Secondary Metabolites Associated with Weed Suppression in Novel Annual Pasture Legumes

Australian farmers and researchers have embraced the use of introduced annual pasture species originating from Mediterranean regions of Europe and northern Africa for pasture diversity and N fixation. Recent research has focussed on the use of novel annual legumes including Ornithopus spp. (French serradella and yellow serradella) and Biserrula pelecinus L. (biserrula) for controlling weeds in rotational cropping systems for sustainable agricultural production. Several mechanisms are associated with the ability of pasture legumes to suppress weeds including 1) competition for resources and 2) release of secondary metabolites into the rhizosphere1 through root exudation. Laboratory bioassays were therefore designed to evaluate the phytotoxic potential of fresh shoot extracts of pasture legumes using germinating seedlings of Lepidium sativum L. (garden cress) and Lolium rigidum Gaud. (annual ryegrass). The I50 of crude extracts was calculated for radicle elongation for both test species. Non-targeted and targeted metabolic profiling was performed using ultra-high pressure liquid chromatography (Agilent, 1200 Infinity) coupled with quadrupole-time of flight mass spectrometry (Agilent 6530). Multi-regression analysis of crude shoot extracts was performed to determine an association between phytotoxicity and the abundance of secondary metabolites. Key bioactive metabolites of interest were identified using a personal compound database library (PCDL) and known standards. Fresh extracts of biserrula (0.78 mg/mL respectively) and yellow serradella (0.84 mg/mL) exhibited the greatest phytotoxic activity against radicle elongation of garden cress and annual ryegrass seedlings respectively. Step-wise multiple regression analysis demonstrated a strong association of between bioactivity and the presence of five metabolites including quercetin and kaempferol and their associated glycosides2. A significantly higher relative abundance of key phytotoxic flavonoids and their glycosides in the rhizosphere of field – collected soil samples further confirmed the phytotoxic potential of biserrula and yellow serradella.