Abstract

If watermelons are exposed to ethylene, which can easily occur in distribution centers or in mixed loads, the fruit rapidly deteriorate, developing flesh and rind water-soaking and otherwise appearing over-ripe or chilled.  We recently tested the ethylene antagonist 1-methylcyclopropene (SmartFresh^®, AgroFresh, Inc. Philadelphia, PA) on harvested, commercially ripe watermelons (cvs. Mardi Gras and Abbott and Cobb 5244, a seedless type) stored in the presence of ethylene or under commercial holding conditions.  SmartFresh^® treatment gave complete protection against the bad effects of added ethylene.  Watermelon fruit in commercial holding conditions (air storage at 55°F without added ethylene) also benefited significantly from prior treatment with SmartFresh, indicating that the low levels of ethylene accumulating in storage facilities (below 50 ppb) can potentially damage watermelon fruit.  Our results show that SmartFresh^® has potential to prolong the quality of watermelon fruit stored under a range of postharvest conditions.

Introduction

Harvested watermelon (Citrullus lanatus Thunb. Matsum and Nakai) fruit are sensitive to ethylene gas, with injury occurring at concentrations of 1 ppm and lower.  Symptoms of ethylene damage in watermelon fruit include flesh watersoaking and rind softening, rendering the fruit unfit for marketing and consumption.  Since the initial injuries occur internally, the effects of ethylene are not easily determined from visual inspection of intact fruit.  Interestingly, the symptoms of ethylene exposure bear visual similarities to over-ripeness or low-temperature injury.  Accordingly, inadvertent exposure to ethylene, such as could occur during mixed-load shipments or during holding in distribution centers containing ethylene-producing commodities (eg., tomatoes, avocados) might result in injury to watermelon mistakenly attributed to causes other than ethylene.

The recent introduction to the market of SmartFresh^® (AgroFresh, Inc. Philadelphia, PA), an antiethylene product used commercially with apple and other ethylene-producing commodities, has proven effective in prolonging the shelf-life of many fruits.  SmartFresh works by interfering with the ability of commodities to ‘see’ ethylene, reducing the rate of ripening and extending shelf life.  In these cases, the objective is to reduce the rate of ripening and softening of the targeted fruits, while eventually allowing recovery from the effects of SmartFresh and resumption of normal ripening and quality characteristics.

In contrast to the climacteric fruits apples, tomatoes, and avocados that are harvested prior to full ripeness, watermelon fruit are ideally harvested at a ready-to-eat, fully ripe stage of development.  It is at this time that the fruit possess optimum sweetness, aroma, texture, and other quality parameters. For this reason, the goal with watermelon fruit is to reduce or arrest continued development during storage.  Harvested watermelon fruit produce very low levels of ethylene (Elkashif et al., 1989), yet they can be damaged if exposed to ethylene of external sources (Elkashif and Huber, 1988). The first study described in this report was conducted to determine the benefits of SmartFresh treatment on watermelon fruit stored in the presence of added ethylene.

Since it is not uncommon for watermelon fruit to be held at 55°F for up to 2 weeks and longer prior to marketing http://www.ba.ars.usda.gov/hb66/contents.html, a second study was performed to determine if SmartFresh would afford protection against the low levels of ethylene that can accumulate naturally in storage facilities. 

Materials and Methods

Plant Material

Watermelon (Citrullus lanatus cv. Mardi Gras (Rogers brand from Syngenta) or Abbott and Cobb 5244, a seedless cultivar) fruit were harvested when commercially ripe in the Spring of 2001 and 2002 from plants grown at the Suwannee Valley Research Station, Live Oak, FL.  A number of fruit were sliced at the time of harvest to confirm uniformity of ripeness, and the fruit were transported directly to the Postharvest Research facilities in Gainesville. Fruit were selected for uniformity of size and washed with sodium hypochlorite (150 µl l^-1 free chlorine) for postharvest decay control.

 SmartFresh^® Application

In the first study, watermelon fruit (Abbott and Cobb 5244) were placed in 174 liter (about 46 US gallons) steel chambers, sealed, and the containers provided with SmartFresh (5 ppm) generated from a commercial formulation (AgroFresh Inc., a division of Rohm and Haas Co.).  Total exposure time to SmartFresh was 18 hours at 20°C (68°F). Afterward, the containers were opened, vented to release excess SmartFresh, and fruit subsequently exposed to either air or ethylene for 8 days at 68°F.  Fruit were analyzed at intervals during storage.

In the second study, harvested ripe watermelon fruit (cv. Mardi Gras) were exposed to either air or 1 ppm SmartFresh (24 h, 13°C/55°F) and then stored in air for up to 21 d at 55°F.  This study involved no application of external ethylene and was designed to study the effects of SmartFresh application on watermelon fruit held under simulated commercial storage/shipping conditions (2 to 3 weeks at 55°F).

Results and Discussion

SmartFresh Effects on Watermelon Fruit Treated with Exogenous Ethylene

The firmness of Abbott and Cobb 5244 watermelon fruit pretreated with air or SmartFresh [18 h at 20°C (68°F)] followed by storage in air or ethylene at 68°F for 8 days is shown in Fig 1.  Firmness was determined directly on the flesh of sliced fruit. This experiment used a relatively high temperature (68°F) in order to accelerate the effects of ethylene exposure over a relatively short storage period.  As illustrated, watermelon fruit exposed to ethylene (G) deteriorated (softened) rapidly and extensively, being unsuitable for consumption after as few as 2 days exposure to ethylene gas. External symptoms were negligible after only 2 days exposure.  In contrast, SmartFresh-treated fruit (#) were effectively protected from the effects of external ethylene, softening at rates comparable to fruit exposed to air ("). Fig. 2 illustrates the appearance of Abbott and Cobb 5244 watermelon fruit following 3 days exposure to ethylene, either without (left) or with prior exposure to SmartFresh.

SmartFresh Effects on Watermelon Fruit Under Simulated Commercial Conditions

The benefits of SmartFresh treatment were very pronounced for fruit exposed to added ethylene; however, close examination of the data for day 8 of storage (see Fig. 1) shows that the flesh of fruit treated with SmartFresh followed by storage in air only (!) was significantly firmer than fruit stored in air without prior exposure to SmartFresh (").  These results prompted a study to examine the benefits of SmartFresh application for fruit destined for holding under simulated commercial conditions (55°F, 85 to 90% RH) in the absence of exogenously provided ethylene.

Flesh firmness values of watermelon fruit, cv. Mardi Gras, during storage for up to 21 days at 55°F are shown in Table 1.  Firmness of control fruit (no SmartFresh) declined continuously during storage, reaching after 13 days values 24% lower than values at the start of the experiment.  In contrast, fruit pretreated with SmartFresh had lost only 4% of their original firmness.  After 21 days, control and SmartFresh-treated fruit had softened nearly 40% and 11%, respectively.  At 21 days, firmness differences were readily evident upon applying slight finger pressure to intact fruit (data not shown).

Fruit utilized in the second experiment, though not provided with exogenous ethylene, clearly benefited from treatment with SmartFresh (see Table 1 and Fig. 3).  Ethylene was detected in the storage facilities at concentrations of 40 to 50 ppb (parts per billion).  These levels are nearly 1000 times lower than those applied to the ‘Mardi Gras’ fruit in the first experiment, indicating that watermelon fruit can respond negatively to extremely low levels of ethylene.  Since these fruit were stored in the absence of other commodities, the ethylene in the storage facilities likely originated from the low levels of ethylene produced by the watermelon fruit (Elkashif et al., 1989).  Evidently, the levels detected (40 to 50 ppb) are sufficient to inflict injury on unprotected (no SmartFresh) fruit, particularly during the longer holding periods used in the second experiment.  The adverse influence of ultra-low levels (ppb range) of ethylene on a number of fruits and vegetables at distribution and retail facilities was reported by Wills et al. (2000).

In summary, a single, short-term exposure of watermelon fruit to SmartFresh protected fruit from injury caused by external ethylene and also maintained quality of fruit held under simulated commercial storage conditions.  In the latter case, the low levels of ethylene produced by the watermelon fruit were apparently sufficient to inflict damage during the longer storage durations.  It is expected that the occurrence of ethylene-related damage could be of major concern in situations where watermelon fruit are stored or shipped with commodities producing high levels of ethylene.  Future studies will address the use of SmartFresh-treated watermelon in fresh-cut operations.  Fresh-cut watermelon fruit is extremely fragile compared with other fruits. The successful application of SmartFresh technology to fresh-cut operations with watermelon would be of immense benefit to the fresh-cut industry. 

Literature Cited

Elkashif, M. E. and Huber, D. J.  1988.  Electrolyte leakage, firmness, and scanning electron microscopic studies of watermelon fruit treated with ethylene.  J. Amer. Soc. Hort. Sci. 113:378-381.

Elkashif, M. E., D. J. Huber and J. K. Brecht.  1989.  Respiration and ethylene production in harvested watermelon fruit: evidence for nonclimacteric respiratory behavior.  J. Amer. Soc. Hort. Sci. 114:81-85.

Karakurt, Y., and Huber, D. J., 2002. Cell wall-degrading enzymes and pectin solubility and depolymerization in immature and ripe watermelon (Citrullus lanatus) fruit in response to exogenous ethylene. Physiol. Plant. 116, 398-405.

Wills, R. B. H., Ku, V. V. V., Shohet, D., and Kim, G. H. 2000. Importance of low ethylene levels to delay senescence of non-climacteric fruit and vegetables. Aust J Exp Agric 39, 221-224.

For further information on the commercial applications and availability of SmartFresh, contact AgroFresh, Inc., Philadelphia, PA.  URL http://www.rohmhaas.com/agrofresh/

(Don Huber, James Lee, and Lin-Chun Mao, Horticultural Sciences Department - Vegetarian 04-04)