Graduate Thesis Or Dissertation

Bioherbicide use of plant defensive compounds in meadowfoam seed meal

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  • Meadowfoam (Limnanthes alba Hartw. ex Benth) seed meal (MSM), a by-product of meadowfoam oil extraction, has a plant defensive compound known as glucosinolate glucolimnanthin (GLN). Myrosinase enzymes present in soil microbes and meadowfoam seeds can convert GLN to glucosinolate breakdown products (GBPs), which demonstrate herbicidal activity and have the potential to be used as bioherbicides. The goals of this research were to evaluate the effectiveness of MSM on weed control and to explore the optimal timing, rate, and application method for further use of MSM as a bioherbicide. Adding active myrosinase from freshly ground meadowfoam seeds to MSM increased its phytotoxicity. In a greenhouse study, no lettuce emergence was observed for six days in soil amended with 3% by weight activated MSM. In a field application, MSM provided a nitrogen source and promoted lettuce growth when lettuce seedlings were transplanted seven days after MSM incorporation. Co-occurrence of herbicide and fertilizer effects was observed with all MSM concentrations. MSM concentrations of 5% and 7% provided greater weed emergence suppression than the 3% concentration but no difference in weed biomass was observed between MSM concentrations. A split MSM application resulted in a significant benefit for weed control, similar to a single MSM application; however for the split application, the concentration and time should be adjusted to prevent residual crop injury. Activated MSM inhibited spiny sowthistle (Sonchus asper (L.) Hill) greater than 95% for emergence and 80% for biomass compared to the untreated control. Soil microbes reallocated carbon input from MSM application to biomass and enzyme production. The reallocation occurred quickly, within 7 to 14 days, after MSM application. Microbial biomass increased by at least 85% for carbon and 95% for nitrogen with MSM application compared to the untreated control. β-N-acetylglucosaminidase activity was highly correlated with microbial biomass nitrogen and was involved in the acquisition of nitrogen from organic sources. Isothiocyanate showed potent herbicidal activity and was detected only in activated MSM. 3-Methoxyphenylacetic acid (MPAA), a previously unidentified GBP with herbicidal activity, was discovered in soil amended with non-activated and activated MSM. A single MSM application at 2.86 kg m⁻² as a pre-emergent soil amendment benefited crop yield, weed suppression, and soil carbon and nitrogen inputs.
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