Abscission in Phaseolus vulgaris the positional differentiation and ethylene-induced expansion growth of specialised cells

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Abstract
Cell separation in abscission zones of explants of Phaseolus vulgaris L. is preceded by the enlargement of a single row of cells. Unlike the other cells surrounding them, the cells of this row enlarge in response to ethylene and not to auxin. Enlargement follows an increase in the activity of carboxymethyl 1: 4-glucanase in the abscission zone; also a response to ethylene and not to auxin. Eventual cell separation occurs between the enlarging cells and the non-enlarging tissue distal to them. It is proposed that the formation of cells at the abscission zone, with growth properties dissimilar to those of the rest of the pulvinus and petiole, is an example of precise positional differentiation. The presence and enlargement of such cells may be a general prerequisite for the abscission of organs or tissues distal to them.