The chemical composition of Astragalus: a comparison of seleniferous and non-seleniferous plants growing side by side

Abstract

COWGILL, U. M. & LANDENBERGER, B.D., 1992. The chemical composition of Astragalus: a comparison of seleniferous and non‐seleniferous plants growing side by side. This paper describes how non‐seleniferous plant species coexist with seleniferous ones and what chemical changes occur in non‐seleniferous species that allow the toleration of large quantities of volatilized Se‐bearing compounds. These compounds are known for their phytotoxicity as well as for their toxicity to mammals and insects. Twenty‐three sites (Colorado, Utah, New Mexico, U.S.A.) were examined over a 6‐year period. Plants collected from these sites were divided into four groups: seleniferous astragali, seleniferous Brassicaceae, non‐seleniferous astragali and non‐seleniferous associated genera. Furthermore, it was possible to categorize these sites: Type 1 sites supported all four groups of plants, Type 2 sites contained only seleniferous astragali, whereas Type 3 sites sustained only non‐seleniferous astragali. When concentrations of Cu, Zn (P <0.0001), Si (P<0.02), As, Pb (P<0.03) and Cs (P<0.05) were measured, the values for those seleniferous astragali that coexisted with other species were significantly different from the values found for those seleniferous astragali that grew alone. There were four areas that in some years supported all four groups of plants (Type 1 site) and in other years contained only seleniferous astragali. In this way, the latter may be used as a chemical control for the former. The seleniferous astragali collected at Type 1 sites always contained more Cu, As, Si and Pb than the solitary astragali of Type 2 sites. When seleniferous astragali of Type 1 sites that support all four groups of plants develop in a year where the usually associated species of past years are absent, they contain in their tissues concentrations of Cu, As, Si, Pb, Zn and Cs typical of Type 2 sites where the seleniferous astragali grew alone. Non‐seleniferous astragali of Type 1 sites that support all four groups of plants have a chemical composition that differs significantly in the quantity of 15 elements from non‐seleniferous astragali of Type 3 sites where the latter grew alone. Since allelochemicals such as phenolic acids and flavonoids are well known to be able to alter mineral absorption by plants and since the astragali are known to produce such substances, it is suggested that the possible production of phenolic acids and flavonoids may permit coexistence of seleniferous plants with non‐seleniferous ones and thus explain the chemical differences among the plants of the four types of sites.