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Induced Abscission Sites in Internodal Explants of Impatiens sultani: A New System for Studying Positional Control: with an Appendix: A Mathematical Model for Abscission Sites |
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| Authors: | WILSON P M WARREN; WILSON J WARREN; ADDICOTT F T; McKENZIE R H |
| Institution: | Department of Developmental Biology, Research School of Biological Sciences, Australian National University GPO Box 4, Canberra, ACT 2601, Australia Department of Botany, Faculty of Science, Australian National University GPO Box 4, Canberra, ACT 2601, Australia Department of Physics, Joseph Henry Laboratories, Jadwin Hall P.O. Box 708, Princeton University, Princeton, New Jersey 08544 U.S.A. |
| Abstract: | Intemodes from Impatiens sultani shoots, explanted into sterileculture, often developed a transverse separation layer afterone to two weeks and the top then abscised from the bottom ofthe explant. Such abscission occurred more rapidly and in agreater proportion of explants when 00001 per cent auxin (IAA)was provided basally and when younger intemodes and shorterexplants were used. The distance of the separation layer fromthe base of the explant varied little with explant length, butincreased with the concentration of auxin applied basally. It seems that in this adventitious abscission the processesof positional definition and differentiation proceed withoutpause, whereas in normal abscission the position is definedearly in development but the final stage of differentiationof the separation layer is delayed until much later when theorgan senesces. To account for the results from the internodal explants andfrom surgical operations on shoots as well as for the characteristicposition of abscission sites of leaves and fruits, we suggestthat the position of abscission is controlled primarily by auxinacting as a morphogen: abscission sites occur at Y-junctionsjust above the base of the arm with the lower activity and auxinstatus, or in single axes above a region of higher auxin status.In both sites, the auxin concentration decreases in the apicaldirection. This hypothesis is supported by a mathematical model (see Appendix)of the interaction of diffusive and polar transport in controllingthe concentration gradient along intemodes with specified auxinconcentrations maintained basally. The model allows predictionsconcerning the site and timing of abscission which accord withobservations on intemodal explants. Impatiens sultani Hook., abscission, auxin, differentiation, diffusion coefficient, IAA, morphogen, polar transport coefficient, positional control, separation layer |
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