322 



J. Bonner 



ACTIVE AND PASSIVE ASPECTS OF lAA-INDUCED GROWTH 



It has already been shown that cell extension in final analysis is 

 stretching of the cell wall clue to osmotically controlled water up- 

 take. In this sense cell extension itself is a passive process. Cell ex- 

 tension in the Avena coleoptile section, however, is accompanied and 

 in fact controlled by at least three distinguishable, metabolically 

 powered and hence active processes. The first is the active accumula- 

 tion of solute molecules or ions which, by maintaining the osmotic 

 concentration of the cell contents, maintain the turgor-induced load 

 on the cell wall. The second is the deposition of new cell wall ma- 

 terial which normally keeps pace with cell extension. The third is 

 the auxin-induced plasticization of the wall. That the plasticization 

 process may be experimentally separated in time from actual cell 

 extension has been shown over the years by Heyn (15) , Thimann (25), 

 and by Cleland and Bonner (11). In this type of experiment, sections 

 are supplied with lAA but restrained from growing by the presence of 

 a suitably high external concentration of nonabsorbable solute. The 

 sections are then allowed to expand in water under conditions in 

 which the action of auxin is blocked by the presence of a suitable in- 

 hibitor. An effect of the auxin pretreatment is manifested by growth 

 of the auxin-treated sections greater than that of nonauxin-treated 

 control sections. Alternatively, the cell wall deformability under ex- 

 ternal load of auxin-treated (but nongrowing) sections may be 

 measured directly (Cleland, 10). 



0.3 

 1-0.2 



H 

 O 



z 

 u 



^ 0.1 



o 



I 0.0 



X 



o 



0.1 



lAA 

 AIR 



No I AA 

 No AIR 



Mannitol , 



_L 



+ IAA^L_ 



I AA 



I 2 3 



TIME. HOURS 



0.2 - 



.0.1 - 



20 40 60 80 



I AA PRE- TREATMENT, MIN 



Fig. 10. Experimental separation of lAA-induced wall softening from tlic act 

 of turgor-controlled cell extension. Left, Avena coleoptile sections placed in 0.3Af 

 mannitol with or without IA.\ for an initial pretreatment inider aerobic conditions 

 and then transferred (with an intermediate treatment) to water under anaerobic 

 conditions. Right, residual auxin effect showing increase in length directly propor- 

 tional to length of treatment, .\fter Cleland and Bonner (11). 



