Assimilation and biological functions
N needs to be reduced to its −3 valence state. Two important enzymes, nitrate reductase and nitrite reductase ensure that the prevalent form in which N is taken up (NO3−) is converted to ammonium. Nitrate reduction can take place in both roots and shoots but is spatially separated between the cytoplasm where nitrate reduction takes place and plastids/chloroplasts where nitrite reduction happens (Figure 1a) [6 and 10].
The foremost function of N is to provide amino groups in amino acids. N is also prolific in nucleotides, where it occurs incorporated in the ring structure of purine and pyrimidine bases. Nucleotides form the constituents of nucleic acids but also have many important functions in their own right such as in energy homeostasis, signalling and protein regulation. In addition, N is essential in the biochemistry of many non-protein compounds such as co-enzymes, photosynthetic pigments, secondary metabolites and polyamines. When in ample supply, NO3− is deposited in the vacuole where it significantly contributes to turgor generation.