2:4-dichlorophenoxyacetic acid, growth, ion absorption 



for between 1 and 2 hours. Subsequently the rate falls off to zero and this is 

 followed at higher concentrations by a phase when there is a net loss of the 

 growth regulator to the external solution. By analogy with ion absorption 

 (see for example review by Broyer, 1951), after the first few minutes the 

 accumulation of the growth regulator will be energy-dependent while the 

 final transference to the external solution is unlikely to be dominated by an 

 exchange process, since the rate of transference is the same into water as it 

 is into a solution containing the unlabelled growth regulator. On this basis, 

 it would be expected that within the 'metabolic' phase uptake would be 

 sensitive to changes of temperature and the nature of the transfer mechanism 

 would be indicated by the magnitude of the Q^^q. Accordingly the rate of 

 uptake from solutions of 16 and 48 p. p.m. in the first 0-75 hours was measured 

 at 24 and 3°C and it was found that the Q,io was of the order of 2-4. In the 

 complementary transference experiment, batches of Lemna minor were placed 

 in a mixture of 16 p.p.m. of the growth regulator and 1 p. p.m. of rubidium 

 chloride labelled with Rb^^ for two hours, and then transferred to water and 

 the rate of loss measured at 0-5, 1-5, and 4-5 hours. Over the initial half 

 hour the loss of rubidium was not temperature-dependent while for the 

 growth regulator the Q^j^ was approximately 1-6. It was also observed that 

 when at the end of 4-5 hours only a small proportion of the rubidium had 

 been transferred to the external solution the loss of the growth regulator, 

 even at 1-25°C, was more than half Before any final conclusions can be 

 drawn it will be necessary to repeat these experiments over a wider range of 

 concentrations and smaller time and temperature intervals, but the evidence 

 suggests that the mechanism of transference has a metabolic component. 



There is one other aspect of the mechanism of absorption which has 

 received preliminary study, namely the influence of pH on the rate of 

 absorption. On the basis of other work with Lemna minor (Blackman and 

 Robertson-Cunninghame, 1953) it had been concluded that above the pK 

 value uptake is accelerated by lowering the pH and for the initial 2 hours this 

 supposition has now been confirmed since between pH 6-1 and 4-6 the 

 amount absorbed may change by a factor of 17. It cannot, however, be 

 concluded that entry is wholly as undissociated molecules since the rate of 

 absorption, although highly correlated with the calculated molecular con- 

 centration in the external solution, is not directly proportional. 



To date, only a few experiments have been carried out on the effects of the 

 dichlorophenoxyacetic acid on ion absorption in the first 24 hours. Enough 

 has, however, been done to reveal that the effects are complex. For example, 

 although relative to the control the growth regulator at 16 p.p.m. has only a 

 small effect on the absorption of Rb^^ at the end of 24 hours, after 1 and 4 hours 

 the rate of uptake is greatly depressed. When samples are first treated with 

 the growth regulator and then transferred to the rubidium solution the 

 uptake is different from that of samples placed in a solution containing both 

 the rubidium and the growth regulator. Moreover, such differences are 

 dependent on the time of pretreatment. 



Contemporaneously another series of experiments were started to 

 investigate some of the factors controlling the absorption of the same growth 

 regulator by coleoptile segments of Avena and Triticum. Because the con- 

 centrations which stimulate extension growth are so small and because the 



V 257 



