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Abstract
Senescence has long been a concern of postharvest scientists. The essential interests are two-fold. On the one hand, the premature senescence of many commodities, often accelerated due to adverse handling and storage conditions, renders them useless to the consumer. On the other hand, for many other commodities, notably fruits and cut flowers, the early manifestations of senescence form the basis of desirable quality attributes. The objectives of postharvest handling practices in the latter case are to: a) arrest continued developm ent during storage and b) ensure that normal development occurs following storage. For these reasons, an appreciation of the underlying causes of senescence becomes crucial to our understanding of how best to control and regulate the process. The symposium speakers will focus on the mechanisms and control of senescence in three horticultural commodity groups: leafy vegetables, fruits, and cut flowers.
Banana production is geographically isolated from consumer markets in temperate regions. This disparity has prompted study of ethylene antagonists such as 1-methylcyclopropene (1-MCP) to extend marketable shelf life. Banana fruit (Musa acuminata v. Cavendish) were treated with ethylene (100 ppm) in sea containers (24 h,14.4 °C, 90% RH). After venting, one container was provided with 300 ppb 1-MCP (12 h, 15 °C). Controls were maintained in similar containers without 1-MCP. After treatments, ripening was monitored at 18 °C. Color was graded from values of 2 (green) to 7 (yellow, with sugar spots). During storage at 18 °C, control fruit remained within the color range of 4 to 6 (considered the most marketable) for 3 d compared with 6 d for 1-MCP-treated fruit. The time to reach stage 7 occurred at 8 and 13 d, respectively, in control and 1-MCP treated fruit. Sugar spots occurred after 6 to 7 d for both control and 1-MCP-treated fruit. Thereafter, incidence diverged significantly, with 1-MCP-treated fruit remaining below 10% for 11 d and control fruit exceeding 30% by 10 d. Through the first 5 d, firmness (initially 75 N) declined at comparable rates in both treatments, with control fruit declining to 20 N after 15 d. 1-MCP-treated fruit remained near 40 N throughout storage. Ripening variability did not differ within the treatments. Informal sensory analysis showed that some participants preferred the taste and firmness properties of 1-MCP-treated fruit while others preferred the lower firmness of traditionally ripened fruit. The sweetness of treated fruit was only slightly lower than that of control fruit, yet was still considered acceptable. Work in progress is addressing sugar transformations in 1-MCP-treated banana fruit.
The present study was performed to characterize the physiological responses of cantaloupe [Cucumismelo (L.) var. reticulates`Athena'] fruit harvested at preripe (1/4 slip), half-slip, and full-slip stages of development and treated with 1-methylcyclopropene (1-MCP) prior to storage at 13 or 15 °C. Cantaloupe fruit (1/4 to full-slip stage) were treated with 1-MCP (0.01 and 1 μL·L-1) for 18 hours at 20 °C and then stored at 15 °C (pre-ripe fruit) or 13 °C (half- and full-slip fruit). The firmness of pre-ripe `Athena' fruit was significantly retained in response to 1 μL·L-1 1-MCP, but did not differ greatly from control fruit in response to 0.01 μL·L-1 1-MCP. Control fruit reached an edible condition (≈70 N) after 6 days of storage at 15 °C and persisted until day 12 (50 N), whereas 1 μL·L-1 1-MCP-treated fruit reached an edible stage after 17 days and persisted through 21 days (over 60 N). Fruit treated with 1-MCP exhibited slightly (0.01 μL·L-1) or dramatically (1 μL·L-1) lower electrolyte leakage throughout storage. 1-MCP (1 μL·L-1) significantly suppressed ethylene production and respiratory rates of pre-ripe cantaloupe during storage at 15 °C. Firmness retention was also highly significant for cantaloupe harvested and treated with 1-MCP (1 μL·L-1) at the half-slip and full-slip stages of development. 1-MCP treatment had a significant effect at reducing decay incidence and the occurrence of depressed or sunken regions of the fruit surface.
The discovery of 1-methylcyclopropene (1-MCP) as an inhibitor of ethylene action has provided yet another effective tool for understanding the role of this hormone in the development of higher plants. In the nearly 12 years since the description of the effects of this growth regulator on ethylene action, the subsequent introduction of stable formulations has resulted in an explosive increase in 1-MCP-targeted research, particularly, although not exclusively, in the discipline of postharvest science. The vicinal relationship between 1-MCP and postharvest biology is understandable in view of the established roles of ethylene, both beneficial and detrimental, in the handling and storage behaviors of a vast majority of harvested fruit and vegetative organs. The use of 1-MCP is proving to be supplemental to molecular approaches for identifying and understanding the spectrum of senescence and ripening processes under the direct control of ethylene perception. Climacteric fruits have served as the predominant target for investigations of 1-MCP, and the responses of these fruits have confirmed that the antagonist operates in opposition to ethylene. Studies of nonclimacteric fruits challenged with 1-MCP and other ethylene action inhibitors have identified both ethylene-dependent and ethylene-independent ripening processes and have posed interesting questions regarding the canonical distinctions between climacteric and nonclimacteric fruits.
Abstract
Cellulose contents in acetone powders derived from tomato (Lycopersicon esculentum Mill. ‘Sunny’) fruit pericarp and locular gel were measured. In the pericarp, cellulose increased throughout development, whereas Cx-cellulase activity increased during ripening. Locular gel cellulose content was lower than that in pericarp at all developmental stages, increasing through the breaker stage. Beyond the breaker stage, during the terminal period of gel formation, locular gel cellulose decreased. Levels of Cx-cellulase were high in gel tissue, and maximum activity was attained prior to the decrease in gel cellulose indicating that Cx-cellulase and cellulose may be important features of gel formation.
Abstract
Hemicelluloses and polyuronides from the cell wall of ripening tomato (Lycopersicon esculentum Mill. cv. Rutgers) fruit were examined using gel-filtration chromatography. Gel filtration of polyuronides revealed that these polymers were extensively degraded during ripening, as evidenced by the increase in the quantity of polymers that fractionated on the gel. Low molecular weight polyuronides were first evident in fruit harvested at the turning stage and they constituted the major portion of the polyuronides obtained from fruit at more advanced stages of ripening. The appearance of degraded polyuronides corresponded well with the activity of endo-D-galacturonanase, which appeared to be solely responsible for the degradation of these wall polymers. The cell wall hemicelluloses were also affected during ripening; gel-filtration analyses revealed marked changes in the molecular-weight distribution of these polymers. Hemicelluloses from immature green and mature green fruit were similar chromatographically, whereas those from fruit harvested during ripening showed progressively lower quantities of high molecular weight polymers and higher quantities of low molecular weight polymers (<40,000). These changes coincided with the degradation of the pectic polysaccharides; however, in vitro studies using isolated cell wall showed that pectin degradation occurred independently of the changes in hemicelluloses.
Ethylene induces postharvest problems in many fruits, including members of the Cucurbitaceae. The effects of ethylene and its antagonist 1-methylcyclopropene (1-MCP) were studied on cucumber fruit, a cucurbit harvested immature. Beit Alpha cucumbers (cv. Manar) were harvested and exposed to 0.5 or 1.0 μL·L-1 1-MCP prior to exposure to ethylene at 10 μL·L-1 at 15 °C. Fruit exposed to ethylene exhibited significant quality loss upon exposure to ethylene, including epidermal yellowing, a 40% reduction in firmness and, following longer exposure (6 d), epidermal sloughing and placental watersoaking. After 8 d, cucumbers exhibited a 75% reduction in firmness and acute pathogen incidence. Control fruit (no ethylene, no 1-MCP) were visually acceptable but firmness had declined 30% compared with initial values. Fruit exposed to 1-MCP were resistant to applied ethylene, with firmness declining 10 (1.0 μL·L-1 1-MCP) to 20% (0.5 μL·L-1) during storage for 14 d at 15 °C. Firmness of fruit treated with 1-MCP at 1.0 μL·L-1 was 28% higher than controls receiving only air, suggesting that ethylene production contributes to softening of cucumber during storage. The softening and watersoaking of fruit treated with ethylene was accompanied by downshifts in pectin mol mass. Fruit exposed to 1-MCP showed significantly reduced mol mass downshifts. The data indicate that cucumber show a PCD response to ethylene that is largely prevented by suppression of ethylene perception. The response of cucumber to ethylene parallels that for other cucurbits, and provides additional information that the benefits of 1-MCP extend beyond application to climacteric fruits.
Having been hold in 10 μL·L-1 1-MCP or air for 18 h, seedless watermelon (Citrullus lanatus Matsum and Nakai, cv. Millionaire) fruits were cut to obtain tissue cylinders which were rinsed with 2% CaCl2 or deionized water. Respiration rate, ethylene production, firmness, electrolyte leakage, total soluble solids, titratable acidity, microbial growth rates (aerobic bacteria and yeast counts), and activities of ACS, ACO, PLC, PLD, LOX were determined during 7 days at 10°C to investigate the effects of 1-MCP and CaCl2. Ethylene was not detected in cylinders, while wound-induced respiration rates increased over time. Although 1-MCP stimulated ACS activity, it completely inhibited ACO activity and lowered respiration rate. CaCl2 had little effect on ACS activity, but stimulated ACO activity and maintained tissue firmness throughout storage. 1-MCP abolished the effect of CaCl2 in retaining or stimulating PLC, PLD and LOX activities, but inhibited aerobic bacteria synergically with CaCl2. Results suggest that 2% CaCl2 stimulated activities of PLC, PLD and LOX, which are key enzymes catalyzing phospholipid degradation. Results also provide evidence indicating that 1-MCP counteracts CaCl2 in aspect of lipolytic enzymes through unknown mechanisms. Furthermore, we provide results that 1-MCP and CaCl2 have a synergic effect in inhibiting the growth of aerobic bacteria. We suggest that CaCl2 may be applied together with 1-MCP as a complex treatment to extend shelf life of fresh-cut products.
The catalytic capacity of tomato polygalacturonase (PG) toward soluble pectic polymers is in excess of activity expressed in vivo; however, in vitro assays of PG have traditionally been performed under conditions (pH 4.0 to 4.5, 150 mM NaCl) that likely do not reflect the apoplastic environment of ripening tomato fruit. In this study, hydrolysis of pectin by purified tomato PG (isozyme 2) was examined in response to K+ (the predominate apoplastic cation) and over the pH range from 3.0 to 6.0. In the presence of K+, PG activity toward polygalacturonic acid measured reductometrically increased nearly 3.5-fold from pH 4.0 to pH 5.5. In the presence of Na+, activity decreased 90% over the same pH range.
PG-mediated degradation of cell wall from mature-green fruit showed divergent hydrolytic patterns in response to pH and K+. At pH 4.5 in the presence of K+ (as KCl), catalysis resulted in both solubilization and extensive depolymerization of cell wall pectin, with oligomers accounting for a significant portion of the hydrolysis products. At pH 5.5, the total quantity of wall pectin released in response to PG2 was similar to that at pH 4.5; however, oligomer production was strongly suppressed at the higher pH. At pH values favoring extensive depolymerization, low mol mass products were produced at 5 mM K+ and increased to a maximum at 100 mM K+. At higher pH, hydrolysis patterns were not affected by [K+]. pH and ionic effects may contribute to the distinctive patterns of pectin hydrolysis observed for different fruits.
In previous studies, 1-methylcyclopropene (1-MCP) was shown to significantly suppress peel degreening and appearance of senescent spotting of banana fruit (Stanley and Huber, 2004). In the present study, the effect of the ethylene antagonist on banana pulp soluble sugar levels and on peel soluble and total phenolics was measured. One hundred and sixty hands (10 boxes) of banana fruit (Musaacuminata cv. Cavendish) were treated with ethylene (300 μL·L-1, 24 h, 15 °C, 90% RH) at a commercial ripening facility in Bradenton, Fla., and transported by truck (15 °C) to the University of Florida. Fruit were sorted and placed in 174-L ripening chambers, where 80 hands received 500 nL·L-1 1-MCP for two 12-h periods at 18 °C, while the other 80 hands (controls) were maintained in identical containers without 1-MCP for equal time periods at 18 °C. Mean whole fruit firmness in both treatment groups was 140 N and decreased to 15 N (controls) and 30 N (1-MCP) by day 12. Soluble sugars in the pulp of control fruit achieved levels between 160–180 mg·g-1 fresh weight by day 8, while 1-MCP treated fruit required about 12 days to achieve similar soluble sugar levels. Total phenolic compounds present in peel tissue of control and 1-MCP treated fruit required 10 and 14 days, respectively, to achieve levels of about 4000 μg·g-1 fresh weight. Chlorogenic acid levels, a subset of total peel phenolic compounds, peaked above 500 μg·g-1 by day 10 in control fruit and by day 12 in 1-MCP treated fruit. Maintenance of fruit firmness along with the achievement of acceptable sugar levels of 1-MCP treated fruit demonstrate possible benefits of suppression of ethylene action for retail and processing markets for banana fruit.