51 



E. G. Mulder: Copper and Molybdenum 



In this connection, did I understand that the test for nitrate 

 reduction in the higher plants involved the measurement of 

 organic nitrogen? If so, then would it not be justifiable to inter- 

 pret the results as suggesting that Mo^ may be necessary for the 

 formation of organic compounds of nitrogen? 



Dr. Erkama:— 



In a preliminary culture experiment at Helsinki Biochemical 

 Institute we have observed that the presence of molybdenum in 

 the nutrient solution somewhat prevents the poisonous effect of 

 copper on peas. 



In the experiment the nutritive salts were freed from heavy 

 metals by dithizone treatment. The peas were inoculated with 

 Rhizohium strains and grown in sterilized N-free nutrient solu- 

 tion. The same basal nutrition was used with varying amounts 

 of copper. An addition of molybdenum (1 mg Na2Mo04.2H20 

 per litre) increased the yield and, on the whole, also the fresh 

 weight of the root nodules, as indicated by the following figures. 



Dr. Erkama's answer to Prof. Arnon concerning the mecha- 

 nism of the symbiotic nitrogen fixation: According to Prof. Vm- 

 TANEN it is to be expected from the energetic point of view that the 

 first steps in nitrogen fixation are oxidation-reduction processes pro- 

 vided that hydroxylamine is formed as an intermediate product. 

 If, however, nitrogen fixation leads to ammonia without the forma- 



1 Though molybdenum deficient plants were low in organic nitrogen after treat- 

 ment with nitrate (Table 8), this has been probably a result of the poor nitrate 

 reduction. Since this Symposium was held, many investigations on molybdenum and 

 copper were made in our laboratory and I am drawing the attention of the reader to 

 these by the addendum on page 49. (Author's Note.) 



