Seasonal variation of two phenolic amines in Acacia berlandieri

The low growing, multi-stemmed shrub Acacia berlandieri Benth. is widely distributed through the Rio Grande Plains of Texas, where it is an important component of the forage supply to both deer and domestic herbivores. The plant is known to contain a number of phenolic amines, including tyramine and N-methyl-â-phenethylamine (NMP). Phenolic amines, like other secondary compounds, may be used as a defence against herbivory, and NMP has been implicated in the occurrence of a hind-limb ataxia in animals consuming A. berlandieri forage over extended periods. Current hypotheses advanced to explain the effects of environmental changes (including herbivory) on phenotypic exhibition of secondary compound metabolism may not apply to plants adapted to semi-arid environments. We examined the seasonal changes in concentrations of phenolic amines in mature and regrowth leaves of A. berlandieri during the 1989 and 1990 growing seasons. Tyramine concentrations, as measured by high performance liquid chromatography, were consistently higher than NMP concentrations in both mature (4·4 ± 0·14 vs. 1·5 ± 0·12) and regrowth (5·4 ± 0·35 vs. 1·8 ± 0·31mg.g DM–1) leaf. Tyramine and NMP concentration in mature leaves did not appear to be greatly influenced by rainfall amount or distribution or by seasonal variation. Regrowth leaf contained higher concentrations of both tyramine and NMP than mature leaves, with tyramine and NMP concentrations increasing by 40 and 35% (3·7 ± 0·15 and 1·5 ± 0·13 to 6·2 ± 0·33 and 2·3 ± 0·28 mg.g DM–1), respectively, with additional defoliations. The apparent induction of both tyramine and NMP in regrowth leaf may adversely impact nutrition and reproduction of wild and domestic livestock browsing A. berlandieri, especially under conditions of heavy utilization.