Methyl jasmonate induces the formation of secondary abscission zone in stem of Bryophyllum calycinum Salisb

Methyl jasmonate (JA-Me) at a concentration of 0.5 % induced the formation of secondary abscission zone and senescence in several types of stem explants (only internode segment, internode segment with nodes and without leaves, internode segment with nodes and debladed petioles) of Bryophyllum calycinum when it was applied in various places of the stem or the debladed petiole as lanolin paste. In the presence of small leaves in stem explants methyl jasmonate also induced the formation of secondary abscission zone and senescence but the presence of larger leaves completely inhibited methyl jasmonate-induced processes. Auxin, (indole-3-acetic acid, IAA), at a concentration of 0.1 % extremely prevented the formation of secondary abscission zones and senescence in the stem tissues induced by methyl jasmonate. Similar relationship between auxin and methyl jasmonate to induce the formation of secondary abscission zone and senescence was found in decapitated shoot of the intact plant. Mechanisms of the formation of secondary abscission zone are also discussed in terms of the interaction of methyl jasmonate with auxin.     

Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L.) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls. Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole. The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour-[1-¹⁴C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-¹⁴C]PAA occurred.Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles. The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem. Following the application of [1-¹⁴C]IAA or [1-¹⁴C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem. Proximal applications of either compound stimulated the acropetal transport of [¹⁴C]sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip. However, PAA was considerably less active than IAA in this response.It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone. Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed.     

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.     

Palm snorkelling: leaf bases as aeration structures in the mangrove palm (Nypa fruticans)

Mangrove species have evolved specialized structures, such as pneumatophores, to supply oxygen to the roots, but, in Nypa fruticans, the only mangrove palm, no such structure has been reported. This study aimed to determine the adaptations of N. fruticans to the mangal habitat with special reference to the air‐supplying structure. Following senescence, the rachis is abscised at the zone of junction with the leaf base. Simultaneously, lenticels develop so that, when abscission is completed, a network of mature lenticels covers the leaf base. Expansigenous aerenchyma with increasing porosity towards the stem junction occurs in the leaf base. The first two root branching orders present a subero‐lignified rhizodermis and exodermis, and the cortex consists of schizo‐lysigenous aerenchyma with wide lacuna, limiting radial oxygen loss and facilitating longitudinal oxygen transport to living tissues. Lifespan estimation suggests that leaf bases can live for up to 4 years following abscission, ensuring the persistence of aeration structures. This study provides structural evidence indicating that N. fruticans has evolved a unique type of air‐supplying structure in the mangal habitat. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 257–270.     

Protective mechanism of desiccation tolerance in <Emphasis Type="Italic">Reaumuria soongorica</Emphasis>: Leaf abscission and sucrose accumulation in the stem

Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub, is widely found in semi-arid areas in northwestern China and can survive severe desiccation of its vegetative organs. In order to study the protective mechanism of desiccation tolerance in R. soongorica, diurnal patterns of net photosynthetic rate (Pn), water use efficiency (WUE) and chlorophyll fluorescence parameters of Photosystem II (PSII), and sugar content in the source leaf and stem were investigated in 6-year-old plants during progressive soil drought imposed by the cessation of watering. The results showed that R. soongorica was characterized by very low leaf water potential, high WUE, photosynthesis and high accumulation of sucrose in the stem and leaf abscission under desiccation. The maximum Pn increased at first and then declined during drought, but intrinsic WUE increased remarkably in the morning with increasing drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PSII(Φ_PSII) decreased significantly under water stress and exhibited an obvious phenomenon of photoinhibition at noon. Drought stressed plants maintained a higher capacity of dissipation of the excitation energy (measured as NPQ) with the increasing intensity of stress. Conditions of progressive drought promoted sucrose and starch accumulation in the stems but not in the leaves. However, when leaf water potential was less than −21.3 MPa, the plant leaves died and then abscised. But the stem photosynthesis remained and, afterward the plants entered the dormant state. Upon rewatering, the shoots reactivated and the plants developed new leaves. Therefore, R. soongorica has the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation.     

Protective mechanism of desiccation tolerance in Reaumuria soongorica: Leaf abscission and sucrose accumulation in the stem

Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub, is widely found in semi-arid areas in northwestern China and can survive severe desiccation of its vegetative organs. In order to study the protective mechanism of desiccation tolerance in R. soongorica, diurnal patterns of net photosynthetic rate (Pn), water use efficiency (WUE) and chlorophyll fluorescence parameters of Photosystem II (PSII), and sugar content in the source leaf and stem were investigated in 6-year-old plants during progressive soil drought imposed by the cessation of watering. The results showed that R. soongorica was characterized by very low leaf water potential, high WUE, photosynthesis and high accumulation of sucrose in the stem and leaf abscission under desiccation. The maximum Pn increased at first and then declined during drought, but intrinsic WUE increased remarkably in the morning with increasing drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PSII(Φ_PSII) decreased significantly under water stress and exhibited an obvious phenomenon of photoinhibition at noon. Drought stressed plants maintained a higher capacity of dissipation of the excitation energy (measured as NPQ) with the increasing intensity of stress. Conditions of progressive drought promoted sucrose and starch accumulation in the stems but not in the leaves. However, when leaf water potential was less than −21.3 MPa, the plant leaves died and then abscised. But the stem photosynthesis remained and, afterward the plants entered the dormant state. Upon rewatering, the shoots reactivated and the plants developed new leaves. Therefore, R. soongorica has the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation. Supported by the Program of the Research of Vegetation Restoration in Arid Areas of Lanzhou (Grant No. 03-2-27) and the National Natural Science Foundation of China (Grant No. 30270243)     

A Comparison of Abscission of Rubber (Hevea brasiliensis) Leaves Infected with Microcyclus ulei with Leaf Abscission Induced by Ethylene Treatment, Deblading and Senescence

Studies on the histology and on effects of growth substancesand phenols as well as changes in activities of pectinmethylesterase indicated that the mechanism of abscission of Hevealeaflets infected with Microcyclus ulei differed from the mechanismof abscission of debladed, ethylene treated and senescent leaves.An abscission layer which was formed during abscission of debladed,ethylene-treated and senescent leaves was absent during abscissionof heavily diseased leaves. The ratio of pectinmethyl esteraseactivities in tissues distal to the abscission zone to activitiesin tissues proximal to the zone decreased in debladed and ethylenetreated leaves but such decreases were not detected during abscissionof Hevea leaves infected with M. ulei. Hevea brasiliensis Muell. Arg., rubber, leaf abscission, Microcyclus ulei, ethylene, indol-3-ylacetic acid, kinetin     

Mechanism of Action of Abscission Accelerators

Abscission zone explants of Gossypium hirsutum L., Cassia fistula L., and Coleus blumei Benth. were used to investigate correlations between endogenous rates of ethylene evolution and time of abscission. Additions of 0.1 nl/ml ethylene to the explants markedly accelerated abscission; continuous aeration of the explants, to prevent accumulation of small amounts of endogenously produced ethylene, inhibited abscission compared with that of sealed controls. Substances that stimulated abscission simultaneously accelerated ethylene evolution on all three species and at any position of application. The positional effects of auxin are explained as being due to differences in transport in the explant. Thus, distally applied auxin inhibits abscission, regardless of the accelerated rate of ethylene evolution, by being rapidly transported to the abscission zone. Auxin applied proximally stimulates abscission because it is unable to move as rapidly to the abscission zone and the ethylene effect becomes dominant. Ethylene was found to be most effective on aged tissues, and it is concluded that abscission rates are determined by an increase in sensitivity of the tissue to the ethylene that is already being produced.     

Translatable mRNA changes in ethylene induced abscission zones of Phaseolus vulgaris (Red Kidney)

Abstract The wheat germ translation system was programmed with soluble RNA extracted from foliar abscission zones of Phaseolus vulgaris, These extracts were taken at various times after the induction of abscission. A translation product with a molecular weight of 42 kilodalton (kD) was only present after this treatment, though three other species 32, 27 and 17 kD increased substantially. The isozyme of cellulase with a pi of 9.5 could not be conclusively identified amongst the products though the 32 kD protein is probably chitinase. Comparison of the abscission zone translatable RNA with that from adjacent petiole and stem tissues showed the 17 kD protein developed in all these location. The 42, 32 and 27 kD bands were found predominantly in the zone and petiole.     

ABSCISSION AND THE AXILLARY FROND IN SPIRODELA OLIGORHIZA (LEMNACEAE)

Two abscission zones are present in fronds of Spirodela oligorhiza. In one zone the abscission layer separates the daughter frond and its connecting stalk from the mother frond. The second abscission layer separates the daughter frond from the connecting stalk. An axillary frond is asymmetrically positioned at the base of each daughter frond where it joins the mother frond. In right-handed physiological clones the axillary frond is to the right of the connecting stalk and in left-handed clones, to the left of the stalk. When a daughter frond abscises it leaves behind its axillary frond in the pocket of the mother frond. Histological features of abscission and treatments that induce abscission in Spirodela oligorhiza are described.     

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.     

Breakdown of middle lamella pectin by ●OH during rapid abscission in Azolla

Azolla, a small water fern, abscises its roots and branches within 30 min upon treatment with various stresses. This study was conducted to test whether, in the rapid abscission that occurs in Azolla, breakdown of wall components of abscission zone cells by ^●OH is involved. Experimentally generated ^●OH caused the rapid separation of abscission zone cells from detached roots and the rapid shedding of roots from whole plants. Electron microscopic observations revealed that ^●OH rapidly and selectively dissolved a well‐developed middle lamella between abscission zone cells and resultantly caused rapid cell separation and shedding. Treatment of abscission zones of Impatiens leaf petiole with ^●OH also accelerated the separation of abscission zone cells. However, compared with that of Azolla roots, accelerative effects in Impatiens were weak. A large amount of ^●OH was cytochemically detected in abscission zone cells both of Azolla roots and of Impatiens leaf petioles. These results suggest that ^●OH is involved in the cell separation process not only in the rapid abscission in Azolla but also in the abscission of Impatiens. However, for rapid abscission to occur, a well‐developed middle lamella, a unique structure, which is sensitive to the attack of ^●OH, might be needed.     

Spontaneous phloem exudation accompanying abscission in Lupinus mutabilis (Sweet)

Like some of the Mediterranean members of the genus Lupinus, the New World lupin, Lupinus mutabilis (Sweet; cv. Inti), exhibits prolonged (20-40 min) exudation of phloem sap from incisions made in stems, in the raceme and at the tips and sutures of developing fruits. Just prior to or immediately following abscission of flowers of L. mutabilis there was also spontaneous exudation from the proximal face of the abscission zone at the base of the pedicel. This is not a recorded feature of other lupins. Analysis of solutes in this exudate was consistent with its having been derived directly from phloem. The major solutes were sucrose (0.940.04 M), amino acids (18811 mM, 45% as asparagine and 15% as glutamine), K ion (52 mM), and total phosphorus (17 mM). Microscopic examination of the proximal face of the pedicel abscission zone at or following abscission showed little or no breakage of the cells at the zone. The major solutes of spontaneous exudate were similar to those in exudates collected from incisions made in the supporting raceme, upper stem and branches, at the tips and sutures of developing fruits and in the mid- and basal stem regions. However, there were significant compositional differences among minor constituents. The spontaneous exudate had a higher level of Ca ion and, consequently, a narrower Mg/Ca ratio (2.8) than exudate from incisions in the adjacent raceme (9.3) or fruits (15.7). There were also higher concentrations of trace elements (Mn, Zn and Cu) but lower concentrations (3 ng m^-1) of cytokinins compared to exudates collected from incisions (20 ng ml^-1). The relative contents of K and Na ions in exudates from incisions at different sites on the plant showed evidence of selective phloem loading and downward translocation of Na ion and selective loading and upward translocation of K ion.     

Autotomy in a polychaete: Abscission zone at the base of the tentacular crown of Sabella penicillus

Summary Under various circumstances the tentacular crown of some sabellid polychaetes becomes detached from the body. Separation occurs always at a preestablished zone of abscission at the base of the crown. We used electron microscopy to study the abscission zone of Sabella penicillus, both in specimens whose crown was intact and in those whose crown had separated.The abscission zone is within the intermediate layer, between the crown skeleton and the body wall musculature, and only structures supported by the crown skeleton separate from the animal's body. Abscission involves a rupture of the paramyosin muscle cells which form bridges connecting extensions from the epimysium of the body wall musculature and from the cartilage matrix of the crown. After abscission the anterior and posterior ends of the cells remain in place on the crown and body respectively. Sabella penicillus appears able to control the loss of its tentacular crown, so this abscission is a kind of autotomy. Under some circumstances autotomy of the crown may permit escape or confer some surgical benefit to the animal. Using standard histology we found the same anatomical provision for crown abscission in a variety of sabellids. We conclude that differences in their capacities to autotomize the crown have a behavioral/physiological basis rather than an anatomical one.     

Relationships between jasmonates and auxin in regulation of some physiological processes in higher plants

Growth and development of plants are regulated by interactions among different plant growth substances. During stress conditions, both abiotic and biotic, interaction of the some hormones activates defense responses. The present review describes the interaction between jasmonates and auxin in regulation of some physiological processes in plant growth and development. Some jasmonate-induced processes reduced by auxins and some auxin stimulated physiological processes inhibited by jasmonates are the focus of this review. Therefore, the following physiological processes are described: stem cell growth, abscission, secondary abscission zone formation, tendril coiling, opening of the pulvinules in Mimosa pudica, wounding and induced gene expression, nicotine biosynthesis and auxin biosynthesis in Brassicaceae.     

SHOOT TIP ABORTION AND SYMPODIAL BRANCH REORIENTATION IN BROWNEA ARIZA (LEGUMINOSAE)

The growth of the tropical tree Brownea ariza Benth. is modular and conforms to Troll's model. Distinctive anatomical features of its shoot development were investigated. Each module consists of from 6–10 compound leaves and terminates its growth by shoot tip abortion. Sympodial branch systems are formed by renewal growth from the most distal (pseudoterminal) 1–2 buds. New modules are wholly preformed within large (15–28 cm) buds. The flush occurs without a resting period and full shoot expansion is completed within one day. A distinct abscission zone develops in the stem just distal to the node of the last expanded leaf. Abortion of the shoot apex and 5–6 embryonic leaves occurs 2–3 days after the flush begins. This tissue vacuolates and begins to become necrotic prior to actual abscission. New flushes are pendent but are reoriented to a plagiotropic or upright position to create an arborescent form. Reorientation begins quickly (10° within 2 days) due to maturation of primary and secondary tissues and continues throughout the life of the branch by means of reaction wood formation on the upper surface.