8153

• UCREFRP Catalog Number: 8153
• Author: Jacobs, e. L. W.
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1989
• Copyright Material: YES

28 MAYLAND ET AL.
Selenium-containing transfer ribonucleic acids (tRNA) have been found
in a number of aerobic and anaerobic bacteria, mammalian cells, and in Se-
accumulating and nonaccumulating plants (Wen et al., 1988). The Se-
containing tRNA appears to be ubiquitous in the plant kingdom. However,
the distribution of selenonucleoside(s) in tRNA species varies both qualita-
tively and quantitatively in different plants. This information provides evi-
dence for a possible biological role of Se in higher plants. Nevertheless, the
possibility that Se substituted for S in the nucleoside has not been evaluated.
The Se metabolites in plants are, generally, analogs of S compounds.
Nevertheless, Se metabolism in non-Se-accumulator plants cannot be identi-
fied from known mechanisms involving Se metabolism (Shrift, 1973). Our
understanding of the metabolic pathways for Se in plants remains very limited.
Many microorganisms can reduce selenite to elemental Se. Some bac-
teria and fungi have been found that reduce selenite or selenate to volatile,
organic Se compounds (Shrift, 1973). Sulfur-Se antagonism occurs in these
microorganisms. The reduction of selenite is enzymatically mediated, and
many isolated enzyme systems can utilize S and Se analogs interchangeably.
However, what was traditionally thought to be the mechanism of Se toxici-
ty, namely, a general interference with the enzymes involving S assimilation,
has proved to be more complex.
Selenium Toxicity in Plants
The toxicity of Se on the growth of grasses was noted as early as 1880
(Brown & Shrift, 1982). However, the concern about selenosis in plants did
not really develop until the identification of selenosis in livestock and the
probable Se-associated reductions in wheat production in the seleniferous
areas (O.E. Olson, 1986 personal communication).
Selenium toxicity to nonaccumulator plants was described by Singh and
Singh (1978). They treated soil in the greenhouse with 0, 2.5, 5, or 10 mg
Se/kg as selenite and measured relative wheat-grain production of 100, 72,
17, and 11%, respectively. The grain contained 0.6, 10.2, 24.5, and 36.8 mg
Se/kg, respectively. The application of Se to the soil decreased the growth
of wheat and sunflower (Helianthus annuus L.); the absorption of Zn, Cu,
Fe, Mn, S, and N; and the synthesis of nucleic acids, chlorophyll, and pro-
tein, whereas additions of 50 or 100 mg P/kg as KzPO4 decreased the harm-
ful effect of Se and increased the values of each of the parameters tested.
Other plants, namely those classified by Rosenfeld and Beath (1964) as
primary or secondary indicator plants, tolerate and possibly require Se for
growth. These indicator plants differ from the nonaccumulator species in
several ways. Large quantities of Se-methylselenocysteine and selenocystathio-
nine, two nonprotein selenoamino acids rarely detected in nonaccumulators,
have been isolated from the tissues of Se accumulators (Brown & Shrift, 1982).
Also, Se is kept from entering proteins so that the Se level in proteins of
accumulator plants is significantly lower than the level in Se-sensitive plants.
Exclusion of Se from the proteins of accumulators is thought to be the basis