UPTAKE OF GROWTH SUBSTANCES .X. ACCUMULATION OF PHENOXYACETIC ACID AND 2,4-DICHLOROPHENOXYACETIC ACID BY SEGMENTS OF AVENA MESOCOTYL

Segments of Avena mesocotyl were placed in buffered solutions of phenoxyacetic acid (POA) or 2,4-dichlorophebnoxyacetic acid (2,4-D), containing carbon-14 in the carboxyl group, and the quantities of radioactivity taken up by the tissues measured. With freshly cut segments in solutions of 2,4-D there is accumulation of carbon-14, but the course of uptake is interrupted by a temporary phase when some of the accumulated 2,4-D is released to the external solution. If after cutting the segments are first pretreated by placing them for about 15 h in buffer, and then transferred to 2,4-D, there is progressive accumulation with no phase of net loss.Pretreated segments absorb greater quantities of either 2,4-D or POA than freshly excised tissues. Following pretreatment in buffer the course of uptake of POA is linear but for 2,4-D the course is curvilinear. However, after pretreatment with non-radioactive 2,4-D the subsequent rate of uptake of radioactive 2,4-D is constant over long periods. The uptake of radioactive 2,4-D is largely independent of the concentration of non-radioactive 2,4-D given during pretreatment.When segments which have absorbed 2,4-D-1-14C are transferred to buffer, a relatively small proportion of the carbon-14, the 'mobile fraction', is released. The amount released following different periods of uptake is constant whereas the level of non-mobile carbon-14, the 'residual fraction', rises progressively in step with accumulation.The uptake of POA and 2,4-D is accompained by the formation within the tissues of other radioactive substances. It is concluded that the residual fraction is composed, at least in part, of these metabolic products and that accumulation and metabolic conversion are inter-connected.Dinitrophenol (DNP) slowly and progressively depresses the uptake of POA whereas the uptake of 2,4-D is very rapidly arrested. However, after about 2 h, in the continued presence of DNP, uptake of 2,4-D restarts but the rate never attains that of the control. These divergent effects of DNP indicate that POA and 2,4-D are accumulated by different pathways. © 1968 Oxford University Press.