Metabolism and mode of action 



respiration diminishes however with increasing external osmotic concentra- 

 tion and disappears at an external osmotic concentration of 0-4 M in which 

 elongation is reduced to a low value. These results parallel those obtained 

 with Jerusalem artichoke storage tissue (Bonner, Bandurski, and Millerd, 

 1953). 



It has been concluded that the Avena coleoptile is at all times essentially 

 in diffusion pressure deficit equilibrium with the external medium. No 

 movement of water against a d.p.d gradient appears to take place in coleop- 

 tile sections (Burstrom, 1953b). When non-absorbable solutes are used to 

 constitute a hypertonic solution no elongation of the tissue takes place. 

 Adjustments of osmotic concentration take place in the coleoptile provided 

 that an absorbable solute is present in the external medium. These osmotic 



^0- 



I 



so 



20- 



10 



Figure 10. Rate of respiration of 

 Avena coleoptile sections in the presence 

 or absence of lAA (5 mgjl.) as a 

 function of external solute concentration. 

 After Ordin et al. (1955). 



0-1 0-2 0-3 



Molarity of mannifol 



OH 



adjustments are, however, independent of auxin and occur in the absence as 

 well as in the presence of added growth substances. Since auxin-induced 

 water uptake by the section is a purely osmotic phenomenon it must take 

 place in response to a d.p.d. gradient into the tissue, which is, however, so 

 small as to be not measurable by the present methods. Classical osmotic lore 

 tells us that the d.p.d. of a cell equals osmotic concentration less cell wall 

 pressure. Auxin does not appear to directly influence internal osmotic 

 concentration. It must be concluded therefore that auxin in some way 

 decreases cell-wall pressure. We have arrived by a circuitous route at a 

 conclusion reached by Heyn 24 years ago (Heyn, 1931). The hypothesis that 

 auxin causes a metabolism mediated plasticization of cell walls of the coleop- 

 tile has found some experimental support, as for example in the work of 

 Bonner (1935). The fact that auxin-induced water uptake is metabolism- 

 dependent has, however, tended to focus our attention on the initial metabolic 

 role of auxin. Net uptake of water by the coleoptile section is dependent 

 upon auxin, on the metabolism of the section, and on the availability of 

 water to the section. In the absence of any of these three factors auxin- 

 induced elongation does not take place. It will now be shown that the 

 metabolism-dependent auxin mediated effect upon the cell wall can be 



268 



