Auxin-induced water uptake 



experimentally separated from the actual act of net water uptake (Cleland, 

 1955). For this purpose coleoptile sections were first treated with or without 

 auxin for 45 minutes in aerated solution. The osmotic concentration of the 

 solution was however so adjusted (0-3 M mannitol) that the sections showed 

 no net water uptake. They were next transferred for 30 minutes to anaerobic 

 conditions in 0-3 M solution. They were now transferred to water under 

 anaerobic conditions. In this final treatment the sections can of course take 

 up water and expand. The presence of auxin in the final solution does not 



Auxin 



pre'fneafed 



Non-auxin 

 ;?>'^* pnefneafed 



Aux/n added during 

 expansion pen led 



No auxin added during 

 expansion period 



1 2 



Time, expansion period 



3 



hns 



Figure 11. Effect of auxin during the anaerobic expansion period. 



increase significantly this expansion {Figure 11) since the conditions are 

 anaerobic. It is however evident from the data of Figure 11 that pretreatment 

 with auxin under aerobic circumstances markedly increases subsequent 

 elongation under anaerobic conditions. Auxin brings about a change in the 

 non-elongating section which can be subsequently expressed as a d.p.d. 

 gradient and lead to a purely passive, net water uptake. The auxin-induced 

 d.p.d. gradient has been shown above not to be due to changes in osmotic 

 concentration. We must conclude again that auxin decreases cell-wall 

 pressure and that this effect is to some degree a persistent one, separable from 

 net water uptake itself. 



CONCLUSION 



The present work has shown that auxin-induced net water movement in the 

 Avena coleoptile section follows osmotic principles and that net water move- 

 ment into the section from truly hypertonic solution does not take place. 

 The role of sucrose and of inorganic salts such as KCl, long known to 

 increase auxin-induced elongation of coleoptile tissue, has been shown to be 

 in part an osmotic one. These solutes by their absorption into the cells of the 

 section serve to maintain or increase the osmotic concentration of the tissue 

 and hence to maintain the turgor pressure upon which elongation rate is 



269 



