Secondary metabolite distribution in guayule (Parthenium argentatum A. Gray): Differences between exuded and extracted resin and plant tissues

Abstract

Known best as an alternative source of natural rubber, guayule ( Parthenium argentatum A. Gray) is also a source of sesquiterpene esters (guayulins A-D), and triterpenoids (argentatins A-B, isoargentatins A-B) which show promise as larvicides, fungicides, and arthropod and mollusk antifeedants. These non-rubber secondary metabolites are isolated as guayule resin, the acetone-soluble component extracted from ground guayule tissues. Within the intact shrub, resin is found in a system of ducts separate from the rubber-bearing tissue. Analysis of these secondary metabolites revealed clear compositional variations in the resin extracted from different parts of the plant because the stems tend to cluster around argentatins and guayulin A, whereas roots tend to cluster around guayulin D and resin. The age (expressed as stem diameter) of the woody tissue showed no correlation between rubber and resin, but there are clear correlations with the accumulation of argentatins and guayulins C and D in the stems. Resin exuded from the duct system was isolated by cutting into stem tissue. Analyte contents slowly increased over 30 days, most probably due to the loss of volatile terpene and sesquiterpene components. Compared to resin isolated by conventional solvent extraction, the exuded resin contained substantially higher total analytes levels. Unexpectedly, guayulins C and D, likely degradation products of guayulins A and B, were absent in exuded resin.