Hoagland — 42 — Plant Nutrition 



the time when the plants were considered to be at a 

 suitable stage of growth samples of leaf tissue were 

 taken from low and high zinc plants. 



One of the indications from the experiments was 

 that the incipient zinc deficiency resulted in a marked 

 effect in retarding protein synthesis. Especially in- 

 teresting was the rapid resumption of protein syn- 

 thesis when zinc was supplied to the low zinc plants. 

 Starch synthesis was likewise retarded by zinc de- 

 ficiency although the sugar content of the plant was 

 not diminished. This is in contrast to the effects of 

 boron deficiency. With this deficiency the tomato plant 

 may greatly increase its percentage content of both 

 sugar and starch. 



Again in a speculative vein one might attempt to 

 explain these responses in protein and starch synthesis 

 on the assumption that zinc is a component of a 

 catalytic system necessary for the phosphorylation of 

 glucose (known to be a step in starch synthesis) or 

 possibly of an amino acid. We must acknowledge, 

 however, that the only secure proof of the function 

 of an element like zinc is to isolate an enzyme system 

 for the operation of which the element is essential. 

 So far as I am aware there is evidence of this kind 

 for only one reaction in which zinc is concerned. 

 Keilin and Mann (1939) have reported the isolation 

 of a zinc protein system (from blood corpuscles) that 

 catalyzes the reversible reaction of H.COa^^COa + H2O. 

 It is conceivable that a similar enzyme system may 

 function in respiratory or photosynthetic processes of 

 the plant, but adequate proof of this is lacking. 



As one other observation, Reed has demonstrated 

 that partial zinc deficiency has special effects in inhibit- 

 ing seed formation or development. 



The possibility exists, according to the views of 

 Reed and Dufr^noy, that deficiency of zinc may bring 

 about a disturbance in oxidation-reduction systems in 

 the plant. Zinc does not undergo reversible valence 

 changes, so any function it may have in oxidation- 



