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Some nonmelting flesh (NMF) peaches develop a characteristic off-flavor during postharvest ripening. A study was conducted using NMF genotypes from the Univ. of Florida breeding program to investigate the off-flavor development in melting flesh (MF) and NMF peach genotypes and to determine the compositional changes associated with the development of off-flavor during postharvest ripening at 8 °C. The study revealed that there were certain chemical components that were consistently associated with the occurrence of off-flavor. Generally, there was a significant increase in total soluble phenolics, polyphenoloxidase (PPO) activity and ethanol content with the increase in the percentage of off-flavored fruit with time in storage at 8 °C in NMF genotypes examined. However, total sugars and total soluble solids decreased significantly during the storage period. These changes in chemical composition of NMF genotypes were not observed in MF genotypes, which did not show off-flavor development. Moreover, highly significant linear correlations were detected between off-flavor development and soluble phenolics, PPO activity, ethanol content, total soluble solids, and sugars in Fla. 92-21C and USDA 87P285, which had the highest percentage of off-flavored fruit. Specifically, soluble phenolics, chlorogenic acid, PPO activity, and ethanol were positively correlated, but soluble sugars and soluble solids were negatively correlated with the off-flavor development. Thus, it is suggested that the accumulation of soluble phenolic compounds and ethanol, and the reduction of soluble solids and sugars contribute to the of off-flavor in NMF genotypes.

Temperatures above 30 °C may delay or inhibit germination of most of commercial lettuce cultivars. Ethylene enhances lettuce seed germination at high temperatures. Enzyme-mediated degradation of endosperm cell walls appears to be a crucial factor for lettuce germination at high temperature. The galactomannan polysaccharides in lettuce endosperm cell wall are mobilized by endomannanase. The role of endo-mannanase during germination of lettuce seeds at high temperature (35 °C) and the possible role of etlene in enzyme regulation were investigated. Seeds of thermotolerant (`Everglades'-EVE) and thermosensitive (`Dark Green Boston'-DGB) lettuce genotypes were incubated at 20 and 35 °C in water, 10 mM of 1-aminocyclopropane-1-carboxylic acid (ACC), or 20 mM of silver thiosulphate (STS). Also, seeds were primed in an aerated solution of polyethylene glycol (PEG), or PEG+ACC, or PEG+STS. Untreated seeds germinated 100% at 20 °C. At 35 °C, EVE germinated 100%, whereas DGB germinated only 33%. Seed priming or adding ACC during imbibition increased germination of DGB to 100% at 35 °C. Adding STS during imbibition led to a decrease in germination at 35%C in EVE and completely inhibited germination of DGB. Priming with STS led to reduced germination at 35%C of both genotypes. EVE produced more ethylene than DGB during germination at high temperature. Providing ACC either during priming or during germination led to an increase in endo-mannanase activity, whereas STS inhibited mannanase activity. Higher endo-mannana activity was observed in EVE than DGB seeds. The results suggest that ethylene might overcome the inhibitory effect of high temperature in thermosensitive lettuce seeds via weakening of endosperm due to increased endo-mannanase activity.

Ethylene is integrally involved in the ripening of climacteric fruit. The ability to prevent ethylene action, or manipulate fruit sensitivity to ethylene, would provide a powerful means of extending postharvest storage life of these fruit, particularly for those that ripen rapidly and/or that are not tolerant of low-temperature storage. In this study, 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, was used to investigate ripening, respiration, and ethylene production in avocado fruit. `Monroe' avocados were treated with 1-MCP (Ethylbloc®) for 24 h at 20 °C. The fruit were subsequently stored at 13 or 20 °C. Some fruit were exposed to 100 ppm ethylene at 13 and 20 °C before or after MCP treatment. As evaluated by flesh firmness, respiration rate, and ethylene evolution, 1-MCP completely inhibited the ripening of avocado fruit stored at 13 and 20 °C and 85% relative humidity. Ethylene evolution and respiration rates were dramatically depressed, greater than 95% and 52%, respectively, by 1-MCP. Whereas firmness of control fruit decreased from over 100 N to 10 N in as few as 7 days, fruit treated with 1-MCP remained firm (>45 N) for periods of up to 3 weeks at 13 °C. Treatment of avocado fruit with 100 ppm ethylene at 20 °C for 12 h did not overcome the influence of MCP treatment. Similarly, treatment with ethylene before MCP exposure did not circumvent the effects of the cyclic olefin on ripening. Current studies are addressing the effects of MCP concentration and exposure time on avocado ripening.

Roma tomatoes (`Sunoma') were hand-harvested at the mature-green color stage and treated with 100 μL·L^-1 ethylene for 60 h at 20 °C and 90% RH. Tomatoes at breaker ripeness stage (<10% red coloration) were sorted by weight (about 100 g) and half of the fruits were treated with 1-methylcyclopropene (1-MCP; 1 μL·L^-1 for 24 h at 22 °C). After 1-MCP treatment, individual fruits were subjected to double impacts over the marked locular surface with force equivalent to a 40-cm height drop using a pendulum impactor. In non-1-MCP treated fruit, impacts increased the maximum respiration rate by 27% (to 39.1 mL·kg^-1 per h) and ethylene production by 24% (to 5.5 μL·kg^-1 per h). Treatment with 1-MCP decreased relative production of both CO₂ (56%) and ethylene (54%) over non-1-MCP treated fruit, while the ripening period (as measured by softening and color development) was extended 2.5 times, to about 8 d. Fruits treated with 1-MCP had increased TTA (about 40%; 0.58% citric acid equivalent), decreased pH (5%), and no difference in soluble solids content (3.7 °Brix); double impacts did not affect these values. Double impacts accelerated the onset of polygalacturonase (PG) activity by about 100% (to 99.8 mol·kg^-1 per min*10^-5 D-galacturonic acid) at day 6 over non-impacted control fruit. 1-MCP treatment delayed the onset of increased PG activity by 10 d over non-1-MCP treated fruit. Although 1-MCP alleviated the impact-induced increase in PG activity, PG activity recovered to rates similar to those of non-1-MCP treated fruit during the final 4 d of ripening.

The present study was conducted to explore the process of watersoaking seen previously in beit alpha-type cucumber fruit treated with ethylene. Fruit were harvested at four levels of maturity: Immature (4 to 8 days after anthesis, DAA), Mature (10 to 14 DAA), Breaker (16 to 20 DAA), and Yellow (35 to 40 DAA). Fruit were then stored at 13 °C in the presence of air (control) or either 10 μL·L^-1 ethylene or 1300 μL·L^-1 propylene for up to 12 days. The physiological response to ethylene treatment varied with fruit maturity. Immature-stage fruit treated with ethylene for 9 days had mesocarp watersoaking, epidermal sloughing, and lower hue (118°, control 124°), endocarp pH (4.4, control 5.4), and whole fruit firmness (23 N, control 46 N). Mature-stage fruit behaved similarly to Immature-stage fruit, but lacked mesocarp watersoaking. In contrast, after 9 days of ethylene exposure, the Breaker- and Yellow-stage fruit exhibited no watersoaking, accumulated beta-carotene in peel tissue (13.6 μg·g^-1 F.W, control 0.35 μg·g^-1 F.W.) and had a “melon”-like aroma. Ethylene exposure for all maturities increased respiration rate and decay incidence compared to air-treated fruit. Ethylene evolution was only detectable in fruit with visible decay. Decay incidence in response to ethylene treatment was inversely proportional to maturity at harvest. Watersoaking, exhibited exclusively in Immature fruit, spread inward from the epidermis starting after about 6 days of ethylene treatment. Cells in watersoaked tissue stained negatively for viability with fluorescein diacetate and cells proximal to watersoaked cells stained weakly compared to air-treated controls. Current work is focused on identifying the mechanism of cell death.

Grape tomatoes (Lycopersicon esculentum Mill. `Santa') harvested at light-red (>90% color) and full-red stages were treated with 1 μL·L^–1 1-methylcyclopropene (1-MCP) for 24 hours at 20 °C and stored at 20 °C. After 1 day of storage, fruit harvested at light-red stage treated with 1-MCP had a 56% lower respiration rate than untreated fruit. By day 7, respiration rates of the two treatments had converged at about 2 mL·kg^–1·h^–1. Ethylene production of light-red stage tomatoes treated with 1-MPC was 24% lower than untreated during storage, with rates converging by day 11. For fruit harvested full-red, 1-MCP had similar effects on respiration and ethylene production, although convergence occurred earlier, by day 5. Subsequent tests were conducted only with fruit harvested at full-red stage, since fruit harvested at the light-red stage had lower soluble solids content (4.3%) than fruit harvested at the full-red stage (5.5%). Several combinations of 1-MCP concentrations and exposure times were applied at 20 °C: 1 μL·L^–1 for 24 h, 5 μL·L^–1 for 6 or 12 h, 25 μL·L^–1 for 6 or 12 h, and 50 μL·L^–1 for 6 or 12 h; following the respective pretreatment fruits were stored at 20 °C. 1-MCP pretreatment extended marketable life by 1 d, irrespective of pretreatment regime, where untreated and pretreated fruit remained marketable (<15% of fruit soft, decayed and/or shriveled) for 6 and 7 d, respectively. However, 1-MCP did not affect whole fruit firmness, epidermal color, internal color, soluble solids content (6.5%), total titratable acidity (0.64%), or pH (4.3). In a third test simulating commercial handling procedures, full-red harvested tomatoes were treated with 1 μL·L^–1 1-MCP for 24 h at either 13 or 20 °C, stored for 4 d at 13 °C, and then transferred to 20 °C. Under these conditions, marketable life for untreated and 1-MCP-treated tomatoes was 7 and 8 d, respectively.

The purpose of the present study was to investigate the role of ethylene action, via use of the ethylene antagonist 1-methylcyclopropene (1-MCP), on the senescence and quality of fresh-cut ripe papaya (Carica papaya L. `Sunrise Solo') fruit. Ripe papaya fruit were treated with 2.5 μL·L^-1 1-MCP and immediately processed into fresh-cut slices or left intact. At 2-day intervals over 10 days at 5 °C, continuously stored slices were monitored for ethylene production, firmness, electrolyte leakage, color, sensory changes, and pathogen incidence. Slices freshly prepared from intact fruit stored under identical conditions were measured similarly. Ethylene production did not differ significantly between the treatments, although production rates were slightly but consistently higher in slices from intact control compared with intact 1-MCP-treated fruit. Mesocarp firmness of continuously stored slices and slices from fruit stored intact was significantly retained by 1-MCP. Firmness of continuously stored slices from 1-MCP-treated fruit declined 50% compared with 75% for control slices. Firmness of fresh-cut slices prepared from intact control and 1-MCP-treated fruit at each sampling interval declined 26% and 15%, respectively. Electrolyte leakage remained low and changed little in slices freshly prepared from fruit stored intact. Leakage from continuously stored papaya slices increased after 4 days, and after 6 days controls increased significantly compared with stored slices derived from papaya fruit initially treated with the ethylene antagonist. The flesh color of continuously stored slices or slices prepared from fruit stored intact was influenced by 1-MCP only during the later periods of storage. Microbial counts in stored slices or slices prepared at each sampling were generally unaffected by 1-MCP. Informal sensory analysis indicated that the edible shelf life was 6 days in stored slices from 1-MCP-treated fruit compared with 2 to 3 days for stored slices from control fruit.

Mature green and pink tomato (Lycopersicon esculentum Mill.) fruit were subjected to ionizing irradiation in the range of 0.7 to 2.2 kGy from gamma-or X-ray sources. Firmness of whole fruit and pericarp tissue, pericarp electrolyte leakage, and pericarp cell wall hydrolase activities were measured following irradiation and during postirradiation ripening at 20 °C. Irradiation-induced softening was evident in mature-green and pink fruit within hours following irradiation, and differences between irradiated and control fruit persisted throughout postirradiation storage. Trends of firmness loss were much more consistent and showed much greater dose dependency in pericarp tissue than whole fruit. Irradiation enhanced electrolyte efflux in fruit of both maturity classes. Fruit irradiated at the mature-green stage softened during postirradiation storage but exhibited an apparently irreversible suppression in polygalacturonase activity, with levels remaining <10% of those of nonirradiated fruit. Polygalacturonase activity was less strongly affected in irradiated pink fruit than in mature-green fruit, but activity remained reduced relative to the controls. Pectinmethylesterase and β-galactosidase activities were significantly enhanced in irradiated fruit of both ripening stages in the early period following irradiation, but reductions were noted after prolonged storage.

Weakening of the endosperm tissue around the radicle tip before radicle protrusion and a potential role of endo-β-mannanase during germination of lettuce seeds (Lactuca sativa L.) at high temperature (35 °C) were investigated. Seeds from the thermotolerant genotypes `Everglades' and PI 251245 had greater endo-β-mannanase activity before radicle protrusion at 35 °C than the thermosensitive genotypes `Dark Green Boston', `Valmaine' and `Floricos 83'. Thermotolerant genotypes also generated more ethylene at high temperature. At 35 °C, germination of `Dark Green Boston' and `Everglades' seeds produced at days/nights of 20/10 °C was 10% and 32%, respectively, whereas germination of seeds produced at days/nights of 30/20 °C was 67% and 83%, respectively. Higher endo-β-mannanase activity was observed before radicle protrusion in `Dark Green Boston' seeds produced at 30/20 °C compared with those produced at 20/10 °C. A relationship between seed germination at high temperature, ethylene production, and an increase in endo-β-mannanase activity before radicle protrusion was confirmed.

Abstract

The postharvest behavior of watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] fruit harvested at selected stages of development and stored in air or exposed to 50 μl ethylene/liter or 6500 μl propylene/liter was investigated. Characteristics measured included the effects of ethylene or propylene on ripening, respiration, ethylene production, and fruit firmness. Ethylene treatment induced a rapid deterioration of fruit at all maturation stages, as evidenced by the acute placental tissue softening and watersoaking. Melons of all maturation stages held in air showed little textural change throughout storage and produced only trace quantities of ethylene. Respiratory activity of fruit at each maturation stage was enhanced in the presence of ethylene or propylene and returned to normal rates upon removal of the gases. Ethylene production was not initiated by exposure of fruit to propylene, and was detected only in fruit exhibiting symptoms of decay. The results support the conclusion that watermelon fruit exhibit a nonclimacteric pattern of ripening.