Rothamsted Symposium on Trace Elements 



48 



tion whether this element is required for the nitrogen fixation of 

 this bacterium only or is needed for growth in a medium with 

 combined nitrogen as well has been discussed by various authors. 

 Although some of these authors contended that molybdenum is 

 essential for the nitrogen fixation only, most of them observed a 

 stimulating effect of this element when Azotohacter was grown 

 in a nitrate medium. 



In an experiment with Azotohacter chroococcum the effect of 

 increasing amounts of molybdenum was determined in nutrient 



Table 8 : Effect of molybdenum on the nitrate reduction in tomato plants:- 



Leaves 



* Plants haryested before adding the nitrate. 



t Fresh weight of leaves and stem. 



" Average values of six separately analysed plants. 



solutions with nitrate, ammonium sulfate and without combined 

 nitrogen, respectively. It appeared that molybdenum was essen- 

 tial for growth in gaseous N2 and in a nitrate medium. In a 

 medium with (NH4)2S04 no response to molybdenum was ob- 

 served. With nitrate-N about ten times smaller amounts of 

 molybdenum were required than in the absence of combined nitro- 

 gen when the bacterium had to fix the nitrogen of the air. This 

 result indicates that the molybdenum requiring processes involved 

 in N2 and nitrate assimilation are not similar. 



