Effects of Applied IAA on the Position of Abscission Sites Induced in Wounded Explants from Impatiens sultani Internodes

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Abstract
Previous work established that if segments of Impatiens sultani internodes are explanted and incubated on a suitable medium, they tend to undergo abscission by a transverse separation layer that differentiates a short distance above the explant base. The present study has shown that the position of the abscission site can be modified experimentally. When an explant was split down to midlength and auxin (IAA) was applied to the top of one of the two arms, abscission often occurred at or near the base of the other arm. Again, when IAA was applied to the explant laterally midway along its length, abscission often occurred just above the application point. These two modifications of abscission sites had been predicted by a hypothesis stating that separation layers tend to be positioned where auxin concentration decreases in the morphologically upward direction. Studies with [14C]IAA confirmed that the separation layers above the explant base, and in the two experimentally modified sites, did indeed arise where the concentration decreased upwards. Also, wounding altered the position of abscission in these explants in ways that can be interpreted in terms of the above hypothesis coupled with the destruction of auxin that occurs at wound surfaces. In this system, auxin is acting as a morphogen: its concentration gradients provide positional information.