Characterization of allelochemicals isolated from parthenium weed (Parthenium hysterophorus L.) for inhibition of Echinochloa colona (L.) Link and Oldenlandia verticillata L.

Study on the isolation and characterization of phytotoxic allelochemicals in parthenium (Parthenium hysterophorus L.), an invasive weed, was conducted to evaluate its herbicidal potential on Echinochloa colona (L.) Link and Oldenlandia verticillata L. herbicide-resistant weeds in Malaysia. The study aimed to isolate, identify, characterize and evaluate the persistence of the most phytotoxic allelochemical with herbicidal potentials from the leaves, stems, and roots of P. hysterophorus. Phytotoxicity tests such as seed germination, seedling growth, and leaf disc discolouration were conducted on E. colona and O. verticillata in the laboratory using four extraction solvents: methanol, ethyl acetate, hexane, and aqueous. The laboratory experiment on Petri plates was carried out at a series of concentrations: 0.0, 2.5, 5.0, 7.5, and 10.0 %, and distilled water was used as a control. The result showed that the ethyl acetate leaf crude extract was the most phytotoxic, with 100% inhibition on seed germination, plumule length, and radicle length at 2.5% concentration. The ethyl acetate extracts also significantly (p<0.05) reduced the total colour difference between O. verticillata and E. colona leaf disc to 71-78%, respectively at 10.0% concentration compared to the control. The isolation of the ethyl acetate leaf extract further yielded nine fractions. The highest suppressive ability was exerted by fractions five and six with 100% inhibition on seed germination and seedling growth. This indicated that the compounds responsible for the phytotoxic effect resided within these two fractions. Five major compounds: stigmasterol (ST), ursodeoxycholic acid (UDCA), naphthalene (NAP), bis (2-ethylhexyl) adipate (DEHA), and bis (2- thylhexyl) phthalate (DEHP) were identified from fractions five and six. The results of the dose-response studies in the nursery showed that DEHP, DEHA and UDCA exhibited strong toxicity on weed germination of E. colona under nursery conditions at the higher dosage of 2.4 - 4.8 kg ai ha-1 with the value of inhibition ranging from 18-54%. Further, the result of the phytotoxicity studies of DEHP, DEHA, and UDCA on E. colona and the tolerance of rice (MR220-CL2) seedlings to the compounds under aerobic conditions in the nursery showed that DEHP at the application rate of 1.2 - 4.8 kg ai ha-1 was able to inhibit germination and reduce the shoot fresh weight of E. colona with 78-81% and 58- 64%, respectively without injury to rice seedlings. The effect of DEHP on E. colona in the field-like experiment further revealed that significant inhibition was achieved at 2.4 - 4.8 kg ai ha-1 with suppression of 72-86% on seed germination of the weed. At these similar rates, DEHP significantly affected the rice yield and yield components (except number of panicles per plant and panicle length) compared to the controls. The persistence study of DEHP incubated in sterilized and non-sterilized soil revealed that ED50 value of DEHP on seed germination in non-sterilized soil was lower (1.2 kg ai ha- 1 ) compared to sterilized soil (2.39 kg ai ha-1 ). The results of this study also showed that incubation time of DEHP with soil between 12-28 days had no major effect on the DEHP persistence and further, suggesting that the microbes did not affect much of the degradation of DEHP in the soil.