| Abstract: | When segments excised from the etiolated hypocotyls of Gossypiumhirsutum are pretreated in buffer, the subsequent uptake ofradioactive 2,4-dichlorophenoxyacetic acid (2,4-D-1-14C) isdepressed and the net loss of radioactivity which normally followsa phase of positive uptake by freshly excised segments doesnot take place. Uptake by fresh segments, in contrast with uptakeafter pretreatment, has a high Q10 and is markedly depressedby both 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 3-indolylaceticacid. On these grounds it is proposed that net loss resultsfrom the release of material accumulated by a specific mechanismwhich, with time, becomes inoperative. Additional experimentssuggest that part of the 2,4-D taken up by stem segments ofTriticum vulgare and Avena sativa is accumulated by a similarmechanism. For 1-cm segments, entry is most rapid through the cut ends,and the effects of pretreatment exert their maximal effectsin the tissue near the ends. Therefore very short segments havebeen used to compare the courses of uptake of phenoxyaceticacid (POA) and its 2-, 4-, 2,6-, 2,4- and 2,4,5- chloro- derivatives.The patterns observed are similar to those previously reportedfor 1 -cm segments, although the differences between compoundsare greater. The courses of uptake of 2,4-D and 2,4,5-T, bothterminate in a phase when there is a net loss. POA and the 2-chloro-substitutedacid (2-CPA) are both continuously accumulated. No net lossis found with either the 2,6- (2,6-D) or the 4- chloro (4-CPA)compounds but the rates of uptake progressively decrease toa low level. It is proposed that the processes which determine the patternof uptake of chlorinated phenoxyacetic acids include two typesof accumulation. With Type I accumulation the mechanisms involvedrapidly become disorganized after tissues are excised from theplant. Type 2 accumulation, on the other hand, is stable. Theavailable data indicate that Type I accumulation is peculiarto compounds with marked auxin-like properties. |
