A Morphogenetic Study of Leaf Abscission in Phaseolus

Abstract

Leaf abscission in Phaseolus vulgaris L., the red kidney bean, is presumed to involve a precisely regulated set of interlocking reactions, all of which are subject to control and many of which are detectable morphologically and histochemically. Four sequential stages which culminate in separation are distinct in the laminar abscission region. These include 1) pith cell breakdown, which may not be related to abscission; 2) cell division; 3) cellular differentiation; and 4) cortical and vascular cell breakdown. Correlation of the time and the nature of these structural changes with the period of effectiveness of certain applied growth regulators provides some interesting insights into the nature of the regulatory mechanisms in abscission. A reevaluation of some contemporary concepts of abscission, particularly the characterization of abscission as a senescence phenomenon, is suggested.

Programmed cell death during plant growth and development

“…Despite the occurrence of pith autolysis during Phaseolus leaf abscission (Webster, 1970), other anatomical and ultrastructural changes occurring in the layer of cells where organ separation occurs are not consistent with pcd directly participating in abscission. On the contrary, abscission zone cells are apparently metabolically active at the time of organ shedding.…”

“…The competence of stem pith to initiate autolysis following cellulase or ethylene treatment (described above) indicates that pith autolysis observed in the abscission zones of Phaseolus may have been a secondary effect induced by external factors, such as cellulase, originating in the separation layer. Pith autolysis, therefore, may not be necessary for organ shedding in Phaseolus (Webster, 1970). That dissolution of cell wall material appears to be sufficient to ensure organ shedding does not exclude the possibility that examination of additional experimental systems may reveal conclusive examples of pcd in the separation layer of abscising organs.…”

“…Abscission zone cells contain more organelles (Goren et al, 1984) and exhibit greater secretory activity (Ramina et al, 1989) than neighboring cells. Moreover, in the case of Phaseolus leaves (Webster, 1970) and citrus fruit (Goren et al, 1984), events leading to abscission include an increase in cell division and cell enlargement in the abscission zone. In Phaseolus, it is the thin walled, newly dividing cells through which separation takes place, and nuclei in these cells are intact and often still dividing when separation occurs and cell contents are lost (Webster, 1970).…”

“…Moreover, in the case of Phaseolus leaves (Webster, 1970) and citrus fruit (Goren et al, 1984), events leading to abscission include an increase in cell division and cell enlargement in the abscission zone. In Phaseolus, it is the thin walled, newly dividing cells through which separation takes place, and nuclei in these cells are intact and often still dividing when separation occurs and cell contents are lost (Webster, 1970). Separation in peach leaf abscission zone occurs when digestion of the outer thicker wall, rather than thin walls produced by secondary cell division, leads to cell breakage and the release of cytoplasm (Ramina et al, 1989).…”

The Anatomy and Histochemistry of Fruit Abscission

“…In other plants similar starch increases have been correlated with increase in number of cells in the abscission region (Bornman et al 1966). This is accompanied by a disappearance of starch from pith cells during abscission (Webster 1968(Webster , 1970. This is accompanied by a disappearance of starch from pith cells during abscission (Webster 1968(Webster , 1970.…”