Coix lacryma-jobi root extract exhibits self-toxicity, with butyl palmitate being the primary toxic substance

Coix lacryma-jobi growth is often inhibited by continuous cropping due to autotoxic effects. The autotoxic effects and decomposition of aqueous extracts from different parts of C. lacryma-jobi plants and impact of their decomposed matter are unclear. The present study tested aqueous extracts from shoots, roots, and decomposed residues at four concentrations to assess their autotoxic properties. The extracts and decomposition products inhibited seed germination and seedling growth in a concentration-dependent manner. At concentrations of ≥ 0.1 g mL^-1, the extracts significantly suppressed seed germination and impaired seedling growth. Specifically, root and shoot lengths, plant height, stem diameter, leaf area, and root activity were all reduced. Concurrently, leaf superoxide dismutase, peroxidase, and catalase activities decreased significantly, whereas proline and malondialdehyde contents increased significantly. The inhibitory potency of the materials tested followed this order: roots > shoots > decomposition products. To isolate the specific autotoxic compounds, we employed a series of solvents for extraction and fractionation. The acidic fraction obtained from an n-butanol partition of a methanol root extract is strongly inhibitory to C. lacryma-jobi seed germination. Gas chromatography-mass spectrometry analysis revealed 22 allelochemical compounds. Subsequent bioassays on seven selected candidates revealed that five compounds-hexadecane, dibutyl phthalate, vanillin, butyl palmitate and palmitic acid -exhibited significant, dose-dependent autotoxic effects on seed germination. Their order of inhibitory potency was: butyl palmitate > vanillin > palmitic acid > hexadecane > dibutyl phthalate.