Spatial and temporal aspects of cell separation in the foliar abscission zones ofImpatiens sultani Hook

Summary The abscission of leaves fromImpatiens sultani Hook. occurs as the direct result of the weakening of a narrow band of cells running transversely across the base of the petiole. This loss of strength of the abscission zone is due to the breakdown of the central cell wall in two or three layers of cells. The process of wall degeneration is first visible 13 hours after the induction of abscission in a small group of cells found just below the concave groove on the adaxial side of the petiole. As the abscission zone gets progressively weaker the area of cells showing wall breakdown expands, spreading through the parenchyma to the lower side of the stele. The walls of the collenchyma and epidermis along the sides and base of the petiole and the central vascular tissues are the last to break down. Experiments in which the abscission zone was dissected into small pieces were undertaken to investigate whether cell wall hydrolysis was a contagious phenomenon, spreading from cell to cell by direct contact. The results of these investigations indicated that there was little requirement for cell to cell contact in either the temporal or spatial integration of cell wall breakdown.     

ULTRASTRUCTURAL STUDIES OF ABSCISSION IN PHASEOLUS: ETHYLENE EFFECTS ON CELL WALLS

This paper reports light and electron microscope observations of changes in the walls of cortical cells in the laminar abscission region of red kidney bean (Phaseolus vulgaris L.). In intact plants two or three rows of cells comprise the abscission zone. Pectic substances are not present in the walls of these cells when wall breaks occur. The separation cavity involves breaks in both radial and longitudinal cell walls. In ethylene-treated explants pectic substances are present in the cell walls when breaking occurs. The separation cavity involves breaks in longitudinal walls only, and breaking is confined to a single row of cortical cells. Prior to cell wall break the plasma membrane frequently invaginates. In intact plants this may be associated with plasmolysis and with the formation of secondary vacuoles. In ethylene-treated explants it may also be related to plasmolysis. At the time of cell wall break many unidentifiable inclusions of varying sizes and shapes are present in the cell wall region. Chloroplasts and mitochondria are structurally altered but recognizable in the cell at the time of wall break. Plasmodesmata are frequently observed in abscission cells and may be structurally elaborate. The observations of the nature of cell wall changes during abscission in ethylene-treated material fail to confirm physiological studies of other workers suggesting that pectin dissolution is necessary and may be sufficient for formation of a separation layer.     

Histochemical changes in the developing abscission layer in fruits of Prunus cerasus L.

Summary Abscission layer formation in the sour cherry (Prunus cerasus L.) during fruit maturation occurred in the transition zone between the fruit and the pedicel. The abscission layer, consisting of 5–8 rows of cells, was first identified by its low affinity for haematoxylin. The walls of cells in the abscission layer contained less total polysaccharides than adjacent cells. The pectins were degraded and the cellulose was partially broken down resulting in cell separation. The Ca level in the abscission zone decreased and Ca and Mg were lost from the walls of cells in the layer during abscission. After the abscission layer formed, cells associated with the layer had a lower capacity to bind ⁴⁵Ca than cells distal or proximal to the layer.Michigan Agricultural Experiment Station Journal Article No. 4607     

Hormonal control of floral abscission in zucchini squash (<Emphasis Type="Italic">Cucurbita pepo</Emphasis>)

In the zucchini squash, Cucurbita pepo, a well coordinated abscission of the female flower during fruit set is essential to obtain a fruit of commercial value. In Spain zucchini is mainly produced in greenhouses in Almería, where high temperatures during the spring-summer period provoke a cultivar-dependent defect in fruits known as the “sticky flower” syndrome. This disorder is characterised by an arrest in growth and maturation of floral organs, and a lack of female floral abscission, thus diminishing fruit shelf-life, commercial quality and value. The aim of the present work was to improve knowledge of the abscission process in C. pepo to better understand the fundamental causes of this disorder. The anatomical analysis of abscission shows a well defined male floral abscission zone (AZ), few hours after anthesis, which differs from the female zone which is not differentiated from the adjacent tissue until the abscission process has begun, and which occurs as a consequence of AZ cell enlargement and the dissolution of their cell walls. To evaluate the role of ethylene and auxins in the regulation of floral abscission in zucchini we performed several treatments, with: ethylene, added as 0.25% ethrel solution; AVG, the inhibitor of ethylene synthesis, at 100 μM; indol-3-acetic acid, 100 μM; and TIBA, the inhibitor of auxin polar transport, at 10 mM. These treatments show that ethylene is an accelerator of zucchini floral abscission, and also promotes abscission in isolated AZs of sticky flowers. On the other hand, IAA delays abscission of the female flowers, whilst the inhibitor of auxin polar transport promotes it. The activity of the cell wall hydrolytic enzymes, polygalacturonase and cellulase, sharply increased just before the shedding of zucchini floral organs (72 h after anthesis). Moreover, both enzyme activities were induced by ethylene, which partly explains the ethylene promoting effect.     

A HISTOCHEMICAL STUDY OF ABSCISSION LAYER FORMATION IN THE BEAN

In debladed bean petioles calcium and dry weight increased in the abscission zone during an induction period of 14 hr. Before the microscopic appearance of the abscission layer calcium decreased in the abscission zone and increased in the petiole. Dry matter began to decrease in both the abscission zone and the petiole 24 hr after deblading. The first visual change in the cells of the abscission zone was a swelling of the pectic materials of the cell walls. This was followed by breakdown of other cell wall components, i.e., non-cellulosic polysaccharides and cellulose. The cellulose of the cell walls adjacent and distal to the abscission layer was found to be altered; however, no lignin was present during abscission layer development. The alteration of pectic materials, coupled with breakdown of cell wall components, resulted in the collapse of cells of the abscission layer just prior to separation. Auxin delayed abscission and also delayed the initial increase in calcium, the movement of calcium from the abscission zone to the petiole, and the decrease in dry weight.     

Fine structure of abscission zones

Summary The 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).     

Poor stomatal control of water balance and the abscission of green needles from a declining stand of spruce trees [Picea abies (L.) Karst.] from the northern Alps

On a site on the west slope of the Wank in the northern Alps changes in water potential, osmotic potential and transpiration rate were measured in spruce trees during the dry summer months of 1991. The pattern of decrease in water potential and osmotic potential on days of varying evaporative demand from trees of widely different decline conditions was used to describe the relative ability of the trees to withstand drought stress. Stress diagrams served as a tool for interpreting the state of health of each tree. The criterion is independent of the water situation of the tree and the other external conditions of the respective experiment. These diagrams clearly show that the foliage of spruces with high needle losses reaches the limit of endurance relatively early. For equal evaporative demand much lower turgor levels were observed in spruces with high needle loss compared to undamaged trees. Associated with the occurrence of low turgor values was the shedding of green needles. The abscission zone was shown in sections. The accumulation of highly fluorescent substances in the bundle sheath cells of the same material was described. Incomplete to non-existent stomatal control over water loss was attributed to modifications in the cell walls of the stomata which appear to alter the ability of the guard cells to sense changes in either atmospheric or cellular hydration. Our studies point to the following situation: air pollution directly affects stomatal control, the loss of stomatal control changes the drought avoidance abilities of the foliage and, as a consequence, low turgor levels occur and premature needle abscission is induced. As site water balance decreases, either due to a dry year or to poor moisture holding abilities of the soil, these conditions become apparent.     

ABSCISSION OF LEAVES AND LEAFLETS IN ACER NEGUNDO AND FRAXINUS AMERICANA

A comparative study of leaf and leaflet abscission in Acer negundo and Fraxinus americana was undertaken with special emphasis on leaflet abscission. Leaf fall in both species is accomplished by orderly, fragmentary abscission of leaflets followed by petiole abscission. Leaflet fall was presaged by differentiation of a separation layer at leaflet bases 10–15 days prior to leaflet fall, without an accompanying protective layer. Anatomical studies of petiole abscission revealed early differentiation of a protective layer followed by differentiation of a separation layer at petiole bases just prior to petiolar fall. Abscission at both sites was facilitated by cell division and dissolution of cell walls within separation layers.     

The positional differentiation of abscission zones during the development of leaves of Sambucus nigra and the response of the cells to auxin and ethylene

Summary Abscission in the leaf rachis of Sambucus nigra L. is preceded by a positional differentiation of zone cells that enlarge and separate in response to ethylene but not to auxin. These cells are absent from youngest leaves, and such leaves do not abscind even in ethylene; other cells of the immature rachii will enlarge in response to auxin. These two classes of target cells are always recognisable by their opposing responses to auxin and ethylene. Prior to separation zone cells exposed to ethylene show considerable activation of the cytoplasm, many polysomes, elongate endoplasmic reticulum and highly dilated dictyosomes with many associated vesicles. Treatment with auxin precludes these changes, and abscission is always retarded: high levels of ethylene must be added to overcome the auxin inhibition. The differentiation of zone cells and their ethylene-stimulated growth and activation are prerequisites for rachis abscission in Sambucus. Such cell development may be of general occurrence prior to organ abscission in plants.Abbreviation IAA indole-3yl-acetic acid     

The Anatomy of Fruit Abscission in Loganberries

Loganberry fruits abscise at the base of the receptacle, justdistal to the sepals. As the fruit ripens, all cells of theabscission zone expand. The central parenchyma cells, due totheir position, appear to be the driving force behind abscission.Their expansion causes early cell-separation within a superficialzone of small cells and rupture of the epidermis at the sepal/receptaclejunction without prior dissolution of cell walls. However, othercells within the abscission zone have their walls degraded,mostly in the region of the middle lamella, as ripening progresses. Xylem transfer cells are found in abundance in the vascularbundles supplying the sepals. The outward curve of these bundlesinto the sepals brings the transfer cells into close proximitywith the abscission zone. A comparison of their distributionin loganberries with that in raspberries (MacKenzie, 1979),which are closely related but abscise at a different site, suggeststhat transfer cells may be implicated in the abscission process. The inevitable structural weakness brought about by the paucityof vascular tissue in the abscission zone relative to the morerobust pedicel may also predispose this area to separation. Anatomy, abscission, loganberry, Rubus idaeus x R, ursinus, Mailing Sunberry, transfer cells structure, fruit     

INCOMPLETE ABSCISSION OF NEEDLE CLUSTERS AND RESIN RELEASE FROM ARTIFICIALLY WATER-STRESSED LOBLOLLY PINE (PINUS TAEDA): A COMPONENT FOR PLANT-ANIMAL INTERACTIONS

A selected Pinus taeda L. tree was rapidly and permanently water-stressed by severing and sealing the bole and observed daily for seven wk and then weekly for six months. Insects were captured before and during treatment from the stressed and adjacent control trees. Tissue samples were extracted at selected intervals for comparison of the stressed and control trees. Drying of the crown of the stressed tree commenced immediately. By the tenth day the previous yrs' needle clusters were red and dropping from the tree while the current yr's needle clusters remained green and did not fall. By the eleventh day beads of resin, predominantly α- and β-pinene, formed and dripped from the abscission scars of the youngest needle clusters of the previous yr. At this time and for several days thereafter, insects were attracted to the stressed tree. Microscopy of abscission sites of the stressed and control trees shows that the sites where resin flowed had incomplete abscission layers, lysigenous collapse of adjacent cortical cells to form a cavity that coalesced the resin ducts associated with the needle cluster, and had mechanically ruptured the tissue connections so that resin normally contained within the duct system was released to the exterior of the tree. Volatilized α-pinene is attractive to the insects associated with pines. Failure of abscission in the stressed tree leading to abnormal resin release is thus proposed as a means by which insects are attracted to stressed trees.     

Die induktion eines trenngewebes bei früchten von Prunus avium L. durch 2-Chloräthylphosphonsäure

Summary 2-Chloroethylphosphonic acid (CEPA) facilitates the separation of the fruit from the pedicel significantly. The application of 2,000 and 4,000 ppm CEPA in four sweet cherry varieties during maturation resulted in the formation of a complete abscission layer in the transition zone between pedicel and fruit. In contrast, in the untreated fruit no abscission layer was evident at maturity. The walls of the cells in the abscission layer contained less total polysaccharides than adjacent cells. Cellulose was partially broken down, and the pectins were degraded. The Ca and Mg content in the cell walls decreased. Thus the same histochemical changes are involved in natural and CEPA induced abscission.     

The study of a monocotyledon abscission zone using microscopic, chemical, enzymatic and solid state 13C CP/MAS NMR analyses

We have investigated distinguishing features in cells of the abscission zone of a monocotyledon fruit, the oil palm Elaeis guineensis. The cell walls of the abscission zone and the subtending mesocarp and pedicel have been analysed by light and transmission electron microscopy, by chemical methods and by solid state 13C CP/MAS NMR spectroscopy. Results show that these abscission zone cells have specific characteristics which include high levels of unmethylated pectin in the walls and an inducible (x35) polygalacturonase enzyme expression. Together these findings help to explain the localised precision of cell separation events.     

In vitro initiation of adventitious structures in relation to the abscission zone in needle explants of Picea abies: anatomical considerations

A comparison of the regeneration potential of needles of different ages in Picea abies emphasized the importance of taking into account the manner of explantation as well as the state of differentiation of the abscission zone. Generally, response in terms of initiation of adventitious structures decreased progressively not only with needle age (5 to 10 to 15 mm long) but also with the distance from the abscission zone towards the tip, that is, distally. On a bud-induction medium adventitious shoot but primordia were produced proximally and distally (except for the very tip) of the weakly-developed abscission zone in needles ca. 5 mm and shorter. In needles between 5 and 10 mm long the various cell types of the abscission zone commenced differentiation, and adventitious structures (shoot bud and other primordia) were formed proximal and immediately distal to it. In needles ca. 15 mm long, in which the abscission zone's hyaline, separation and protective layers became well-developed and where senescence of the distal part commenced, response was limited to proximal tissues. Divisions giving rise to adventitious primordia distal to the abscission zone arose from epidermal cells proper and, in a more acropetal position, also from subsidiary cells of the stomata. Cells of the hypodermis contributed only in the near-distal region of the abscission zone in younger needles, and before commencing differentiation into fibres.
When excised carefully, the explant consists of a leaf plus a peg-like cushion of axillary, meristematic tissue proximal to the abscission zone and capable of ready regeneration. In view of the apparent relationship between age and the stage of development of the abscission zone, it was concluded that there exists a critical needle length which should not be exceeded when attempting in vitro induction of organ primordia.     

Sequential cell wall transformations in response to the induction of a pedicel abscission event in Euphorbia pulcherrima (poinsettia)

Alterations in the detection of cell wall polysaccharides during an induced abscission event in the pedicel of Euphorbia pulcherrima (poinsettia) have been determined using monoclonal antibodies and Fourier transform infrared (FT-IR) microspectroscopy. Concurrent with the appearance of a morphologically distinct abscission zone (AZ) on day 5 after induction, a reduction in the detection of the LM5 (1→4)-β- d -galactan and LM6 (1→5)-α- l -arabinan epitopes in AZ cell walls was observed. Prior to AZ activation, a loss of the (1→4)-β- d -galactan and (1→5)-α- l -arabinan epitopes was detected in cell walls distal to the AZ, i.e. in the to-be-shed organ. The earliest detected change, on day 2 after induction, was a specific loss of the LM5 (1→4)-β- d -galactan epitope from epidermal cells distal to the region where the AZ would form. Such alteration in the cell walls was an early, pre-AZ activation event. An AZ-associated de-esterification of homogalacturonan (HG) was detected in the AZ and distal area on day 7 after induction. The FT-IR analysis indicated that lignin and xylan were abundant in the AZ and that lower levels of cellulose, arabinose and pectin were present. Xylan and xyloglucan epitopes were detected in the cell walls of both the AZ and also the primary cell walls of the distal region at a late stage of the abscission process, on day 7 after induction. These observations indicate that the induction of an abscission event results in a temporal sequence of cell wall modifications involving the spatially regulated loss, appearance and/or remodelling of distinct sets of cell wall polymers.     

Transdifferentiation of Mature Cortical Cells to Functional Abscission Cells in Bean

Abscission explants of bean (Phaseolus vulgaris L.) were treated with ethylene to induce cell separation at the primary abscission zone. After several days of further incubation of the remaining petiole in endogenously produced ethylene, the distal two-thirds of the petiole became senescent, and the remaining (proximal) portion stayed green. Cell-to-cell separation (secondary abscission) takes place precisely at the interface between the senescing yellow and the enlarging green cells. The expression of the abscission-associated isoform of β-1,4-glucanhydrolase, the activation of the Golgi apparatus, and enhanced vesicle formation occurred only in the enlarging cortical cells on the green side. These changes were indistinguishable from those that occur in normal abscission cells and confirm the conversion of the cortical cells to abscission-type cells. Secondary abscission cells were also induced by applying auxin to the exposed primary abscission surface after the pulvinus was shed, provided ethylene was added. Then, the orientation of development of green and yellow tissue was reversed; the distal tissue remained green and the proximal tissue yellowed. Nevertheless, separation still occurred at the junction between green and yellow cells and, again, it was one to two cell layers of the green side that enlarged and separated from their senescing neighbors. Evaluation of Feulgen-stained tissue establishes that, although nuclear changes occur, the conversion of the cortical cell to an abscission zone cell is a true transdifferentiation event, occurring in the absence of cell division.     

LOCALIZATION OF DEHYDROGENASE AND ACID PHOSPHATASE IN THE ABSCISSION ZONE OF BEAN LEAVES

Leaf abscission in Phaseolus vulgaris L. cv. ‘Contender’ is associated with enzymatic changes during and prior to separation. Deblading resulted in a localized increase in dehydrogenase and acid phosphatase in the abscission zone. Increased enzyme activities were observed 24–48 hr after deblading. In debladed plants separation was complete in 6–8 days. At separation, dehydrogenase activity appeared to decrease and localization was specific to the protective layer, while the petiole side had no activity. In contrast, acid phosphatase activity was observed in some layers of cells on the petiole side after separation. Ethylene treatment promoted abscission and separation occurred in 24–48 hr in both debladed and intact plants. No protective layer was formed during ethylene-induced abscission. Enzymatic changes similar to those observed in debladed control plants were observed with ethylene treatment. Ethylene induced an additional abscission layer between the pulvinus and petiole, where an abscission layer normally does not form. In this ethylene-induced abscission layer, similar enzyme activities were detected.     

Gibberellin-induced formation of tension wood in angiosperm trees

After gibberellin had been applied to the vertical stems of four species of angiosperm trees for approximately 2 months, we observed eccentric radial growth that was due to the enhanced growth rings on the sides of stems to which gibberellin had been applied. Moreover, the application of gibberellin resulted in the formation of wood fibers in which the thickness of inner layers of cell walls was enhanced. These thickened inner layers of cell walls were unlignified or only slightly lignified. In addition, cellulose microfibrils on the innermost surface of these thickened inner layers of cell walls were oriented parallel or nearly parallel to the longitudinal axis of the fibers. Such thickened inner layers of cell walls had features similar to those of gelatinous layers in the wood fibers of tension wood, which are referred to as gelatinous fibers. Our anatomical and histochemical investigations indicate that the application of gibberellin can induce the formation of tension wood on vertical stems of angiosperm trees in the absence of gravitational stimulus.     

The dibenzodioxocin lignin substructure is abundant in the inner part of the secondary wall in Norway spruce and silver birch xylem

A specific condensed lignin substructure, dibenzodioxocin, was immunolocalized in differentiating cell walls of Norway spruce (Picea abies (L.) H. Karsten) and silver birch (Betula pendula Roth) xylem. A fluorescent probe, Alexa 488 was used as a marker on the dibenzodioxocin-specific secondary antibody. For the detection of this lignin substructure, 25-m cross-sections of xylem were viewed with a confocal laser-scanning microscope with fluorescein isothiocyanate fluorescence filters. In mature cells, fluorescence was detected in the S3 layer of the secondary wall in both tree species, but it was more intense in Norway spruce than in silver birch. In silver birch most of the signal was detected in vessel walls and less in fiber cell walls. In very young tracheids of Norway spruce and vessels and fibers of silver birch, where secondary cell wall layers were not yet formed, the presence of the dibenzodioxocin structure could not be shown.Abbreviation CLSM confocal laser-scanning fluorescence microscopy     

The use of immunological methods to study the activity of cellulase isozymes (B 1:4 glucan 4-glucan hydrolase) in bean leaf abscission

Abstract The changes in the levels of two different isozymes of cellulase (EC 3.2.1.4) have been followed during the abscission of the primary leaves of bean (Phaseolus vulgaris c.v. Red Kidney), using antibodies raised against the 9.5 form of the enzyme. Data from both radioimmune and direct assay show that the 9.5 form of cellulase is undetectable prior to the induction of abscission. After a 12 h lag this isozyme increases in activity, the increase preceding a decrease in integrity of the abscission zone cell walls. The results are consistent not only with the view that this specific isozyme is involved in wall hydrolysis but also with previous data which showed that cellulase is synthesized ‘de novo’. The 4.5 isozyme of cellulase is more widely spread throughout the plant, being most active in young tissues. During abscission the activity of this isozyme in the abscission layer falls and consequently it is not thought to be involved directly in the abscission process.