54 THE PLANT CELL WALL 



concentration has a more striking inhibitory effect on the con- 

 version of sucrose into polyuronides, but increases both sucrose 

 and acetate incorporation into non-cellulosic polysaccharides 

 by about 50 per cent. Later studies in Arena indicate that 

 2.8xl0- 5 M IAA accelerates incorporation of methionine CH 3 

 into cell wall pectic fractions, but has no effect on acetyl con- 

 tent. 



In wheat roots and leaves 10- 5 M 1-naphthaleneacetic acid 

 inhibits growth. The naphthaleneacetic acid-inhibited leaf shows 

 increased hemicellulose and cellulose. In the inhibited root, pectin 

 is higher, hemicellulose slightly increased, and cellulose unchanged. 

 Similar effects can be produced in wheat by calcium deficiency. 

 In contrast with the specific implication of auxin in pectic 

 metabolism, the behavior of wheat has led to the view that 

 growth regulators change the structural pattern of wall carbo- 

 hydrate deposition rather than their biosyntfntic reactions them- 

 selves. 



A functional regulatory relationship between the pectic sub- 

 stances and 3-indoleacetic acid may occur in the abscission 

 process. The physiology and chemistry of abscission arc yet to be 

 analyzed in detail, but it is clear that a breakdown of intercellular 

 substances is involved in the loss of tissue integrity at the abscis- 

 sion zone. The cell wall chemistry of the abscission zone, when 

 it is morphologically distinguishable, is sometimes destructive. In 

 Citrus, for example, the cells bordering the abscission layer are 

 highly suberized. In many species abscission may be prevented 

 by applications of IAA or synthetic auxins and enhanced by 

 substances or conditions which lower the internal IAA (auxin) 

 level. Thus, directly or indirectly the hormone has an influence 

 upon the intercellular layer. 



The chemistry and biochemistry of lignins and lignification 

 differ markedly from the transformations involved in the synthesis 

 of wall polysaccharides. Accordingly, it is to be expected that 

 the regulation of lignin synthesis and depositions will exhibit 

 unique features. The control of lignin deposition by polysaccha- 

 rides will be treated subsequently as a problem in surface biochem- 

 istry. Thus, the present discussion will deal primarily with control 



