Response of Conifer Seedlings to Potting Medium Amendment with Meadowfoam Seed Meal

Meadowfoam (Limnanthes alba L., Limnanthaceae) is an herbaceous winter-spring annual, grown as an oilseed crop in the Willamette Valley of Oregon, US and in New Zealand. The exceptionally high quality seed oil, consisting almost entirely of long-chain fatty acids, is used in cosmetics and lubricants. The seed meal (MSM) remaining after extraction of the oils with hexane solvent under elevated temperature is a granular by-product with little apparent commercial value, at present, except as an organic fertilizer or as an additive to livestock feed. However, MSM has some characteristics that suggest its potential utility in agriculture as a soil amendment to enhance plant growth and possibly suppress soil pests. For example, MSM contains the glucosinolate glucolimnanthin (Bartlett and Mikolajczak 1989; Vaughn et al. 1996) that, when hydrolyzed in the presence of the enzyme thioglucosidase glucohydrolase (myrosinase), can produce allelochemic compounds (aglycones), including isothiocyanates, thiocyanates, nitriles, and ionic thiocyanate (Van Etten and Tookey 1983), which have been shown to be potent biocides in soil (Bartlett and Mikolajczak 1989; Brown and Morra 1997; Deuel and Svensen 1999). Furthermore, MSM has been shown to have plant growth-enhancing properties of a phytohormonal nature (W.A. Deuel and S.E. Svensen, 1998, unpubl.; Deuel, US Patent No. 6,596,323 B1, 2003), but the nature of the responsible compound(s) remains unknown. At concentrations of MSM amendment to soil >2% by volume, inhibition of seed germination and other herbicidal properties occurs. Vaughn et al. (1996) demonstrated the herbicidal component in MSM to be 3-methoxyphenyl acetonitrile. Deuel and Svensen (1999) demonstrated that soil amendment with MSM reduced the incidence and severity of clubroot of cabbage and cauliflower in either soil or potting medium. The enzyme myrosinase needed to hydrolyze glucosinolates (Rosa et al. 1997) is present in the seed, and is produced by some soil microbes (Tani et al. 1974), but it and the glucosinolate are denatured during the hot hexane process of oil extraction, leaving only acetonitrile as a by-product in the MSM. Thus there is little or no potential to hydrolyze glucosinolate to S-containing volatile biocides in soil, such as isothiocyanates, as is known to occur with cruciferous crop residues (Vaughn 1999; Gamliel and Stapleton 1983). The unusual growth-enhancing properties of MSM prompted an examination of it as an amendment to soilless potting medium on the growth of conifer seedlings commonly grown in container nurseries in the Pacific Northwestern US. The findings from those studies are reported here.