Patterns of Ethylene and Carbon Dioxide Evolution during Cotton Explant Abscission

The relationship between abscission and the evolution of ethylene and CO₂ was examined in explants and explant segments of cotton seedlings (Gossypium hirsutum L. cv. Acala SJ-1) under both static and flow system conditions, and in the presence and absence of mercuric perchlorate. Explant excision was immediately followed by increased ethylene evolution (wound ethylene); senescence was also accompanied by increased ethylene evolution (senescence ethylene). One or two ethylene peaks were found to interrupt the low background rate of ethylene evolution during the period between excision and senescence. The first intermediate ethylene peak coincided with a rise in CO₂ evolution; however, precedence could not be established. No statistical correlations were discovered between either intermediate ethylene peak and abscission. The best statistical correlation was found between wound ethylene and abscission at 12 hr after excision. No positive correlations were found between senescence ethylene and abscission. Implications of these results for the understanding of the role of ethylene in explant abscission are discussed.     

Auxin Transport as Related to Leaf Abscission during Water Stress in Cotton

Plant water deficits reduced the basipetal transport of auxin in cotyledonary petiole sections taken from cotton (Gossypium hirsutum L.) seedings. A pulse-labeling technique was employed to eliminate complications of uptake or exit of ¹⁴C-indoleacetic acid from the tissue. The transport capacity or the relative amount of radioactivity in a 30-minute pulse which was basipetally translocated was approximately 30% per hour in petioles excised from well watered seedlings (plant water potentials of approximately -4 to -8 bars). No cotyledonary leaf abscission took place in well watered seedlings. Plant water potentials from -8 to -12 bars reduced the transport capacity from 30 to 15% per hour, and although the leaves were wilted, cotyledonary abscission did not increase appreciably at these levels of stress. The threshold water potential sufficient to induce leaf abscission was approximately -13 bars and abscission increased with increasing stress while the auxin transport capacity of the petioles remained relatively constant (15% per hour). The basipetal transport capacity of well watered petioles tested under anaerobic conditions and acropetal transport tested under all conditions were typically less than basipetal transport under the most severe stress conditions. Cotyledonary abscission took place during and 24 hours after relief of stress with little or no abscission taking place 48 hours after relief of stress. Although the water potential returned to -4 bars within hours after rewatering the stressed plants, partial recovery of the basipetal transport capacity of the petioles was not apparent until 48 hours after rewatering, and at least 72 hours was required to return the transport capacity to near normal values. These data support the view that decreased levels of auxin reaching the abscission zone from the leaf blade influence the abscission process and further suggest that the length of time that the auxin supply is maximally reduced is more critical than the degree of reduction.     

Water Stress Enhances Ethylene-mediated Leaf Abscission in Cotton

Abscission of cotyledonary leaves from cotton (Gossypium hirsutum L. cv. Stoneville 213) seedlings occurred following relief from water stress. The amount of abscission was related to the magnitude of the plant water deficit. Leaf abscission promoted by exogenous ethylene was enhanced in seedlings subjected to water stress. Treatment with ethylene (2.0 to 3.2 microliters of ethylene per liter of air for 24 hours) raised the threshold plant water potential required to induce abscission from —17 to —7 bar, indicating that the stress caused the tissue to become predisposed to ethylene action. Based on the abscission response curve for seedlings treated with ethylene while under water stress, this apparent predisposition was developed as the plant water potentials reached the —7 to —10 bar range. The abscission-promoting effects of ethylene in combination with water stress were reversed with 15% CO₂ at plant water potentials above —12 bar, but the CO₂ reversal was lost at lower water potentials. These results are compatible with the concept that ethylene plays a regulatory role in leaf abscission induced by water stress.     

Movement of Kinetin and Gibberellic Acid in Leaf Petioles during Water Stress-induced Abscission in Cotton

Movement of [¹⁴C]kinetin and [¹⁴C]gibberellic acid was examined in cotton (Gossypium hirsutum L.) cotyledonary petiole sections independent of label uptake or exit from the tissue. Sections 20 millimeters in length were taken from well watered, stressed, and poststressed plants. Transport capacity was determined using a pulse-chase technique. Movement of both kinetin and gibberellic acid was found to be nonpolar with a velocity of 1 millimeter per hour or less, suggesting passive diffusion. Neither water stress nor anaerobic conditions during transport of labeled material affected the transport capacity of the petioles.     

Involvement of Hydrogen Peroxide in Cotton Leaf Abscission Induced by Thidiazuron

Journal of Plant Growth Regulation - Thidiazuron (TDZ) has been extensively applied as chemical defoliant in cotton production, but the physiological mechanisms for its defoliating activity are...     

Synthesis of Cellulase during Abscission of Phaseolus vulgaris Leaf Explants

When abscission in leaf explants from Phaseolus vulgaris, cultivar Red Kidney, was allowed to proceed while the explants were in ²H₂O, a 1.25% increase in the buoyant density of cellulase in a cesium chloride gradient was observed. These data indicate that the increase in cellulase activity during abscission is a result of the synthesis of new protein. Two differentially soluble forms of cellulase are present in the abscission zone. The form which is soluble only in a high salt buffer seems more closely related to the abscission process than the form which is soluble in dilute buffer. The correlation between changes in pull force and increase in cellulase activity and the effects of several hormones on cellulase activity are discussed.     

Leaf Age and Ethylene-induced Abscission

Ethylene has been generally credited with promoting the abscission of the oldest leaves on a plant first. Vegetative cotton (Gossypium hirsutum L.) seedlings are an exception to this generalization. Under some conditions the younger, apical, unexpanded, or partially expanded leaves abscise before the less young, basal leaves or cotyledons. The degree or extent of apical leaf abscission increases with ethylene concentration and with plant age from 2 to 5 weeks. The response is promoted by auxin transport inhibitors. Usually the leaves which abscise first are those which have just unfolded and ones apical to the opened but unexpanded leaves. With plants with eight or nine leaves and macroscopic leaf buds, after the initial loss of unexpanded leaves, abscission tends to progress downward from the youngest remaining leaves and upward from the oldest leaves. The findings indicate that some characteristic(s) of apical leaves increases their sensitivity to ethylene. The characteristic may be differences in the abscission process between expanded and unexpanded leaves or differences in the hormone complement of the different leaves. Work is under way to modify this young leaf abscission response in an effort to determine its cause.     

Movement and Endogenous Levels of Abscisic Acid during Water-Stress-induced Abscission in Cotton Seedlings

In an effort to investigate possible involvement of abscisic acid (ABA) in foliar abscission processes, its movement and endogenous levels were examined in cotyledons taken from cotton seedlings (Gossypium hirsutum L.) subjected to varying degrees of water deficit, a condition which initiates leaf abscission. Using a pulse-labeling technique to avoid complications of uptake and exit from the tissue, ABA-1-¹⁴C movement was observed in both basipetal and acropetal directions in cotyledonary petioles taken from well watered, stressed, and rewatered plants. The label distribution patterns obtained after 1 and 3 hours of transport under all situations of water supply were diffusive in nature and did not change when tested under anaerobic conditions. The transport capacity of the petioles ranged from 3.6 to 14.4% ABA-1-¹⁴C transported per hour at estimated velocities of 0 to 2 millimeters per hour. Comparison of basipetal and acropetal movement indicated a lack of polarity under all conditions tested. These low transport capacities and slow velocities of movement, when compared to the active transport systems associated with auxin movement, as well as the lack of anaerobic effects and polarity, suggest that ABA movement in cotton cotyledonary petiole sections is facilitated by passive diffusion. Increases in free and bound ABA in the lamina with increased water stress did not correlate with patterns of cotyledonary abscission. Thus, no evidence was found to suggest that ABA is directly involved in stress-induced abscission processes.     

Role of IAA-Oxidase in Abscission Control in Cotton

The potential role of indoleactic acid (IAA)-oxidase as an in vivo abscission regulating system in the cotton (Gossypium hirsutum L.) cotyledonary explant was investigated. Phenols (usually monophenols), which are cofactors of cotton IAA-oxidase in vitro, accelerated abscission. Phenols (usually orthodihydroxyphenols), which inhibit cotton IAA-oxidase in vitro, inhibited abscission. Inhibition or stimulation of abscission was accomplished by phenols both with and without IAA. Results were similar when treatments were applied as lanolin pastes to the cut petiole ends or as solutions in which explants were submerged. An abscission accelerating phenol stimulated the decarboxylation of IAA-1-¹⁴C by explants and an abscission inhibiting phenol inhibited the decarboxylation of IAA-1-¹⁴C.     

Leaf Abscission Induced by Ethylene in Water-Stressed Intact Seedlings of Cleopatra Mandarin Requires Previous Abscisic Acid Accumulation in Roots

The involvement of abscisic acid (ABA) in the process of leaf abscission induced by 1-aminocyclopropane-1-carboxylic acid (ACC) transported from roots to shoots in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings grown under water stress was studied using norflurazon (NF). Water stress induced both ABA (24-fold) and ACC (16-fold) accumulation in roots and arrested xylem flow. Leaf bulk ABA also increased (8-fold), although leaf abscission did not occur. Shortly after rehydration, root ABA and ACC returned to their prestress levels, whereas sharp and transitory increases of ACC (17-fold) and ethylene (10-fold) in leaves and high percentages of abscission (up to 47%) were observed. NF suppressed the ABA and ACC accumulation induced by water stress in roots and the sharp increases of ACC and ethylene observed after rewatering in leaves. NF also reduced leaf abscission (7-10%). These results indicate that water stress induces root ABA accumulation and that this is required for the process of leaf abscission to occur. It was also shown that exogenous ABA increases ACC levels in roots but not in leaves. Collectively, the data suggest that ABA, the primary sensitive signal to water stress, modulates the levels of ethylene, which is the hormonal activator of leaf abscission. This assumption implies that root ACC levels are correlated with root ABA amounts in a dependent way, which eventually links water status to an adequate, protective response such as leaf abscission.     

The Role of Auxin in the Apical Regulation of Leaf Abscission in Cotton (Gossypium hirsutum L.)

The petiole abscission induced by deblading cotyledonary leavesof cotton (Gossypium hirsutum L. cv. Delta Pine) was acceleratedby the presence of the intact shoot apex or, in decapitatedplants and explants, by application to the stem (proximal application)of indol-3yl-acetic acid (IAA) or 1-aminocyclopropane-l-carboxylicacid (ACC). IAA and ACC accelerated the abscission of debladedpetioles whether applied above or below the cotyledonary node.Transport of IAA to the node was not required for the responseto proximal IAA. [2,3-¹⁴C]ACC was readily transported to thenodal region whether applied to the stem above or below thenode. Application of IAA or ACC to the stem did not induce theabscission of intact leaves or of debladed petioles treateddistally with IAA The acceleration of abscission by proximal IAA, but not thatcaused by ACC, was prevented if explants were treated with a-aminooxyaceticacid (AOA), an inhibitor of ACC-synthase. AOA also preventedthe acceleration of abscission caused by the shoot apex. Theprogress of abscission in debladed explants was greatly delayedby silver thiosulphate (STS—an inhibitor of ethylene action),whether or not the explants were treated with IAA or ACC. Itis suggested that the speeding effects of the shoot apex andof proximal auxin on the abscission of debladed petioles requiresauxin-induced ACC synthesis. The possibility is discussed thatACC may function as a mobile abscission promoter Key words: Abscission, ACC, ACC-synthase, cotton (Gossypium), proximal auxin     

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     

Abscisic Acid, Auxin, and Ethylene in Explant Abscission

Experiments with explants of Phaseolus vulgaris L., cv. CanadianWonder, show that abscission and the associated rise in oarboxymethyl-cellulaseactivity in the separation zone are initiated by a peak in ethyleneproduction during senescence of pulvinar tissue distal to thezone. Distal applications of abscisic acid (ABA) induce an earlierpeak in ethylene production, increase cellulase activity, andpromote abscission. ABA is more effective in these ways if treatmentis delayed from 0 to 24 h after excision. With increasing concentrations of ABA the maximum rate of ethylene production is achievedsooner. Indol-3yl-acetic acid (IAA) and ABA are antagonisticin this system and have opposing effects. IAA retards the timeof peak ethylene-production and delays abscission. Explantsmay be retained for long periods without abscinding if incubatedin an ethylene-free atmosphere: the addition of ethylene forany one 24-h period (except the first 24 h after excision) willinduce abscission. The initial period of insensitivity to ethyleneis extended by distal applications of IAA. Ethylene-inducedabscission can be inhibited by IAA applied up to 72 h afterexcision provided the ethylene is not applied first. It is proposedthat abscission in the explant is controlled at two levels:(1) an auxin-dependent stage determining the duration of insensitivityto ethylene; (2) the timing of a rise in ethylene productionin senescing tissue distal to the separation zone. An auxin-ethylenebalance-mechanism at the separation zone is discussed.     

Changes in Polygalacturonase Activity and Solubility of Polyuronides during Ethylene-stimulated Leaf Abscission in Sambucus nigra

Leaflet abscission in Sambucus nigra is precipitated by cellwall degradation which is restricted to the site of cell separation.Accompanying wall breakdown is an increase in the activity ofthe enzyme polygalacturonase (PG) (E.C. 3.2.1.15 [EC] ) and this riseis primarily confined to the abscission zone tissue. The polygalacturonasehas a pH optimum of 4·4 and has the characteristics ofan endo-acting enzyme. The elevation in enzyme activity is theresult of an increase in at least two isoforms of PG as revealedby polyacrylamide gel electrophoresis of the natured protein.Leaflet abscission in S. nigra is associated with an increasein the solubility and depolymerization of polyuronides fromthe cell wall. These observations are discussed in relationto the mechanism of cell separation during ethylene-stimulatedleaf abscission. Key words: Elder, Sambucus nigra, abscission, polygalacturonase, polyuronides, ethylene     

Mercury-induced Ethylene Formation and Abscission in Citrus and Coleus Explants

Mercury vapor induces ethylene formation and abscission in Citrus and Coleus explants. Both responses are markedly greater in the absence of CO₂. The stimulation of these metabolically complex processes indicates that the action of mercury vapor is not consistent with the more popular conception of mercury toxicity. This was manifested in its complete failure to disturb respiratory gas exchange, and in the total absence of any necrosis. Accordingly, the effect of mercury appears to be highly specific. The overall significance of these findings is discussed with respect to physiological, environmental, and methodological aspects.