oxygen species in Rhizoctonia solani mycelia. This study laid the foundation for the future development of emodin and plumbagin as alternative, environmentally friendly bioinsecticide and biofungicide, respectively. Rhubarb and its residues can be used as renewable sources of anthraquinone-based products.