Nitrate Reductase and Nitrite Reductase Activity in Dark‐Grown Radish Seedlings: Relation to the Supply of NADPH

Abstract

Abstract Functioning of nitrate reductase and nitrite reductase was measured in intact cotyledons from radish seedlings ( Raphanus sativus L.) grown in the dark in a nitrate medium. Reduction of nitrate to nitrate did proceed during the whole period of 45 h, whereas the reduction of nitrite in the intact cotyledons dropped abruptly between 20 and 23 h after exposing the roots to nitrate. The activity of the enzymes glucose‐6‐P dehydrogenase (G6PDH) and 6‐P‐gluconate dehydrogenase (6PGDH), measured in cotyledon extracts, showed a sharp decline simultaneously with the drop in nitrite reductase activity of the intact cotyledons. It was concluded that the amount of NADPH generated by the enzymes G6PDH and 6PGDH is not sufficient to allow continuous functioning of nitrite reductase after 20 h in cotyledons of seedlings grown in the dark. Therefore, the results from our experiments point to the functioning of nitrite reductase as the rate limiting step in the reduction pathway of nitrate in the dark.