248 PLANT BIOCHEMISTRY 



becomes unavailable to plants under alkaline conditions. Once inside 

 a plant, this element is generally loc.itctl in the more physiologically 

 active parts, particularly the leaves. Since it is a relatively immobile 

 element, manganese is not readily redistributed throughout the plant. 

 Manganese undoubtedly plays a number of roles in the plant. It 

 has been shown that Mn++ is an activator of some enzyme systems, 

 including some carboxylases and dehydrogenases. This element is 

 also related in some way to chlorophyll synthesis, since a deficiency of 

 Mn++ leads to a chlorosis distinctly different from that obtaining as 

 a result of iron or magnesium deficiency. Manganese also probably 

 plays a part in oxidation-reduction reactions in the plant. It is known, 

 for example, that excess manganese acts as an oxidizing agent, con- 

 verting ferrous ion to Fe+ + +, thereby effectively inducing symptoms 

 of iron deficiency by converting Fe++ to the physiologically inactive 

 ferric state. 



Copper 



This element is highly toxic to plants except in very low concentra- 

 tion. However, there is no doubt that traces of copper are essential 

 for plant development. This element acts as part of certain oxidizing- 

 reducing enzymes such as tyrosinase and ascorbic acid oxidase. Aside 

 from this particvdar function, no specific role for copper has been 

 established in the plant. 



Zinc 



Zinc is known to be a part of the enzyme, carbonic anhydrase. Al- 

 though highly toxic even in moderate concentrations, traces of this 

 element are required for normal plant growth. For instance, it is 

 fairly well established that zinc is necessary for the synthesis of one 

 of the plant-growth hormones, indoleacetic acid. Zinc also functions 

 in the active DPN+ complex during reactions catalyzed by alcohol 

 dehydrogenase. 



Molybdenum 



Of all the essential elements, molybdenum is required in the 

 smallest concentration for plant growth and development. For exam- 

 ple, it has been demonstrated that 1 part in 100,000,000 parts of cul- 

 ture solution is sufficient to prevent molybdenum deficiency in tomato 

 plants. Although the complete role that molybdenum plays in plant 

 development is not understood, it is generally agreed that this element 

 functions in the reduction of nitrates. 



