Endogenous and exogenous ethylene induces needle abscission and cellulase activity in post-harvest balsam fir (Abies balsamea L.)

Post-harvest needle loss is a major problem for balsam fir and other Christmas tree species. Recent evidence has implicated ethylene as a signal responsible for post-harvest needle abscission, but enzymological changes remain unknown. The objective of this study was to identify and quantify cellulase activity associated with endogenous and exogenous ethylene-induced abscission. An experiment was designed with three treatments (control, endogenous ethylene, or exogenous ethylene) with five replicates. Key response variables include needle retention duration, xylem pressure potential, ethylene evolution rate, and cellulase activity. Two complimentary methods were used to assess cellulase activity: a cellulose plate digestion and zymography. The results confirm ethylene as a signal for post-harvest abscission and identify ethylene-induced cellulase. Ethylene evolution was typically between 15 and 16 μL g⁻¹ h⁻¹, but there was no difference among the three treatments. However, exogenous ethylene significantly decreased needle retention by 60% and resulted in a sixfold decrease in xylem pressure potential. In addition, cellulase activity increased by 8- and 12-fold in endogenous and exogenous ethylene-induced abscission, respectively, compared to the control. Identification of ethylene-induced cellulase activity has increased our understanding of the post-harvest needle abscission process and confirms ethylene’s role as a signal molecule.     

Effect of 1-methylcyclopropene on ethylene-induced abscission in citrus

Pre-treatment of citrus leaves and leaf explants ( Citrus sinensis [L.] Osbeck cv. Shamouti), with 1-methylcyclopropene (1-MCP), induced endogenous ethylene production when leaves were further incubated in air. The induction of ethylene production was 1-MCP concentration-dependent. Abscission was concomitantly delayed. In leaves pre-treated with 1-MCP followed by exposure to ethylene, abscission was significantly delayed in comparison with those without 1-MCP pre-treatment. When leaf explants were co-treated for 24 h with ethylene and 1-MCP, abscission was delayed quite efficiently. The Lineweaver-Burke plot yielded a half-maximal value of 0.234 μl l⁻¹ for the effect of ethylene on abscission. 1-MCP⁻¹ competed kinetically with ethylene with a K_i value of approximately 1.4−5.5 nl l⁻¹ 1-MCP. Under these experimental conditions there was some competition between 1-MCP and ethylene. However, ethylene was not able to completely counteract the inhibitory effect of 1-MCP. Pre-treatment with 1-MCP, followed by exogenous ethylene treatment, suppressed the induction of endo- β -glucanase (EG) activity at the laminar abscission zone. The ethylene-dependent accumulation of the hydrolyse gene was demonstrated by blocking the accumulation of CsCel a1 mRNA by 1-MCP. Six hours of exposure of leaves to 1-MCP at various times during a total of 24 h ethylene treatment efficiently reversed ethylene induction of CsCel a1 gene at mRNA level up to 18 h. The results demonstrate that the induction of abscission by ethylene is controlled at mRNA level at the abscission zone.     

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.     

Nucellar Senescence and Ethylene Production as they Relate to Avocado Fruitlet Abscission

Events preliminary to avocado (Persea americana Mill) fruitletabscission include senescence of the nucellus and seed coat.The dynamics of nucellar deterioration and ethylene productionleading to seed abortion and abscission in avocado was examined.Excised branches bearing clusters of fruit from 1.0–2.5cm diameter were placed in humid chambers to reduce transpirationalwater loss. Fruitlets synchronously began nucellar and seedcoat deterioration 27–33 h after excision and rapidlyprogressed through stages of increasing degradation culminatingin abscission approximately 2 days later. The nucellus-seedcoat produced a temporary burst of ethylene at the first visiblesign of nucellar senescence followed by less ethylene productionin the mesocarp approximately 12 h later. All fruit underwentnucellar degradation prior to abscission. Exogenously appliedethylene accelerated fruitlet abscission with concentrationsas low as 1.0µ 1^–1 and with maximum response at100µl^–1 or greater. Maximal response took 2 days.Aminoethoxyvinyl-glycine (AVG) at 30 µ M inhibited ethyleneproduction and fruitlet abscission. The senescence process,however, was not af fected in any way by ethylene or AVG treatments.Observations of attached fruit suggest that nucellar-seed coatsenescence, concomitant ethylene production, and resulting abscissiontake place in a manner and within a time period similar to thatobserved on detached branches. It is concluded that nucellarand seed coat senescence is prerequisite to avocado fruitletabscission, and the time required from the first indicationof nucellar breakdown to abscission of that fruitlet appearsto be approximately 2 days. The senescence process is responsiblefor a large, transient rate increase in ethylene productionby the nucellus and perhaps seed coat. Ethylene is consideredto be the result rather than the cause of nucellar-seed coatsenescence. The ethylene thus produced induces fruit abscission.     

Ethylene- and dark-induced flower abscission in potted Plectranthus: Sensitivity,prevention by 1-MCP,and expression of ethylene biosynthetic genes

Prevention of ethylene- and shipping-induced flower abscission is necessary to maintain the quality of both cut flowers and potted plants during handling, transport and retail display. The aims of the present work were to determine the sensitivity of Plectranthus cultivars to applied ethylene, to alleviate ethylene- and shipping-induced flower abscission in intact potted plants using 1-methylcyclopropene (1-MCP), and to investigate the possible causes of dark-induced flower abscission. All cultivars were sensitive to ethylene in a concentration-dependent manner, and complete abscission occurred within 24 h with 1 and 2 μl l^− 1 ethylene. Unopened buds were more sensitive to applied ethylene, and exhibited greater abscission than open flowers. Ethylene synthesis remained below detection limits at all time points under control and continuous dark conditions. Dark treatment significantly increased flower abscission in Plectranthus cultivars, and like ethylene-induced flower abscission, this could be prevented by continuous 1-MCP treatment. Gene expression of ethylene biosynthetic enzymes ACS and ACO was examined as possible causes for the accelerated flower abscission observed in plants kept in continuous darkness. Expression patterns of ACS and ACO varied between different cultivars of Plectranthus. In some cases, increased expression of ACS and ACO led to increased flower abscission. Gene expression was higher in open flowers when compared to unopened flowers suggesting a cause for the observed preferential shedding of open flowers in some cultivars. Although the cause of dark-induced abscission in Plectranthus remains elusive, it can be effectively controlled by treatment with 1-MCP.     

Stamen abscission and water balance in Metrosideros flowers

Cymules (3-flowered units borne on single pedicels) were cut from inflorescences of Metrosideros collina J.R. & G. Forst. cv. Tahiti and used to test the effects of ethephon and ethylene on stamen abscission in the presence of silver thiosulphate (STS) and 1-methylcyclopropene (1-MCP), and to test the effects of holding solutions on cymule water balance and the progression of floral development. Flower bud and stamen abscission occurred in response to 0.5–5.0 and 0.1 μl l⁻¹ ethylene, respectively. Ethylene effects were partially negated by scrubbing exogenous ethylene, and more completely negated by STS (2.0 m M ). 1-MCP caused greater ethylene production and inhibited stamen abscission for only 1–2 days after treatment. Ethephon (10-10 000 mg l⁻¹) induced stamen wilting rather than abscission, an effect that was not negated by STS. Stamen wilting was negatively correlated with stamen relative water content, and the increase in stamen wilting was generally reduced by treatments that enhanced cymule mass. Stamen wilting was least using a 100 g l⁻¹ sucrose pulse or holding solutions containing 30–40 g l⁻¹ sucrose, with hydroxyquinoline citrate (200 mg l⁻¹) maintained at pH 5. Our results indicate that 1-MCP may be relatively ineffective in blocking the effects of ethylene on the abscission of organs, such as the stamens of M . collina , which are highly sensitive to this hormone.     

Ethylene perception inhibitor 1-MCP decreases oxidative damage of leaves through enhanced antioxidant defense mechanisms in soybean plants grown under high temperature stress

Ethylene is a stress hormone involved in early senescence and abscission of vegetative and reproductive organs under stress conditions. Ethylene perception inhibitors can minimize the impact of ethylene-mediated stress. The effects of high temperature (HT) stress during flowering on ethylene production rate in leaf, flower and pod and the effects of ethylene inhibitor on ethylene production rate, oxidative damage and physiology of soybean are not understood. We hypothesize that HT stress induces ethylene production, which causes premature leaf senescence and flower and pod abscission, and that application of the ethylene perception inhibitor 1-Methyl cyclopropene (1-MCP) can minimize HT stress induced ethylene response in soybean. The objectives of this study were to (1) determine whether ethylene is produced in HT stress; (2) quantify the effects of HT stress and 1-MCP application on oxidative injury; and (3) evaluate the efficacy of 1-MCP at minimizing HT-stress-induced leaf senescence and flower abscission. Soybean plants were exposed to HT (38/28 °C) or optimum temperature (OT; 28/18 °C) for 14 d at flowering stage (R2). Plants at each temperature were treated with 1-MCP (1 μg L⁻¹) gas for 5 h or left untreated (control). High temperature stress increased rate of ethylene production in leaves, flowers and pods, production of reactive oxygen species (ROS), membrane damage, and total soluble carbohydrate content in leaves and decreased photosynthetic rate, sucrose content, F_v/F_m ratio and antioxidant enzyme activities compared with OT. Foliar spray of 1-MCP decreased rate of ethylene production and ROS and leaf senescence traits but enhanced antioxidant enzyme activities (e.g. superoxide dismutase and catalase). In conclusion, HT stress increased ethylene production rates, caused oxidative damage, decreased antioxidant enzyme activity, caused premature leaf senescence, increased flower abscission and decreased pod set percentage. Application of 1-MCP lowered ethylene and ROS production, enhanced antioxidant enzyme activity, increased membrane stability, delayed leaf senescence, decreased flower abscission and increased pod set percentage. The beneficial effects of 1-MCP were greater under HT stress compared to OT in terms of decreased ethylene production, decreased ROS production, increased antioxidant protection, decreased flower abscission and increased pod set percentage.     

Characterization of phytohormonal and postharvest senescence responses of balsam fir (Abies balsamea (L.) Mill.) exposed to short-term low temperature

To fulfill the US Thanksgiving and Christmas tree markets, balsam fir (Abies balsamea (L.) Mill.) is generally harvested before the cold season, anecdotally leading to premature needle senescence. Accordingly, we tested the hypothesis that LT exposure before harvest induces specific hormonal changes and delays postharvest senescence and/or abscission in balsam fir. Two hundred and six seedlings exposed to two temperature treatments for 48?h, LT at 5?°C and controls at 22?°C were severed off roots and monitored for their postharvest needle senescence. Root and shoot (needles and buds) tissues were examined for major endogenous hormone metabolites. LT increased shoot ABA (2,007?ng?g^?1 DW) by 2.5× and decreased GA₄₄ (9.84?ng?g^?1 DW) by 3.5× over those in roots. LT did not alter cytokinins, auxins or any root hormonal concentration. With auxins, only IAA, IAA-Asp, IAA-Leu and IAA-Glu were detected and the concentrations of IAA and IAA-Asp in shoots were lower than those found in roots. Among cytokinins, shoot c-ZR (58.95?ng?g^?1 DW) and t-ZR (4.17?ng?g^?1 DW) were 3× higher than those in roots. Apart from GA₄₄, GA₉ (136.76?ng?g^?1 DW) was abundant in shoots. The PBL and PNL were 46 and 1.2?%, irrespective of treatments. LT seedlings held needles 11?days longer than the controls (122?days). In balsam fir, short-term LT exposure augmented ABA and decreased GA₄₄ levels in shoots and delayed postharvest needle senescence.     

Effects of 1-MCP and exogenous ethylene on fruit ripening and antioxidants in stored mango

Mango (Mangifera indica L. cv. Tainong) fruits were harvested at the green-mature stage in Hainan and air-freighted to the laboratory at Peking. The fruits were treated with either 1 μl l⁻¹ 1-MCP or 5 μl l⁻¹ ethylene for 24 h and stored at 20°C for up to 16 days. 1-MCP maintained fruit firmness, whereas exogenous ethylene decreased fruit firmness. Exogenous ethylene accelerated the increase in ethylene and 1-aminocyclopropane-1-carboxylate (ACC) oxidase, whereas 1-MCP reduced both. Exogenous ethylene stimulated and 1-MCP inhibited the production of H₂O₂ of mango fruit during storage. Ascorbic acid was maintained at a high concentration in 1-MCP-treated fruit but was low in ethylene-treated fruit. 1-MCP inhibited activities of antioxidant enzymes including catalase, superoxide dismutase and ascorbate peroxidase. These results suggest that 1-MCP could play a positive role in regulating the activated oxygen metabolism balance. Baogang Wang and Jianhui Wang contributed equally to this work.     

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.     

Disturbance of ethylene biosynthesis and perception during somatic embryogenesis in <Emphasis Type="Italic">Medicago sativa</Emphasis> L. reduces embryos’ ability to regenerate

Effects of non-specific ethylene biosynthesis inhibitors: salicylic acid (SA) and aminoethoxyvinylglycine (AVG), and of specific inhibitors of ethylene binding to receptors: 1-methylcyclopropene (1-MCP) and 2,5-norbornadiene (NBD) applied during proliferation and differentiation phases of indirect somatic embryogenesis (SE) of Medicago sativa L. cv. Rangelander on embryogenic suspension growth, embryo production, development, and ability to germinate and convert were studied. Application of SA and AVG alone or together at concentrations from 1 to 500 μM in B₅g liquid medium during the proliferation phase had an inhibitory effect on ethylene production and embryogenic suspension growth. Additionally, it caused a drastic reduction in production of embryos and their development on BOi2Y solid differentiation medium. The inhibitory effect of SA was more visible than that of AVG. In addition, disturbance of ethylene biosynthesis during the proliferation phase of SE resulted in diminished lateral germination and conversion of cotyledonary embryos on MS solid medium. Moreover, blocking of ethylene receptors by 1-MCP during the proliferation phase also inhibited ethylene production and embryogenic suspension growth and reduced embryo production during differentiation. MCP almost completely inhibited development of cotyledonary embryos. At the same time, development of more embryos was arrested at the globular stage, and the number of abnormal embryos almost doubled. Similarly, addition of 1-MCP or NBD to the ambient atmosphere during the differentiation phase evidently arrested the development of embryos and, consequently, their ability to germinate and convert on MS regeneration medium. All the results presented above demonstrated that not only ethylene biosynthesis, but also ethylene action is involved in the control of individual phases of SE in Medicago sativa L. cv. Rangelander. And what is more, disturbance of these processes during distinct phases of SE adversely affects vigor of the somatic embryos obtained.     

Effect of octanoic acid on ethylene-mediated processes in Arabidopsis

We have examined whether octanoic acid (OA) one of the short chain saturated fatty acids (SCSFA), increases ethylene response in the following three ethylene-mediated processes: a) hypocotyl growth in darkness; b) formation of new flowers; c) flower abscission. These processes were examined in the presence or absence of exogenous ethylene in Arabidopsis wild type (WT) and in the ethylene-insensitive mutants, etr1-3 and ein2-1 and in the ethylene over-producer mutant eto1-1. Our results show that OA decreased hypocotyl length of WT in the absence or presence of exogenous ethylene, apparently showing that OA acts via augmentation of ethylene action. However, the hypocotyl growth inhibition could not be ascribed to increased ethylene sensitivity since application of inhibitors of ethylene synthesis (aminoethoxyvinylglycine; AVG) or action (1-methylcyclopropene;1-MCP) to WT seedlings did not prevent specifically the OA-induced growth inhibition. Also, OA inhibited hypocotyl growth in the mutants etr1-3 and ein2-1 in a similar pattern to that obtained in WT. On the other hand, OA had no effect on flower formation neither in WT, etr1-3 and eto1-1, in which ethylene reduced flower formation, nor in the ein2-1 mutant, in which ethylene had no effect. OA also did not increase flower abscission in WT or in the mutants etr1-3 and ein2-1 neither in the absence nor in the presence of ethylene. However, OA has augmented flower abscission in the mutant eto1-1 only in the absence of exogenous ethylene. This result might indicate that the effect of OA on eto1-1 is specific to this mutant and is not due to general deleterious effects inflicted by OA. Taken together, our results show that in general OA does not augment ethylene response in Arabidopsis, but it might affect ethylene action in flower abscission of the ethylene-overproducer mutant.     

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.     

IN VITRO ALLELOPATHIC INHIBITION OF NITRIFICATION BY BALSAM POPLAR AND BALSAM FIR

A soil percolation apparatus was used to study the effect of plant extracts on nitrification. Foliar leachates, leaf extracts, and bud extracts of balsam fir (Abies balsamea) and balsam poplar (Populus balsamifera) were added to the ammonium percolation solution. Color tests and quantitative measurement of residual ammonium and formed nitrate were applied. Foliar leachates of balsam fir strongly inhibited nitrification; leachates of balsam poplar were slightly less inhibitory. Balsam fir needle and balsam poplar dormant bud extracts (5% w/v) completely prevented the oxidation of ammonium. The same estracts (2% w/v) still prevented the oxidation of ammonium to nitrate to a large extent. Once again balsam fir extracts inhibited the process more strongly than balsam poplar. No evidence was obtained for the presence of a specific inhibitor of nitrifier microorganisms.     

Ethylene-promoted tomato flower abscission and the possible involvement of an inhibitor

The abscission zone in tomato (Lycopersicon esculentum (L.) Mill. flower pedicels is morphologically distinguishable prior to separation and is delineated by an indentation of the epidermis. Exposure of excised pedicels with the flower attached to ethylene results in abscission within 12 h and this can be accelerated by flower removal. Abscission of excised pedicels with the flower removed takes place in the absence of exogenous ethylene but this is delayed by pretreatment with aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis. The data presented support the hypothesis that flower tissue is the source of an abscission inhibitor.Abbreviations AVG aminoethoxyvinyl glycine - IAA indole-3-acetic acid     

Does ethylene play a role in thidiazuron-regulated somatic embryogenesis of geranium (Pelargonium × hortorum bailey) hypocotyl cultures?

Summary The accumulation of ethylene in headspace of hypocotyl cultures of geranium (Pelargonium × hortorum Bailey) and its possible role in thidiazuron-mediated somatic embryogenesis was investigated. The action of ethylene as determined by various ethylene synthesis and action inhibitors was varied. Silver nitrate (AgNo₃), aminoethoxyvinylglycine (AVG), and silver thiosulphate (STS) had no significant influence on the embryogenic response, while 1-methylcyclopropene (1-MCP) applied during the initial 3 d of induction or the expression phase, significantly increased the number of somatic embryos formed. Thidiazuron-treated tissues accumulated large quantities of ethylene within 6 h of culture, but the levels decreased after 12 h and reached very low levels after 3 d in culture. In the presence of acetylsalicylic acid (ASA), the levels of ethylene decreased by 20 to 50% during the first 48 h of culture. Analysis of endogenous auxin, cytokinins, and abscisic acid (ABA) indicated possible interactions of ethylene with other phytohormones during the induction of somatic embryos on geranium hypocotyl explants. Thidiazuron (10 μM) increased, while ASA decreased the levels of endogenous auxin, cytokinins, and abscisic acid during this period of induction.     

Mechanical wounding and abscission in citrus

Fruit detachment force (FDF), ethylene evolution, fruit and leaf drop were determined in Citrus sinensis for periods up to 96 h after mechanical wounding. Injury by removing a thin section of mature fruit flavedo reduced FDF, increased ethylene evolution and promoted abscission. Injuring flavedo 1 cm below the calyx was more effective at reducing FDF than injuring flavedo at the equator or the blossom‐end of mature fruit. Injuring the calyx or peduncle of mature fruit, or injuring three leaves closest to the mature fruit did not reduce FDF. Immature fruitlets either did not abscise or underwent low rates of abscission in response to mechanical wounding, depending on age. Inhibiting ethylene binding in wounded mature fruit with 1‐methylcyclopropene (1‐MCP) increased ethylene evolution compared with wounded fruit alone, but the reduction in FDF was similar. When an ethylene biosynthesis inhibitor (aminoethoxyvinylglycine, AVG) was used, reduction in FDF of wounded mature fruit exposed to AVG was similar to that of wounded fruit alone but ethylene production was markedly reduced. Wounding mature leaf blades in the presence or absence of 1‐MCP resulted in elevated but equal ethylene evolution up to 48 h after wounding, however, no leaf drop occurred. Thereafter, ethylene evolution was higher in 1‐MCP‐treated wounded leaves. Removing up to 77% of the total mature leaf area did not cause leaf drop, nor did wounding tissue across the laminar or petiolar abscission zones. Leaflets of 5 mm length reached nearly 100% abscission after mechanical wounding, whereas wounding leaves 20 mm length resulted in 15% abscission. The data suggest that mechanical wounding of flavedo results in mature fruit abscission, and ethylene binding may not be mandatory to initiate abscission in citrus fruit. The differential response of fruit and leaves at different ages to wounding may be related to potential contribution to carbohydrate accumulation, and production and sensitivity of tissues to an abscission signal(s).     

1-Methylcyclopropene and ethylene as regulators of in vitro organogenesis in kiwi explants

In this paper, we study the influence of ACC, AVG and 1-MCP on in vitro organogenesis of kiwi (Actinidia deliciosa) explants and on ethylene metabolism. Results indicated that increasing ethylene production and accumulation in the head space of the culture vessel by adding ACC to the culture medium inhibited organogenesis, except for rooting, which increased and stimulated ACC oxidase activity threefold, whereas AVG increased the length and number of shoots and leaves and inhibited about 80% ACC synthase activity compared with the reference explants. 1-MCP exerts a stimulatory effect analogous to AVG, increasing ACC synthase activity and organogenesis of kiwi explants. This effect is not reverted by the addition of ACC to the culture medium. Therefore, ethylene production and its accumulation in the headspace of the culture vessels seems to be responsible for the inhibition of shoot development as well as increasing rooting in in vitro cultured kiwi explants.