Herbicidal β-triketones are compartmentalized in leaves of Leptospermum species: localization by Raman microscopy and rapid screening

Summary The New Zealand mānuka shrub, Leptospermum scoparium, and the Australian L. morrisonii produce herbicidal β-triketones in their leaves. The localization of these potential self-toxicants has not been proven. We investigated the localization of these compounds in leaves using Raman microscopy. The results are presented as heat maps derived from principal component analysis (PCA) of the Raman spectra from sampling grids of leaf sections. This approach used undirected, data-driven analysis to qualitatively distinguish localized plant chemistry. The presence of β-triketones and lipophilic flavonoids was confirmed by GC-MS and 1H NMR spectroscopy. Grandiflorone was compartmentalized within the leaf oil glands of L. morrisonii. Leptospermum scoparium also contained high concentrations of grandiflorone, previously reported as only a trace component in essential oils, localized in the oil glands in the leaves of varieties from diverse geographical locations. Raman microscopy was used to probe the chemistry of oil glands in several ornamental mānuka varieties, revealing high concentrations of bioactive flavonoids localized in these glands. The compartmentalization of β-triketones within oil glands inside leaves of Leptospermum shrubs may defend the plants against herbicidal activity.