2.1.1. Forms of nitrogen
Nitrogen is essential to plant life. It is a constituent of both proteins and nucleic acids and also occurs in chlorophyll. Most animals cannot assimilate inorganic nitrogen or synthesize many of the amino acids unless assisted by bacteria (e.g. in the rumen of cattle). Nitrogen is available in the atmosphere as N2 but only legumes have the capacity to utilize this nitrogen using Rhizobium bacteria in the root nodules. In most plants, nitrate (NO3-) is the sole source of nitrogen. After uptake, NO3- is reduced to NH4+ prior to incorporation into organic molecules. (The removal of oxygen from a chemical compound and its replacement by hydrogen, is termed reduction.) The relevance of nitrogen is illustrated by the vast amounts of nitrogen reserves in seeds (as storage proteins).
Both growth and morphogenesis in tissue cultures are markedly influenced by the availability of nitrogen and the form in which it is presented. Compared to the nitrate ion, NO3- (which is a highly oxidized form of nitrogen), the ammonium ion, NH4+, is the most highly 'reduced' form. Plants utilise reduced nitrogen for their metabolism and internally, nitrogen exists almost entirely in the reduced form. As a source of reduced nitrogen, plant cultures are especially able to use primary amines:
R-NH2 and amides: R-CO-NH-R- (where R and R- are functional groups)
The primary amines which are most commonly employed in culture media are ammonia (NH4+) and, occasionally, amino acids.
Amides are less commonly added to culture media: those which can be used by plants are particularly
NH2-CO-NH2 (urea) and ureides, which include allantoin and allantoic acid (Kirby, 1982) (Fig. 3.1). Allantoin or allantoic acid are sometimes more efficient nitrogen sources than urea (Lea et al., 1979).
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