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Department of Plant Sciences, University of Rhode Island, Kingston, RI 02881
* Corresponding author (rhu6441u{at}postoffice.uri.edu).
Turfgrass roots play an important role in the overall metabolism of NO–3 partly because leaves of turfgrasses are partially lost during mowing. Nitrate transported to shoots stimulates shoot growth and decreases N use efficiency through clipping removal. This study was conducted to quantify NO–3 reduction by shoots and roots of Kentucky bluegrass (Poa pratensis L.). Two cultivars, Livingston, which performed better under high N fertility, and Merit, which performed better under low N fertility, were grown in aerated nutrient solution containing 0.1 to 5.2 mM NO–3. These grasses were analyzed for relative growth rate (RGR), shoothoot ratio (S/R),in situ NO–3 uptake rate (NUR), in vivo NO–3 reductase activity (NRA), metabolic NO–3 pool (MNP), and storage NO–3 pool (SNP) in shoots and roots. Under low NO–3 levels, Merit exhibited a higher NUR, a larger root SNP, a smaller shoot SNP and a greater RGR than Livingston. Under high NO–3 levels, Livingston exhibited a greater shoot growth rate and reduced a greater proportion of NO–3 in its shoots than did Merit. In both cultivars, root contribution to the plant total NO–3 reduction (PTNR), estimated from NRA, was <5%. When dissolved O2 was decreased and root carbohydrate content increased, root contribution to PTNR increased to 40 and 15%, respectively. Our results suggest that root carbohydrate status and root-zone O2 levels strongly influence root contribution to PTNR and N use efficiency of Kentucky bluegrass in response to N fertility. This in turn is a function of the partitioning of NO–3 assimilation between shoots and roots.
Received for publication July 7, 1998.
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