FIXATION OF NITROGEN 



57 



globin-like pigment ('leghaemoglobin') in the effective 

 nodules [44]. The pigment is only formed after the nodules 

 have been established and concurrent with the change to a 

 green colour, their ability to fix N2 declines. The pOg in the 

 nodules is relatively low and the pigment may serve to store 

 and transport Og to the Rhizobia, which, it will be remem- 



•NEN 



+ H 



nitrous oxide 



+ H2P 



nitrogen 



+ HP 

 / 

 (NOH), HO. N.N. OH 

 hyponitrous acid /H H 



nitrogen 



+ H 



NH^OH_ 

 hydroxylomine 



+ COCOOH 



I 



CH^COOH 

 oxaloacetic 

 acid 



hooc.ch2c(noh) cooh 



oximinosuccinic acid 



H00C.CH2CH(NH2.)C00H 

 ospgrtic acid 



(CH2)2C00H + 



COCOOH 

 oC-ketoglutaric 

 ocid 



(CHp)2COOH 



ch(nh2)cooh 



glutamic acid 



FIG. 4.2. — ^Hypothetical pathways for the fixation of 

 molecular nitrogen 



bered, possess a high rate of respiration. Some workers have 

 suggested that the haemoglobin participates directly in the 

 ifixation mechanism. For example, NHgOH is decomposed 

 by haemoglobin (Hb) in vitro, thus [14]: 



NH20H+2Hb+++H20 -> 2MetHb++++NH3+20H- 

 2NH20H+2MetHb+++ -> 2Hb+++2H20+N2+2H+ 

 and it is possible that the fixation of nitrogen is accom- 

 plished by the reverse of these reactions. However, claims 

 for the natural occurrence of methaemoglobin in the nodules 



