Aqueous extracts of fresh leaves and organic soil of northern sheep laurel (Kalmia angustifolia var. angustifolia) were found to be inhibitory to the growth of black spruce (Picea mariana) germinants. Primary root growth of black spruce was more affected by the extracts than was shoot growth. The growth inhibition caused by the leaf extract was most pronounced under acidic conditions (pH 3-4). The aqueous extract of Kalmia leaves contained ferulic, vanillic, syringic, gentisic, m-coumaric, p-coumaric, o-hydroxyphenylacetic, and p-hydroxybenzoic acids as well as some other unknown compounds. These compounds were isolated from the aqueous extract of Kalmia leaves by ethyl acetate extraction and identified using thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). Bioassay indicated that the overall toxicity of the phenolic compounds to black spruce appeared to increase in the order of o-hydroxyphenylacetic, p-hydroxybenzoic, vanillic, p-coumaric, gentisic, syringic, ferulic, and m-coumaric acids.
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