Auxin-induced water uptake 



Ursprung (1923) was used for determination of the d.p.d. of coleoptile tissue. 

 This method consists in the measurement of lengths of sections which have 

 been equilibrated in a graded series of mannitol concentrations. The 

 sections are held submerged in the solution in stainless steel baskets. Anaero- 

 bic conditions are maintained with argon in order to ensure that metabolic 

 processes are kept at a minimum and do not contribute to the measured 

 d.p.d. That group of sections which show no change in length is said to have 

 been in a solution whose osmotic concentration equals the internal diffusion 

 pressure deficit of the tissue. 



According to classic osmotic theory, the simplified method of Ursprung 

 should also be capable of yielding values of internal osmotic concentration 

 for the tissue measured, since the curve which relates tissue length to external 

 osmotic concentration should show a sharp inflection at the osmotic con- 

 centration of incipient plasmolysis. The data of Figure 4 show, however, that 

 with the coleoptile sections used in the present experiments such a sharp 

 inflection is not found. The method is however suitable for the measurement 

 of diffusion pressure deficit. The data of Table 1 show that within the error 



Table 1 



Diffusion pressure deficits of Avena coleoptile sections as a function of external osmotic concentration. 



{After Ordin et al. (1955)) 



of measurement (^^J^O'O^ M) the diffusion pressure deficit of the tissue is equal 

 to that of the external solution in which the tissue has been incubated. This 

 is true both in the presence and in the absence of auxin. 



The present method of measurement of tissue d.p.d. excludes any possible 

 metabolism dependent component of d.p.d. That coleoptile sections do 

 not exhibit any such d.p.d. component to a measvirable degree has been 

 shown by transferring elongating sections to anaerobic conditions. The 

 sections do not decrease in length under these conditions {Figure 5). lAA- 

 induced elongation occurs in the absence of any apparent d.p.d. gradient 

 over the entire range of external osmotic concentrations used. Since water 

 enters the cell under the influence of auxin it must do so under the influence 

 of a diffusion pressure deficit gradient. This is apparently infinitesimal and 

 not detectable by present methods of measurement. 



When sections are placed in hypertonic solution their cells are rapidly 

 plasmolysed. The cells of such sections subsequently deplasmolyse under 

 aerobic conditions. One might therefore conclude that absorption of solutes 

 or production of solutes within the tissue has taken place. According to this 

 view the section is deplasmolysed because internal osmotic concentration is 



263 



