Nitrate reductase (NR) (EC 1.6.6.1) activity, nitrogen content and soluble protein content in shoots of timothy (Phleun pratense L., cultivars Bodin and Grindstad) were measured during a three- and a two-week daylength treatment (12 h photosynthetically active radiation plus day-lengthening light of low intensity). The protein and nitrogen contents relative to dry matter in shoots and the NR activity in crude extracts from shoots were significantly lower in 18 and 24 h photoperiods than in 12 and 14 h photoperiods. The nitrogen content of root dry matter and the total amount of nitrogen per plant were not influenced by daylength. In an experiment with nitrate and ammonium as nitrogen sources no significant interaction between the effects of daylength and nitrogen source on nitrogen and protein content was observed. The results from this experiment therefore do not support the hypothesis that NR is the photoperiodically sensitive control point for the ratio of assimilated nitrogen to assimilated carbon in the timothy plant.
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Anne Kjersti Bakken (1993) studied this question.
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