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In vivo nitrate reductase activity in ryegrass (Lolium perenne) and white clover (Trifolium repens): Differences due to nitrogen supply, development, and plant part

Abstract Differences in response to nitrogen (N) exhibited by different plants might be explained by nitrate reductase activity (NRA), but drawing conclusions about NRA is difficult because of the large range in NRA, and the variation in sampling techniques reported in the literature. This experiment was established to determine a sampling regime for ryegrass and white clover which would allow the effect of experimental treatments to be observed. Ryegrass and white clover were grown in sand culture in a glasshouse. Plants were supplied with a complete nutrient solution containing either a low‐N (0.5 mol m−3) or high‐N (5.0 mol m−3) concentration. NRA was assessed weekly, for 8 weeks. Ryegrass was separated into leaf blades, sheaths, and roots; white clover was separated into laminae, petioles, and roots. In both species, NRA decreased (P < 0.05) with time and was greater in the high‐N treatment than the low‐N treatment. Ryegrass had significantly (P < 0.05) less NRA than white clover (1.32 cf. 2.20 μmol NO2 – gfwt−1 h−1, respectively), and at most harvests there was significantly (P < 0.05) more in vivo NRA in the leaf sheaths than in the blades or roots. In white clover, there was significantly (P < 0.05) more NRA in the roots than in the petioles or laminae. In both species, analysing components of plants separately at 6 weeks resulted in most sensitivity for in vivo NRA analysis.