Abscission Sites in Nodal Explants of Impatiens sultani

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Abstract
When Impatiens sultani stem segments including a node are explanted and cultured on a suitable medium, they often undergo abscission about a week later just above the node, and in many cases a second abscission subsequently occurs some millimetres above the base of the explant. These two separation layers differ; that formed above the node is oblique, its position is related to the stem morphology and is not modified by basal application of auxin, whereas that formed above the base is transverse, its position is unrelated to stem morphology and can be modified by auxin application. Separation layers above the node form more frequently and rapidly than those above the base, and their frequency is less sensitive to explant length, nodal age and sugar concentration. Surgical experiments indicate that the abscission site above the node, despite its regular position, is not determined prior to explanting but becomes committed in the first few days after explanting. In line with a hypothesis that abscission sites are positioned where auxin concentration decreases in the apical direction, it is suggested that the separation above the node reflects increased auxin at the node, due to impeded basipetal flow, and that the separation above the base reflects basal accumulation of auxin.