PLANT NUTRITION 247 



importance in the formation of chlorophyll, iron does not become a 

 part of this complex molecule. It seems clear, however, that in its 

 stimulatory role iron is most effective as the Fe + + ion. Although this 

 element is frequently absorbed in the ferric state, it is rather rapidly 

 reduced within the cells to the ferrous condition, in which form it is 

 said to be physiologically active. 



Iron is known to be intimately associated with a number of the 

 enzymes and carriers which function in the oxidative mechanism of 

 living cells. For example, iron is an important ingredient of catalase, 

 peroxidase, and the cytochromes. 



Despite its importance, the total amount of iron found in plant 

 tissues is very low and much of that which is present is in organic com- 

 bination. Unlike a number of the other elements, iron is quite im- 

 mobile in the plant; hence very little redistribution of iron reserves 

 takes place. This can be demonstrated quite readily by removing a 

 plant from an environment in which it has been supplied with iron 

 and placing it in a cultural solution deficient in this element. Whereas 

 the old leaves retain their normal green color, new leaves will show 

 distinct symptoms of chlorosis. Hence no appreciable transfer of iron 

 takes place from older to younger tissue. 



Boron 



The exact role that boron plays in plant metabolism is unknown. 

 The accumulation of carbohydrates, ammonium compounds, and 

 other soluble nitrogenous substances in boron-deficient plants suggests 

 a block of the protein-synthesis mechanism. Leaf-roll of potato, brown- 

 ing of cauliflower, and brown-heart of turnip are diseases of these 

 plants which have been identified with boron deficiency. 



While boron is necessary only in very small quantities, there seems 

 to be quite a variation in the requirements of some species when com- 

 pared with others. For example, whereas tomatoes seem to develop 

 best on boron concentrations of less than 1 p. p.m., asparagus exhibits 

 its greatest development when boron is available to the extent of 10 to 

 15 p. p.m. Curiously enough, the range between the amount of boron 

 necessary for optimal plant development and the amount causing 

 injury is very small and the quantities sometimes overlap. Thus, 

 when some plants are supplied with sufficient boron for optimal devel- 

 opment, they may exhibit boron-toxicity symptoms at the same time. 



Manganese 



Except in extremely low concentrations, manganese is very toxic 

 to plants. Like iron, manganese is not often deficient in soils but 



