A homoisoflavonoid and a fatty acid in common purslane (Portulaca oleracea L.) synergistically inhibit growth of Spodoptera litura larvae

Portulaca oleracea L., common purslane, is an insecticidal plant that has been documented as a 'Chinese indigenous pesticide', and it is seldom visited by insects in the field. However, identification of anti-insect compounds and mechanisms of action are still unclear.Interplanting purslane with Chinese cabbage demonstrated that purslane may contain secondary compounds that S. litura avoids eating. Four compounds were isolated from P. oleracea by directed anti-insect activity, and their chemical structures were identified by NMR spectra as (9Z,11E,15Z)-13-hydroxyoctadeca-9,11,15-trienoic acid (1), portulacanone A (2), portulacanone D (3), and a new natural product 2,4'-dihydroxy-3',5'-dimethoxychalcone (4). A combination of compound 1 and 2 possessed stronger activity than other combinations (compounds 1 + 3; 1 + 4; 2 + 3; 2 + 4; 3 + 4). Both active compounds were detected in all samples from 23 regions in China, and concentrations in samples collected from 17 regions were generally above 500 μg/kg. Concentrations of compounds 1 and 2 fluctuated greatly from April to November, and reached maximum concentrations of 45 951.44 μg/kg for compound 1 and 3739.09 μg/kg for compound 2 in November. The combination of these compounds (1 + 2) caused mid-gut structural deformation and tissue decay as determined by mid-gut histopathology of S. litura.In general, these active compounds coexisting contributed to partly protect purslane from insects. This research also provides new insights into the use of purslane as important ingredient of botanical pesticide alternatives to traditional chemical pesticides. © 2019 Society of Chemical Industry.