Jack G. Valdovinos

Fine structure of abscission zones : I. Abscission zones of the pedicels of tobacco and tomato flowers at anthesis.

SummaryElectron micrographs of the zone of separation in flower pedicels of Lycopersicon esculentum and Nicotiana tabacum are presented with particular reference to the indentation of epidermal tissue in the abscission zone, subcellular organelles, and the cell wall. The indentation or groove which delineates the abscission zone extends some distance into the pedicel with branchings off the main groove. These branches are approximately 20 mμ in width while the main groove averages approximately 200 mμ in width. Invaginations of the plasmalemma are observed with considerable frequency. within these invaginations one observes a material of about the same density as the cell wall except that it is more fibrillar. Plasmodesmata are also observed, with considerable branching into middle lamellae of cells comprising the abscission zones. Microbodies with crystalloid cores appear with considerable frequency in cells of the abscission zone. The crystalloids appear to be cubical in shape and are composed of parallel sheets of osmiophilic material. The sheets average about 6 mμ in thickness and are spaced at 4 mμ intervals. The microbodies with crystalloid cores are observed to be characteristically of two size groupings. In tobacco the microbodies average 900 mμ and 1,500 mμ in profile. In tomato they average 300 mμ and 500 mμ. Chloroplasts contain a granular component which is membrane-enclosed. The component is large in comparison with the plastid in which it occurs, averaging 1.2–1.4 μ in diameter in chloroplasts ranging from 1.6 μ to 2.2 μ in diameter. The inner membrane of the chloroplast is highly invaginated, and DNA- and phytoferritin-like materials are observed within the plastids. Microtubules with an average diameter of 20 mμ are observed adjacent and parallel to the plasmalemma, primarily in the corners of the cells. Micrographs of other normally occurring cytoplasmic inclusions are also presented.

Fine structure of abscission zones : III. Cytoplasmic changes in abscising pedicels of tobacco and tomato flowers.

SummaryThe fine structure of the abscission zones of Lycopersicon esculentum and Nicotiana tabacum flower pedicels was studied, with special reference to structural changes in the walls of cells during the abscission process. The separation of cells appeared to be initiated primarily in the middle-lamella region of the cell walls. Disintegration of the primary wall, which usually followed breakdown of the middlelamella region, also occurred concurrently with the lysis of the middle-lamella region. During cell-wall degradation, the walls appeared to swell and became highly flexible. The walls of at least some cells in the zone of separation invaginated during the advanced stages of cell-wall disintegration, and ultimately collapsed. Cell-wall changes in abscising pedicels are almost identical to those which occur in abscising cotton and Coleus leaves, as described by Bornman (1967).

An Ultrastructural and Physiological Study of the Effects of Actinomycin D and Cycloheximide on Abscission of Tobacco Flower Pedicels

The effects of Actinomycin D (Act D) and cycloheximide (CHI) on ethylene (ETH)-mediated abscission were studied, using detached panicles of tobacco flowers. CHI reversed ETH-mediated abscission; Act D did not. Both inhibitors caused most mitochondria to develop large electron-transparent areas and some cristae disorganization. Act D caused the development of numerous membrane whorls that averaged 2 μm in diameter and caused areas of the plasma membrane to pull away from the cell wall, revealing possible sites of attachment of the wall to the plasma membrane. Many microbodies in Act D-treated tissue also became polymorphic and developed electron-dense matrices. Cells of control, CHI-, CHI-and-ETH-, and Act D-treated tissue contained small amounts of rough endoplasmic reticulum (RER). The ETH- and Act D-and-ETH-treated abscission cells had substantially elevated levels of RER; with both treatments, abscission was accelerated.