2:4-dichlorophenoxyacetic acid, growth, ion absorption 



the experimental period. However, on the basis of dry weight, in the 

 second phase the higher concentrations (12 and 24 p. p.m.) depress the 

 content so that the final percentage of potassium is less than that of the 

 control. 



When the external concentrations of nitrogen, phosphorus, and potassium 

 are varied, for the uptake of phosphorus and potassium in the initial 4 days 

 there are no significant interactions of concentration on the order of the effect 

 induced by growth regulator. However, the depression caused by the 

 phenoxyacetic acid is greater if the level of nitrogen in the culture solution is 

 high. A similar interaction is found for the decrease in the nitrogen content, 

 but with phosphorus and potassium there is no evidence of any such 

 interactions. 



From the two series of experiments it is apparent that the influence of 

 2:4-dichlorophenoxyacetic acid on the absorption processes in the two species 

 are markedly divergent. On a priori grounds it would be expected that the 

 uptake of mineral nutrients would be correlated with the growth rate and 

 comparisons between the two species will be most valid when the effects of 

 the phenoxyacetic acid on growth are of the same magnitude. Taking the 

 whole experimental period, a concentration of 12 p. p.m. brings about a 

 similar depression in growth in both species and yet the effects on uptake 

 are quite different. For Lemna minor the inhibition of absorption is greatest 

 for potassium and least for phosphorus, while for Salvinia nutans the suppres- 

 sion is again least for phosphorus but in this instance maximal for nitrogen. 

 Turning to the results for mineral content, it is clear that the growth 

 regulator increases the nitrogen content of Lenma minor whereas it depresses 

 the content in Salvinia nalans. Again, although the potassium content of 

 Salvinia nutans is at first increased and then later decreased, the content in 

 Lemnu minor is initially reduced but the degree of reduction diminishes with 

 time. From these contrasting results it must be concluded that the inter- 

 acting effects of 2:4-dichlorophenoxyacetic acid on the processes responsible 

 for growth and those relating to absorption are different in the two species. 



The balance between the effects on growth and the effects on absorption 

 will determine whether the content of a given nutrient will be increased or 

 diminished and any shift in the balance with time may alter or reverse the 

 trend. From the experimental evidence it seemed reasonable to suppose that 

 this balance is correlated with the concentration of the growth regulator or of 

 some derivative at cell level, and that this in turn will be linked with the rate 

 of its absorption from the external solution. It was therefore decided that 

 the next step was to investigate the absorption of 2:4-dichlorophenoxyacetic 

 acid by Lemna minor. 



The cultures of Lemna minor were grown under the same conditions as 

 before with 2:4-dichlorophenoxyacetic acid labelled with C^"* in the carboxyl 

 group. All measurements of uptake were made by estimating the C^^ in the 

 tissue, and the following is a brief account of the technique. After the material 

 had been washed and dried (12 hours at 95°C) it was oxidized with Van 

 Slyke reagents in an evacuated system, and the collected carbon dioxide 

 absorbed in a known amount of baryta. The barium carbonate was then 

 filtered off and the filter paper after drying transferred to a planchette and 

 counted under constant geometry. The recovery is of the order of 94 per cent. 



V_ 255 



