Induced Leaf Abscission Research Articles

Thidiazuron Promotes Leaf Abscission by Regulating the Crosstalk Complexities between Ethylene, Auxin, and Cytokinin in Cotton.

Thidiazuron (TDZ) is widely used as a defoliant to induce leaf abscission in cotton. However, the underlying molecular mechanism is still unclear. In this study, RNA-seq and enzyme-linked immunosorbent assays (ELISA) were performed to reveal the dynamic transcriptome profiling and the change of endogenous phytohormones upon TDZ treatment in leaf, petiole, and abscission zone (AZ). We found that TDZ induced the gene expression of ethylene biosynthesis and signal, and promoted ethylene accumulation earlier in leaf than that in AZ. While TDZ down-regulated indole-3-acetic acid (IAA) biosynthesis genes mainly in leaf and IAA signal and transport genes. Furthermore, the IAA content reduced more sharply in the leaf than that in AZ to change the auxin gradient for abscission. TDZ suppressed CTK biosynthesis genes and induced CTK metabolic genes to reduce the IPA accumulation for the reduction of ethylene sensitivity. Furthermore, TDZ regulated the gene expression of abscisic acid (ABA) biosynthesis and signal and induced ABA accumulation between 12–48 h, which could up-regulate ABA response factor genes and inhibit IAA transporter genes. Our data suggest that TDZ orchestrates metabolism and signal of ethylene, auxin, and cytokinin, and also the transport of auxin in leaf, petiole, and AZ, to control leaf abscission.

Preparing seedlings for dry spells: Drought acclimation in the seedlings of two tree species of a seasonal tropical dry forest

The effect of two drought acclimation treatments—high (HS) or moderated (MS) drought stress—on seedling growth was assessed in the two tree species Caesalpinia platyloba S.Watson (Fabaceae) and Ipomoea wolcottiana Rose (Convolvulaceae) in order to prepare them for dry spells. These species are mainly distributed in mature and disturbed sites, respectively, in a seasonal tropical dry forest in Jalisco, Mexico. Without acclimation, the seedlings of C. platyloba showed low values in both Specific Leaf Area (SLA) and Net Assimilation Rate (NAR), suggesting a conservative biomass allocation, while in I. wolcottiana, the high Relative Growth Rate (RGR), Root:Shoot ratio (R:S), and low SLA suggested seedling drought evasion. After the acclimation period, the seedlings of both species showed reduced biomass allocation to leaves, as indicated by the decrease in Leaf Mass Ratio (LMR). Additionally, increases in R:S suggested greater allocation to water capture during the short growth season. After the dry spell simulation, the control seedlings of C. platyloba showed negative NAR, but the relatively high NAR observed in the seedlings subjected to HS and MS suggested that these treatments were sufficient to endure drought stress. In I. wolcottiana, a sigmoid relationship described that seedlings’ RGR rapidly increased when NAR was raised. Contrastingly, in C. platyloba seedlings, the constant RGR increase related to the raise in NAR suggested a conservative use of water. In the I. wolcottiana seedlings, the HS treatment induced leaf abscission. Both species were plastic enough to acclimate to short periods of drought, but different levels of drought stress were necessary to acclimate the seedlings of each species. Suggested drought-acclimation treatments are to water the C. platyloba seedlings at 25% (HS) and I. wolcottiana seedlings at 50% (MS) of field capacity.

Impact of Chemical Defoliants on Chlorophyll Fluorescence, Biochemical Parameters, Yield and Fiber Quality of High Density Cotton

Background: The Cotton growing farmers in India are facing major problem at the time of harvest due to labor shortages, asynchronized boll opening, leaf trash in the cotton, which is reducing the boll picking efficiency and fiber quality. Presently, the Indian cotton industry is moving towards mechanical harvesting by cotton harvester. Defoliation or leaf abscission is induced in cotton as a natural physiological process which usually is inadequate for a complete mechanical harvest of cotton. Defoliation before harvest is often induced by managing the plants so that senescence, abscission (separation) layer development and leaf drop are encouraged. Chemical defoliants induce leaf abscission, hasten mature boll dehiscence and inhibit re­ growth. Selection of appropriate defoliants is one of the critical decisions in cotton production. Their use can result in increased manual as well as machine harvest efficiency, reducing boll rot and the trash in seed cotton. The physiological basis of defoliation in cotton is essential to understand the role of appropriate defoliant with time of application. Hence, the objective of the study was to evaluate the defoliation efficiency, boll opening percentage and to know the physiological basis of defoliation in response to different defoliants and time of application in high density Cotton cultivation which eases the mechanical harvesting.Methods: In this field investigation during 2018 - 2019 at Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore was carried with seven different defoliants and three different time of defoliants application were fixed based on crop maturity and duration of the crop. In this study, high density cotton variety CO 17 was used as experimental material. A series of lab and field analysis were determined on defoliation process, physiological attributes, boll opening percentage and seed yield and quality. The collected data were subjected to statistical analysis.Result: Our investigation exploring the effect of different defoliants on physiological, biochemical, yield and quality parameters of high density Cotton. Among the different defoliants and time of applications, two defoliants and two times of applications works better in terms of better defoliation, boll opening rate, physiology, yield and quality. The physiological and biochemical effects of defoliants on defoliation process were discussed. The present work will be useful for mechanized picking with these effective defoliants without loss of yield and quality loss in cotton.

Gummosis and leaf abscission in Yoshino cherry (Prunus yedoensis): relevance to hormonal regulation and chemical composition of gums

Gummosis is widely spread in the plant kingdom, especially in stone-fruit species of the Rosaceae, which resulted from an interaction with biotic and abiotic environmental factors. In this study, gummosis in shoots of 'Yoshino' cherry (Prunus yedoensis) was studied in relation to hormonal status, leaf abscission and chemical composition of gums. An ethylene-releasing compound, ethephon (1 to 2%, w/w in lanolin) substantially induced gummosis when it was applied in shoots as lanolin paste from the end of July to late August. Methyl jasmonate (JA-Me, 2.5%) had little effect on gummosis. Simultaneous application of ethephon with JA-Me caused total leaf abscission at 4 days after treatment and no gummosis. Our studies also showed that simultaneous application of ethephon (1.0%) with low concentrations of JA-Me (0.05 and 0.1%) in the first half of September greatly induced gummosis, but ethephon alone was not effective. On the contrary, ethephon (0.1 and 1.0%) and JA-Me (0.05, 0.1, 1.0 and 2.5%) applied independently on petioles in early August did not induce leaf abscission. Ethephon (0.1 and 1%) applied simultaneously with JA-Me induced almost total leaf abscission. Analyses of chemical composition of gums formed by ethephon revealed that gums were mainly polysaccharides consisting of ca. 85% of neutral sugars (Ara: ca. 55%; Gal: ca. 20%; Xyl: ca. 10%) and 15% of uronic acids. On the other hand, in sweet cherry (P. avium) gummosis was induced by the application of ethephon and JA-Me, their simultaneous application substantially enhancing gummosis. These results suggest that gummosis and leaf abscission in shoots of 'Yoshino'O cherry are regulated by a signal network and/or a crosstalk between ethylene and jasmonates, and that the presence of leaves plays an important role in gummosis.

Tau GSTs involved in regulation of leaf abscission by comparison the gene profiling of MeGSTs in various abscission-promoting treatments in cassava abscission zones

BackgroundGlutathione S-transferases (GSTs) have been reported to regulate the plant tolerance to environmental stresses. Many plant GSTs exhibited the roles on promoting tolerance to drought stress, oxidative stress and plant hormones. The biological function of GSTs has been well characterized in Arabidopsis thaliana in response to exogenous environmental stresses. However, their regulation function under exogenous environmental stresses regulating leaf abscission in cassava (Manihot esculenta Crantz) remained unknown.ResultsHere, 83 GSTs were identified from tropical plant cassava. The amino acid motifs and phylogenetic analyses indicated that MeGSTs were divided into 9 classes. The global expression analyses were carried out to analyze the gene expression patterns of MeGST in cassava abscission zones by comparing the MeGST genes expression patterns in both ethylene and drought induced cassava leaf abscission. Totally, 34 GSTs were detected to express in both ethylene and drought induced leaf abscission in cassava abscission zones. Comparison of GST expression profiling between ethylene and drought induced leaf abscission suggested that Tau GST genes showed with the similar expression in both treatments induced leaf abscission in cassava abscission zone. GO annotation indicated that all 17 Tau GST genes participated in the pathway of toxin catabolism (GO: 0009407). The expression levels of 17 Tau MeGST genes were analyzed in two cassava cultivars, ‘SC124’ and ‘Arg7’, the two cultivars exhibit different levels of leaf abscission when suffered from the same environmental stress. Higher expression levels of Tau MeGSTs were detected in the precocious abscission Arg7 cultivar, while lower expression levels in delayed abscission SC124 cultivar. All the results indicated that Tau MeGSTs have the function in regulation the cassava leaf abscission under environmental stresses.ConclusionAnalysis of the expression patterns of GSTs in various abscission-promoting treatments in cassava abscission zones helps us to understand the possible roles of GSTs in cassava leaf abscission.

Inhibition of ethylene response by 1-methylcyclopropene in potted ornamental pepper

In the face of great potential of 1-MCP in blocking ethylene action, this study aimed to evaluate the efficacy of 1-MCP on ethylene action and extend the post-production of ornamental peppers Capsicum annuum potted plants. Ornamental pepper cultivar Calypso shows complete abscission of leaves when exposed to 10µL L-1 ethylene. By exposing plants to ethylene, significant drops on a, b and total chlorophyll were found at the end of the treatment. Nevertheless, no changes on leaf total carotenoid were observed after the treatment with ethylene. When the plants were treated with 1-methylcyclopropene (1-MCP), followed or not by treatment with ethylene, contents of a, b, total chlorophyll and carotenoid remained unchanged. Control plants showed no changes in chlorophylls or carotenoid contents after 48 hours from moving the pots from greenhouse to indoor conditions illuminated with white fluorescent light. The 1-MCP was able to prolong the post production shelf life, by inhibiting the abscission of leaves. Plants treated with 1-MCP, followed by ethylene had 38% accumulated leaf abscission, after 18 days of post production life, similar to control plants. After the same period, plants treated only with 1-MCP had 22% leaf abscission. The 1-MCP partially blocked ethylene and induced leaf abscission. Regardless which treatment was applied during post production life, there was a continuous decrease on leaf chlorophyll content beyond 18th day. Thus, 1-MCP was effective in preventing leaf abscission even in the presence of ethylene, avoiding deleterious effects of ethylene by blocking its receptor sites.

Effect of aminoethoxyvinylglycine and 1-methylcyclopropene on leaf abscission and root formation in Corymbia and Eucalyptus cuttings

Auxins such as indole-3-butyric acid (IBA) promote adventitious root formation in many woody-plant cuttings but auxins also induce leaf abscission by stimulating ethylene production. This study determined whether the ethylene synthesis inhibitor, aminoethoxyvinylglycine (AVG), or the ethylene perception inhibitor, 1-methylcyclopropene (MCP), could reduce leaf abscission, promote adventitious root formation, and increase growth of cuttings of two eucalypt hybrids treated with a high dose (8g/kg) of IBA. In Corymbia torelliana×Corymbia citriodora, AVG and MCP alleviated IBA-induced leaf abscission in a dose-dependent manner, with the most effective AVG dose (375mg/L) also increasing the percentage of cuttings that formed roots, and the most effective MCP dose (1200nL/L) increasing the shoot weight of rooted cuttings. In Eucalyptus pellita×Eucalyptus grandis, most AVG and MCP doses did not alleviate leaf abscission, and no AVG or MCP dose significantly increased rooting percentage or shoot weight. However, rooting percentage was correlated with leaf retention in both eucalypt hybrids. A lower IBA dose is recommended for propagation of these eucalypts because AVG and MCP did not increase leaf retention, rooting percentage, root weight or shoot weight of cuttings treated with 8g/kg IBA above those of cuttings treated with 3g/kg IBA. Compared with untreated cuttings, 3g/kg IBA increased rooted cutting production by 153% and 56%, adventitious root number by 3.6 and 1.0, and root weight by 29% and 86% in C. torelliana×C. citriodora and E. pellita×E. grandis, respectively.

Activity of cell wall degrading glycanases in methyl jasmonate-induced leaf abscission in Kalanchoe blossfeldiana

It was found previously that methyl jasmonate (JA-Me) induced leaf abscission in <em>Kalanchoe blossfeldiana</em>. In present studies it was shown that JA-Me markedly increased the total activities of cellulase, polygalacturonase, pectinase and xylanase in petioles, but did not affect activities of these enzymes in the blades and apical part of shoots of <em>K. blossfeldiana</em>. These results suggest that methyl jasmonate promotes the degradation of cell wall polysaccharides in the abscission zone and in this way induces leaf abscission in <em>Kalanchoe blossfeldiana</em>.

Fatty acid and sterol contents during methyl jasmonate-induced leaf abscission in Kalanchoe blossfeldiana

It was found previously that methyl jasmonate (JA-Me) induced leaf abscission in <i>Kalanchoe blossfeldiana</i>. In present studies it was showed that JA-Me did not affect or only slightly affected the content of free and bound fatty acids in petioles and blades. ß-Sitosterol, campesterol and ß-amyrin were identified in petioles and blades of <i>K. blossfeldiana</i>; JA-Me decreased the content of campesterol in petioles and increased the content of ß-sitosterol in blades. In blades of plants treated with JA-Me disappearance of olean-12-one was indicated but appearance of 2H-cyclopropa[a]-naphthalen-2-one,l, la, 4, 5, 6, 7, 7a, 7b-octahydro-l, 1, 7, 7a-tetramethyl (aristolone) was documented. The significance of these findings in leaf abscission induced by methyl jasmonate in <i>K. blossfeldiana</i> is discussed.

ABSCISSION OF SWEET BASIL LEAVES INDUCED BY ETHYLENE UNDER MODIFIED ATMOSPHERE PACKAGING

Sweet basil (Ocimum basilicum L.) is an aromatic culinary herb used for flavour addition and decoration in many foods and drinks. A crucial postharvest problem of sweet basil is short shelf life due to water loss and abscission of leaves. Sweet basil branches were packed in 60 μm polyethylene (PE) bags (20x45 cm dimension) and stored at 13°C and 95% RH. The number of abscised leaves was highest in sealed PE bags containing an initial concentration of 2500 ppm ethylene lowest in sealed PE bags containing an ethylene adsorbent. Ethylene accumulating in the bags also induced leaf abscission. Basil stored in PE bags lost less weight resulting in extended storage life compared to non-packed branches. Packing basil in non-perforated PE bags resulted in decay after 6 days. Perforated PE bags with 16 holes (12.56 cm 2 /bag) provided the best condition in maintaining the quality of basil for 9 days.

Effects of temperature on fruit thinning with ethephon in ‘Golden Delicious’ apples

The effect of temperature on the ability of 2-chloroethylphosphonic acid (ethephon), when applied at 20 mm stage of fruit development, to induce ethylene evolution of fruit and leaves and abscission of fruit and leaves was determined using 9-year-old root-bagged ‘Golden Delicious’/M.27 apple ( Malus domestica Borkh.) trees in environment-controlled growth rooms. Ethephon at 400 μL L −1 effectively thinned apples, and its thinning effect was not affected by increasing day/night temperature from 21.1/10 to 32.2/21.1 °C. Fruit ethylene evolution was enhanced by application of ethephon. Peak fruit ethylene evolution occurred 1 day after application of ethephon when day/night temperature was 32.2/21.1 °C whereas it occurred 2 days after application of ethephon at a day/night temperature of 21.1/10 or 26.7/15.6 °C. Ethephon increased leaf ethylene evolution drastically but it did not induce leaf abscission regardless of temperature.

(146) Minimal Role of Ethylene on Leaf Abscission in Water-stressed Betula papyrifera Marsh.

It has been observed that paper birch (Betula papyrifera Marsh.) has significant leaf abscission under mild to severe water stress. One-year-old paper birch seedlings were exposed to water deficit, ethylene, or inhibitors of ethylene action under controlled conditions to study water-stress induced leaf abscission. Exposing well-watered and water-stressed paper birch to 20 ppm of ethylene for 96 hours resulted in more than 50% leaf abscission regardless of plant water status, while water stress alone did not cause leaf abscission. However, the application of 1 ppm ethylene did not cause leaf abscission. Exposure to 1 ppm 1-methylcyclopropene or treatment with 0.1 mM of silver thiosulfate did not affect predawn water potential, gas exchange, and chlorophyll fluorescence. The measured evolved ethylene did not significantly increase in water-stressed paper birch prior to leaf abscission. Based on these observations, ethylene would appear to play a minor role in water-stress induced leaf abscission in paper birch.

Responses of Papaya Seedlings (Carica papaya L.) to Water Stress and Re-Hydration: Growth, Photosynthesis and Mineral Nutrient Imbalance

The effects of water stress and subsequent re-hydration on growth, leaf abscission, photosynthetic activity, leaf water potential and ion content were investigated in papaya seedlings (Carica papaya L.) cv. “Baixinho de Santa Amalia”. Water stress was imposed by suspending irrigation during 34 days. Thereafter, plants were regularly re-watered. Drought arrested plant growth, induced leaf abscission and drastically decreased photosynthetic rate. However, leaf water potential was hardly reduced. Water deficit also induced sodium, potassium and chloride accumulation in leaves and roots, and did not modify nitrogen levels in both organs. Re-hydration stimulated growth, promoted emergence of new leaves, reactivated photosynthetic machinery function and reduced ion content to control levels. The results indicated that the ability of papaya plants to improve drought tolerance is not mediated through the reduction of leaf abscission, the detention of growth or the decrease of net CO2 assimilation. In contrast, the data suggested that under water stress conditions these plants appear to posses a certain capacity to increase ion content, which might contribute to osmotic adjustment.

Abscisic Acid Reduces Leaf Abscission and Increases Salt Tolerance in Citrus Plants

This paper describes the physiological effects of abscisic acid (ABA) and 100 mM NaCl on citrus plants. Water potential, leaf abscission, ethylene production, photosynthetic rate, stomatal conductance, and chloride accumulation in roots and leaves were measured in plants of Salustiana scion [Citrus sinensis (L) Osbeck] grafted onto Carrizo citrange (Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf) rootstock. Plants under salt stress accumulated high amounts of chloride, increased ethylene production, and induced leaf abscission. Stomatal conductance and photosynthetic rates rapidly dropped after salinization. The addition of 10 mM ABA to the nutrient solution 10 days before the exposure to salt stress reduced ethylene release and leaf abscission. These effects were probably due to a decrease in the accumulation of toxic Cl- ions in leaves. In non-salinized plants, ABA reduced stomatal conductance and CO2 assimilation, whereas in salinized plants the treatment slightly increased these two parameters. The results suggest a protective role for ABA in citrus under salinity.

Phosphorus-induced Leaf Abscission in Detached Shoots of Olive and Citrus

Previous studies have demonstrated that phosphorus, which stimulates ethylene biosynthesis, induces abscission of olive leaves directly without the involvement of ethylene. In the present study this possibility was further explored by comparing the effects of an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), and an ethylene action inhibitor, 2,5-norbornadiene (NBD), in olive [Olea europaea (L.) `Manzanillo'] and citrus [Citrus sinensis (L.) Osbeck `Shamouti']. In olive, leaf abscission was always induced in the presence of KH2PO4 with or without AVG and NBD (alone or in combination), but it was much more pronounced when KH2PO4 was applied alone. In citrus, KH2PO4 did not induce leaf abscission in the presence of NBD during the first 48 (detached shoots) or 60 hours (leaf explants) despite the high levels of ethylene production by the tissues. Our results demonstrate that phosphorus can, at least partly, act independently of ethylene action in inducing leaf abscission in olive but not in citrus.

Phosphorus-induced Leaf Abscission in Olive and Citrus Explants

Previous studies, in which the role of phosphorus in abscission of olive leaves was examined in the presence of ethylene biosynthesis inhibitors, have suggested that phosphorus induces abscission directly, without involvement of ethylene. In the present study, this possibility was further explored by comparing the effects of an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), and an ethylene action inhibitor, 2,5-norbornadiene (NBD), in olive [Olea europaea (L) cv. Manzanillo] and citrus [Citrus sinensis (L.) Osbeck cv. Shamouti]. In olive, leaf abscission was always induced in the presence of KH2PO4, with or without AVG and NBD (alone or in combination), but was more pronounced when KH2PO4 was applied alone. In citrus, the effect of KH2PO4 alone on the induction of leaf abscission and ethylene production was much stronger than that observed in olive. However, in the presence of NBD, KH2PO4 did not induce leaf abscission in citrus during the first 60 hr. Similar results were obtained when NBD was replaced by AVG, but, in this case, abscission was inhibited for only 48 hr. In both cases, ethylene was detected after the inhibitory period had ended. The results obtained with citrus indicate that the observed effect of KH2PO4 on the ethylene-independent induction of leaf abscission in olive is not a general phenomenon and may differ in different species.

Physiology of Olive Leaf Abscission Induced by Phosphorus

Adding Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene. Exogenous ethylene at 791 nl·liter-1 for 72 to 120 hours and at 193 nl·liter-1 for 120 hours induced leaf abscission, whereas no leaf abscission occurred at 47 nl·liter-1 for 72 to 120 hours. Ethylene at 791 nl·liter-1 for 72 to 120 hours increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough to induce leaf abscission. Three different ethylene inhibitors—aminooxyacetic acid (AOA), CoCl2, and aminoethoxyvinylglycine (AVG)—were used to determine whether P-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2PO4, AVG inhibited ethylene evolution. Each inhibitor, except 5 mm CoCl2, promoted leaf abscission when administered alone or with P. Our results reveal that P-induced olive leaf abscission may occur without elevated ethylene evolution. At 40 or 75 mm NaH2PO4, abscission did not occur until explants were removed from N2 and placed in ambient air.

Phosphorus Effects on Olive Leaf Abscission

When continuously stem-fed with 75 mm NaH2PO4, `Manzanillo' olive explants showed significant leaf abscission after 48 hours; by that time 1.042 mg·g-1 fresh weight P had accumulated in the abscission zone (AZ). The potential contribution of ethylene to phosphate-enhanced abscission was investigated using aminooxyacetic acid (AOA), an ethylene-synthesis inhibitor, and by measuring ethylene evolution in phosphate-treated explants. In combination with NaH2PO4, AOA did not affect leaf abscission. Though ethylene evolution from explants increased as leaf abscission was initiated, it was about two orders of magnitude less than the concentration necessary to induce leaf abscission as judged by exogenous treatments. Based on leaf-abscission kinetics, we have concluded that the mechanism of P-induced abscission is independent of gross measurement of evolved ethylene, but we cannot rule out ethylene confined to the AZ itself. When evaluated for P-induced leaf abscission, leaves of `Manzanillo' and `Sevillano' abscised earlier than `Ascolano' and `Mission'.

754 PB 146 Physiology of Olive Leaf Abscission Induced by Phosphorus

The addition of Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene treatment. Exogenous ethylene at 791 nl·l-1 for 72 to 120h and at 193 nl·l-1 for 120h induced leaf abscission whereas at 47 nl·l-1 for 72 to 120h no leaf abscission occurred. Ethylene treatment at 791 nl·l-1 for 72 to 120h increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough for leaf abscission induction. Three different ethylene inhibitors, aminooxyacetic acid (AOA), CoCl2 and am inoethoxyvinylglycine (AVG), were used to determine whether phosphorus-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2P O4, AVG inhibited ethylene evolution. Each of the inhibitors except for 5 mM CoCl2 promoted leaf abscission when administered alone or with phosphorus. Our results reveal that phosphorus induced olive leaf abscission occurs without elevated ethylene evolution, but that oxygen is required.

Abscisic acid- and ethylene-induced defoliation of Radermachera sinica L.

Radermachera sinica L. is an ornamental plant with demonstrated sensitivity to ethylene-induced leaf abscission. In this study, we examine the relationship between abscisic acid (ABA) and ethylene in initiating the abscission response. Treatment with 1 μl L\s-1 of ethylene, 1 mM 1-aminocyclopropane-1-carboxylic acid (ACC) or 1 mM ABA resulted in complete defoliation of leaf explants. Application of 0.125 mM silver thiosulfate (STS) inhibited ethylene- and ACC-induced abscission but had no effect on explants treated with ABA. The ABA-induced abscission was unaffected by treatment with aminoethoxyvinylglycine (AVG) or aminooxyacetic acid (AOA). Treatment of explants with 1 mM cobalt chloride (CoCl2) or 2000 μl L\s-1 of norbornadiene (NBD) completely inhibited abscission in explants treated with 1 μl L\s-1 ethylene or 1 mM ACC but they were only marginally effective in blocking ABA-induced abscission despite the lower level of endogenous ethylene. ABA appeared to increase the sensitivity of explants to ethylene. However, the evidence suggests that ABA may also function independent of ethylene to induce leaf abscission in R. sinica.