Allelopathic effects of rapeseed (Brassica juncea) on invasive weed Phalaris minor Retz

Abstract Background Growing allelopathic crops to control invasive weeds is considered as an environmental friendly, and economically sustainable approach, understanding the reasons are essential for current and future green control of invasive weeds. Phalaris minor Retz. is one of the most troublesome invasive weeds worldwide. Our previous studies found that rapeseed(Brassica juncea)exhibited strong inhibitory effects of this weed in agroecosystems, but it is unknown whether allelopathic effects are involved in this process. In this study, we assessed the allelopathic effects of B. juncea to P. minor, then, allelochemicals were separated and identified from its leaves, further, biological activity and physiological responses by some individual compounds were investigated. Results Our results showed that water extracts from roots, stems, and leaves of B. juncea have strong allelopathic potential which can significantly inhibit the seedling growth of P. minor, and leaves have the strongest allelopathic inhibitory effects. Different solvents extracts from leaves water extracts of B. juncea significantly inhibited seed germination and seedling growth of P. minor at higher concentration (20 mg/mL), but no significantly inhibited at lower concentration (1.25 mg/mL), allelopathic effects of different solvents extracts were ranked in order from petroleum ether (strongest), ethyl acetate (moderate), N-butyl alcohol, and residual water (least). The petroleum ether extracts of B. juncea affected the protective oxidase activity and chlorophyll content, resulting in a significant increase of malondialdehyde (MDA) content and a significant reduced superoxide dismutase (SOD) activity and chlorophyll content of P. minor. Twenty-one components were identified by GS-MS, with proportion of 96.47% of petroleum ether extracts of B. juncea. The major components were erucic acid (EA) (19.08%), palmitic acid(PA) (14.16%), dibutyl phthalate(DP) (13.19%), ethyl palmitate(EP) (12.31%), linoleic acid(LA) (9.43%), and oleic acid(OA) (6.24%), among which components PA, EP, and LA exhibited strong inhibitory activity against P. minor. Conclusions Overall, these relults suggested that B. juncea has strong allelopathic inhibitory effect on P. minor, and PA, EP and LA were major compounds of petroleum ether extract of B. juncea and exhibited strong inhibitory activity against P. minor, suggested that B. juncea can be used for ecological control and developing environmentally friendly herbicides to control P. minor.