Induced Abscission Sites in Internodal Explants of Impatiens sultani: A New System for Studying Positional Control

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Abstract
Intemodes from Impatiens sultani shoots, explanted into sterile culture, often developed a transverse separation layer after one to two weeks and the top then abscised from the bottom of the explant. Such abscission occurred more rapidly and in a greater proportion of explants when 00001 per cent auxin (IAA) was provided basally and when younger intemodes and shorter explants were used. The distance of the separation layer from the base of the explant varied little with explant length, but increased with the concentration of auxin applied basally. It seems that in this adventitious abscission the processes of positional definition and differentiation proceed without pause, whereas in normal abscission the position is defined early in development but the final stage of differentiation of the separation layer is delayed until much later when the organ senesces. To account for the results from the internodal explants and from surgical operations on shoots as well as for the characteristic position of abscission sites of leaves and fruits, we suggest that the position of abscission is controlled primarily by auxin acting as a morphogen: abscission sites occur at Y-junctions just above the base of the arm with the lower activity and auxin status, or in single axes above a region of higher auxin status. In both sites, the auxin concentration decreases in the apical direction. This hypothesis is supported by a mathematical model (see Appendix) of the interaction of diffusive and polar transport in controlling the concentration gradient along intemodes with specified auxin concentrations maintained basally. The model allows predictions concerning the site and timing of abscission which accord with observations on intemodal explants.