Comparative effects of selenite and selenate on nitrate assimilation in barley seedlings.
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-3 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 SO3=, 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 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 the 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.
Aslam
M
M
Department of Agronomy and Range Science, University of California, Davis 95616.
Harbit
K B
KB
Huffaker
R C
RC
D001692:Biological Transport; D001467:Hordeum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D018527:Hydroponics; D009565:Nitrate Reductases / Q000378:metabolism; D009566:Nitrates / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009572:Nitrite Reductases / Q000378:metabolism; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism; D018517:Plant Roots / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D064586:Selenic Acid; D018036:Selenium Compounds / Q000494:pharmacology* / Q000633:toxicity; D018038:Sodium Selenite / Q000494:pharmacology* / Q000633:toxicity; D013997:Time Factors
D009566:Nitrates; D010940:Plant Proteins; D018036:Selenium Compounds; D009565:Nitrate Reductases; D009572:Nitrite Reductases; D018038:Sodium Selenite; D064586:Selenic Acid