Comparative effects of selenite and selenate on nitrate assimilation in barley seedlings

Abstract. The effect of SeO₃ and SeO₄ on NO₃ assimilation in 8-d-old barley (Hordeum vulgare L.) seedlings was studied over a 24-h period. Selenite at 0.1 mol. m^? in the uptake solutions severely inhibited the induction of NO₃ uptake and active nitrate reductases. Selenate, at 1.0 mol m^?3 in the nutrient solution, had little effect on induction of activities of these systems until after 12 h; however, when the seedlings were pretreated with 1.0 mol m^?3 SeO₄ for 24 h, subsequent NO₃ uptake from SeO₄-free solutions was inhibited about 60%. Sulphate partially alleviated the inhibitory effect of SeO₃ when supplied together in the ambient solutions, but had no effect in seedlings pretreated with SeO₃. By contrast, SO₄ partially alleviated the inhibitory effect of SeO₄ even in seedlings pretreated with SeO₄. Since uptake of NO₃ by intact seedlings was also inhibited by SeO₃, the percentage of the absorbed NO₃ that was reduced was not affected. By contrast, SeO₄, which affected NO₃ uptake much less, inhibited the percentage reduced of that absorbed. However, when supplied to detached leaves, both SeO₃ and SeO₄ inhibited the in vivo reduction of NO₃ as well as the induction of nitrate reductase and nitrite reductase activities. Selenite was more inhibitory than SeO₄; approximately a five to 10 times higher concentration of SeO₄ than SeO₃ was required to achieve similar inhibition. In detached leaves, the inhibitory effect of both SeO₃ and SeO₄ on in vivo NO₃ reduction as well as on the induction of nitrate reductase activity was partially alleviated by SO₄. The inhibitory effects of Se salts on the induction of nitrite reductase were, however, completely alleviated by SO₄. The results show that in barley seedlings SeO₃ is more toxic than SeO₄. The reduction of SeO₄ to SeO₃ may be a rate limiting step in causing Se toxicity.