Auxin-induced water uptake 



mass spectrometer. Data are presented as mole per cent DHO. Auxin has no 

 influence on the course of either the inward or the outward diffusion of 

 heavy water into Avena coleoptile sections. The data of Figure 8 show that the 

 half time of the diffusion in either direction is approximately 9 minutes. 

 Other data similarly show that the rate of diffusion of water into the tissue 

 is not detectably different as between plasmolysed and unplasmolysed tissue. 

 In all of these cases water concentration approaches equilibrium in a 

 logarithmic manner and we are apparently dealing with a diffusion process. 

 In order to compare the diffusion constants measured by the isotopic method 

 with the filtration constants as ordinarily obtained by osmotic methods, 

 osmotic determinations of permeability have also been made (Ordin, 1955). 



Of 



I 



1 



/ \ 



J. • 



V. 



y^-^fntn 



_i_ 



f^ m 180 s'^o 



Time f"'" 



Figure 9. Time course of net water movement into and out of Avena. coleoptile sections as followed by 

 changes in length of sections after transfer from 0-4 M mannitol to water and from the latter solution back 

 to mannitol. After Ordin (1955). 



This involves measuring the rate of tissue elongation or shrinkage when 

 sections are transferred from 0-4 M mannitol to distilled water and vice versa. 

 The data of Figure 9 show that the curves obtained are similar in shape to 

 those obtained with DHO and that the half times are of the same order 

 of magnitude, approximately 7-5 minutes in the present case. 



The data oi' Figure 2 indicate that for the Avena coleoptile section to increase 

 by half in volume of water requires about 13 hours, i.e. the half time of net 

 water movement is about 13 hours. This is to be contrasted with the half 

 time for absolute water movement which is about 8 to 9 minutes. The rate of 

 exchange is therefore approximately 90 times as great as the rate of net 

 movement. It would appear that water can move relatively freely in and out 

 of coleoptile cells and tissue but that net movements are governed primarily 

 by classical osmotic relations. 



Net water movement into the coleoptile under the influence of auxin 

 is readily inhibited by varied metabolic inhibitors. Suitable experiments 

 with isotopically labelled water have shown that this is not the case for water 

 diffusion per se. That the net water movement into the coleoptile section is 

 attended by an increase in respiratory rate of the tissue is indicated by the 

 data oi" Figure 10. These data show that auxin, as is well known, increases rate 

 of respiratory metabolism of the tissue. The auxin-induced increment in 



267 



