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