Auxin-induced water uptake 



however, permits very large increases in internal osmotic concentration to 

 take place. This is true both for solutions which are initially hypotonic and 

 for those which are initially hypertonic. The presence of sucrose in the 

 medium evidently permits the tissue to adjust its internal osmotic concentra- 

 tion. This is clearly shown by the results of experiments in which sections 

 were plasmolysed in mannitol and incubated either in mannitol alone or in 

 mannitol and sucrose. Only in the presence of the sucrose did deplasmolysis 

 take place and this was accompanied by an increase in tissue osmotic 

 concentration. 



It has been shown above {Figure 2) that when sections are incubated in 

 solutions more concentrated than approximately 0-2 M there is a detectable 

 lag period in establishment of steady-state elongation rate. The lag period 



Figure 6. Effect of A-hour pretreatment 

 with mannitol alone (0-2 M) or with 

 mannitol plus absorbable solute on sub- 

 sequent rate of coleoptile section elongation 

 in 0-2 M medium containing marmitol and 

 sucrose (total concentration 0-2 M). lAA 

 5-0 mgll. added at first arrow. After 

 Ordin et al. (1955). 



appears to represent the period needed by the cell to accumulate solutes and 

 to increase in internal osmotic concentration. To study this matter, sections 

 were placed in 0-2 M solutions containing lAA and mannitol or mixtures of 

 mannitol and sucrose or KCl for 4 hours. The sections were then all trans- 

 ferred to a 0-2 M solution containing lAA, mannitol, and sucrose. The data 

 oi' Figure 6 show that the lag period in the second treatment is shortened or 

 eliminated only if an absorbable solute is present externally in the pretreat- 

 ment solution. The lag period is not shortened by a pretreatment in solu- 

 tions containing indoleacetic acid and mannitol alone. 



It is of interest to determine the effect of auxin upon the osmotic readjust- 

 ment which occurs during the lag period. Sections were grown in 0-3 M 

 solution containing mannitol and sucrose. Auxin was added periodically 

 to successive lots of sections and the growth rates measured both before and 

 after addition of the indoleacetic acid. The data o^ Figure 7 show that the 

 lag period, which is of about 7 hours duration under these conditions, is 

 independent of the time of addition of auxin. 



The important conclusion of this work is then that the auxin-induced 

 uptake of water by Avena coleoptile sections follows osmotic principles and is 

 not attended by any detectable metabolically controlled component of 



265 



