UPTAKE, DISTRIBUTION, AND TURNOVER RATES OF SELENIUM IN BARLEY

The present communication elucidates initially the topographic distrib ution of selenium in barley grains. Then by the fluorimetric method th e uptake of selenium (selenite) in 8-16 d old germinating barley was e stimated. Finally by means of Se-75 the anabolic and catabolic rates ( turnover) of Se-75 (selenite) was compared. The distribution of seleni um in barley was evaluated after microdissection of barley grains. In dried grains the highest concentration was found in husk and pericarp with about 0.6 ppm Se. Then followed Scutellum with 0.4 and 0.3 ppm in embryon. The aleurone layer, embryonic leaves, and initial root did o nly have 0.2 ppm Se. In order to know more about the uptake and distri bution of selenium in 8-d-old barley, the plants were cultivated for a further 8 d in the culture medium with variation in selenite concentr ation. In roots and leaves, the uptake did not arrive at saturation du ring the period studied since the dose-response curve increased up to 0.34 mM selenite in the medium, whereas the selenium levels were about 200 ppm in roots and 30 ppm in leaves. However, the uptake was linear , with concentration during 8 d of cultivation up to 0.84 mu M selenit e for grain and stem. At higher concentrations the dose-response curve diminished its slope. At 0.34 mM selenite the concentration in grain increased to 6.87 ppm and in the stem to 8.13 ppm. The uptake, distrib ution, and catabolic rate of selenium components in germinating barley were further evaluated by exposing the plants to 0.0492 mu Ci(75)Se ( 12.6 mu M selenite) for up to 4 d. Then the plants were moved to a sel enium deficient medium for further 4 d. Then finally the medium was su pplemented with high doses of cold selenite (0.126 mM selenite) for fu rther 4 d. The first third period made it possible to estimate the rat e of uptake. It was highest in roots (313 fmol/h/mg dw), i.e., about 1 0 times those of grains, stems, and leaves. The intermediate period wh ere the barley was transferred to a selenium deficient medium made it possible to estimate the kinetics and eventual sparing mechanisms. The selenium losses were highest for leaves (39%), then followed by roots and stems (22 and 25%, respectively). The losses were lowest in grain with 9% Se losses. The losses were three times more pronounced during the first day than in the following 3 d. These data may argue that th e selenium is distributed into different pools and that sparing mechan isms may be in function. The last period, i.e., the chase experiment, revealed the rate of elimination of selenium under conditions with sur plus selenium. The catabolic rate was about 10 times faster in roots ( 169 fmol/h/mg dw) than in grains and about 8 times faster than in leav es.