Low Levels Of Ethylene Research Articles

Growth of transgenic canola (Brassica napus cv. Westar) expressing a bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene on high concentrations of salt

Canola, Brassica napus cv. Westar, was transformed to express a bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase (EC 4.1.99.4) gene under the transcriptional control of (a) the constitutive and strong 35S promoter from cauliflower mosaic virus, (b) the root-specific promoter of the rolD gene within the T-DNA from the Ri plasmid of Agrobacterium rhizogenes, and (c) the promoter for the pathogenesis-related prb-1b gene from tobacco. Following the growth of transformed and non-transformed canola plants in the presence of 0–200 mM NaCl, the fresh and dry weights of plants, leaf protein concentration, and leaf chlorophyll contents were measured. The data suggest that the presence of ACC deaminase provides the transgenic canola lines with tolerance to the inhibitory effects of salt stress, compared to the non-transformed canola plants, with the rolD transformants being the most effective. The improved salt tolerance of these transgenic plants is likely the consequence of the decreased synthesis of stress ethylene. This data is consistent with previous studies with transgenic tomato plants expressing bacterial ACC deaminase which showed that lowering ethylene levels partially protected plants against growth inhibition by metals, phytopathogens and flooding.

Pawpaw [Asimina triloba (L.) Dunal] Fruit Ripening. I. Ethylene Biosynthesis and Production

Pawpaw fruit ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities, and tissue content of the ethylene precursor ACC and conjugate malonyl-ACC (MACC) were measured during postharvest ripening. Fruit were harvested near the advent of the ripening process and were ripened at room temperature. The fruit displayed increases in ethylene production and respiration rate during ripening with maxima for both 3 days after harvest. Mean ethylene maxima on a fresh weight basis were 4.7 and 7.6 μg·kg-1·h-1 and mean respiratory (CO2 production) maxima on a fresh weight basis were 220 and 239 mg·kg-1·h-1 in 1999 and 2001, respectively. The increase in ethylene evolution coincided with an increase in respiration and a rapid decline in fruit firmness. Internal and external fruit firmness declined in a parallel manner. The ethylene climacteric peak occurred after the greatest decline in fruit firmness, indicating that low levels of ethylene may be sufficient to initiate the ripening process. The ethylene climacteric peak also coincided with the highest activities of both ACS and ACO as well as the maximum tissue ACC content. As ACC content increased, MACC content declined, suggesting a regulation of ethylene production via free ACC levels by malonylation of ACC. Thus, the climacteric development of ethylene production may be regulated by an increase of ACS activity and a decrease in ACC malonyltransferase activity, making more free ACC available for the production of ethylene by increased activity of ACO.

Expression of genes encoding cell wall modifying enzymes is induced by cold storage and reflects changes in pear fruit texture

Preclimacteric 'Rocha' pears stored under chilling conditions, had a larger increase of ACO (1-aminocyclopropane-1-carboxylate oxidase) activity and softened faster than those treated with ethylene. Non-treated fruit did not ripen or soften, acquired a rubbery texture, and showed barely detectable levels of ACO activity. The transcript accumulation of seven genes encoding cell wall modifying enzymes was followed during fruit growth, ripening, and senescence, and in fruit that failed to ripen, by quantitative real-time PCR. Transcripts from 'Rocha' pear polygalacturonase1 and 2 (PcPG1, PcPG2), beta-galactosidase (PcbetaGAL) and beta-xylosidase (PcXYL) genes accumulated up to 1000-fold at the climacteric onset, while low transcript levels were detected in growing fruit. In fruit that did not ripen, this transcript accumulation was lower compared with fruits that ripened normally. Transcripts for expansin1 and 2 (PcEXPA1, PcEXPA2) accumulated in growing fruit, but about 10-fold more in fruit after rewarming. Xyloglucan endotransglucosylase/hydrolase (PcXTH) had the highest basal expression levels in all samples, showing only a small increase during fruit growth and ripening. PcEXPA2 and PcXTH transcripts accumulated in untreated fruit, 21 d after harvest, to levels similar to those of fruit that ripened normally. Since in untreated fruit ACO activity was barely detectable, it is likely that the activation of these genes might occur at very low ethylene levels. Results suggest that PcXTH and PcEXPA2 gene induction might be associated with cell wall maintenance during 'Rocha' pear development and ripening, while PcEXPA1, PcPG1, PcPG2, PcbetaGAL, and PcXYL expression is likely to be related to cell wall disassembly and loosening.

Effects of ethylene and 1-MCP on ripening and senescence of European seedless cucumbers

The effects of exogenous and endogenous ethylene combined with or without the ethylene action inhibitor, 1-methylcyclopropene (1-MCP), on the shelf-life of European seedless cucumbers (Cucumis sativus L.) were studied. Exogenous ethylene (3–5 μl l−1) accelerated the breakdown of chlorophyll but the degradation pathway was not altered by ethylene treatment. Chlorophyll catabolism in cucumbers seems to follow the same pathway as in parsley with replacement of Mg2+ as the first step with a minor role for chlorophyllide, followed by oxidative ring cleavage of the pheophytin a formed. Degreening was accompanied by an increasing amount of citric acid in the endocarp with a concomitant fall of malic acid but with the onset delayed until 50% of the chlorophyll had disappeared. Fresh cucumbers, fumigated with 1-MCP before continuous exposure to ethylene, did not deviate from those untreated, with inhibited chlorophyll degradation for 14 days in one experiment, but for only 9 days in another experiment, after which 1-MCP + ethylene resulted in a patchy degreening of the peel. However, in pre-stored fruit either continuously exposed to propylene or air, fumigation with 1-MCP prior to exposure did not prevent degreening indicating that breakdown of chlorophyll in aged fruit may be triggered and regulated by developmental factors and not by the low level of endogenous ethylene produced. It is therefore concluded that cucumbers probably show little benefit from fumigation with 1-MCP unless exogenous ethylene is present. Although the cucumbers were harvested before one quarter of the final weight was reached, they were able to develop the same ripening characteristics, i.e. yellowing, accumulation of citrate and tissue softening as if left on the plant until growth has ceased. These results suggest that cucumber ripening and senescence are overlapping processes and internal factors activating senescence-associated genes probably determine the keeping quality of cucumbers.

Changes in gene expression in canola roots induced by ACC-deaminase-containing plant-growth-promoting bacteria.

The technique of RNA arbitrarily primed-polymerase chain reaction (RAP-PCR) was used to study changes in gene expression over time in canola roots treated with the 1-aminocyclopropane-1-carboxylate (ACC) deaminase-containing plant-growth-promoting bacterium Enterobacter cloacae UW4 and to compare the changes with those in a mutant of E. cloacae UW4 in which the ACC deaminase structural gene acdS was replaced by homologous recombination with acdS with an intentional knockout containing a tetracycline resistance gene. Genes that were either up- or down-regulated over a three-day period in canola plants treated with wild-type or mutant bacteria were isolated, cloned, and sequenced; all appeared to have high homology with Arabidopsis thaliana genes. The upregulated genes included a cell division cycle protein 48 homolog and a eukaryotic translation initiation factor 3 subunit 7 gene homolog. The downregulated genes included one encoding a glycine-rich RNA binding protein with a function in RNA processing or binding during ethylene-induced stress, which is expressed only in roots, and another gene thought to be involved in a defense signaling pathway. All RAP-PCR results were verified using Northern blotting. These data, indicate that roots isolated from canola seeds treated with the ACC deaminase-producing E. cloacae UW4 upregulate genes involved in cell division and proliferation but down-regulate stress genes. This data is in agreement with a model in which ACC deaminase-containing plant-growth-promoting bacteria reduce plant stress and induce root elongation and proliferation in plants, largely by lowering ethylene levels.

Efficacy of Potassium Permanganate Impregnated into Alumina Beads to Reduce Atmospheric Ethylene

A systematic study was conducted on the ability of potassium permanganate absorbent to remove low levels of ethylene from the atmosphere. Absorption of potassium permanganate onto alumina beads by dipping in a saturated solution was maximal at 2 g/100 g after 2 hours at 20 °C and 4 g/100 g after 1 hour at 65 °C. Commercial alumina-based absorbents were found to contain potassium permanganate at 2.7 to 6.0 g/100 g suggesting many are prepared at elevated temperature. Trials in a closed system at 20 °C and 60% to 70% RH with alumina beads containing potassium permanganate at 4 g/100 g showed a logarithmic decrease in ethylene concentration with 90% of the ethylene removed after 2.5 to 3.0 hours. Relative humidity (RH) had a marked inverse effect on ethylene absorption with reactivity at 100% RH calculated to be 15% of that at 0% RH. Performance of potassium permanganate where ethylene was continually generated by a continuous flow of ethylene at 14 μL·h-1 through the container showed a steady state was attained within 1 hour and maintained for 24 hours. Ethylene removal increased linearly with bead weight and ranged from 30% with 1 g to 90% with 50 g. Examination over 20 days showed a continuing decrease in rate of ethylene removal which after 14 days had declined to 10% of incoming ethylene although 44% of the original level of potassium permanganate still remained in the beads. Calculations based on known endogenous ethylene production rates suggest that at 20 °C and 90% RH, use of a potassium permanganate-alumina absorbent would be beneficial with produce having a low level of ethylene generation. Suitability for larger packages of produce generating higher ethylene levels is questionable as >1 kg of absorbent may be required.

Effect of Low Levels of Ethylene on Sprouting of Potatoes in Storage

Potato tubers (Solanum tuberosum cv. Sebago) were stored at 20 °C in air containing ethylene at <0.005, 0.01, 0.1, 1.0, or 10 μL·L-1 and the level of sprouting was measured over 35 days. The time for tubers to develop an average of one sprout per tuber was found to linearly increase as the log10 ethylene concentration decreased with the effect present over the whole range of concentration. After 35 days of storage, the number of sprouts/tuber was inversely related to the ethylene concentration, but the weight of sprouts was only lower for tubers held in <0.005 μL·L-1 ethylene. The more numerous sprouts on tubers held in 10 μL·L-1 ethylene were short and thick, while the less numerous sprouts on tubers in 0.01-1.0 μL·L-1 were long, thin, and branched, and resulted in no significant difference in total sprout weight between these concentrations. Reducing the concentration of ethylene in the atmosphere around stored potatoes thus reduced sprouting, but levels <0.01 μL·L-1 are required to minimize both sprout emergence and sprout growth.

Effects of ethylene and 1-MCP on the quality and storage life of strawberries

Strawberry quality declines rapidly after harvest. Deterioration may be accelerated by ethylene and is potentially increased, decreased or unaffected by the ethylene inhibitor 1-MCP (1-methylcyclopropene). We have examined the effects of 0.01, 0.05, 0.1 and 1 m ll 1 of ethylene and 0.01, 0.1, and 1.0 m ll 1 1-MCP on the quality attributes and respiration rates of strawberries stored at 0 or 5 8C. Ethylene did not affect the rate of rot development. However, calyx quality was significantly reduced by exposure to 0.1 or 1.0 m ll 1 ethylene. Treatment with 1 m ll 1 1-MCP protected the calyx tissue from these effects. Exposure of strawberries to 0.01, 0.1 or 1.0 m ll 1 1-MCP did not affect overall fruit acceptability but did slightly increase the rate of rot development. 1-MCP treatment reduced ethylene production by the fruit. Increased production of CO2 by 1-MCP treated fruit was associated with the earlier onset of rots. Although the results suggest that blocking ethylene perception interferes with disease resistance in strawberries, there was only a small effect on total storage life. It was concluded that neither the removal of low levels of ethylene from the storage environment nor the treatment with 1-MCP are likely to be cost effective methods of extending strawberry storage life. It was concluded that neither removing low levels of ethylene from the storage environment nor treating with 1-MCP are likely to be cost-effective methods of extending storage life. # 2002 Elsevier Science B.V. All rights reserved.

THE EFFECTS OF CHILLING TEMPERATURES ON JUICINESS AND ETHYLENE EVOLUTION IN PEACH FRUIT

During the past three years we conducted a series of studies on peach storageability. Longitudinal studies using incubators set at 0, 2.5, 5, 7.5 and 10°C showed that less juice was extractable after storage at temperatures below 7.5°C. Fruits held at 7.5C° or greater were capable of fully softening in storage, while fruit held at 5°C softened less. Studies demonstrated that storage at 2.5°C or less suppressed ethylene production dramatically. Shipping-mature 'Redhaven' fruits were successfully held for up to six weeks in storage at 0°C. After holding for seven days at room temperature, fruit still softened and became juicy. Ethylene evolution also increased dramatically at room temperature. Six weeks of storage at 0°C did not appear to damage the fruit's ability to evolve ethylene. Short-term studies of the effect of temperature on ethylene were conducted at temperatures from 0 to 20°C. After equilibration, ethylene evolution was measured. Preliminary plots of ethylene production appeared to be log-linear and did not show an inflection point at chilling temperatures. High Q(10) values were measured for ethylene production in peach and nectarine. This temperature response may contribute to the development of storage disorders. Our working hypothesis was that flesh mealiness resulted from 'chilling injury.' While it was an attractive hypothesis, it was difficult to prove. Commercial observations of mealiness in late-harvested fruit would also argue against chilling injury. In hindsight, storage recommendations of 0°C rather than 5°C also seem illogical if mealiness is a symptom of chilling injury. We postulated that peach and nectarine disorders that occur in storage at 5°C are caused by the production of low levels of ethylene that induce some, but not all, ripening-related genes. Mealiness and the lack of juiciness may occur in response to senescent breakdown, rather than chilling injury.

Ethylene biosynthesis in Botrytis cinerea

Ethylene is often released during plant pathogenesis. Enhanced ethylene biosynthesis by the attacked plant, and formation of ethylene by the attacking pathogen may be involved. We defined the biosynthetic pathway of ethylene in the pathogenic fungus Botrytis cinerea, and characterized the conditions that affect ethylene production in vitro. During the first 48 h of culture the fungus uses methionine to produce alpha-keto gamma-methylthiobutyric acid (KMBA) and secretes it to the medium. In darkness, KMBA accumulates in the medium. In light KMBA is photo-oxidized and ethylene is released. The photo-oxidation reaction is spontaneous and does not involve any enzymatic activity. Low levels of ethylene are produced in darkness between 48 and 96 h of culture. Adding peroxidase to dark-grown cultures induced ethylene formation. The results suggest that formation and secretion of KMBA by B. cinerea may affect ethylene levels during plant infection.

Sensitivity of Wheat and Rice to Low Levels of Atmospheric Ethylene

Ethylene (C2H4) gas is produced throughout the life cycle of plants and can accumulate in closed growth chambers to levels 100 times higher than in outside environments. Elevated atmospheric C2H4 can cause a variety of abnormal responses, but the sensitivity to elevated C2H4 is not well characterized. We evaluated the C2H4 sensitivity of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) in five studies. The first three studies compared the effects of continuous C2H4 levels ranging from 0 to 1000 nmol mol−1 (ppb) in a growth chamber throughout the life cycle of the plants. A short‐term 1000 nmol mol−1 treatment was included in which exposure was stopped at anthesis. Yield was reduced by 36% in wheat and 63% in rice at 50 nmol mol−1 and both species were virtually sterile when continuously exposed to 1000 nmol mol−1 However, the yield reductions were much less with exposure that stopped at anthesis, suggesting the detrimental effect of C2H4 on yield was greatest around the time of seed set. Two additional studies evaluated the differential sensitivity of two wheat cultivars (Super Dwarf and USU‐Apogee) to 50 nmol mol−1 C2H4 at three CO2 levels [350, 1200, 5000 μmol mol−1 (ppm)] in a greenhouse. Yield of USU‐Apogee was not significantly reduced by C2H4 but the yield of Super Dwarf was reduced by 60%. Elevated CO2 did not influence the sensitivity to C2H4 A difference in the C2H4 sensitivity of USU‐Apogee between greenhouse and growth chamber trials suggests that C2H4 sensitivity is dependent on the environment. Collectively, the data suggest that relatively low levels of C2H4 could induce anomalous plant responses by accumulation in greenhouses and growth chambers with inadequate ventilation. The data also suggest that C2H4 sensitivity can be reduced by both genetic and environmental manipulations.

Ethylene biosynthesis in Botrytis cinerea

Ethylene is often released during plant pathogenesis. Enhanced ethylene biosynthesis by the attacked plant, and formation of ethylene by the attacking pathogen may be involved. We defined the biosynthetic pathway of ethylene in the pathogenic fungus Botrytis cinerea, and characterized the conditions that affect ethylene production in vitro. During the first 48 h of culture the fungus uses methionine to produce α-keto γ-methylthiobutyric acid (KMBA) and secretes it to the medium. In darkness, KMBA accumulates in the medium. In light KMBA is photo-oxidized and ethylene is released. The photo-oxidation reaction is spontaneous and does not involve any enzymatic activity. Low levels of ethylene are produced in darkness between 48 and 96 h of culture. Adding peroxidase to dark-grown cultures induced ethylene formation. The results suggest that formation and secretion of KMBA by B. cinerea may affect ethylene levels during plant infection.

Phytoremediation of arsenate contaminated soil by transgenic canola and the plant growth-promoting bacterium Enterobacter cloacae CAL2

It was previously observed that transgenic tomato plants that express the Enterobacter cloacae UW4 1-aminocyclopropane-1-carboxylate (ACC) deaminase (EC 4.1.99.4) gene, and thereby produce lower levels of ethylene, were partially protected from the deleterious effects of six different metals. However, since tomato plants are unlikely to be utilized in the phytoremediation of contaminated terrestrial sites, transgenic canola ( Brassica napus) plants that constitutively express the same gene were generated and tested for their ability to proliferate in the presence of high levels of arsenate in the soil and to accumulate it in plant tissues. The ability of the plant growth-promoting bacterium E. cloacae CAL2 to facilitate the growth of both non-transformed and ACC deaminase-expressing canola plants was also tested. In the presence of arsenate, in both the presence and absence of the added plant growth-promoting bacterium, transgenic canola plants grew to a significantly greater extent than non-transformed canola plants.

Aminoethoxyvinylglycine, Combined with Ethephon, Can Enhance Red Color Development without Over-ripening Apples

AVG, as ReTain™, an inhibitor of ethylene biosynthesis, was used alone or with a subsequent application of ethephon (Ethrel™), an ethylene-releasing chemical, to determine if red color development could be enhanced without over-ripening `Gala' and `Jonagold' apples. Treatments included: 1) AVG alone; 2) AVG followed by ethephon; 3) ethephon alone; and 4) control. Silwet L-77 surfactant was included in all treatments. Application of AVG delayed the onset of the ethylene climacteric and red color development of both cultivars. Application of AVG followed by ethephon similarly delayed the onset of the ethylene climacteric, but red color development at the commercial harvest date was only marginally reduced or not affected. The results were similar in both 1998 and 1999, although environmental stress during the growing seasons differed (1998—heat; 1999—moderate temperatures). The delay of fruit maturation and ripening observed at harvest following AVG +/- ethephon treatments improved storability of fruit in controlled atmosphere (CA) storage, as demonstrated by low internal ethylene levels after storage, and high retention of flesh firmness and shelf-life, while control fruit and those treated only with ethephon entered the ethylene climacteric during storage, and flesh firmness subsequently declined during shelf-life evaluation. Chemical name used: aminoethoxyvinylglycine (AVG).

Ripening-Associated Microstructural Changes in Antisense ACC Synthase Tomato Fruit

The ultrastructural impact of low ethylene biosynthesis (less than 0.5% of normal levels) was evaluated in transgenic (A11.1) tomatoes ( Lycopersicon esculentumMill.) expressing an antisense 1-aminocyclopropane-1-carboxylic acid synthase (ACC-S) transgene by means of transmission and environmental scanning electron microscopy. In 48-day mature green fruit, no significant ultrastructural differences were found between transgenic and control tomatoes. In 78-day control fruit, which were overripe and showed deteriorated texture, many areas of the cytoplasm were devoid of structures, and micrographs showed cell collapse with folding and dissolution of the cell wall. On the other hand, in 90-day transgenic fruit, which were firm and not ripe, the cytoplasm showed a relatively high electron density. Plastids retained remnants of chloroplast thylakoids along with significant amounts of osmiophylic plastoglobuli, but lycopene was not detected. Conspicuous starch granules were observed in mature green transgenic tomatoes, but were not detected in 90-day chlorochromoplasts. Electron-dense regions reflecting the integrity of the middle lamella alternated with other partially degraded regions. This incipient dissolution of the middle lamella pectic polymers may be attributable to nonenzymatic deaggregation or to cell-wall hydrolases which could be ethylene independent or responsive to very low levels of ethylene. Besides, cells were attached along extended contact areas and appeared turgid. This feature may provide an explanation of firmness retention that does not solely involve cell walls. Disruption of the middle lamella and development of lycopene crystalloids were observed when exogenous ethylene (12 ppm) was applied.

The Arabidopsis eer1 mutant has enhanced ethylene responses in the hypocotyl and stem.

By screening for enhanced ethylene-response (eer) mutants in Arabidopsis, we isolated a novel recessive mutant, eer1, which displays increased ethylene sensitivity in the hypocotyl and stem. Dark-grown eer1 seedlings have short and thick hypocotyls even in the absence of added ethylene. This phenotype is suppressed, however, by the ethylene biosynthesis inhibitor 1-aminoethoxyvinyl-glycine. Following ethylene treatment, the dark-grown eer1 hypocotyl response is greatly exaggerated in comparison with the wild type, indicating that the eer1 phenotype is not simply due to ethylene overproduction. eer1 seedlings have significantly elevated levels of basic-chitinase expression, suggesting that eer1 may be highly sensitive to low levels of endogenous ethylene. Adult eer1 plants display exaggerated ethylene-dependent stem thickening, which is an ethylene response previously unreported in Arabidopsis. eer1 also has enhanced responsiveness to the ethylene agonists propylene and 2,5-norbornadiene. The eer1 phenotype is completely suppressed by the ethylene-insensitive mutation etr1-1, and is additive with the constitutive ethylene-response mutation ctr1-3. Our findings suggest that the wild-type EER1 product acts to oppose ethylene responses in the hypocotyl and stem.

Ripening of climacteric fruits initiated at low ethylene levels

Mature, unripe mango, peach, custard apple, kiwifruit and tomato were stored at 20˚C in air containing ethylene at <0.005, 0.01, 0.1 1.0 and 10 L/L. The time to ripen of all the climacteric fruits increased linearly with logarithmic decrease in ethylene concentration over the whole concentration range examined. Similar observations were also obtained with kiwifruit and custard apple held at 0 and 14˚C, respectively. However, the sensitivity of fruits to ethylene varied with banana and kiwifruit > custard apple and mango > tomato, avocado and peach. Since the ethylene level around horticultural produce during marketing is always >0.005 L/L, the time climacteric fruit can be held in an unripe condition is currently less than optimal but intervention to limit ethylene action would appear to be only warranted for the most sensitive fruits.

Non‐invasive photoacoustic spectroscopic determination of relative endogenous nitric oxide and ethylene content stoichiometry during the ripening of strawberries Fragaria anannasa (Duch.) and avocados Persea americana (Mill.)1

Employing non-invasive photoacoustic spectrometry, emissions of nitric oxide (NO) and ethylene in post-harvest strawberries and avocados were monitored. A clear-cut stoichiometric relationship was found between the two gases: unripe fruit manifesting high NO and low ethylene levels-the converse in ripe fruit. Findings are discussed in the light of putative control of ethylene-promoted fruit senescence by endogenous NO.

Non‐invasive photoacoustic spectroscopic determination of relative endogenous nitric oxide and ethylene content stoichiometry during the ripening of strawberries Fragaria anannasa (Duch.) and avocados Persea americana (Mill.) 1

Employing non‐invasive photoacoustic spectrometry, emissions of nitric oxide (NO) and ethylene in post‐harvest strawberries and avocados were monitored. A clear‐cut stoichiometric relationship was found between the two gases: unripe fruit manifesting high NO and low ethylene levels—the converse in ripe fruit. Findings are discussed in the light of putative control of ethylene‐promoted fruit senescence by endogenous NO.

Soil compaction. A role for ethylene in regulating leaf expansion and shoot growth in tomato?

The role of ethylene in regulating growth in tomato (Lycopersicon esculentum Mill.) during compaction stress was examined using wild-type (cv Ailsa Craig) and transgenic (ACO1(AS)) genotypes; the latter has a reduced capacity to produce ethylene. Ethephon or silver ions were applied to increase ethylene production or block its action. Shoot growth in both genotypes was comparable in uncompacted (1.1 g cm(-3)) and uniformly compacted soil (1.5 g cm(-3)). However, a 1.1/1.5-g cm(-3) split-pot treatment invoked marked genotypic differences: growth was reduced in cv Ailsa Craig but was comparable to uncompacted control plants in ACO1(AS). As xylem sap abscisic acid levels were similar, abscisic acid was not responsible for inhibiting growth in cv Ailsa Craig. These genotypic differences in growth were accompanied by increased ethylene evolution in cv Ailsa Craig, suggesting that the ability of ACO1(AS) to maintain growth in the split-pot treatment reflected its lower ethylene levels, a view supported by the observation that excising the roots in the compacted compartment reduced ethylene evolution and restored shoot growth in cv Ailsa Craig. Treatment with silver restored shoot growth in cv Ailsa Craig, whereas treatment with ethephon reduced growth in ACO1(AS). Thus, ethylene apparently has a key role in determining growth when tomato plants encounter differential soil compaction.