The Uptake of Growth Substances

Abstract

If segments from the mesocotyls of Avena sativa are first kept in buffer then the initial rates of uptake of radioactive 2,3,6-trichlorobenzoic acid (2,3,6-TCBA) and 2,4- and 2,5-dichlorobenzoic acids (2,4-DCBA and 2,5-DCBA) are less than those of freshly excised segments. No such effect of pretreatment is found for benzoic acid or for 2-chlorobenzoic acid (2-CBA). Uptake of 2,3,6-TCBA normally becomes negative between two and six hours after excision, and this phase of net loss is prevented by the addition of streptomycin, which also offsets the decline in the rates of uptake of 2,5-DCBA and 2,4-DCBA. In contrast, streptomycin inhibits accumulation of 2-CBA. From a comparison of these results with similar and prior findings for substituted phenoxyacetic acids, it is concluded that the initial uptake of 2,3,6-TCBA, 2,5-DCBA, and 2,4-DCBA is governed by an unstable accumulatory system (Type 1), whose breakdown can result either in a phase of net loss during the course of uptake, or in a decline in uptake following pretreatment. Net loss of 2,3,6-TCBA is also prevented by synthalin (decamethylene diguanidine dihydrochloride), cetyl trimethyl ammonium bromide (CTAB) and by 2,3,5-triiodobenzoic acid (TIBA). During pretreatment, the presence of streptomycin, synthalin or TIBA prevents a fall in the subsequent uptake of 2,3,6-TCBA, while the addition of CTAB causes a dramatic increase in uptake. We have proposed for Type 1 accumulation a biochemical mechanism capable of accounting for the unstable nature of the accumulation and for the protective action of the compounds with cationic nitrogen groups, such as streptomycin, synthalin, and CTAB.