An Arabidopsis mutant (rnc1) with a mutation at the 313th nucleotide from the translational start site of AtNRT3.1 was isolated. The mutation resulted in the replacement of aspartate by asparagine at the 105th amino acid in a region conserved among higher plants. In the rnc1 mutant, both the nitrate concentrations in plants and the nitrate uptake from the medium were <13% compared with those of the wild type, while AtNRT3.1 mRNA was accumulated similarly and both AtNRT1.1 and AtNRT2.1 mRNA were decreased. These results suggest that the replacement of Asp105 in AtNRT3.1 markedly reduces nitrate uptake and accumulation.

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A simple technique for estimating the nitrate nitrogen (NO3‐N) concentration of crop in a field was developed for diploid Italian ryegrass (Lolium multiflorum Lam.) at the heading stage, using a hand‐held chlorophyll meter (SPAD‐502). A rapid method for estimating the NO3‐N concentration in the crop is required in order to feed cattle safely, since high levels of NO3‐N in crops cause the nitrate poisoning of ruminants. The SPAD reading was determined as an average for the middle part of 10 flag leaves. SPAD readings for a variety of Nioudachi were correlated with reduced nitrogen concentrations with the following equation: reduced N concentration (g kg−1 DM) = 0.563 × SPAD reading −11.492 with a coefficient of determination of 0.715. SPAD readings also increased with NO3‐N concentrations. However, the relationship did not seem to fit a linear curve. A critical value for the SPAD reading was 47 for a NO3‐N concentration of 2.0 g kg−1 DM, a critical value for the acute nitrate poisoning of ruminants. The critical SPAD reading value was applicable for eight of 10 other varieties tested. Yields for Nioudachi were increased with the increase in SPAD readings and maximal yields within a year were obtained with SPAD readings over 45, which were less than the critical value for NO3‐N accumulation. Therefore, it is possible to manage nitrogen levels to obtain sufficient yields and prevent acute nitrate poisoning.

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