The Uptake of Growth Substances: VIII. ACCUMULATION OF CHLORINATED BENZOIC ACIDS BY AVENA SEGMENTS: A POSSIBLE MECHANISM FOR THE TRANSIENT PHASE OF ACCUMULATION

If segments from the mesocotyls of Avena sativa are first keptin buffer then the initial rates of uptake of radioactive 2,3,6-trichlorobenzoicacid (2,3,6-TCBA) and 2,4- and 2,5-dichlorobenzoic acids (2,4-DCBAand 2,5-DCBA) are less than those of freshly excised segments.No such effect of pretreatment is found for benzoic acid orfor 2-chlorobenzoic acid (2-CBA). Uptake of 2,3,6-TCBA normallybecomes negative between two and six hours after excision, andthis phase of net loss is prevented by the addition of streptomycin,which also offsets the decline in the rates of uptake of 2,5-DCBAand 2,4-DCBA. In contrast, streptomycin inhibits accumulationof 2-CBA. From a comparison of these results with similar andprior findings for substituted phenoxyacetic acids, it is concludedthat the initial uptake of 2,3,6-TCBA, 2,5-DCBA, and 2,4-DCBAis governed by an unstable accumulatory system (Type 1), whosebreakdown can result either in a phase of net loss during thecourse of uptake, or in a decline in uptake following pretreatment. Net loss of 2,3,6-TCBA is also prevented by synthalin (decamethylenediguanidine 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 fallin the subsequent uptake of 2,3,6-TCBA, while the addition ofCTAB causes a dramatic increase in uptake. We have proposed for Type 1 accumulation a biochemical mechanismcapable of accounting for the unstable nature of the accumulationand for the protective action of the compounds with cationicnitrogen groups, such as streptomycin, synthalin, and CTAB.