Partitioning of Nitrate Assimilation between Shoots and Roots of Kentucky Bluegrass

Abstract

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.