The Uptake of Growth Substances

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Abstract
An analysis has been made of the factors controlling the uptake of 2:4-dichloro-phenoxyacetic acid (2:4-D) and related compounds by Lemna minor, grown under controlled conditions. All compounds were labelled with 14C in the carboxyl group and the 14C in the plants, after liquid combustion, assayed as barium carbonate. With 2:4-D the rate of entry from concentrations of 8 to 32 mg./l. is maximal in the first 20 minutes, then falls progressively until the rate is zero between 1 and 2 hours and in the third phase (up to 24 hours) there is a net loss from the plants due to an outward movement of the compound back into the external solution. When the chlorine substitutions are in the 2:6-position the course of uptake is similar, save that loss to the solution takes place later. For the 2-chloro-substitution, after the initial phase of uptake there is some loss but this is followed by a period of recovery and uptake again proceeds, but slowly. In contrast, the course of uptake over 24 hours of phenoxyacetic acid is normal, i.e. there is a steady accumulation. When plants are pretreated with labelled 2:4-D for 30–60 minutes and transferred to water or culture solution then over 90 per cent. of the 14C is found in the external medium after 4:5 hours and this release cannot be accounted for in terms of exchange processes since the addition of unlabelled compound to the medium retards the rate of loss. This loss is slowed but not stopped at 1.25°C. and up to 22.5°C. the range of the Q10 is 1.6–1.9. Uptake in the first 30 minutes is temperature sensitive between 7.5 and 30°C. (Q10 2.3–2.6) and in general is positively and curvilinearly related to the external concentration. Pretreatment with unlabelled compound up to 2 hours progressively depresses the subsequent initial uptake of labeled growth regulator. It is concluded that initially the rate of entry greatly exceeds the rate of loss but that with time the ratio steadily diminishes to less than unity. Initial uptake of 2:4-D is markedly dependent on the pH of the solution and closely but not completely correlated with the external concentration of undissociated molecules. On the other hand, the outward movement is relatively unaffected by the external pH. Combinations of concentration and time of exposure which bring about loss to the solution need not cause any permanent retardation of growth. In fact, exposure for 1 hour to 8 mg./l. significantly accelerated growth in the next 8 days. These results are discussed in relation to previous findings and it is concluded that the pattern of uptake is determined on the one hand by the chemical structure of the growth substance and on the other by specific physiological differences at cell level.