NITRATE UPTAKE RATE BY SOYBEAN AND WHEAT PLANTS DETERMINED BY EXTERNAL NITRATE CONCENTRATION AND SHOOT-MEDIATED DEMAND

When NO3- is maintained at concentrations greater than those required to sustain maximum cumulative uptake by wheat (Triticum aestivum) and nonnodulated soybean (Glycine max [L.] Merrill), the net rate of uptak e is not constant but oscillates between maxima and minima. The amplit udes of both the maxima and the minima increase with increases in NO3- concentration of the solution. To determine if the amplitudes of osci llations decline when NO3- in solution is maintained at suboptimal con centrations, net rates of NO3- uptake were monitored daily during 22 d of vegetative growth under controlled environmental conditions for wh eat from solutions containing 0.1, 0.25, 0.4, 0.5, 0.8, 1.5, 3.0, and 5.0 mM NO3- and for soybean from solutions containing 0.05, 0.1, 0.5, and 1.0 mM NO3-. The amplitudes of variations in daily net rates of NO 3- uptake increased with the increases in concentration of NO3- in the nutrient solutions. The differences in magnitude of the amplitudes in oscillation, expressed as coefficients of variability for daily uptak e rates, were positively related to calculated rates of export of nitr ogen from root to shoot. To explain these oscillations in the long-ter m patterns of net rates of NO3- uptake, acquisition of nitrogen by pla nts is viewed as the summation of two separate mechanisms. One mechani sm, which is dependent on NO3- concentration in solution, regulates ne t movement of nitrogen from the external solution into the root sympla sm. The second mechanism, which is contingent both on availability of nitrogen in the symplasm and on shoot activity via phloem transport of substrates such as amino acids and carbohydrates, regulates movement of nitrogen from the symplasm into the xylem for translocation to the shoot.