Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 27, 1997
Publication Date: N/A
Interpretive Summary: The occurrence of toxic elements in irrigation drainage waters necessitates that these waters be properly managed to limit their impacts on the environment. A major problem with disposal of irrigation return waters from the west-central San Joaquin Valley of California is the inadvertent cycling and concentration of the trace element selenium (Se) in evaporation pond sediments. The toxicity of Se in nature has been suggested to result from the alteration of the three-dimensional structure of proteins by the incorporation of Se analogs of S-containing amino acids. Little is known on the introduction of the selenoamino acids into the environment. This manuscript details the assimilation of inorganic Se into selenoamino acids by alfalfa and distribution of the selenoamino acids upon alfalfa decomposition in soil.
Technical Abstract: The toxicity of selenium (Se) in nature has been suggested to result from the alteration of the three-dimensional structure of proteins by the incorporation of Se analogs of S-containing amino acids. The occurrence of Se-containing amino acids in irrigation evaporation pond systems has been attributed to microbial synthesis and food-chain accumulation patterns but little research has been conducted on Se accumulation by plant tissue assimilation and decomposition cycling. This study investigates the assimilation of soluble inorganic Se (selenate) by alfalfa (Medicago sativa) into Se amino acids and the resulting Se oxidation state distribution after microbial decomposition of alfalfa residue. Alfalfa growth studies indicate that the majority of nonprotein Se assimilated (25% of total Se assimilated) was as free Se-amino acids, Se-methyl-cysteine, selenomethionine and selenocyst(e)ine. Aerobic alfalfa decomposition studies (60 d) showed that 30% of the assimilated Se was mineralized to water-soluble, nonamino acid selenide-Se, while the remaining organic selenide-Se persisted in protein form. The data suggests that each plant growth-decomposition cycle results in a substantial reduction of soluble inorganic Se and a large increase in organic Se levels in biological systems. A 3.5-fold decrease in the C/Se ratio was determined in the alfalfa residue after decomposition compared to the initial C/Se ratio suggesting that Se enrichment noted in evaporation pond systems may partially result from plant Se cycling.