Auxin-induced water uptake 



concentration (0-0025 M), and to this mannitol was added as the non- 

 permeating, osmotically active solute. It is clear from the data o^ Figure 1 

 that sections elongate only in the hypotonic solution and that no elongation at 

 all occurs in the hypertonic solution. 



The rate of elongation of the sections o^ Figure 1 decreases with time. It is 

 known that the response of coleoptile sections to auxin is prolonged in time 

 by the addition of sucrose or certain other substances to the medium. 

 Figure 2 gives data on the time course of the elongation response to auxin in 

 medium containing an optimum concentration of sucrose (0-09 M) and 

 mannitol added in appropriate concentration to produce varying total 

 osmotic concentrations. The sections were first equilibrated in the medium 

 for one hour after which auxin was added. In basal medium alone elongation 



Figure 2. Elongation of Avena 

 coleoptile sections as a function of time 

 in media containing 0-09 M sucrose 

 and mannitol in varying concentration. 

 lAA (5-0 mgll) added at arrow. After 

 Orrfm etal. (1955). 



,• 009 M 



0-20 n 



0-30 n 



starts at once when lAA is added and continues at a constant rate through 

 the 20 hours of the experiment. As the value of external osmotic concentra- 

 tion is increased a lag period in attainment of steady-state elongation rate 

 becomes evident. This lag period increases in length as the external osmotic 

 concentration is increased. The final steady-state rate of elongation also 

 decreases regularly as external osmotic concentration is increased. Figure 3 

 summarizes data on the relation of external osmotic concentration to elonga- 

 tion rate of coleoptile sections. The auxin-induced increment in elongation 

 decreases sharply with increasing external osmotic concentration. None the 

 less, elongation of tissue does occur in sections placed in solutions of osmotic 

 concentration as high as 0-6 M. The cells of sections placed in such solutions 

 are rapidly plasmolysed as will be discussed below. At the end of the 20-hour 

 incubation period of the experiment o^ Figure 2, deplasmolysis has occurred. 

 Elongation has then taken place in the presence of solutions initially hyper- 

 tonic to the 0-4 M osmotic concentration of the initial section. 



Burstrom (1953a) has shown that the extent to which wheat roots are 

 irreversibly stretched by growing (irreversible extension) is independent of 



261 



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