INITIAL KINETICS OF N-15-NITRATE LABELING OF ROOT AND SHOOT N FRACTIONS OF BARLEY CULTURED AT DIFFERENT RELATIVE ADDITION RATES OF NITRATE-N

The reduction of absorbed nitrate in the root and N transport to the s hoot were studied in young barley (Hordeum vulgare L.) plants growing at low external nitrate levels. Plants were grown at three relative ad dition rates (RA) of nitrate: 0.04, 0.09, and 0.14 d(-1), which repres ent different degrees of growth limiting nitrate supply. Root nitrate reduction and N transport in situ were estimated using N-15 labelled n itrate exposures ranging from 5 to 60 min. With increasing RA, nitrate uptake in absolute terms, increased, but the proportion of absorbed N -15-nitrate that was reduced in the root decreased markedly. After 10- 20 min of exposure to the label, 75, 49, and 27 % of the N-15 taken up was recovered as reduced N-15 in the root at RAs 0.04, 0.09, and 0.14 d(-1), respectively. The response pattern was supported by root nitra te reductase activities and xylem sap nitrate measurements. The decrea sing proportion of reduced N-15 in the root with higher nitrate supply rates was matched by a relative increase in N-15-nitrate storage anc, to some extent, an increase in N transport to the shoot. Although sma ll amounts of N-15 were rapidly transported to the shoot, the accumula tion of label in the shoot at the end of the 60 min period remained a relatively small proportion of total N-15 uptake, which indicates dela yed movement of N-15 out of the root. The results clearly indicate tha t differing degrees of adjustment occur in important nitrate assimilat ion processes throughout the N deficiency range.