Salt accumulation and mode of action of auxin 



tissue causes zero extension. It will be seen that a zero initial or negative 

 initial extension is quite rapidly converted into a positive extension. The 

 result is that one can only obtain crude estimates of the true initial s.p. of the 

 tissue. The data oi Figure 1 suggest that in the absence of external lAA the 

 s.p. is about 7-1 atm; with 1 mg/1. lAA it is about 9-8 atm and with 10 mg/1. 

 5-4 atm. In view of the experimental errors involved, it is uncertain how 

 much significance should be attached to these differences in apparent s.p., 



lAA absent lAA 7mg/l. lAA /Oirig/l. 



.0 



i-ifO 



to 



I 



20 



-10 



Tirs 

 Figure 1. Per cent extensions of oat coleoptiles plotted against time in hours: without lAA, with lAA 

 1 mgjl. and lAA 10 mgjl. The figures at the right-hand side of each curve show the osmotic pressures of 

 potassium chloride solutions in which the extension growth took place. 



but one inclines to the view that the increase at low auxin concentrations is 

 possibly due to increased wall plasticity and the reduction at higher concentra- 

 tions may be explained on the basis of reduced activity of pumping 

 mechanisms. 



As it has been suggested that lAA increases water secretion, it may be well 

 to record here that Dr. J. F. Sutcliffe (private communication) has shown 

 that lAA, while having little effect on ion secretion at low concentrations, 

 definitely inhibits ion uptake by actively absorbing beet tissue at higher 

 concentrations such as 10""* M and over. 



The same set of data can be used to obtain estimates of the osmotic 

 pressure of tissue by noting the external o.p. above which further shrinkage 

 does not occur. Unfortunately, although one tends to expect that a plas- 

 molysed tissue will not shrink on being placed in a hypertonic solution, 

 considerable shrinkage does occur when plasmolysed cells are more strongly 

 plasmolysed, especially in young extending tissues, though the increments of 

 shrinkage are much less. One cannot therefore get an accurate estimate, 

 but the data suggest values of around 18 atm at the lower auxin concentra- 

 tions and 13 atm in the case where the concentration was 10 mg/1. 



The difference (s.p.— o.p.) gives an estimate of the wall pressure. The 



285 



19a 



