Anatomical changes induced by ethylene in the abscission zone of citrus leaf and fruit explants

Abscission zones of leaves (LA-AZs) and 2-week-old fruits (AZ-A) of Citrus sinensis (L.) Osbeck cv. ‘Shamouti’ have similar structural and physiological characteristics, hi both, the separation process involved the following events: cell walls swelled and became gelatinous, wall components degraded, and complete wall dissolution resulted in cell separation. The process was induced by ethylene and the reaction of the cell wall was pronounced. There were minor differences between the AZs of leaves and fruits. Abscission of the leaf at the LA-AZ began at the edge of the groove and proceeded inwards, whereas in the AZ-A of young fruits it first occurred in the middle and inner cortex and then advanced into the vascular tissues and outwards to the outer cortex. Six to eight-week-old fruits did not abscise at the peduncle AZ-A when treated either with ethylene or air. However, the marked anatomical changes that took place after ethylene treatment in the bark, pith, and vascular cylinder parenchyma cells led to the swelling of the abscission zone, and the following processes were observed: (a) cell expansion; (b) swelling and breakdown of pith and bark parenchyma cell walls; and (c) cell division of parenchyma, mainly in the outer- and mid-bark and in the vascular cambium. The breakdown of pith and bark parenchyma cell walls, which was accompanied by an external swelling of the whole AZ, was a result of a typical abscission process. However, since the process occurred only in restricted areas of the swelling AZ, total fruit separation at the AZ-A at this stage of development was prevented by the unaffected areas.