Partitioning of nitrate assimilation between shoots and roots of Kentucky bluegrass

Turfgrass roots play an important role in the overall metabolism of NO3/- 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 NO3/- 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 NO3/-. These grasses were analyzed for relative growth rate (RGR), shoot/root ratio (S/R), in situ NO3/- uptake rate (NUR), in vivo NO3/- reductase activity (NRA), metabolic NO3/- pool (MNP), and storage NO3/- pool (SNP) in shoots and roots. Under low NO3/- levels, Merit exhibited a higher NUR, a larger root SNP, a smaller shoot SNP and a greater RGR than Livingston. Under high NO3/- levels, Livingston exhibited a greater shoot growth rate and reduced a greater proportion of NO3/- in its shoots than did Merit. In both cultivars, root contribution to the plant total NO3/- reduction (PTNR), estimated from NRA, was \u3c5%. 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 NO3/- assimilation between shoots and roots