244 PLANT BIOCHEMISTRY 



metabolism arc not completely understood. Whereas often it is rela- 

 tively easy to identify the conspicuous role played by some elements, 

 it is difficult to state that the same element does not have a number ol 

 minor functions in plant metabolism. By the same token, it is reason- 

 able to assume that an element might play a series of important but 

 inconspicuous roles and therefore not be recognized as an essential 

 element. The elements included in the following discussion are known 

 to have specific functions in plant metabolism. 



Phosphorus 



This element, which is absorbed by plants chiefly as the HP04= ion, 

 is not reduced in plant tissues but rather is incorporated into organic 

 compounds in highly oxidized form. Phosphorus, as the phosphate, is 

 combined as part of the phospholipide and the nucleic acid molecules. 

 The significance of phosphate carriers such as ATP and ADP and the 

 importance of phosphorylation reactions in living tissue have already 

 been discussed. 



In the growing plant, a large proportion of the phosphorus is found 

 associated with meristematic tissue, where it is used in the synthesis 

 of nucleoproteins and some other phosphorus-containing compounds 

 used in the respiration mechanism. In growing plants phosphorus is 

 easily redistributed from one organ to another. Such redistribution is 

 most evident under conditions of phosphorus deficiency of the medium 

 in which the plant is grown. At such times, phosphorus, probably as 

 the phosphate, is drained from mature leaves into other tissues. Under 

 such conditions of phosphorus deficiency in the medium, the phos- 

 phate of even the youngest leaves is sacrificed in the formation of the 

 seed or fruit of the plant. 



In mature plants, a large proportion of the phosphorus accumulates 

 in the seed or fruit as a result of their development. In fact it has 

 been shown that phosphorus favors seed formation and maturation 

 of plants. In this respect it is antagonistic to nitrogen, which favors 

 vegetative growth and delayed maturity. Because of its limited solu- 

 bility in the soil solution, an excessive phosphorus content is seldom 

 a problem. However, low phosphorus not only affects plant growth 

 and metabolism but also gives rise to phosphorus deficiency in animals 

 who subsist on such plants. 



Sulfur 



This element is found widely distributed among the plant tissues. 

 It is an essential component of two important amino acids, cystine 



