Delicious Quality Can Be Affected by Ethephon or ReTainTM
Last year, we presented a paper concerning the physiology of ethylene as a review of the properties of ethylene and its role in fruit ripening and storage. This year, we want to extend this to discuss the implications of 1) enhancing ethylene by the use of Ethrel (ethephon); and 2) inhibiting ethylene by using ReTain™. In addition, we will mention the latest technology, a compound known as MCP, and its effects and implications.
In the long run, quality will always bring the greatest returns, both for the grower and for the industry. For longer storage periods, fruit is carefully monitored so that it might be harvested at optimum storage potential. Then, using the proper storage temperature and atmosphere regimes, fruit metabolism and respiration are slowed to suspend further ripening as long as possible without losing quality. Unfortunately, length of storage is often related to some loss in dessert quality because firmness, acidity and soluble solids all decrease.
There are also situations where changing the quality characteristics before harvest can be beneficial. These include ethephon to enhance ripeness, and ReTain™ to delay it. Regardless of which direction is chosen it is still imperative to know the stage of maturity of the fruit under consideration-and to follow changes in fruit quality characteristics after the treatment has been applied. This, then, is the basis for this short report.
Maturity and Ripening
The line of demarcation between a maturing fruit and one beginning its ripening phase is called the preclimacteric minimum. This preclimacteric minimum is the point at which the respiration rate (the production of CO2 from the mitochondria in the cells) and ethylene production rate is lowest. The timing of the preclimacteric minimum is different for different cultivars such as Delicious, Gala, Golden Delicious and Granny Smith. On the other hand, it can also be quite similar for sports of the same cultivar such as Oregon Spur Delicious, Red Chief Delicious and Scarlett Spur Delicious. These differences and similarities are important because the timing of the preclimacteric minimum determines, in large part, when the fruit should be harvested for optimum storage or optimum edibility. For example, in some years, Scarlett Spur Delicious may be completely red in mid-August and thus look ready for harvest, if color were the main criterion. On the other hand, the timing of the preclimacteric minimum is quite similar to that of Red Chief Delicious whose red color often lags weeks behind . Therefore, for optimum harvest based on the timing of the preclimacteric minimum, harvest timing should be similar to that of Red Chief Delicious. Often, market demands are such that Scarlett Spur Delicious is harvested in advance of its edible quality or optimum storability resulting in low dessert quality fruit both at harvest and after long?term storage.
The timing of the preclimacteric minimum is, for the most part, determined by the particular genetics of the apple. True, the weather does influence somewhat when this physiological phenomenon occurs, but by monitoring fruit maturity, one can often predict whether maturity is delayed or advanced, relative to what is considered normal. The changeover from maturation to ripening occurs when the main function of the fruit/tree changes from making sucrose during photosynthesis and transporting it to the fruit vacuole where it is stored in the form of starch, to the conversion of this stored starch back to sugar. Usually, this will also be accompanied by an increase in respiratory CO2 and ethylene production; both signifying an increase in metabolism-or the beginning of ripening. Probably, the transition is gradual; some tissues may become responsive to ethylene earlier than others.
Regulation of Physiological Systems by Ethylene
Ethylene production is also genetically predetermined although researchers don't understand how the time of onset is determined or what is the initiation factor. We do know that it is autocatalytic-meaning that it stimulates its own production. This feature is what distinguishes climacteric fruit such as apple and banana, for which ethylene hastens ripening, from nonclimacteric fruit such as pineapple and strawberry, in which the progress of ripening appears to be independent of exogenous ethylene.
One of the first responses to ethylene, whether exogenous (from external sources) or endogenous (from within the plant tissue) is a change in the nature of the proteins being synthesized; some declining and others increasing. A number of these have been identified in tomato. For example, when ethylene is applied to mature green tomatoes, certain proteins responsible for degrading cell walls begin accumulating, while other proteins are synthesized which inhibit protein degradation. Still other proteins stop being synthesized altogether. The result of this genetic regulation, regarding which proteins are made and which are not, is a turning on of a system that will transform the tomato molecule to a gelatinous mass. We call this ripening which actually is the most desirable period of senescence for consumption.
The tomato provides a good system to study the subtle changes in ripening because of recent advances in the development of genetically transformed fruit. For example, by inserting certain genes that prevent fruit from ripening, researchers can study what is absent, or what is "turned on" when ethylene is applied. In so doing, they have been able to detect certain genes that are expressed just prior to the increase in ethylene. In the future, these signals may well be used to identify the ripening threshold; that is, fruit that have the capacity to ripen but have not been triggered yet. Unfortunately, in apple, the picture is not as well developed. The length of time to determine the effects of genetic alterations on fruit is comparatively long. However, it is likely a matter of time before apples will be studied in the same way as the tomato, thereby enabling researchers to determine what specific genetic signals establish the ripening precedents.
When apple fruit are capable of ripening, ethylene triggers a number of key enzymes which are amplified in the tissue and which promote softening, respiration and ethylene production. Other major changes which accompany this autolytic degradation are: 1) degradation of chlorophyll; 2) increased cellulase activity which results in loss of cell wall structure (particularly important for development of the abscission layer); 3) increased synthesis of anthocyanin and carotenoids; 4) stimulation of a?amylase which promotes starch conversion; and 5) an increase in the metabolism of malic acid, the main organic acid and source of CO2 production during respiration.
Hopefully, the above criteria have made convincing that, commercially, the most important developmental phase in the life of an apple is the period just before and just after the preclimacteric minimum.
Generally, fruit tissue in the preclimacteric state will not respond, autocatalytically, to applied ethylene. Certainly, this is rate dependent. Applying 2 to 3 times the normal rate of ethephon will likely hasten abscission and ripening, and retard fruit growth. On the other hand, application of 1/10 the normal rate would have little effect on any of the physiological processes. There is, however, middle ground. Several years ago, I conducted an experiment that consisted of applying multiple low doses of ethephon to Granny Smith apples. (Ethephon is a compound which, upon contact with the high acidity of the intra? and intercellular fluids, is physically converted to ethylene and phosphate.)
Results of this experiment indicated low rates applied 4 to 6 weeks before commercial harvest increased the amount of antioxidants in the peel without significantly affecting maturity or ripening (Curry, 1993). Curiously, I have also observed organic blocks of Delicious that are redder than similar conventional blocks. In this case, the organic fruit received numerous applications of fish oil that may have had a similar effect as the ethephon in that a short burst of ethylene at a low concentration was being induced with each application. In either case, it is conceivable that the low rate of ethylene generated affected only peel tissue and 'tricked' it into thinking the ripening was underway thereby inducing some of the changes that normally are triggered with natural ripening such as stimulation of anthocyanin production, while being unable to undergo ethylene autocatalasis.
Applying regular, labeled rates of ethephon, for example to Delicious apples in early August, will not normally trigger the ripening phase. On the other hand, a similar application during the first week of September is likely to advance ripening processes including cell wall breakdown that would lead to tissue softening. Therefore, if ethephon is to be used to advance maturity, the fruit should be in the early stages of maturation. Starch clearing should be evident. Whether or not the fruit increases red color will depend on the weather. If application is followed by conditions that favor the development of anthocyanin pigments, red coloration will likely be enhanced concomitant with a hastening of other ripening characteristics. On the other hand, if the application of ethephon is followed by weather conditions unfavorable for red color development, the physiological maturity will be advanced without the development of anthocyanin. Thus, similar to the application of several other growth regulators, the grower must look ahead to the weather predictions. A continued hot spell should warn of hastened maturity with little coloring advantage, whereas the forecast of cooler weather, especially cooler night time temperatures, should offer the likelihood of hastened maturity plus an enhancement of red color.
Inhibiting Ethylene Biosynthesis
In the last few years, Abbott Laboratories has been developing a chemical tool for use in apple production. ReTain™ (AVG, aminoethoxyvinylglycine) was discovered in 1976 by researchers at Hoffmann-LaRoche, but it wasn't until the last few years that it has been successfully developed. This compound is a substituted amino acid produced by a streptococcal mold and is specifically targeted to inhibit the enzymatic production of ACC, which is the precursor of ethylene. In 1980, Williams in Wenatchee showed that AVG could be used to inhibit internal ethylene production thereby reducing fruit abscission, but could not be used to compensate for external ethylene, no matter the source. In 1981, we did some experiments on Golden Delicious apples, in which we took fruit that were harvested at optimum harvest time and dipped them in solutions containing 0, 100, 200, or 400 ppm AVG (Curry and Patterson, 1993). These fruit were stored in Plexiglas chambers and the ethylene and carbon dioxide were measured daily for a period of seven weeks. After about 10 days, the untreated fruit showed an increased rate of respiration that coincided with the climacteric rise. For this treatment, ethylene exceeded 50 ppm for the remainder of the study. Fruit dipped in 100 ppm AVG had a respiratory climacteric delayed about six weeks. At seven weeks, ethylene levels were about 20 ppm and by the ninth week, ethylene had not exceeded 30 ppm. At 200 ppm, it took approximately nine weeks to see a slight increase in carbon dioxide and at that time, ethylene was approximately 4 ppm. Lastly, at 400 ppm, Golden Delicious apples dipped after harvest never showed an increase in respiration and the ethylene levels were near zero, even at nine weeks after harvest. At the end of the nine weeks, the control apples were yellow and shriveled whereas apples dipped in AVG were still green and noticeably firmer.
Recent increases in grower use of ReTain™ as a tool to manage harvest and increase storage life have resulted in an increased number of ReTain™?treated Apple Maturity Program (AMP) samples. In addition to providing members with maturity and storage information on untreated blocks, the program now generates data to monitor maturity and storability of ReTain™-treated blocks. During harvest testing, a portion of each Apple Maturity Program weekly meeting is devoted to discussions of maturity and storability of ReTain™ samples. ReTain™-treated Delicious samples, from approximately twenty sites located throughout the Washington apple growing areas, were included in the 1998 AMP harvest testing. Since the majority of ReTain™-treated Apple Maturity Program samples are Delicious, this report will focus only on that variety.
The past four years of study have taught us that ReTain™ generally slows the apple maturation rate. AMP test results indicate that ReTain™-treated samples averaged approximately 1/2 to 1 lb firmer than untreated samples when the fruit is harvested later in the season. Acidity is also slightly higher, starch scores, soluble solids and ethylene content are lower in treated samples than untreated samples. In many instances, ReTain™ also reduces firmness loss, during and after storage, by 1/2 to 3 lbs. After the fruit is removed from storage and warmed to 68 °F for 10 days, treated samples are still approximately 1/2 to 3 lbs firmer than untreated samples. Watercore suppression is one of the most pronounced effects of ReTain™. If watercore develops in treated fruit, it is generally less severe (rated as initial or slight) than in untreated fruit, which has more watercore rated as moderate or severe. Fruit with watercore severity ratings of initial to slight will be less susceptible to internal browning than fruit with ratings of moderate to severe.
Response to ReTain™ can be different from year to year and from block to block. It is important to remember that each block has different characteristics, resulting in differences in maturation rates and storability of the fruit. Weather variations and cultural practices vary from year to year, which affect maturation rates differently. Maturation differences also can be caused by differences in elevation, soil type, cultivar or airflow patterns.
Determining the optimum application time can be tricky, but accurate timing is critical if the grower is to derive full benefits of ReTain™. ReTain™ must be applied before ethylene production begins; otherwise, it cannot accomplish its function of ethylene inhibition. If ReTain™ is applied too early, efficacy will diminish early in the season and have limited usefulness. Delicious apples should be treated approximately 30 days before the estimated time that untreated fruit would be harvested. The most effective method currently used to determine when to apply ReTain™ is to calculate days from full bloom (dffb). Greatest accuracy in determining days from full bloom at harvest of each block can be attained by maintaining historical records of harvest maturity in relation to dffb. It is important to remember that calendar date is not a reliable method of determining when to harvest apples; therefore, it is also not a reliable method of determining when to apply ReTain™. Apple Maturity Program data indicate that harvest dates can differ by as much as 3 weeks from one year to the next. Harvest dates are different each year because weather patterns are different each year. Since bloom date, apple development and maturation are affected by weather, the fruit reaches optimum maturity on a different date each year.
Using the recommended rates of ReTain™ will produce the best results. Fifty grams of active ingredient per acre are recommended for apples in Washington. If ReTain™ is applied at a lower rate, efficacy might be compromised.
To gain optimum benefits of ReTain™ growers must also harvest at the optimum time. Fruit maturity should be tested each week beginning approximately two weeks before estimated time of harvest. Growers and horticulturists should test for firmness, soluble solids and starch content. The Apple Maturity Program performs all of these tests plus it additionally tests for acid content, internal ethylene content, box size, skin color, watercore, storage scald and other internal and external disorders. This service is a beneficial addition to grower and warehouse testing programs and is available to all Washington apple warehouses, horticulturists and growers. Weekly changes in maturity parameters, plus the rate of change from week to week should be monitored in each block. Generally, ReTain™-treated apples will be less mature by approximately 1 to 2 weeks than untreated fruit. Growers who harvest before the fruit his reached optimum maturity for its planned use will not realize the full benefits of using ReTain™.
In most instances, growers can rely on ReTain™ as a valuable tool for managing harvest timing, reducing watercore incidence and increasing apple storage life. But, it must be applied correctly and the fruit must be harvested at optimum maturity to gain the advantage of its full potential.
Curry, E.A. and M.E. Patterson. 1993. Controlling ethylene biosynthesis with natural compounds. Proc. Wash. State Hort. Soc.: 312-313.
Curry, E.A. 1994. Preharvest applications of ethephon reduce superficial scald of Fuji and Granny Smith apples in Storage. J. of Hort. Sci. 69 (6):1111-1116.
Williams, M.W. 1980. Retention of fruit flesh firmness and increase in vegetative growth and fruit set of apples with aminoethoxyvinylglycine. HortScience 15(1): 76-77.
Dr. Eric Curry and Joyce Thompson
USDA/ARS Tree Fruit Research Laboratory
1104 N. Western Ave., Wenatchee, WA 98801
15th Annual Postharvest Conference
March 9-10, 1999