D. R. HOAGLAND 



at the time of the experiment gave no visible symptomatic deficiency 

 response, yet addition of zinc to the nutrient medium rapidly induced 

 an increased rate of protein synthesis (1). Observations of this kind, 

 however, only show that some unknown link in a chain of reactions 

 is broken. The nature of the enzyme systems existing in the plant, of 

 which zinc is an essential component, or activator, is an unanswered 

 question. From investigations on blood cells comes evidence that 

 zinc is a component of carbonic anhydrase; and there is a suggestion 

 from studies on yeast that zinc is one of the activators of the enzyme 

 aldolase. We have no positive evidence of this character derived 

 from experiments performed directly on enzyme systems of higher 



plants. 



Zinc is often successfully applied in curing zinc deficiency disease 

 by spraying the plant or treating the soil. This discovery, valuable as 

 it is in agricultural practice, does not satisfy the curious investigator 

 who seeks enlightenment on the function of zinc in plant metabolism. 

 Knowledge of why zinc is needed by the plant might improve existing 

 agricultural practice by providing a rational basis for the treatment of 

 zinc deficiency disease; but clearly, as in other fields of research, 

 sound progress in the study of plant biochemistry cannot be hoped for 

 if the direction of research is to be governed by the degree of prob- 

 ability that a given investigation will in itself have a practical outcome. 



These are only illustrations of the wide gaps in fundamental 

 biochemical insight into the functions of the so-called plant foods. 

 One could write of the lack of the kind of experimentation which might 

 elucidate the role of potassium, one of the most important fertilizer 

 elements in enzymic reactions in the plant. At the present time, it 

 is possible to cite data from one source or another that could be in- 

 terpreted in terms of an eff'ect of potassium on almost every general 

 biochemical process of which the plant is capable. The net conclu-^ 

 sion is that potassium is an essential element for plant growth and that 

 its deficiency may impair growth in various ways or alter the compo- 

 sition of the plant, depending upon the factors such as degree of potas- 

 sium deficiency, the concentration in the media of calcium, sodium, 

 or other ions, the species of plant studied, and the physiological age of 

 the plant. The desirability of further basic information on the role of 

 potassium in biochemical processes in the plant is evident. For 

 example, a suggestion has been advanced that potassium has an im- 



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