Relationship between Nitrate Uptake, Flux, and Reduction and the Accumulation of Reduced Nitrogen in Maize (Zea mays L.): II. EFFECT OF NUTRIENT NITRATE CONCENTRATION.

Two maize hybrids were grown under growth chamber conditions on solution or vermiculite medium that contained 2.5, 7.5, or 15 millimolar nitrate. The objectives were to determine: (a) the effect of nitrate supply on N metabolism and growth and (b) the interrelationship between nitrate uptake, flux, and reduction on the accumulation of reduced N and nitrate by the various plant parts and for the whole plant.Increases in nutrient nitrate concentration caused increases in (a) shoot, but not root, dry weight and (b) nitrate uptake, flux, and reduction and accumulation of nitrate and reduced N by the aerial plant parts of both hybrids. Increases in nitrate supply resulted in decreases in nitrate reductase activity and negligible increases in reduced N in the roots of both hybrids. At 2.5 and 15 millimolar, but not at 7.5 millimolar, external nitrate, hybrid B had higher rates of nitrate uptake and flux. Hybrid B also had lower nitrate reductase activity at all levels of external nitrate and accumulated less reduced N than did hybrid C, except when the plants were grown at 2.5 millimolar nitrate. Correlation values between nitrate uptake and flux were significant for each hybrid and for both hybrids, whereas correlations between nitrate reductase activity and flux and nitrate reductase activity and uptake were significant only for a given hybrid. The correlation values (pooling of all data) between (a) nitrate uptake, flux, or reduction and the accumulation of reduced N by the whole plant, and (b) flux x nitrate reductase activity and accumulation of reduced N were all positive and statistically significant. Because nitrate uptake, flux, and reduction (as measured) were all closely associated with the accumulation of reduced N by the plant, all three parameters should be measured in attempts to estimate the genetic potential of a maize genotype to accumulate reduced N.