Identification of Six Phytotoxic Compounds as Plant Growth Inhibitors from Afzelia xylocarpa Leaves

Plant-derived phytotoxins are widely investigated as sustainable alternatives to synthetic herbicides; however, a major limitation is the insufficient chemical characterization of active constituents in many promising candidate species, including Afzelia xylocarpa (Kurz) Craib. In this study, the phytotoxicity of A. xylocarpa leaves and their phytotoxic compounds were investigated to evaluate their potential value as a bioherbicide. The results showed the A. xylocarpa leaf extracts suppressed the seedling growth of Lepidium sativum L., Lactuca sativa L., and Lolium multiflorum Lam. Six compounds were obtained from the A. xylocarpa leaf extracts using bio-guided fractionation and were identified as (+)-dehydrovomifoliol (1), (3R,6R,7E)-3-hydroxy-4,7-megastigmadien-9-one (2), (+)-3-hydroxy-β-ionone (3), (S)-N-(1-hydroxy-3-phenylpropan-2-yl) benzamide (4), isololiolide (5), and (+)-lariciresinol (6). Compounds 1 to 6 significantly reduced seed germination, seedling growth, and dry biomass accumulation into different extents (p < 0.05). L. sativum roots were more susceptible to all the obtained compounds than other growth parameters, except for compound 4. Based on the doses of six compounds required for 50% growth inhibition (defined as EC50 value), compound 3 (EC50 values = 227.4 to 582.3 µM) and compound 5 (EC50 values = 53.8 to 200.8 µM) were the most toxic against all the growth parameters of L. sativum and may be the principal active compounds of the A. xylocarpa leaf extracts. Such phytotoxic effects indicate that these six compounds could be candidates for bioherbicides.