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SELENIUM IN SELENIFEROUS ENVIRONMENTS 27 nutrient cations and the micronutrient metals in interactions which often resulted in yield differences. Where the influences were relatively well balanced, plant growth was favorable and yields were relatively constant. Broyer et al. (1972a) observed floral initiation in 6 of 25 replicate Astragalus bisulcatus grown in 20 µg Se/L as selenite, whereas no flowering occurred in plants grown in the control cultures. This result, indicating a possible beneficial effect of Se, was also noted by Trelease and Trelease (1939) with A. racemosus. However, Broyer et al. (1972a) believed that the relatively large growth benefit from Se application reported by them for A, racemosus was at least in part, possibly related, to corrected phosphate toxicity in solution culture. If either A. bisulcatus (accumulator) or A. crotolariae (nonaccumula- tor) has a requirement for Se, the critical level probably would be < 80 µg Se/kg dry plant tops (Broyer et al., 1972a). This value is of the same order reported for alfalfa and/or subterranean clover (Trifolium subterraneum L.) (Broyer et al., 1966, 1972a). These values are much less than the comparable data suggested by Trelease and Trelease (1939) and Rosenfeld and Beath (1964), which were 3000 times and 15 000 times greater for the two legume crops, respectively. Selenium Compounds in Plants Shift (1973) has summarized the findings on the many chemical com- pounds of Se isolated from plants. Much of the Se in nonaccumulating species is found in the form of protein-bound selenomethionine. In contrast, the Se in accumulator plants is mostly water-soluble and found in nonprotein forms like Se-methylselenocysteine. Only trace amounts of the methylseleno- cysteine are found in the nonaccumulator species. Metabolic pathways of Se in plants probably diverge at the Se-cysteine stage to go toward protein- bound selenomethionine in nonaccumulators and to the nonprotein forms like methylselenocysteine in accumulator plants (Shrift, 1969, 1973). Lewis (1976) summarized the metabolic differences between Se accumu- lators and nonaccumulators. selenomethionine, selenocystine, and possibly Se-methyl selenomethionine and selenonium have been detected in non- accumulators but not in the accumulators that have been tested. Accumula- tor species can form selenocystathionine from selenate or selenite. The subsequent formation of methylselenocysteine has been found to occur only in accumulators and has been suggested as a biochemical basis for distin- guishing accumulators from nonaccumulators. Although little incorporation of the selenoamino acids into proteins oc- curred in the accumulator plants tested (Peterson & Butler, 1962), extensive protein incorporation of selenoamino acids occurred in ryegrass, wheat, and clover. Lewis (1976) suggested that Se accumulator species have evolved a detoxification mechanism whereby Se is excluded from protein incorpora- tion, whereas nonaccumulator species do not have this mechanism. Seleni- um incorporated into proteins could result in alteration of the protein structure, inactivation of the protein, and eventual poisoning of the plant (Peterson & Butler, 1962).