Metabolic profiling of bioactive flavonoids in selected annual pasture legumes

Australian farmers and researchers have embraced the use of introduced annual pasture species originating from Mediterranean regions of Europe and nothern Africa and these have been continuously evaluated in the face of a changing climate to increase crop diversity and fill existing agro-ecological gaps. Recent efforts have focussed on less well-known annual legumes including Ornithopus spp. (French serradella and yellow serradella) and Biserrula pelecinus (biserrula). However there is a need to better understand the performance of each species as well as the bioactive plant secondary compounds associated with weed suppression, produced by these legumes under Australian climatic conditions. Legumes typically produce a diversity of secondary metabolites particularly flavonoids that are involved in nitrogen fixation, rhizobium signalling, and also possess antioxidant and allelopathic effects. Various plant tissue types including leaf, stem, flower and rhizoplane and bulk soil sampels were extracted with methanol using high temperature and pressure speed extractor. Metabolic profiling was performed using high pressure liquid chromatography coupled with quadrupole – time of flight mass spectrometry followed by chemometric ananlysis. Compounds were identified by comparing accurate mass, retention time and mass spectra with available libraries and known standards. Numerous bioactive flavonoids were profiled and these varied significantly with species and plant tissue types. Further data analyses revealed that field and laboratory experimental results were similar, with the highest abundance of key metabolites including bioactive flavonoids in plant tissues including rhizoplane and bulk soil. Stepwise linear regression showed strong correlation between phytotoxicity and presence of key flavonoids quercetin, kaempferol and their glycosides.