Studies on nitrate reduction in higher plants. I

Abstract

1. 1. The distribution of N 15 followed the same pattern in all organs of intact plants, whether the nitrogen source was N 15-labeled ammonium or N 15-labeled nitrate; the ammonia fraction contained the highest atom per cent excess of N 15 in all cases. 2. 2. Plants which received ammonium contained the highest concentration of N 15 in the roots, whereas those which received nitrate contained the highest concentration in the leaves. 3. 3. Tomato leaf disks are able to assimilate nitrate in both the light and dark, but the rate of assimilation in the light is about 50% greater than in the dark. 4. 4. Carbohydrate depletion in leaf tissue results in a decreased nitrate reduction in the dark, whereas it is without effect in the light. 5. 5. Addition of oxidizable substrates to carbohydrate-depleted leaf tissue increased the dark reduction of nitrate, although not to the level of the fresh tissue. 6. 6. Iodoacetate markedly inhibits the reduction of nitrate in leaf disks incubated in the dark, but has no effect on the process in the light. 7. 7. Ammonia isolated from leaf disks incubated 30 min. in the light contained an N 15 enrichment almost two-thirds that of the original nitrate. The ammonia from the dark reaction was not so highly labeled, containing only about one-seventh the enrichment of N 15 of the medium. 8. 8. Evidence for two mechanisms of nitrate assimilation are obtained: one dependent upon respiration as an energy source, and the other involving a photochemical reduction.