No abstract

Intact preparations of plastids from pea (Pisum sativum L.) roots have been used to investigate the metabolism of glucose-6-phosphate and reduction of inorganic nitrite within these organelles. The ability of hexose-phosphates to support nitrite reduction was dependent on the integrity of the preparation and was barely measurable in broken organelles. In intact plastids, nitrite was reduced most effectively in the presence of glucose-6-phosphate (Glc6P), fructose-6-phosphate and ribose-5-phosphate and to a lesser extent glucose-1-phosphate. The Km (Glc6P) of plastid-located Glc6P dehydrogenase (EC 1.1.1.49) and Glc6P-dependent nitrite reduction were virtually identical (0.68 and 0.66 mM respectively) and a similar relationship was observed between fructose-6-phosphate, hexose-phosphate isomerase (EC 5.3.1.9) and nitrite reduction. The pattern of release of CO2 from different carbon atoms of Glc6P supplied to root plastids, indicates the operation of both glycolysis and the oxidative pentose-phosphate pathway with some recycling in the latter. During nitrite reduction the evolution of CO2 from carbon atom 1 of Glc6P was stimulated but not from carbon atoms 2, 3, 4, or 6. The importance of these results with regard to the regulation of the pathways of carbohydrate oxidation and nitrogen assimilation within root plastids is discussed.

(15)N-labelled nitrate was used to show that nitrate reduction by leaf discs in darkness was suppressed by oxygen, whereas nitrite present within the cell could be reduced under aerobic dark conditions. In other experiments, unlabelled nitrite, allowed to accumulate in the tissue during the dark anaerobic reduction of nitrate was shown by chemical analysis to be metabolised during a subsequent dark aerobic period. Leaves of intact plants resembled incubated leaf discs in accumulating nitrite under anaerobic conditions. Nitrate, n-propanol and several respiratory inhibitors or uncouplers partly reversed the inhibitory effect of oxygen on nitrate reduction in leaf discs in the dark. Of these nitrate and propanol acted synergistically. Reversal was usually associated with inhibition of respiration but some concentrations of 2,4-dinitrophenol (DNP) and ioxynil reversed inhibition without affecting respiratory rates. Respiratory inhibitors and uncouplers stimulated nitrate reduction in the anaerobic in vivo assay i.e. in conditions where the respiratory process is non-functional. Freezing and thawing leaf discs diminished but did not eliminate the sensitivity of nitrate reduction to oxygen inhibition.

Applications of 56 and 112 kg N ha‐1 (as KNO3) on April 2 and 23 and May 9 and 224 kg N ha‐1 on April 23 to Arthur and Blueboy (soft red winter) and Parker (hard red winter) wheats increased yield of grain, grain protein and grain protein percentage without exception. The average increases for all treatments and varieties for grain yield, grain protein and percentage grain protein were 21, 63 and 36% over the respective control values. Based only on amino acid composition, quality of the grains was unaffected by the nitrogen treatments. Depletion of soil nitrogen estimated by removal of grain alone for all varieties occurred not only in the control plots but in plots receiving 56 and 112 kg N ha‐1. Only the 224 kg N ha‐1 treatment plots gave an estimated net gain in soil nitrogen. The favorable results obtained for grain and grain protein production, the increasing availability of lodgeresistant wheats, and the concern over nitrate pollution of ground water and streams, all indicate that the practice of split applications of nitrogen made late in the spring to wheat has merit.

SummaryA 14.5 kDa protein with antigenic components in common with pea leaf ferredoxin was detected on transblots of the soluble proteins of pea root plastids. The amount of this protein was found to increase during the induction of nitrate assimilation in pea roots, reaching a maximal level at S 1 2 h. Concurrent with this, a fourfold increase in NADPH-dependent ferredoxin-NADPC oxidoreductase (FNR) activity was obsewed corresponding to an increase in the amount of this protein detected immunologically on transblots using a leaf FNR antibody. These changes were not obsewed in plastids from roots of plants grown on ammonia or depleted of nitrogen. It is suggested that in addition to the already well reported induction by nitrate of nitrate reductase and nitrite reductase, there is a co-induction of a plastid located ferredoxin and FNR. Both these proteins are necessary for the transfer of reductant generated by the oxidative pemtose phosphate pathway to nitrite reductase.