Because of the strategic role of nitrate reductase in nitrogen metabolism, relationships between activity and productivity or quality of the kernel have been the subject of much research. Deckard, Lambert and Hageman (Crop Science 13:343, 1973) found in six genotypes of maize hybrids that the "nitrate reductase activity of the total leaf canopy showed a significant positive correlation with grain protein (kg N/ha), grain yield and total reduced N in the vegetative material and grain and stover at maturity."

In maize seedlings this enzymatic activity reaches a maximal level eight to ten days after sowing, and its measurement might be a successful means for an early selection of genotypes with high nitrate reductase activity. As a first step, we looked for differences between three groups of inbred lines and for variability within each group.

The first group is twelve "current lines" (five from our Station and seven released American lines), which are the check material. The second group is twelve "high protein lines" (eight from our Station, three from Eastern Europe and the origin: Illinois High Protein), which could possibly exhibit high nitrate reductase activity and the third group is twelve "ear prolific lines" (seven from our Station, two from Eastern Europe and three American lines), which were selected to obtain prolific hybrids with broad adaptability and strong productivity in a high plant population. In a high plant population with a reduction of light by mutual shadowing, current hybrids cannot reduce nitrates efficiently enough to insure a high productivity; under the same conditions prolific hybrids might possibly have either a higher nitrate reductase activity or enzymatic activity less sensitive to lowered light intensity.

We used a modified in vitro assay (Hageman and Hucklesby, Methods in Enzymology 23:491, 1970) on leaves of ten-day-old seedlings grown in a growth chamber. Two sets of experiments were conducted, and the results are presented in Table 1.

Table 1.

There are differences in enzymatic activity between the two sets of assays that can be explained by slight differences in growing conditions and by conservation of the samples. However, the correlations between the averages of the two sets are +0.93, +0.95 and +0.97 for the current, high protein and prolific lines, respectively. For all lines, there is considerable variability of the nitrate reductase activity. A wide range of activity (up to six-fold) is observed in the two sets. In the second one, this range goes up to 2.5-fold in current and high protein lines and up to 4.6-fold in prolific lines. Statistical analyses have supported the following conclusions: the activities are significantly different between lines and between groups of lines; the prolific lines have a higher nitrate reductase activity than either the current or the high protein lines; the activities of the high protein lines do not seem different from the current lines.

The low nitrate reductase activity of high protein lines is surprising. However, we must remember that the reduction of nitrates is only one step leading to a high protein content in the kernel. The capacity to mobilize and carry the reduced nitrogen might also be of great importance and could therefore explain the degradation of nitrogen compounds during kernel filling by assaying the protease activity in leaves.

In the prolific lines the high nitrate reductase activity is of great interest. The adaptation to high plant population seems to be analogous in these and certain American lines (Zieserl, Rivenbark and Hageman, Crop Science 3:27, 1963); we assume that this material maintains a sufficient level of nitrate reductase activity for a high productivity even if it is affected by shading when sown densely. With this material we intend to study the relationship between early nitrate reductase activity and ear prolificacy. The variation of nitrate reductase activity with the quantity of reduced nitrogen in the plant during the next stages of growth will also be followed up.

S. Rautou, A. Boyat and P. Robin


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