Relationship between Nitrate Uptake, Flux, and Reduction and the Accumulation of Reduced Nitrogen in Maize (Zea mays L.)

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Abstract
The study presented here was an extension of a preceding field project concerned with changes in N metabolism of four maize hybrids during grain development. The objectives were to relate uptake, flux, and reduction of nitrate to accumulation of reduced N in growth-chamber-grown seedlings of the same four hybrids and to compare these results with those obtained in the field study. Hybrid D took up more nitrate than the other three hybrids, primarily because of a larger root system. The correlations between total N (nitrate plus reduced N plant−1) accumulated by harvest and root dry weight or shoot to root ratios were r = +0.97 and −0.90, respectively. Correlations with shoot dry weight were low. Although the larger root system indicates enhanced partitioning of photosynthate to the root of hybrid D, the observations made do not elucidate the role of photosynthate in increasing nitrate uptake. There was no genetic difference in partitioning of nitrate (per cent of total) among the plant parts; however, the hybrids differed in amounts of nitrate stored in stalks and midribs. Hybrids D and B accumulated more nitrate than A and C. Although two of the hybrids (A and C) with highest nitrate reductase activity had the lowest concentrations of nitrate in all plant parts, nitrate reductase activity was not correlated with accumulation of nitrate or reduced N for the four hybrids. Uptake and flux of nitrate were not numerically related to accumulation of reduced N for the four hybrids. Among the four hybrids, nitrate flux was not associated with level of leaf nitrate reductase activity. None of the individual parameters, as measured, would serve as an index for reduced N accumulation for these four hybrids. When the hybrid pairs were compared separately, it was evident that both rate of nitrate flux and level of nitrate reductase activity affect the accumulation of reduced N by the plant. Relative to the other hybrids, hybrid D that accumulated the most reduced N and nitrate as a 23-day-old seedling had the least reduced N in grain plus stover at maturity under field conditions. Hybrid C that had high nitrate reductase activity as a seedling had low nitrate reductase activity after anthesis under field conditions. These changes in metabolic activities with plant development and different environments illustrate the problems encountered in attempting to develop simple physiological or biochemical screening criteria useful in identifying superior cultivars at the seedling stage.