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

Author
item AGRELL, DAVID - DEPT BOTANY, STOCKHOLM UN
item LARSSON, CARL-MAGNUS - NATL CHEM INSP, SWEDEN
item LARSSON, MARIE - SWEDISH EPA, RES & DEV
item Mackown, Charles
item RUFTY, JR, THOMAS - NO CAROLINA STATE UNIV

Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Nitrogen (N) availability affects plant growth and development as well as the metabolic processes controlling these responses. Growth at low N leads to a greater investment of photosynthate for root growth and increased partitioning of N in the plant to support root growth. Nitrate absorbed by the plant must be assimilated before the absorbed N can support growth; assimilation occurs in both the root and shoot, but is difficult to quantitatively measure the distribution of nitrate assimilation between the root and shoot. We used stable isotope techniques and short term exposures of 5 to 60 minutes to determine the distribution of nitrate assimilation by barley plants grown at different levels of N sufficiency, thereby minimizing uncertainties due to redistribution of absorbed and assimilated nitrate. As the level of N sufficiency increased, the absolute amounts of nitrate absorbed increased, but the proportion of nitrate assimilated and recovered in the root decreased. These measurements were collaborated wit measurements of xylem sap transport of nitrate from the root to the shoot and activity measurements of root and shoot nitrate reductase, the first enzyme catalyzing the conversion of nitrate to forms of N used to support growth. These results indicate that adjustments in nitrate uptake and assimilation forms a continuum in response to nitrate levels in the root environment and implies that regulation of these processes is closely coupled to plant N status. The results will be useful to biologist attempting to manipulate nitrate assimilation of economically important plants. Technical Abstract

Technical Abstract: 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 per day, which represent different degrees of growth limiting nitrate supply. Root nitrate reduction and N transport in situ were estimated using labelled 15N-nitrat exposures ranging from 5 to 60 min. With increasing RA, nitrate uptake in absolute terms increased, but the proportion of absorbed 15N-nitrate that was reduced in the root decreased markedly. After 10 to 20 min. of exposure to the label, 75, 49, and 27% of the 15N taken up was recovered as reduced 15N in the root, at RAs 0.04, 0.09, and 0.14 per day, respectively. The response pattern was supported by root nitrate reductase activities and xylem sap nitrate measurements. The decreasing proportion of reduced 15N in the root with higher nitrate supply rates was matched by a relative increase in 15N-nitrate storage and, to some extent, an increase in N transport to the shoot. Although small amounts of 15N 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 15N uptake, which indicates delayed movement of 15N out of the root. The results clearly indicate that differing degrees of adjustment occur in important nitrate assimilation processes throughout the N deficiency range.