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Nitrate Reductase as a Factor Affecting N Assimilation During the Grain Filling Period in Spring Wheat¹

In 1976 we investigated whether or not nitrate reductase (NR) enzyme activity in shoots of two spring wheat (Triticum aestivum L. em Thell) cultivars limits N assimilation during the grain filling (post anthesis) period. Twice weekly after anthesis NR activities in the flag leaf blade, flag leaf sheath, and first leaf blade below the flag leaf were estimated on field-grown plants with an in vivo assay. Activity declined during the period in apparent association with senescence of the tissues. In contrast, when NR estimates were made by a modified assay in which no exogenous nitrate was supplied, activity was consistently low. This suggests that relatively little nitrate was available for reduction in these tissues although NR enzyme appeared to be there.

The cultivars accumulated approximately 90% of their total reduced N before anthesis, as estimated by sequential sampling of shoots from the same plot area from which the NR samples were taken. Nitrogen accumulation after anthesis was predicted from the enzyme activities measured by both assay methods. Nitrogen accumulation predicted from the NR activities measured when nitrate was included was substantially greater than the accumulation actually observed. Our results suggest that the minimal accumulation of reduced N after anthesis by the cultivars in this study should not be attributed to the level of NR enzyme. It may be more a function of a limitation in a process responsible for supplying nitrate to the enzyme.

Key Words: Post anthesis period • Triticum aestuvum L.

² Research assistant (now assistant professor) and professor, respectively. Present address for Moss: Dep. of Crop Science, Oregon State Univ., Corvallis, OR 97331.

Received for publication August 27, 1977.

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