Hoagland —30— Plant Nutrition 



based on physiological researches on plants or on 

 the results of the biochemist, who often deals with 

 animal tissues. The micronutrient elements must act 

 as catalysts. That statement, of course, is too general 

 to be of much value. We wish to know the nature of 

 the reactions catalyzed. With regard to the metals, 

 copper and manganese, as well as iron, something has 

 been learned of the nature of oxidation — reduction 

 systems in which those metals may be involved when 

 associated with specific protein molecules — for ex- 

 ample, poly- and monophenol oxidases, and ascorbic 

 acid oxidase, in the case of copper. Certain researches 

 also suggest that copper has a role in the synthesis 

 of the porphyrin nucleus and thus in the formation of 

 haem-compounds or of chlorophyll. 



Manganese protein enzymes are not clearly defined, 

 but there can be little doubt that manganese in plants 

 does function in some oxidation system. LundegArdh 

 (1939) considers that manganese is of primary 

 importance in the respiratory system of plants and 

 BURSTROM (1938-39) has reported evidence that is in- 

 terpreted to mean that the reduction of nitrate is de- 

 pendent on an enzyme system in which manganese has 

 an essential role. In some physiological experiments 

 on barley plants in our laboratory by Arnon (1937) 

 an interesting relation was disclosed between the form 

 of nitrogen employed, either NH4 or NO3, the aeration 

 of the culture solution, and the amount of manganese* 

 furnished to the plants. In the solutions containing 

 nitrogen as ammonium salt, increasing the manganese 

 concentration, within certain limits, had a markedly 

 beneficial effect, when the oxygen supplied to the roots 

 was limited. Manganese also may have a role in the 

 photosynthetic process, apart from the synthesis of 

 chlorophyll. Experiments elsewhere on Chlorella 

 suggested that increasing the supply of manganese 

 increased the quantum efficiency but the interpretation 



*Some effects were also found for copper and certain other 

 elements capable of valence changes. 



