Abscission: ethylene and light control

The role of ethylene in light control of leaf abscission im mung bean, Vigna radiata (L.) Wilczek cv Jumbo, cuttings was examined. While red light inhibits and far-red light promotes loss of break strength in abscission zones as compared with dark controls, changes in the rate of abscission could not be associated with changes in the rate of ethylene production. Reducing ethylene synthesis in tissue with aminoethoxyvinylglycine did not alter the effects of red or far-red light on abscission. Far-red light appeared to increase and red light appeared to decrease tissue sensitivity to ethylene.     

Deferral of senescence and abscission by chemical inhibition of ethylene synthesis and action in bean explants

Three compounds known to inhibit ethylene synthesis and/or action were compared for their ability to delay senescence and abscission of bean explants (Phaseolus vulgaris L. cv Contender). Aminoethoxyvinyl-glycine (AVG), AgNO₃, and sodium benzoate were infiltrated into the petiole explants. Their effect on abscission was monitored by measuring the force required to break the abscission zone, and their effect on senescence was followed by measuring chlorophyll and soluble protein in the distal (pulvinus) sections. AVG at concentrations between 1 and 100 micromolar inhibited ethylene synthesis by about 80 to 90% compared to the control during sampling periods of 24 and 48 hours after treatment. This compound also delayed the development of abscission and senescence. Treatment with AgNO₃ at concentrations between 1 and 100 micromolar progressively reduced ethylene production, but to a lesser extent than AVG. The effects of AgNO₃ on senescence and abscission were quite similar to those of AVG. Sodium benzoate at 50 micromolar to 5 millimolar did not inhibit ethylene synthesis during the first 24 hours, but appreciably inhibited ethylene synthesis 48 hours after treatment. It also delayed the development of abscission and senescence. The effects of AVG, Ag⁺, and sodium benzoate suggest that ethylene could play a major role in both the senescence induction phase and the separation phase in bean explants.     

Perianth Abscission in Montbretia (Crocosmia x crocosmiiflora)

The anatomy, histochemistry, kinetics and hormonal regulationof perianth abscission in Crocosmia x crocosmiiflora (Montbretia)has been investigated. The abscission zone is anatomically welldefined, with cell divisions occurring in this region at anthesis.Abscission is first detectable 3 d after perianth opening, whenthe walls of a group of cells beneath the adaxial epidermisshow reduced staining with polyanion-specific stains, and adecline in penanth break strength also occurs. Abscission isachieved by cell wall breakage in thc abscission zone, whichprogresses eccentrically from the adaxial epidermis throughthe abscission zone, rather than the separation of intact cellsas occurs in flowers of dicotyledons. Experiments on detachedflowers suggest similarities in the hormonal regulation of abscissionin Crocosmia to that of dicotyledons, in that an ethylene promotion,and possibly an auxin inhibition, mechanism may exist in Crocosmia.Ovary expansion occurs throughout the development and senescenceof unpollinated flowers, but does not appear to be the solecause of wall breakage in the abscission zone. It is suggestedthat hormonally regulated wall hydrolases weaken the cell wallsin the abscission zone, and allow wall breakage and subsequentabscission to occur. Cdrocosmia x crocosmiiflora, Montbretia, anatomy, breakstrength, cell wall changes, histochemistry, flowers, monocotyledons, perianth, senescence, ethylene, auxin     

Abscission: the phytogerontological effects of ethylene

The role of ethylene in the aging of bean (Phaseolus vulgaris L. cv. Red Kidney) petiole abscission zone explants was examined. The data indicate that ethylene does accelerate aging in addition to inducing changes in break strength. Application of ethylene during the aging stage (stage 1) promoted abscission when followed by a second ethylene treatment during the cell separating stage (stage 2). The half-maximal effective concentration of ethylene to induce aging was around 0.3 microliter per liter; 10 microliters per liter was a saturating dose. CO₂ reversal of ethylene action during stage 1 was incomplete and gave ambiguous results. CO₂ (10%) reversed the effect of 10 microliters per liter ethylene but not 1 microliter per liter ethylene. The possibility that ethylene not only accelerated aging but was also a requirement for it was tested, and experimental evidence in favor of this idea was obtained. It was concluded that ethylene plays a dual role in the abscission of bean petiole explants: a phytogerontological effect and a cellulase-inducing effect.     

Fine structure of abscission zones

Summary The effect of ethylene on abscission of flower pedicels of tobacco plants has been investigated. For the first 2 h of exposure to C₂H₄, the pedicels bend rather than break in response to applied force, but after 2.5 h exposure they break at the abscission zone under an applied force of 40 g. The break strength of the abscission zone decreases exponentially with time to 5 g at 5 h after beginning of the C₂H₄ treatment. An examination of the tissue at the fine structural level 2 h after exposure to C₂H₄ reveals the accumulation of rough endoplasmic reticulum (RER) in the abscission cells. Rough ER becomes increasingly abundant by 3–5 h exposure of the tissue to C₂H₄. There is approximately a 30 fold increase in RER by 5 h of exposure, as compared to untreated tissue.Loss in the integrity of the membranes of microbodies occurs by 5 h exposure of the tissue to C₂H₄. As cell wall degradation proceeds, fibrous material, vesicular structures, and electron dense bodies—the latter often appearing striated—develop in the disintegrating wall. Little change is seen in the structure of nuclei, mitochondria, chloroplasts and in the crystalloid cores of microbodies during the first 5 h of exposure of the tissue to C₂H₄. However, disorganization of cytoplasmic components does occur in cells where cell wall breakdown is at an advanced stage.     

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.     

A novel mechanism of abscission in fronds of Lemna minor L. and the effect of silver ions

Lemna minor L. (duckweed) forms colonies through vegetative propagation because mother fronds remain connected for some time with their daughter fronds by stipes. The colony size is controlled by abscission of stipes at a specific preformed abscission zone. Application of silver ions (Ag(+) ) enhances the rate of frond abscission, thus resulting in smaller colonies. The mechanism behind this process has not yet been identified. Silver caused an abscission response that saturated after 7 h of treatment. The half-maximal effective concentration was 0.72 μm Ag(+) for the standard clone, L. minor St. Other clones of the same species show sensitivities that differ by one order of magnitude. Transmission electron microscopy revealed: (i) large numbers of vesicles close to the plasmalemma in cells adjacent to the abscission zone, which proves a vesicular type secretory activity; and (ii) a moderately electron-dense secretion accumulated in the enlarging intercellular spaces, and seemed to flow from the adjacent cells towards the abscission zone. We assume that increasing pressure causes this material to push apart the cells, thereby causing the break in the abscission zone of the stipe. This is a novel mechanism of abscission that has not previously been described. The same mechanism occurs in stipes of both control and Ag(+) -treated samples. Silver ions only accelerate the process leading to abscission of stipes, without affecting the mechanism involved.     

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.     

Kinetics of abscission in the bean petiole explant

The progress of bean petiole abscission has been followed using quantitative measurements of the mechanical force required to break explants at the separation zone. It is found that the shortest time for a measurable effect of ethylene (1 ppm) in stimulating the development of frangibility is about 1 hr. Removal of the ethylene is followed by a return to the endogenous rate of weakening, the slower rate being established in 1 hr. Application of the inhibitor cycloheximide leads to a cessation of abscission development within one-half hr. As in the stimulations of certain plant processes with auxins, gibberellins, and cytokinins. ethylene stimulation of abscission requires the continuous presence of the regulator.     

Biophysical and Hormonal Changes Linked to Postharvest Needle Abscission in Balsam Fir

Efforts are being made to determine significant biophysical and physiological events related to postharvest needle abscission. It is known that initial postharvest average water consumption is 0.2 mL g^?1 day^?1 (based on dry shoot tissue), but gradually decreases by up to 75 %. It is hypothesized that some degree of water deficit is manifested through changes in several biophysical and hormonal factors. Parameters including needle loss, water use, relative water content, electrical capacitance, membrane injury, and xylem pressure potential were measured once every 5 days on balsam fir branches collected from a clonal orchard. In addition, needles were sampled at the beginning of the experiment and during peak needle abscission which were then subjected to endogenous hormonal analysis. Peak needle abscission occurred within 24 days. During this time water use decreased by 70 %, relative water content decreased by 23 %, capacitance decreased by 64 %, membrane injury increased by 100 %, needle break strength decreased by 50 %, and xylem pressure potential decreased fourfold. Abscisic acid increased by 32-fold and trans-zeatin riboside increased by fourfold during peak abscission compared to fresh branches. Other cytokinins, such as cis-zeatin riboside, isopentenyl adenosine, trans-zeatin-O-glucoside, and dihydrozeatin riboside all doubled during abscission. Finally, there was a 95 % decrease in indole-3-acetic acid. Observed changes in all biophysical parameters, as well as abscisic acid, could be indicative of a possible postharvest water stress or dehydration. It is possible that dehydration-induced changes in biophysical and hormonal factors trigger and/or modulate postharvest needle abscission.     

不同脱落力下蜡梅花梗CpEXP1基因的表达与扩张蛋白活性

为了研究不同脱落力下蜡梅花梗中CpEXP1基因的表达水平以及扩张蛋白活性,采用real-timePcR技术检测不同脱落力下蜡梅花梗组织CpEXP1基因的表达水平;同时采用蛋白质体外重组法以及培养基添加法对花梗组织扩张蛋白活性进行测定。结果表明：CpEXP1基因的表达与扩张蛋白活性变化趋势相同,并在蜡梅花蕾脱落过程中具有阶段特异性。CpEXPI基因在自然脱落组的表达量显著高于其他组（P〈0．05）。此时扩张蛋白活性也最强。扩张蛋白对麻点百合愈伤组织的分化无明显影响,对增殖以及生根有明显促进作用。     

The role of free polyamines in the alternate-bearing of pistachio (<Emphasis Type="Italic">Pistacia vera</Emphasis> cv. Pontikis)

The present study was conducted in order to examine the physiological role of free polyamines in flower bud abscission. For this reason five 15-year old pistachio trees cv. "Pontikis" were selected and half of the main branches were manually defruited in early May. Polyamines were analyzed in three different organs (shoots, leaves and flower buds) from both fruiting and non-fruiting branches, during the period of kernel growing. Five samplings took place and the polyamines putrescine, spermidine and spermine were assayed. The flower bud abscission percentage was recorded every 5–10 days during kernel formation. Polyamine concentration declined during the period that coincides with that of kernel development, in both fruiting and non-fruiting branches, while significant bud abscission occurred from mid-July till late September in fruiting branches. Polyamine concentration in organs from fruiting branches was in most cases lower than that of non-fruiting ones. Most of the individual polyamines exhibited a high and significant negative correlation with bud abscission. By measuring the spermidine content of leaves and the spermine content of buds, it was possible to estimate the forthcoming bud abscission with significant accuracy (approximately 93%). On the other hand, the total polyamine content of the buds exhibited a significant strong negative relationship with bud abscission. Consequently, polyamines could have an important physiological function in the development of flower bud abscission of pistachio.     

Effect of Protein Synthesis Inhibitors, Auxin, and Gibberellic Acid on Abscission

Chloramphenicol, actinomycin D, and other inhibitors of protein synthesis promote abscission in several plant genera. Abscission is accelerated in species where an abscission layer is present, as well as in tissue where no abscission layer develops prior to abscission. The inhibitors promote abscission in species where cell division is reported to precede the separation processes as well as in tissues where no cell division is associated with the initiation of abscission. Indoleacetic acid (IAA) or auxin precursors, when applied with chloramphenicol and aclinomycin D, overcome the promotive effects of the inhibitors on abscission. These inhibitors apparently do not promote abscission through their effects on auxin precursor conversion, IAA transport, and IAA destruction in the petiole. IAA increases the incorporation of leucine-1-¹⁴C into a trichloroacetic acid precipitable fraction of the abscission zone under conditions where abscission is retarded. A low concentration of IAA which accelerates abscission, decreases incorporation of leucine into protein. Other promoters of abscission — chloramphenicol, d-aspartic acid, and gibberellic acid —also decrease the incorporation of leucine into the protein of the abscission zone. The data indicate that enzymes required for the degradative processes associated with abscission are already present in the abscission zone whereas a continuous synthesis of protein is required for the retention of the leaf.     

Regulation of polyamine metabolism and biosynthetic gene expression during olive mature-fruit abscission

Exogenous ethylene and some inhibitors of polyamine biosynthesis can induce mature-fruit abscission in olive, which could be associated with decreased nitric oxide production as a signaling molecule. Whether H?O? also plays a signaling role in mature-fruit abscission is unknown. The possible involvement of H?O? and polyamine in ethylene-induced mature-fruit abscission was examined in the abscission zone and adjacent cells of two olive cultivars. Endogenous H?O? showed an increase in the abscission zone during mature-fruit abscission, suggesting that accumulated H?O? may participate in abscission signaling. On the other hand, we followed the expression of two genes involved in the polyamine biosynthesis pathway during mature-fruit abscission and in response to ethylene or inhibitors of ethylene and polyamine. OeSAMDC1 and OeSPDS1 were expressed differentially within and between the abscission zones of the two cultivars. OeSAMDC1 showed slightly lower expression in association with mature-fruit abscission. Furthermore, our data show that exogenous ethylene or inhibitors of polyamine encourage the free putrescine pool and decrease the soluble-conjugated spermidine, spermine, homospermidine, and cadaverine in the olive abscission zone, while ethylene inhibition by CoCl? increases these soluble conjugates, but does not affect free putrescine. Although the impact of these treatments on polyamine metabolism depends on the cultivar, the results confirm that the mature-fruit abscission may be accompanied by an inhibition of S-adenosyl methionine decarboxylase activity, and the promotion of putrescine synthesis in olive abscission zone, suggesting that endogenous putrescine may play a complementary role to ethylene in the normal course of mature-fruit abscission.     

GIBBERELLIN AND AUXIN RELATIONSHIPS IN ABSCISSION

Gibberellic acid (GA) has no effect on abscission when applied proximally or distally to the abscission zones of debladed petioles of Coleus. Application of GA to the stem apex increases the rate of abscission of debladed petioles. The effect on abscission is accompanied by an increase in the level of endogenous auxin in the stem. Correspondingly proximal applications of indoleacetic acid (IAA) accelerate abscission, whereas the longevity of the debladed petiole approaches that of the intact leaf only in the presence of a continuous distal supply of IAA. No correlation is found between petiole elongation and its longevity. The experimental data support the view that auxin acts at the abscission zone in regulating separation processes and that the effect of GA is through its effect on the level of endogenous auxin.     

The Role of Cellulase and Polygalacturonase in Abscission of Young and Mature Shamouti Orange Fruits

During the first eight weeks after setting young citrus fruits gradually lose their ability to abscise at the abscission zone between the stem and the pedicel; in fruits older than eight weeks abscission occurs at the calyx area. The activity of cellulase and polygalacturonase in the two abscission zones was markedly increased before and during abscission, and was localized mainly in the abscission zone. Ethylene accelerated the increase in enzymic activity after an 8- to 10-h lag period; 2,4-D delayed abscission and enzymic activity when applied during the first 24 h after excision. During this period 2,4-D also partly suppressed the enhancing effect of ethylene. Early application of cyclo-heximide inhibited the formation of the enzymes and thus abscission was delayed to a certain extent. Although there are some indications that the relationship between enzymic activity and abscission is a complex one, the data presented indicate that cellulase and polygalacturonase play a significant role in abscission of citrus fruits at various developmental stages. Both enzymes act almost simultaneously and are equally controlled by ethylene and 2,4-D.     

Studies on the abscission of blue lupin leaves

Summary The responses of three types of explants of blue lupin leaves are considered: pulvinar explants, consisting of the pulvinar region alone, petiolar explants, consisting of the pulvinar region plus petiole and laminar explants consisting of the pulvinar region plus leaflets. Abscission is accelerated by removal of the leaflets; removal of the petiole has much less effect. Pulvinar explants fail to abscise in darkness but are the first to abscise in the light. This is in accordance with previous evidence of high light sensitivity of the pulvinar region. Kinetin applied directly to the pulvinar region delays abscission, as does kinetin supplied via the transpiration stream. As shown by experiment, this is probably due to transported kinetin reaching the abscission zones of the pulvinar region. The effects of photoperiodic treatments on explants or whole leaves are described. Abscission in the whole leaf is delayed by short daily photoperiods; the delay reaches a maximum with 8 hours light per day. However, abscission is more rapid in continuous light than in darkness. Removal of the leaflets greatly accelerates abscission even in darkness. The pulvinar explant fails to abscise with photoperiods of 4 hours or less; although it appears to have a long day response, preliminary attempts failed to demonstrate that this is a true photoperiodic response (replacement of a long day by a short day together with a light break). The complex responses of leaves and explants to day length lend further support to the hypothesis that light has effects on abscission other than in photosynthesis.