41 

 formation of protein and little remains to accumulate in non-protein, 

 organic compounds or as inorganic sulfate. 



Many of the volatile organic S compounds found in plants are 

 characteristic of particular plant families such as the Cruciferae, 

 which include the Brassica (cabbage) , the Amaryllidaceae, which include 

 the Alliae (onion) tribe, and the Tropaeolaceae, which include the 

 nasturtium (Metson, 1973). These plants have a high S requirement, and 

 the volatile S compounds impart a characteristic odor to many of them. 

 A relatively high proportion of the organic S is present as glycosides 

 which yield organic iso-thiocyanates on hydrolysis (Dijkshoorn and 

 Van Wijk, 1976). 



Sulfur has also been found to be important to plants in other ways. 

 Disulfide linkages (-S-S-) have been associated with the structure of 

 protoplasm. Sulfur is required for N fixation by legumes since it is 

 part of the nitrogenase enzyme associated with this reaction (Anderson 

 and Spencer, 1950). Sulfhydryl groups (-SH) in plants have been 

 related to increased cold resistance in some plants (Levitt et al . , 

 1961) . Other quality factors improved by S fertilization are chloro- 

 phyll and vitamin A content of forages. Alfalfa fertilized with P and 

 S contained almost twice as much carotene and vitamin A as alfalfa 

 fertilized with P alone (Tisdale, 1977) . 

 2.2 SULFUR UPTAKE BY PLANTS 

 2.2.1 Root Uptake 



Sulfur may be absorbed by plant roots exclusively as the sulfate 



2- 

 (SO ) ion or absorbed as S0„ directly through plant leaves. Sulfur 



uptake has been shown to be a function of the sulfate concentration of 



the soil solution (Spencer, 1959; Barrow, 1967) or of nutrient 



