246 PLANT BIOCHEMISTRY 



of chlorophyll is inhibited and a condition known as chlorosis ob- 

 tained. Under deficiency conditions, magnesium is readily redis- 

 tributed from the older to the younger plant tissues. 



Magnesium plays a part in the phosphate metabolism of plants and 

 therefore assists in the respiratory mechanism. Magnesiimi ions ap- 

 parently are specific activators for a nimiber of enzymes, such as some 

 carboxylases and transphosphorylases. 



Potassium 



Potassium has a number of important functions in plant metab- 

 olism, although this element is not definitely known to be a con- 

 stituent of any organic compoimds essential to plant life. It is never- 

 theless indispensable and is generally present in soluble inorganic 

 salts as well as in organic salts. Owing to the fact that potassium is 

 highly mobile, it is invariably foimd in all sap and plays an important 

 part in the water economy of plants through the effect of its soluble 

 compounds on osmotic pressure. 



Potassium aids the oxidative reactions of plants by acting as a car- 

 rier for iron, which in turn is required by a number of important oxi- 

 dative enzyines. The assimilation of carbon and the formation of 

 carbohydrates have been shown to be dependent on the presence of 

 potassium. One of its functions in carbohydrate metabolism is to form 

 the potassium salts of the phosphorylated intermediates. High-carbo- 

 hydrate plants such as sugar beets have a high potassium content. 

 Protein metabolism in plants also requires the presence of an ample 

 supply of potassium. 



In the early stages of growth, plants supplied with limited amounts 

 of potassium may show normal development. Later the relatively 

 small amoiuit of potassium migrates from older tissues to younger tis- 

 sues. This is followed by a migration of nitrogen to the younger 

 tissues. Such migrations result in premature aging of older leaves when 

 potassium is present in suboptimal amounts. 



Iron 



A deficiency of iron in soils is usually a result of its insolubility and 

 hence its unavailability, rather than its actual absence. It is far more 

 available in acid soils than in neutral or alkaline soils. Even in the 

 latter case, however, some iron absorption takes place as a residt of 

 intimate contact between roots and soil particles. Thus a deficiency of 

 iron is seldom a limiting factor in plant production. 



Iron is indispensable to the synthesis of chlorophyll. Thus a defi- 

 ciency of iron in plants results in a lack of chlorophyll. In spite of its 



