Postharvest Variation in Cell Wall-Degrading Enzymes of Papaya (Carica papaya L.) during Fruit Ripening

Pectin methylesterase (PME), polygalacturonase (PG), xylanase, cellulase, and proteinase activity were determined and related to respiration, ethylene evolution, and changes in skin color of papaya (Carica papaya L.) fruit from harvest through to the start of fruit breakdown. PME gradually increased from the start of the climacteric rise reaching a peak 2 days after the respiratory peak. PG and xylanase were not detectable in the preclimacteric stage but increased during the climacteric: during the post climacteric stage, the PG declined to a level one-quarter of peak activity with xylanase activity returning to zero. Cellulase activity gradually increased 3-fold after harvest to peak at the same time as PME, 2 days after the edible stage. Proteinase declined throughout the climacteric and postclimacteric phases. A close relationship exists between PG and xylanase and the rise in respiration, ethylene evolution, and softening. Cultivar differences in postclimacteric levels of enzymic activity were not detected.

An inhibitor of cellulase activity was detected in preclimacteric fruit. The inhibitor was not benzyl isothiocyanate (BITC). BITC did inhibit PG activity, though no inhibitor of PG activity was detected in preclimacteric homogenates when BITC was highest. The results indicate that inhibitors did not play a direct role in controlling wall softening.

     

Effects of aminoethoxyvinylglycine and countereffects of ethylene on ripening of bartlett pear fruits

Pear fruits (Pyrus communis L. var. Bartlett) were treated with solutions containing aminoethoxyvinylglycine (AVG) using a modified vacuum infiltration method that introduced 4.3 milliliters solution per 100 grams tissue. At concentrations of 1 millimolar, AVG strongly inhibited ethylene production and delayed for 5 days the respiratory climacteric and accompanying ripening changes in skin color and flesh firmness. AVG was less effective in inhibiting the ripening of more mature fruits. Fruit infiltrated with 5 millimolar AVG had not begun to ripen 12 days after initiation of ripening in the controls. When treated with ethylene the inhibited fruit exhibited a climacteric rise in respiration, softened, and became yellow. Treatment of the AVG infiltrated fruits with ethyelne for 24 hours resulted in no recovery in endogenous ethylene production, but in a stimulation of protein synthesis measured as a 200% increase in leucine incorporation by excised tissue and a 74% increase in the percentage of ribosomes present as polysomes.     

Glycolysis at the climacteric of bananas.

This work was carried out to investigate the relative roles of phosphofructokinase and pyrophosphate-fructose-6-phosphate 1-phosphotransferase during the increased glycolysis at the climacteric in ripening bananas (Musa cavendishii Lamb ex Paxton). Fruit were ripened in the dark in a continuous stream of air in the absence of ethylene. CO2 production, the contents of glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, phosphoenolpyruvate and PPi; and the maximum catalytic activities of pyrophosphate-fructose-6-phosphate 1-phosphotransferase, 6-phosphofructokinase, pyruvate kinase and phosphoenolpyruvate carboxylase were measured over a 12-day period that included the climacteric. Cytosolic fructose-1,6- bisphosphatase could not be detected in extracts of climacteric fruit. The peak of CO2 production was preceded by a threefold rise in phosphofructokinase, and accompanied by falls in fructose 6-phosphate and glucose 6-phosphate, and a rise in fructose 1,6-bisphosphate. No change in pyrophosphate-fructose-6-phosphate 1-phosphotransferase or pyrophosphate was found. It is argued that phosphofructokinase is primarily responsible for the increased entry of fructose 6-phosphate into glycolysis at the climacteric.     

Internal carbon dioxide and ethylene levels in ripening tomato fruit attached to or detached from the plant

The carbon dioxide and ethylene concentrations in tomato fruit ( Lycopersicon esculentum cv. Castelmart) and their stage of ripeness (characteristic external color changes) were periodically measured in fruit attached to and detached from the plant. An external collection apparatus was attached to the surface of individual tomato fruit to permit non-destructive sampling of internal gases. The concentration of carbon dioxide and ethylene in the collection apparatus reached 95% of the concentration in the fruit after 8 h. Gas samples were collected every 24 h. A characteristic climacteric surge in carbon dioxide (2-fold) and ethylene (10-fold) concentration occurred coincident with ripening of detached tomato fruit. Fruit attached to the plant exhibited a climacteric rise in ethylene (20-fold) concentration during ripening, but only a linear increase in carbon dioxide concentration. The carbon dioxide concentration increases in attached fruit during ripening, but the increase is a continuation of the linear increase seen in both attached and detached fruit before ripening and does not exhibit the characteristic pattern normally associated with ripening climacteric fruit. In tomato fruit, it appears that a respiratory climacteric per se, which has been considered intrinsic to the ripening of certain fruit, may not be necessary for the ripening of "climacteric" fruit at all, but instead may be an artifact of using harvested fruit.     

Respiration during Postharvest Development of Soursop Fruit, Annona muricata L

Fruit of soursop, Annona muricata L., showed increased CO₂ production 2 days after harvest, preceding the respiratory increase that coincided with autocatalytic ethylene evolution and other ripening phenomena. Experiments to alter gas exchange patterns of postharvest fruit parts and tissue cylinders had little success.

The respiratory quotient of tissue discs was near unity throughout development. 2,4-Dinitrophenol uncoupled respiration more effectively than carbonylcyanide m-chlorophenylhydrazone; 0.4 millimolar KCN stimulated, 4 millimolar salicylhydroxamic acid slightly inhibited, and their combination strongly inhibited respiration, as did 10 millimolar NaN₃. Tricarboxylic acid cycle members and ascorbate were more effective substrates than sugars, but acetate and glutarate strongly inhibited.

Disc respiration showed the same early peak as whole fruit respiration; this peak is thus an inherent characteristic of postharvest development and cannot be ascribed to differences between ovaries of the aggregatetype fruit. The capacity of the respiratory apparatus did not change during this preclimacteric peak, but the contents of rate-limiting malate and citrate increased after harvest.

It is concluded that the preclimacteric rise in CO₂ evolution reflects increased mitochondrial respiration because of enhanced supply of carboxylates as a substrate, probably induced by detachment from the tree. The second rise corresponds with the respiration during ripening of other climacteric fruits.

     

Comparative effects of acetylene and ethylene gas on initiation of banana ripening

Bananas were exposed to acetylene or ethylene at 0·01, 0·1 and 1 ml/litre, under high humidity, for 24 h at 18 °C. They were then transferred to an atmosphere of air alone for a further 4 days and during this period the respiration rate of three fruit from each treatment was measured. Ripeness was then assessed by colour score and soluble solids content. All levels of ethylene initiated ripening. Treatment with ethylene induced a climacteric rise in respiration, an increase in the soluble solids content of the pulp and degreening of the peel. All levels of acetylene, except 0·01 ml/litre, induced a climacteric rise in respiration. Fruit treated with acetylene at 1 ml/litre had a similar colour score and soluble solids content to those ripened by exposure to ethylene. Fruits treated with acetylene at 0·1 ml/litre had a lower soluble solids content and their peel remained green. Treatment with acetylene at 0·01 ml/litre failed to initiate ripening. Sensory evaluation of fruit ripened by acetylene at 1 ml/litre indicated that the acetylene treated fruit ripened slightly more slowly. When compared at the same stage of ripeness fruits from the two treatments were equally palatable.     

Papaya fruit softening, endoxylanase gene expression, protein and activity

Papaya ( Carica papaya L.) cell wall matrix polysaccharides are modified as the fruit starts to soften during ripening and an endoxylanase is expressed that may play a role in the softening process. Endoxylanase gene expression, protein amount and activity were determined in papaya cultivars that differ in softening pattern and in one cultivar where softening was modified by the ethylene receptor inhibitor 1-methylcyclopropene (1-MCP). Antibodies to the endoxylanase catalytic domain were used to determine protein accumulation. The three papaya varieties used in the study, 'Line 8', 'Sunset', and 'Line 4-16', differed in softening pattern, respiration rate, ethylene production and showed similar parallel relationships during ripening and softening in endoxylanase expression, protein level and activity. When fruit of the three papaya varieties showed the respiratory climacteric and started to soften, the level of endoxylanase gene expression increased and this increase was related to the amount of endoxylanase protein at 32 kDa and its activity. Fruit when treated at less than 10% skin yellow stage with 1-MCP showed a significant delay in the respiratory climacteric and softening, and reduced ethylene production, and when ripe was firmer and had a 'rubbery' texture. The 1-MCP-treated fruit that had the 'rubbery' texture showed suppressed endoxylanase gene expression, protein and enzymatic activity. Little or no delay occurred between endoxylanase gene expression and the appearance of activity during posttranslational processing from 65 to 32 kDa. The close relationship between endoxylanase gene expression, protein accumulation and activity in different varieties and the failure of the 1-MCP-treated fruit to fully soften, supported de novo synthesis of endoxylanase, rapid posttranslation processing and a role in papaya fruit softening.     

Involvement of wound and climacteric ethylene in ripening avocado discs

Avocado (Persea americana Mill. cv Hass) discs (3 mm thick) ripened in approximately 72 hours when maintained in a flow of moist air and resembled ripe fruit in texture and taste. Ethylene evolution by discs of early and midseason fruit was characterized by two distinct components, viz. wound ethylene, peaking at approximately 18 hours, and climacteric ethylene, rising to a peak at approximately 72 hours. A commensurate respiratory stimulation accompanied each ethylene peak. Aminoethoxyvinyl glycine (AVG) given consecutively, at once and at 24 hours following disc preparation, prevented wound and climacteric respiration peaks, virtually all ethylene production, and ripening. When AVG was administered for the first 24 hours only, respiratory stimulation and softening (ripening) were retarded by at least a day. When AVG was added solely after the first 24 hours, ripening proceeded as in untreated discs, although climacteric ethylene and respiration were diminished. Propylene given together with AVG led to ripening under all circumstances. 2,5-Norbornadiene given continuously stimulated wound ethylene production, and it inhibited climacteric ethylene evolution, the augmentation of ethylene-forming enzyme activity normally associated with climacteric ethylene, and ripening. 2,5-Norbornadiene given at 24 hours fully inhibited ripening. When intact fruit were pulsed with ethylene for 24 hours before discs were prepared therefrom, the respiration rate, ethylene-forming enzyme activity buildup, and rate of ethylene production were all subsequently enhanced. The evidence suggests that ethylene is involved in all phases of disc ripening. In this view, wound ethylene in discs accelerates events that normally take place over an extended period throughout the lag phase in intact fruit, and climacteric ethylene serves the same ripening function in discs and intact fruit alike.     

Nitric oxide on/off in fruit ripening

Fruit ripening is a complex physiological process involving significant external and internal modifications. Classic edible fleshy fruits have been classified as climacteric or non‐climacteric according to their dependence on the phyto hormone ethylene; however, data have increasingly confirmed the involvement of the free radical nitric oxide (NO) in this process. Moreover, the exogenous application of NO demonstrates its beneficial effects on fruit quality.     

Role of Brassinosteroids, Ethylene, Abscisic Acid, and Indole-3-Acetic Acid in Mango Fruit Ripening

Rapid ripening of mango fruit limits its distribution to distant markets. To better understand and perhaps manipulate this process, we investigated the role of plant hormones in modulating climacteric ripening of ??Kensington Pride?? mango fruits. Changes in endogenous levels of brassinosteroids (BRs), abscisic acid (ABA), indole-3-acetic acid (IAA), and ethylene and the respiration rate, pulp firmness, and skin color were determined at 2-day intervals during an 8-day ripening period at ambient temperature (21?±?1°C). We also investigated the effects of exogenously applied epibrassinolide (Epi-BL), (+)-cis, trans-abscisic acid (ABA), and an inhibitor of ABA biosynthesis, nordihydroguaiaretic acid (NDGA), on fruit-ripening parameters such as respiration, ethylene production, fruit softening, and color. Climacteric ethylene production and the respiration peak occurred on the fourth day of ripening. Castasterone and brassinolide were present in only trace amounts in fruit pulp throughout the ripening period. However, the exogenous application of Epi-BL (45 and 60?ng?g^?1 FW) advanced the onset of the climacteric peaks of ethylene production and respiration rate by 2 and 1?day, respectively, and accelerated fruit color development and softening during the fruit-ripening period. The endogenous level of ABA rose during the climacteric rise stage on the second day of ripening and peaked on the fourth day of ripening. Exogenous ABA promoted fruit color development and softening during ripening compared with the control and the trend was reversed in NDGA-treated fruit. The endogenous IAA level in the fruit pulp was higher during the preclimacteric minimum stage and declined during the climacteric and postclimacteric stages. We speculate that higher levels of endogenous IAA in fruit pulp during the preclimacteric stage and the accumulation of ABA prior to the climacteric stage might switch on ethylene production that triggers fruit ripening. Whilst exogenous Epi-BL promoted fruit ripening, endogenous measurements suggest that changes in BRs levels are unlikely to modulate mango fruit ripening.     

Immersion of 'Coscia' pear fruit in water at 55 degrees C for 60 sec controls Penicillium expansum decay and delays ripening during short storage

'Coscia' is an early ripening pear with a short postharvest Life (1 month) and chemical treatments to prevent decay are generally not undertaken. This, along with the fast deterioration under shelf-life conditions, makes it difficult to contain postharvest moulds. 1-methylcyclopropene (1-MCP) has been employed with success to delay ripening and, as a co-effect, the development of decay was contained, but this treatment is not allowed for organically grown crops. Here we report the results of an alternative approach employing immersion treatments in water at 45, 50, 55 or 60 degrees C for 0 (control), 15, 30, 45 or 60 sec. Fruit was harvested after the climacteric peak, immediately subjected to the heat treatment and stored for 2 weeks at 1 degrees C followed by a 3 days of a simulated shelf-life at 17 degrees C and 75% RH. Half of the fruit was wounded (3 x 3mm) and inoculated with Penicillium expansum (20 microL of 10(4) conidia mL(-1). Decay inhibition and fruit appearance, rated from 0 to 3 (0 = excellent; 1 = good; 2 = scarce; 3 = not marketable), were monitored and compared to the control after storage and shelf-life. All heat treatments affected the mould development when performed for 45 or 60 sec. The best result in terms of decay control and appearance after storage and shelf-life occurred when fruit was immersed at 55 degrees C for 60 sec with a decay reduction of about 85% compared to control (75% decay) and with a good appearance.     

钙对香蕉采后果实呼吸的影响

本实验对香蕉成熟过程中水溶性钙的变化及CaCl_2溶液真空浸渗处理对香蕉果实呼吸作用的影响进行研究,结果表明,香蕉成熟期间,水溶性钙含量随成熟度的提高而增加,果实呼吸跃变上升前期,水溶性钙的增加尤为明显,果皮和果肉中水溶性钙含量与采后成熟天数呈正相关。以0.05mol/l和0.1mol/l CaCl_2溶液真空浸渗,可使香蕉果实呼吸跃变高峰延迟,但没有明显降低跃变峰值。     

Impact of nitric oxide on shelf life and quality of nectarine (<Emphasis Type="Italic">Prunus persica</Emphasis> var. <Emphasis Type="Italic">nucipersica</Emphasis>)

Nectarine is an emerging fruit crop in India which has immense nutritional quality and fairly good amount of antioxidants. In India, peach orchards are being replaced by nectarine primarily because of fuzzless peel. However, shelf life of nectarine is lean due to its climacteric behaviour. Hence, we attempted to observe the effect of nitric oxide (NO), using its donor compound, namely sodium nitroprusside (SNP) on postharvest life and quality of ‘Silver Queen’ nectarine fruit. In this study, fruit of ‘Silver Queen’ nectarine were treated with various concentrations of SNP (0.25 mM, 0.5 mM, 1.0 mM, 1.5 mM) after harvesting at climacteric stage of maturity. Fruit were stored at ambient condition for further analysis on daily basis, after air drying at room temperature. Our results revealed that among the various concentrations of SNP, 0.5 mM was found to be best in reducing physiological loss in weight (PLW), maintaining firmness and retaining higher phenolics, antioxidant activity, exhibiting slower increase in lipoxygenase (LOX) and pectin methylesterase (PME) activity and better quality fruit up to 8 days than 4 days of untreated fruit. Hence, postharvest dip of ‘Silver Queen’ nectarine fruit in 0.5 mM solution of SNP could be recommended for enhancing the shelf life by 4 days.     

Inhibition of the ethylene response by 1-MCP in tomato suggests that polyamines are not involved in delaying ripening, but may moderate the rate of ripening or over-ripening

Ethylene initiates the ripening and senescence of climacteric fruit, whereas polyamines have been considered as senescence inhibitors. Ethylene and polyamine biosynthetic pathways share S-adenosylmethionine as a common intermediate. The effects of 1-methylcyclopropene (1-MCP), an inhibitor of ethylene perception, on ethylene and polyamine metabolism and associated gene expression was investigated during ripening of the model climacteric fruit, tomato (Solanum lycopersicum L.), to determine whether its effect could be via polyamines as well as through a direct effect on ethylene. 1-MCP delayed ripening for 8 d compared with control fruit, similarly delaying ethylene production and the expression of 1-aminocyclopropane-1-carboxylic acid (ACC)-synthase and some ethylene receptor genes, but not that of ACC oxidase. The expression of ethylene receptor genes returned as ripening was reinitiated. Free putrescine contents remained low while ripening was inhibited by 1-MCP, but increased when the fruit started to ripen; bound putrescine contents were lower. The activity of the putrescine biosynthetic enzyme, arginine decarboxylase, was higher in 1-MCP-treated fruit. Activity of S-adenosylmethionine-decarboxylase peaked at the same time as putrescine levels in control and treated fruit. Gene expression for arginine decarboxylase peaked early in non-treated fruit and coincident with the delayed peak in putrescine in treated fruit. A coincident peak in the gene expression for arginase, S-adenosylmethionine-decarboxylase, and spermidine and spermine synthases was also seen in treated fruit. No effect of treatment on ornithine decarboxylase activity was detected. Polyamines are thus not directly associated with a delay in tomato fruit ripening, but may prolong the fully-ripe stage before the fruit tissues undergo senescence.     

Effect of phenolic compounds in Lycopersicon esculeutum on the synthesis of ethylene

Caffeic, coumaric, sinapic and ferulic acids and naringenin were found in green tomato fruit, Chlorogenic acid accounted for 75% of the total phenolics in mature green fruit but only 35% in ripe fruit. There was very little change in the phenolic composition of the flesh of the fruit during ripening, whereas in the skin, naringenin increased markedly at the onset of the climacteric and three unidentified compounds increased during the climacteric rise. The increase in the concentration of naringenin was accompanied by an increase in the production of ethylene in the skin. Investigation of three systems producing ethylene from 4-methylmercapto-2-oxobutyric acid in the presence of peroxidase, showed that only p`-coumaric acid or naringenin were capable of acting as phenolic substrates, the other phenolic compounds being inhibitory.     

Characterization of ripening-regulated cDNAs and their expression in ethylene-suppressed charentais melon fruit

Charentais melons (Cucumis melo cv Reticulatus) are climacteric and undergo extremely rapid ripening. Sixteen cDNAs corresponding to mRNAs whose abundance is ripening regulated were isolated to characterize the changes in gene expression that accompany this very rapid ripening process. Sequence comparisons indicated that eight of these cDNA clones encoded proteins that have been previously characterized, with one corresponding to ACC (1-aminocyclopropane-1-carboxylic acid) oxidase, three to proteins associated with pathogen responses, two to proteins involved in sulfur amino acid biosynthesis, and two having significant homology to a seed storage protein or a yeast secretory protein. The remaining eight cDNA sequences did not reveal significant sequence similarities to previously characterized proteins. The majority of the 16 ripening-regulated cDNAs corresponded to mRNAs that were fruit specific, although three were expressed at low levels in vegetative tissues. When examined in transgenic antisense ACC oxidase melon fruit, three distinct patterns of mRNA accumulation were observed. One group of cDNAs corresponded to mRNAs whose abundance was reduced in transgenic fruit but inducible by ethylene treatment, indicating that these genes are directly regulated by ethylene. A second group of mRNAs was not significantly altered in the transgenic fruit and was unaffected by treatment with ethylene, indicating that these genes are regulated by ethylene-independent developmental cues. The third and largest group of cDNAs showed an unexpected pattern of expression, with levels of mRNA reduced in transgenic fruit and remaining low after exposure to ethylene. Regulation of this third group of genes thus appears to ethylene independent, but may be regulated by developmental cues that require ethylene at a certain stage in fruit development. The results confirm that both ethylene-dependent and ethylene-independent pathways of gene regulation coexist in climacteric fruit.     

Protein synthesis in relation to ripening of pome fruits

Protein synthesis by intact Bartlett pear fruits was studied with ripening as measured by flesh softening, chlorophyll degradation, respiration, ethylene synthesis, and malic enzyme activity. Protein synthesis is required for normal ripening, and the proteins synthesized early in the ripening process are, in fact, enzymes required for ripening. ¹⁴C-Phenylalanine is differentially incorporated into fruit proteins separated by acrylamide gel electrophoresis of pome fruits taken at successive ripening stages. Capacity for malic enzyme synthesis increases during the early stage of ripening. Fruit ripening and ethylene synthesis are inhibited when protein synthesis is blocked by treatment with cycloheximide at the early-climacteric stage. Cycloheximide became less effective as the climacteric developed. Ethylene did not overcome inhibition of ripening by cycloheximide. The respiratory climacteric is not inhibited by cycloheximide. It is concluded that normal ripening of pome fruits is a highly coordinated process of biochemical differentiation involving directed protein synthesis.     

Variability of responses to 1-methylcyclopropene by banana: influence of time of year at harvest and fruit position in the bunch

To examine the effect of early‐climacteric (postripening) 1‐methylcyclopropene (1‐MCP) exposure on the shelf‐life and quality of green Cavendish bananas (Musa acuminata cv. Williams) from the middle section of the bunch, bananas were harvested bimonthly and treated with 100 μL L^?1 ethylene for 2 consecutive days prior to exposure to 0, 100, 300, 1000, 3000 or 10 000 nL L^?1 1‐MCP for 24 h prior to storage at 22°C. 1‐MCP treatment at a concentration of 300 nL L^?1 or above increased banana shelf‐life significantly compared with the control, regardless of the month in which fruit were harvested except March where a higher concentration was needed (3000 nL L^?1). Fruit harvested in May were the most responsive with a greater than twofold increase in shelf‐life. To examine the effect of fruit position in the bunch on 1‐MCP efficacy, green fruit from the top or bottom of bunches were treated with 100 μL L^?1 ethylene for 2 consecutive days prior to early‐climacteric 1‐MCP (300 nL L^?1) exposure for 24 h at 22°C. In spring and autumn but not in summer, application of 1‐MCP to early‐climacteric fruit was more effective in fruit from the top than in those treated from the bottom of the bunch, increasing shelf‐life. Firmness of 1‐MCP‐treated fruit was up to 19% greater than that of the control across the year, except in fruit from the bottom of the bunch. Given that 1‐MCP is less effective in extending the shelf‐life of summer‐harvested fruit (particularly those from the bottom of the bunch), we conclude that preharvest conditions and fruit position in the bunch affect their responsiveness to ethylene and their behaviour during the ripening process.     

Effect of phytohormones on pectate lyase activity in ripening Musa acuminata.

A differential activity peak of pectate lyase (PEL) was observed during ripening of banana fruits (Musa acuminata Harichhal) receiving different hormone treatments. Exposure of fruits to 25 ppm ethylene for 24 h, as well as dipping of M. acuminata fruits in 1 mM 2,4-dichlorophenoxy acetic acid (2,4-D) for 4 h, hastened fruit ripening. Both PEL activity peak and climacteric peak were observed on the 4th and 10th days of treatment with ethylene and 2,4-D, respectively, compared to the 16th day in control fruits. Gibberellic acid (GA) treatment retarded fruit ripening and both PEL activity and climacteric peaks were observed on the 19th day. Treatment of fruits with ethylene or 2,4-D also advanced the appearance of a polygalacturonase (PG) peak and GA delayed its appearance, but the activity peaks always appeared in post-climacteric fruits, in contrast to PEL activity peaks coinciding with the respiratory peaks.     

Metabolism of 1-aminocyclopropane-1-carboxylic acid in ripening apple fruits

In preclimacteric apple fruits ( Malus × domestica Borkh. cv. Golden Delicious) ethylene production is controlled by the rates of 1-aminocyclopropane-1-carboxylic acid (ACC) synthesis, and by its metabolism to ethylene by the ethylene-forming enzyme and to 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) by malonyl CoA-ACC transferase. The onset of the climacteric in ethylene production is associated with an increase in the activity of the ethylene-forming enzyme in the pulp and with a rise in the activity of ACC synthase. Malonyl transferase activity is very high in the skin of immature fruit, decreases sharply before the onset of the climacteric, and remains nearly constant thereafter. More than 40% of the ACC synthesized in the skin and around 5% in the flesh, are diverted to MACC at early climacteric. At the climacteric peak there are substantial gradients in ethylene production between different portions of the tissue, the inner cortical tissues producing up to twice as much as the external tissues. This increased production is associated with, and apparently due to, increased content of ACC synthase. Less than 1% of the synthesized ACC is diverted to MACC in the flesh of climacteric apples. In contrast, the skin contains high activity of malonyl transferase, and correspondingly high levels [1000 nmol (g dry weight)⁻¹] of MACC.