Stimulated Ethylene Production Research Articles

Effects of exogenous propylene on softening, glycosidase, and pectinmethylesterase activity during postharvest ripening of apricots.

Apricots (Prunus armeniaca L. cv. Boccuccia spinosa) picked at the commercial ripening stage [soluble solids content (SSC) 12.6%] were left to reach full ripening in continuously humidified air at 20 degrees C. Changes in the rate of ethylene production, firmness, soluble solids concentration, and titratable acidity were measured. The alpha-D- and beta-D-glucosidases, alpha-L-arabinofuranosidase, alpha-D- and beta-D-galactosidases, beta-D-xylosidase, and alpha-D-mannosidase activities were assayed. To evaluate the influence of ethylene on glycosidase activity, propylene (500 microL x L(-1)) was applied to apricots for 24 and 48 h. In apricots ripened in air, ethylene production increased on the first day and exhibited a typical climacteric pattern. Good edible quality was reached in 5 days when SSC was at least 14% and acidity was between 1.1 and 1.2% (% malic acid). During postharvest ripening, alpha-L-arabinofuranosidase activity increased from 1.9 to 11.6 nkat until day 7. alpha-D-Galactosidase, alpha-D-mannosidase, and beta-D-galactosidase activity increased continuously but at a lower rate. beta-D-Xylosidase activity also increased, but the level of activity was lower than the other glycosidases assayed. Pectinmethylesterase (PME) decreased during the postharvest ripening, and propylene enhanced this pattern, by stimulating ethylene production. Even the activities of alpha-L-arabinofuranosidase, beta-D-xylosidase, alpha-D-mannosidase, and beta-D-galactosidase were greatly stimulated by the propylene treatment, which consequently induced rapid softening of the fruits.

Effect of 1-methylcyclopropene (1-MCP) on respiration, ethylene production and color change of pineapple fruit after harvest

Involvement of ethylene on changes in gas metabolism and fruit color of pineapple fruit after harvest was studied, using 1-methylcyclopropene (1-MCP), an ethylene action inhibitor. Rates of respiration and ethylene production of fruit harvested at mature green stage continued increasing during storage, showing the pattern increasing towards the end of storage, but the maximum ethylene production rate was less than 1 nl g-1 h-1. 1-MCP decreased respiration and temporarily increased ethylene production from the fruit. Propylene, which transiently increased respiration, did not stimulate ethylene production. When whole fruit was separated into fruit part and crown, the pattern of gas metabolism in whole fruit was completely dependent on fruit part, not on crown. Treatment of immature, mature green and ripe fruits with 1-MCP for several times resulted in temporal stimulation of ethylene production immediately after each treatment, irrespective of harvest maturity. Respiration and ethylene production, which increased on the tree as fruit developed, continued increasing after harvest. Fruit color progressed in all fruits harvested at three different maturity, but was delayed by 1-MCP, in which the younger the fruit, the more prominent was the effect of 1-MCP on inhibition of color change. These results indicated that ethylene production in harvested pineapple fruit is controlled by negative feedback regulation (autoinhibition), and that ethylene is involved in fruit color change. Pineapple fruit at least partially perform postharvest ripening since gas metabolism and fruit color in harvested fruits changed just as on-tree fruit.

Effects of applying stem-shortening plant growth regulators to leaves on root elongation by seedlings of wheat, oat and barley: mediation by ethylene

Several plant growth regulators (PGRs) commonly used in practicalfarming to restrict shoot height and control lodging were examined for theirimpact on root growth in naturally short or tall cultivars of barley (cvs.Kymppi and Saana), oat (cvs. Veli and Pal), and wheat (cvs. Mahti and Tjalve).The possible involvement of ethylene in the responses was also examined. Shootswere sprayed at the two-leaf stage with the gibberellin biosynthesis inhibitorsCycocel (chlormequat chloride) (CCC) or Moddus (Trinexapac-ethyl) (TE), or withthe ethylene-releasing agent Cerone (ethephon) (ETH) at 0, 0.1, 1, 10 or 50times the recommended agricultural rate (RR). Root elongation and ethyleneproduction by roots or shoots were unaffected by CCC at all application ratesorby TE at ×0.1 or ×1.0 RR. At ×10 and ×50 RR, TE wasinhibitory to root extension but did not increase ethylene biosynthesis bytheseroots or the shoots. ETH at ×0.1 or ×1.0 RR did not affect rootextension or ethylene production in roots or shoots. At all higher rates ofapplication ETH stimulated ethylene production strongly in shoots and roots ofall three species, while root elongation was retarded severely in barley,moderately in oat and only slightly in wheat. These differences in elongationresponse are attributed to differences in sensitivity to ethylene released byethephon. Accordingly, root elongation in wheat was only slightly affected whenethylene gas was supplied at concentrations up to 100 ppm for 3d. In contrast, root elongation in barley was strongly inhibitedbyethylene, with oat demonstrating an intermediate responsiveness.

Levels of ACC and related compounds in exudate and extracts of canola seeds treated with ACC deaminase-containing plant growth-promoting bacteria.

It was previously proposed that plant growth-promoting bacteria that possess 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase could utilize ACC that is present in the exudate of germinating canola seeds. The uptake and cleavage of ACC by these bacteria would lower the level of ACC, and thus ethylene within the plant, and reduce the extent of its inhibition on root elongation. To test part of the above mentioned model, ACC levels were monitored in canola seed tissues and exudate during germination. Lower amounts of ACC were present in the exudate and tissues of seeds treated with the plant growth-promoting bacterium Enterobacter cloacae CAL3, than in control seeds treated with MgSO4. The ACC-related compounds, alpha- and gamma-aminobutyric acids, both known to stimulate ethylene production, were also measured in the canola seed exudate and tissues. Approximately the same levels of alpha-aminobutyric acid were present in the exudates of the bacterium-treated seeds and the control seeds, but the amount of alpha-aminobutyric acid was lower in the tissues of the bacterium-treated seeds than in the control seeds. Smaller quantities of gamma-aminobutyric acid were seen in both the exudate and tissues of the E. cloacae CAL3-treated seeds than in the control seeds.

Role of the gynoecium in natural senescence of carnation (Dianthus caryophyllus L.) flowers

Although the role of the gynoecium in natural senescence of the carnation flower has long been suggested, it has remained a matter of dispute because petal senescence in the cut carnation flower was not delayed by the removal of gynoecium. In this study, the gynoecium was snapped off by hand, in contrast to previous investigations where removal was achieved by forceps or scissors. The removal of the gynoecium by hand prevented the onset of ethylene production and prolonged the vase life of the flower, demonstrating a decisive role of the gynoecium in controlling natural senescence of the carnation flower. Abscisic acid (ABA) and indole-3-acetic acid (IAA), which induced ethylene production and accelerated petal senescence in carnation flowers, did not stimulate ethylene production in the flowers with gynoecia removed (-Gyn flowers). Application of 1-aminocyclopropane-1-carboxylate (ACC), the ethylene precursor, induced substantial ethylene production and petal wilting in the flowers with gynoecia left intact, but was less effective at stimulating ethylene production in the -Gyn flowers and negligible petal in-rolling was observed. Exogenous ethylene induced autocatalytic production of the gas and petal wilting in the -Gyn flowers. These results indicated that ethylene generated in the gynoecium triggers the onset of ethylene production in the petals of carnation during natural senescence.

Action of malformin A1 on gravitropic curvature in primary roots of maize (Zea mays L.)

Malformins, a small family of cyclic pentapeptides, are active plant growth regulators isolated from the fungusAspergillus niger. We purified malformin A1 from the crude malformin A mixture, and studied its action in the gravitropic response of maize roots. Intact primary roots that had been pretreated vertically with malformin A1 were placed in a humidified box in the horizontal position. Positive curvature (downward) was inhibited in the pretreated roots compared with the control. In addition, we measured the lateral transport of IAA in primary roots. Roots pretreated with malformin A, did not show asymmetric distribution of IAA between the upper and lower sides of the elongation zone. Malformin A, also stimulated ethylene production in maize root segments. Our results had suggested that malformin A1 might inhibit the lateral transport of IAA across the roots from the upper to the lower side because of an increased level of ethylene. Therefore, we placed more IAA on the upper side at the initial phase of gravistimulation. These results were consistent with malformin A1-pretreated roots showing inhibited positive gravitropic curvature.

113 Ethylene Mediates Responses of `Gala' Apple Fruit to Ionizing Irradiation

Pre-climacteric `Gala' apple fruit treated with air (control) or 2 μmol·L–1 1-methylcyclopropene (MCP) were exposed to gamma irradiation at 0, 0.5, 1, or 1.5 kGy at 23 °C. Fruit were held at 20 °C for 3 weeks after treatment during which respiration rate, production of ethylene and other volatile compounds, fruit firmness, soluble solid content, titratable acidity, and irradiation injury were determined. MCP treatment reduced respiration and ethylene production and slowed loss of fruit firmness and titratable acidity. Irradiation induced increased respiration of MCP-treated fruit in a dosage-dependent fashion. Irradiation caused a decrease in ethylene production by control (non-MCP) fruit, and the magnitude of the decrease was dependent on irradiation dosage. Irradiation at 0.5, 1, and 1.5 kGy stimulated ethylene production of MCP-treated fruit for only 1 day after treatment. Irradiation induced internal browning and the injury severity increased with dosage. The severity and incidence of irradiation damage were higher in MCP-treated fruit than control fruit. The results indicate that ethylene is involved in mediating apple fruit responses to irradiation.

Boll openers in cotton: effectiveness and environmental influences

The indeterminate growth of cotton ( Gossypium hirsutum L.) often results in a boll population at various stages of maturity when the crop is harvested. Boll opening compounds can be used to facilitate harvest by accelerating the opening of green bolls. Field trials were conducted at two locations in 1996 and 1997 to test the efficacy of four commercially available boll openers that stimulate ethylene production or desiccate the plant. Two of these boll openers are new to the marketplace: Finish™, ethephon+cyclanilide (2-chloroethyl phosphonic acid+1-(2,4-dichlorophenylaminocarbonyl)-cyclopropane carboxylic acid), and CottonQuick™, ethephon+AMADS (2-chloroethyl phosphonic acid+1-aminomethanamide dihydrogen tetraoxosulfate). Open bolls were hand harvested at 3–7 days intervals after application of the boll openers, and environmental conditions in conjunction with these harvests were recorded. The desiccant, paraquat (1,1′-dimethyl-4,4′-bipyridinium dichloride), was found to have erratic boll opening activity. Two products that contain ethephon plus the synergists cyclanilide or AMADS were found to have equal or higher rates of boll opening than ethephon alone. The rate of boll opening of all compounds was influenced more by the accumulation of heat units than by the amount of time following the application of boll openers. Although erratic, boll opening activity from paraquat application can be significant. Moreover, the reliability of ethephon based products, which are consistent boll openers, may be enhanced by formulations that contain a synergist. Heat unit accumulation may be used to predict and model the efficacy of boll opening with these harvest aides.

The role of ethylene in enhancing the initial ovary growth of Dendrobium ‘Pompadour’ following pollination

SummaryPollination stimulated ethylene production concomitant with the increase in ovary growth of Dendrobium ‘Pompadour’. Emasculation stimulated ethylene production but did not increase ovary growth. Emasculation synergistically increased ovary growth of pollinated orchid flowers. 1-Aminocyclopropane-1-carboxylic acid (ACC) and 1-naphthylacetic acid (NAA) treatments stimulated ethylene production of orchid flowers but only NAA fully substituted for pollination in enhancing ovary growth, while ACC application synergistically increased ovary growth of pollinated orchid flowers. Aminooxyacetic acid (AOA) treatment inhibited pollination- and NAA-induced ovary growth. ACC application restored the ovary growth response with added AOA prior to pollination. The results suggested that ethylene was required for initial growth of the ovary following pollination in Dendrobium ‘Pompadour’.

Stripped plant oils maintain fruit quality of ‘Golden Delicious’ apples and ‘Bartlett’ pears after prolonged cold storage

SummaryEffects of stripped (α-tocopherol reduced) plant oils (corn, soybean, peanut, cottonseed, and linseed) on ethylene, fruit firmness, colour, titratable acidity (TA), and soluble solids content (SSC) were evaluated in ‘Golden Delicious’ apples after six months’ and ‘Bartlett’ pears after six weeks’ storage at 08C. All oils similarly inhibited ethylene production and accumulation in the first two weeks in ‘Bartlett’ and in the first three months in ‘Golden Delicious’. Compared with the untreated controls, oil-treated fruit were firmer, greener, and contained a higher level of TA after six weeks or six months at 0°C plus 7.d at 20°C. Treating with 200.μl.l–;1 propylene for 6.h at harvest stimulated ethylene production, accelerated fruit softening and degreening, and reduced acidity after cold storage in untreated apples and pears. In oil-treated fruit, however, propylene treatment was ineffective. SSC was not affected by oil or propylene treatment. During one week of storage at 20°C, ethylene production and changes in fruit firmness, colour, and acidity were reduced by oil treatments applied to fruit at harvest (preclimacteric) or applied to fruit after one month of storage at 08°C (climacteric). The effectiveness of oil treatment, however, was higher in preclimacteric than in climacteric fruit.

Induction by ozone of ethylene production and an ACC oxidase cDNA in rice (Oryza sativa L.) leaves

Exposure to ozone at 1 µl l−1 for 6 h induced ethylene production in rice (Oryza sativa L. cv. Hitomebore) leaves. The stimulation of ethylene production was detectable 2 h after the start of the exposure to ozone, and lasted for 6 h after the exposure. A 429-bp cDNA fragment encoding ACC oxidase was obtained by RT-PCR from ozone-treated rice leaves. Its nucleotide sequence and deduced amino-acid sequence had 97.2% and 94.4% identity, respectively, to those of OS1A1COX, which was previously obtained from deepwater rice. The abundance of the cDNA increased in accordance with the induction of ethylene production by the exposure to ozone.

Gum Formation by Methyl Jasmonate in Tulip Shoots is Stimulated by Ethylene

The promotive effect of methyl jasmonate (JA-Me) on the induction of gum in tulip shoots (Tulipa gesneriana L. cvs. Gudoshnik and Apeldoorn) was studied in the presence of ethylene. Gum formation in the stem and the basal part of the leaves was induced by JA-Me (1% w/w in lanolin) and stimulated strongly by the simultaneous application of 1 or 5 mm 1-aminocyclopropane-1-carboxylic acid (ACC). JA-Me at a concentration of 0.1% did not induce gum, but that together with ACC at a concentration of 1 or 5 mm induced it substantially. Although JA-Me stimulated ethylene production substantially in the stem of intact tulips, ethephon (1% w/w) or ACC (1 or 5 mm) did not induce gum formation in tulip shoots. JA-Me induced gum formation in tulip shoots even in the presence of aminooxyacetic acid or cobalt ions. Moreover, gum formation was also observed in the cut shoot applied with JA-Me as a solution at concentrations of 0.23 mm or more. These results strongly suggest that JA-Me is required for gum formation in tulip shoots, and ethylene probably makes the tissues of shoots sensitive to JA-Me.

DNA methylation levels and ethylene production in senescent, suspension‐cultured pear fruit cells: Implications for epigenetic control?

Auxin‐deprived, senescent, suspension‐cultured pear (Pyrus communis cv. Passe Crassane) fruit cells that can be stimulated to produce ethylene were employed in the search for a possible interdependence between DNA methylation levels and ethylene production. Neither short‐term stimulation of ethylene production by CuCl2, nor longer‐term stimulation by auxin, nor the inhibition of ethylene biosynthesis had a significant effect on the cellular level of DNA methylation. However, short‐term exposure to S‐adenosylhomocysteine enhanced cellular ethylene production and long‐term exposure to azacytidine resulted in the reduction of both DNA methylation levels and stress‐induced ethylene production. These and other correlative findings, or lack thereof, are discussed in the context of ethylene physiology, DNA methylation/demethylation, and epigenetic control.

Effects of Ethephon on Growth and Oil Yield of Palmarosa (Cymbopogon martinii Wats.)

ABSTRACT Ethephon [(2-chloroethyl) phosphonic acid, Ethrel, Composan] was evaluated in field trials as a chemical hybridizing agent on four varieties of palmarosa (Cymbopogon martinii): PRC-1, Tripta, Trishna, and Composite. Application of 500 μg/ml ethephon to foliage either as a mist or by wrapping ethephon-soaked cotton around the stem during the vegetative and anthesis stages initiated increases in growth and yield attributable to heterosis. A positive correlation (r2 = 0.81) between ethephon application and yield of dry matter and ethephon application and essential oil with enhanced geraniol levels was obtained, suggesting ethephon treatment is an effective chemical hybridizing agent in palmarosa. Ethephon also appeared to stimulate ethylene production in flowers of palmarosa, resulting in male sterility and premature anther-petal senescence.

Characterization of Carbon Dioxide Stress-Induced Ethylene Biosynthesis in Cucumber (Cucumis sativus L.) Fruit

In the present study, we have investigated the mechanism through which carbon dioxide induces ethylene biosynthesis in cucumber {Cucumis sativus L.) fruit. A series of inhibitors were tested in order to determine the involvement of de novo protein synthesis, new mRNA synthesis, protein kinase activity, and phosphoprotein phospbatase activity in carbon dioxide stress-induced ethylene biosynthesis. Carbon dioxide stimulated ethylene production through the induction of both 1-aminocyclopropane-l-carboxylate (ACC) synthase and ACC oxidase activities. Induction of ACC synthase activity led to accumulation of ACC. Cycloheximide blocked carbon dioxide stress-induced ethylene biosynthesis while cordycepin had no effect. Dibucaine and 6-dimethylaminopurine, inhibitors of protein kinases, blocked carbon dioxide stress-induced increases in ethylene production, ACC synthase activity, and ACC content but not the increase in ACC oxidase activity. Cantharidin, a potent inhibitor of protein phosphatase type 1 and type 2A, stimulated carbon dioxide stress-induced ethylene production, ACC synthase activity, and ACC accumulation but had no additional effect on carbon dioxide stress-induced ACC oxidase activity. In the presence of dibucaine or cycloheximide, the stimulation of carbon dioxide stress-induced ethylene biosynthesis by cantharidin was blocked. These data suggest that carbon dioxide stress-induced ethylene biosynthesis may be regulated posttranscriptionally and that the induction involves the synthesis of novel protein(s) in the cytosol. The results further identify protein phosphorylation and dephosphorylation as requirements in one or more of the steps involved in the carbon dioxide signal transduction pathway that leads to induction of ethylene biosynthesis and, in particular to the induction of ACC synthase, presumably the key enzyme in the ethylene biosynthetic pathway.

Stimulated ethylene production in tobacco ( Nicotiana tabacum L. cv. Ky 57) leaves infected systemically with cucumber mosaic virus yellow strain

Ethylene production was stimulated in the leaves of tobacco ( Nicotiana tabacum L. cv. Ky 57) infected systemically with cucumber mosaic virus yellow strain (CMV-Y). This observation was made with the 9th leaf of tobacco plants which had been inoculated with CMV-Y on the 6th leaf at the middle stage of growth. A transient peak of ethylene production per fresh-weight base appeared 2 weeks after inoculation when the mosaic symptoms of green, yellow and white sectors covered about 40% of the leaf area. The increase in ethylene production was accompanied by an increase in 1-aminocyclopropane-1-carboxylate (ACC) content and activities of ACC synthase and ACC oxidase in systemically-infected leaves. Among three sectors with different colors in CMV-Y-infected mosaic leaves, ACC content was highest in the green sector, implying ethylene production took place most actively there.

Gamma-Aminobutyric acid stimulates ethylene biosynthesis in sunflower.

gamma-Aminobutyric acid (GABA), a nonprotein amino acid, is often accumulated in plants following environmental stimuli that can also cause ethylene production. We have investigated the relationship between GABA and ethylene production in excised sunflower (Helianthus annuus L.) tissues. Exogenous GABA causes up to a 14-fold increase in the ethylene production rate after about 12 h. Cotyledons fed with [14C]GABA did not release substantial amounts of radioactive ethylene despite its chemical similarity to 1-aminocyclopropane-1-carboxylic acid (ACC), indicating that GABA is not likely to be an alternative precursor for ethylene. GABA causes increases in ACC synthase mRNA accumulation, ACC levels, ACC oxidase mRNA levels, and in vitro ACC oxidase activity. In the presence of aminoethoxyvinylglycine or alpha-aminoisobutyric acid, GABA did not stimulate ethylene production. We therefore conclude that GABA stimulates ethylene biosynthesis mainly by promoting ACC synthase transcript abundance. Possible roles of GABA as a signal transducer are suggested.

Modifications to polyamines provoked by short-chain saturated fatty acids in Cicer arietinum seeds

Short-chain saturated fatty acids (SCSFA), ranging from pentanoic (C 5) to decanoic acid (C 10), when added at micromolar concentrations to the germination medium of chick-pea seeds, did not significantly affect radicle emergence, although fresh weight diminished. Only heptanoic acid (C 7) and octanoic acid (C 8) stimulated ethylene production and free spermidine and spermine content, whereas putrescine reached a maximum with nonanoic acid (C 9). All the SCSFA tested increased the polyamines (PAs) bound to substances of low M r ; however, only hexanoic acid (C 6) and C 9 acid induced a strong appearance of PAs bound to substances of high M r . Proline synthesis was stimulated in the presence of C 7 acid and C 9 acid, but values lower than control were obtained with the other SCSFA studied.

Comparative Physiological Effects of Quinclorac and Auxins, and Light Involvement in Quinclorac-Induced Chlorosis in Corn Leaves

The physiological effects of the herbicide quinclorac (3,7-dichloro-8-quinolinecarboxylic acid) in corn and the involvement of light on the herbicide-induced chlorosis were investigated. Quinclorac was highly inhibitory to corn growth. However, different from the tested auxins, quinclorac showed no promotion in an auxinic activity assay (corn coleoptile elongation). Quinclorac induced significantly lower amounts of ethylene in corn coleoptile sections compared with IAA, NAA, and 2,4-D under darkness. Furthermore, quinclorac did not bind with a putative corn membrane-bound auxin receptor. Quinclorac caused chlorotic discoloration in the youngest leaf under light. When the quinclorac-treated corn leaf disks were continuously exposed to light, a significant decrease in total chlorophyll content was observed. No decrease in total chlorophyll content of the herbicide-treated leaf disks under dark conditions was detected. There was a significant correlation between light intensity and the reduction in total chlorophyll content of the leaf disks induced by quinclorac. Moreover, quinclorac considerably increased ethylene production in leaf disks under light conditions, whereas no enhancement of ethylene production in the dark was observed. 2,4-D slightly stimulated ethylene production under both the light and dark conditions. There was a high negative correlation between quinclorac-induced ethylene production and total chlorophyll content in quinclorac-treated leaf disks. These results suggest that the physiological actions of quinclorac applied to corn are not necessarily the same as 2,4-D and other auxins. Ethylene biosynthetic reaction may be closely related with the major herbicidal action of quinclorac, similar to that of 2,4-D. However, our data indicate that the quinclorac-enhanced ethylene biosynthetic reaction and the herbicide-induced chlorosis in corn leaves are light dependent.

Bioregulatory Effects of the Fungicidal Strobilurin Kresoxim-methyl in Wheat (Triticum aestivum)

Apart from its fungicidal effect, the strobilurin kresoxim-methyl (BAS 490 F) was found to induce physiological and developmental alterations in wheat (Triticum aestivum L.) which are seen in connection with improved yield. In a series of biotests including heterotrophic maize and photoautotrophic algal cell suspensions, duckweed, isolated mustard shoots and germinating cress seeds, kresoxim-methyl showed a similar response pattern to standard auxins (e.g. indol-3-ylacetic acid, IAA; 2-(1-naphthyl)acetic acid, α-NAA). Auxin-like activity of kresoxim-methyl was also found when stem explants of tobacco were cultured on a hormone-free medium. Kresoxim-methyl stimulated shoot formation, particularly at 10-7 M. The same effect was induced by 10-8 M IAA. The determination of phytohormone-like substances in shoots of wheat plants foliar-treated with 7×10-4 M kresoxim-methyl revealed only slightly changed levels of endogenous IAA, gibberellins and abscisic acid. In contrast, the contents of dihydrozeatin riboside-type cytokinins increased to 160% of the control, while trans-zeatin riboside- and isopentenyladenosine-type cytokinins remained nearly unchanged. The most remarkable alterations were the reductions in 1-aminocyclopropane-1-carboxylic acid (ACC) levels and ethylene formation which were demonstrated in intact plants, leaf discs and the shoots of wheat subjected to drought stress. Kresoxim-methyl affected the induction of ACC synthase activity which converts S-adenosyl-methionine to ACC in ethylene biosynthesis. In shoots from foliar-treated wheat plants, 10-4 M kresoxim-methyl inhibited stress-induced increases in endogenous ACC synthase activity, ACC levels and ethylene formation by approximately 50%. Reductions in ACC synthase activity and ACC levels of 30% were also obtained at low concentrations of α-NAA (10-6 M). In contrast, ACC synthase activity in vitro was not influenced by adding the compounds. In wheat leaf discs, the inhibiting effect of kresoxim-methyl, α-NAA and IAA on ethylene formation was accompanied by delayed leaf senescence, characterized by reduced chlorophyll loss. However, in contrast to kresoxim-methyl which showed only inhibitory activity on ethylene synthesis over a wide range of concentrations applied, the auxins stimulated ethylene production at high concentrations of about 10-4 M. The inhibition of ethylene biosynthesis by kresoxim-methyl, together with an increase in endogenous cytokinins could explain the retardation of senescence and the intensified green leaf pigmentation in wheat exposed to this strobilurin. © 1997 SCI.