Rothamsted Symposium on Trace Elements 52 



tion of hydroxylamine the reaction is probably a reduction process 

 (c/. Biol. Rev. 22:239, 1947). 



Dr, Hewitt:— 



There are three points that arise in discussion of the results 

 described by Dr. Mulder with regard to the effects of molyb- 

 denum deficiency in plants and microorganisms. In the first 

 place it is interesting to hear that Dr. Mulder found that accumu- 

 lation of nitrate occurs in plants when molybdenum is withheld, 

 as a similar result has been obtained with molybdenum deficient 

 plants grown in sand culture at Long Ashton. In these experi- 

 ments it was found that considerable nitrate accumulation oc- 

 curred in tomato, mustard and cauliflower when molybdenum was 

 lacking, but not in savoy cabbage, although all the plants men- 

 tioned showed marked pathological effects of molybdenum de- 

 ficiency, which were not those caused by nitrogen deficiency. 

 It thus seems that the influence of molybdenum in nitrate reduc- 

 tion may differ according to species and also that molybdenum 

 may have more than one function in plants. 



In the second place the relative effects of molybdenum on 

 the metabolism of the nitrogen fixing Azotohacter and of denitrify- 

 ing bacteria in the presence of different nitrogen compounds re- 

 quires comment. It is reasonable to assume that the reduction 

 of nitrate prior to its final assimilation proceeds through a number 

 of steps and that the fixation of atmospheric nitrogen also involves 

 several steps from a lower initial energy level before either ultimate 

 assimilation or entry at some link in the general chain of reactions 

 involved in nitrate reduction. The fact that the amount of molyb- 

 denum needed for fixation of atmospheric nitrogen is about ten 

 times that required for reduction of a corresponding amount of 

 nitrogen as nitrate by the same organism may imply that molyb- 

 denum is involved in two distinct reactions and not in one only 

 that is common to both processes. Further studies on the assimila- 

 tion of nitrogen from likely intermediate compounds would be of 

 value in the elucidation of this problem. 



Finally the apparent unimportance of molybdenum for Azoto- 

 hacter when nitrogen is supplied as the ammonium ion raises the 

 question of its essentiality in the terms of the criteria proposed 

 earlier by Dr. Arnon, since the substitution of one metabolite by 

 another may apparently eliminate the need for this element in the 

 normal development of the organism. 



