The Effect of Short-Term H2S Fumigation on Nitrate Reductase Activity in Spinach Leaves

Short-term exposure of spinach plants to 250 ppb H₂S at a photonfluence rate of 35µmol m^–2s^–1 (within the400–700 nm range) in the ambient air did not affect invitro nitrate reductase activity (NRA) in the leaves. Likewise,H₂S exposure did not significantly affect in vivo NRA measuredunder anaerobic conditions. In vivo NRA of untreated plantswas apparently inhibited in the presence of oxygen. However,shortterm H₂S exposure increased in vivo "aerobic" NRA up tofive fold of that of untreated plants. H₂S induced increaseof in vivo "aerobic" NRA depended on the sulfide concentration.After 24 hours of exposure maximal increase (two to five fold)of in vivo NRA "aerobic" was observed at 220 ppb H₂S. It isproposed that H₂S inhibited NADH oxidizing enzymes, which resultedin an increase in NADH supply to nitrate reductase (NR) in thepresence of oxygen. It was unlikely that the increase in invivo "aerobic" NRA in sulfide exposed plants was due to an alteredcompetition between mitochondrial respiration and NR since leafrespiration was not affected by an exposure to 250 ppb H₂S (Received February 12, 1986; Accepted June 27, 1986)