Mechanics of Auxin-induced Groiuth 



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lAA CONCENTRATION, M. 



rig. 7. lAA concentration dependence of (left) plastic deformation and of 

 (right) growth of Avena coleoptile sections. In plastic deformation experiment, 

 sections were equilibrated with lAA for 60 min. and plastic deformation then 

 measured for 4 min. In growth experiment, sections were incubated in sucrose 

 0.09M, K-maleate buffer 0.0025M, pH 4.5, and lAA for 18 hrs. 



of lAA which ehcit half maximal response are identical within the 

 errors of measurement. 



That calcium ions increase the resistance of section cell walls to 

 deformation under external load is shown in Figure 8. The calcium 

 ions responsible for this effect appear to be exchangeably bound, as 

 is shown by the fact that the cell wall stiffening effect persists when 

 the tissue is transferred to water but is discharged when the tissue is 

 transferred to K+ ion-containing solution. The effects of calcium and 

 of potassium on cell wall deformability as measured by rate of de- 

 formation under external load are then similar to the effects of the 

 same ions upon cell wall deformability as measured by rate of cell 

 extension in response to internal (osmotically induced) load. 



CHEMICAL BASIS OF CELL WALL PROPERTIES 



The facts adduced above lead to the conclusion that lAA increases 

 cell wall deformability and that cell wall deformability is decreased 

 in the presence of calcium ions. lAA increases cell wall deformability 

 even in cell walls made stiff by the presence of calcium ions. In such 



