Analysis of the Auxin Control of Bean Leaf Abscission

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Abstract
An analysis of the control of leaf abscission by auxin was carried out using petiole explants and debladed intact petioles of Phaseolus vulgaris var. Red Kidney. By adding a-naphthaleneacetic acid (NAA) at various intervals following leaf removal, the presence of 2 abscission stages was revealed - an induction period of about 6 hours or more which is inhibited by auxin and a later stage which is stimulated by auxin. Low temperatures were shown to separately retard each of the abscission stages, permitting a further distinction between them. The amino acid alanine which stimulates abscission was found to act on the 2nd stage and to have no apparent effect on the induction period. The 2-phase concentration effects of auxin on abscission are described as the consequence of actions on 2 stages with separate and roughly opposite auxin sensitivities. Experiments with radioactive NAA indicate that under the conditions of the bean tests, the abscission response is correlated with the time of auxin application and not with the gradient of auxin about the abscission zone nor with the total concentration extractable from that region.
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