Nitrate uptake and reduction in C3 and C4 grasses

Nitrate uptake and enzymatic reduction are important processes in regulating nitrogen (N) utilization of the plant and in reducing N losses from the plant-soil ecosystem. In turf-type and forage grasses, N can be lost by tissue removal through mowing or grazing and the efficiency of N utilization must be significantly affected by nitrate uptake and reduction in the roots. In the present study, we compared nitrate uptake by the roots and nitrate reduction in roots and shoots of one C-4 grass, bermudagrass (Cynodon dactylon [L.] Pers.), and three C-3 grasses, Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue (Festuca arundinacea Schreb.). Grasses were grown in a hydroponic system supplied with modified Hoagland's nutrient solutions containing 5.0 or 0.5 mM nitrate. Nitrate uptake was determined by a nutrient depletion method, and nitrate reductase activity (NRA) was determined by an in vivo method. Contrary to a hypothesis that C-4 grasses have more active nitrate uptake and reduction than do C-3 grasses because of the C-4 pathway for CO, fixation, nitrate uptake rate in bermudagrass was not higher, and NRA of bermudagrass shoots was consistently lower than that of the three C-3 grasses. However, bermudagrass exhibited a greater N use efficiency in the shoots, which was defined as dry matter per unit of N present in the tissue, and partitioned more of the plant total fresh weight and total NRA to its roots than did the three C-3 grasses. These results suggested that nitrate reduction in the roots significantly influenced the efficiency of N utilization of the regularly mowed or grazed grasses.