Partitioning of previously-accumulated nitrate to translocation, reduction, and efflux in corn roots

The effect of nitrate uptake, or its absence, on the utilization of nitrate previously accumulated by dark-grown, decpitated maize (Zea mays L., cv. DeKalb XL-45) seedlings was examined. Five-d-old plants that had been pretreated with 50 mM (14)NO 3 (-) for 20 h were exposed for 8 h to nutrient solutions containing either no nitrate or 50 mM (15)NO 3 (-) , 98.7 atom % (15)N. The ambient solution, xylem exudate, and plant tissue were analyzed to determine the quantities of previously-accumulated (endogenous) (14)NO 3 (-) that were translocated to the xylem, lost to the solution, or reduced within the tissue during the 8-h period. Energy was continuously available to the roots from the attached endosperm. In the absence of incoming nitrate, appreciable reduction and translocation of the endogenous (14)NO 3 (-) occurred, but efflux of (14)NO 3 (-) to the external solution was minimal. In contrast, during (15)NO 3 (-) uptake, there was considerable efflux of (14)NO 3 (-) as well as translocation of (14)NO 3 (-) to the xylem, but little (14)NO 3 (-) was reduced. Thus there appeared to be an inverse relationship between (14)NO 3 (-) efflux and reduction. The data are tentatively interpreted on the basis of a model which envisages (a) two storage locations within roots, one of which primarily supplies nitrate for translocation and the other of which primarily supplies nitrate for outward passage through plasmalemma, and (b) the majority of nitrate reduction as occurring during or immediately following influx across the plasmalemma, with endogenous (14)NO 3 (-) initially moving outward being recycled inward and thereby being reduced.