The compounds ReTain (1-aminoethoxyvinylglycine or AVG) and SmartFresh (1-methylcyclopropene or 1-MCP) are growth regulators that allow the orchardist and storage operator, respectively, to manipulate the responses of apple fruit to the natural hormone ethylene.

　　On apples, ReTain is a naturally occurring amino acid that is sprayed pre-harvest to inhibit ethylene formation and thereby slow ripening. A single application at the labeled rate can delay harvest by five to 10 days or more, depending on the variety. It is used to improve harvest management, provide protection against preharvest fruit drop in susceptible varieties, and can provide some improvement of fruit firmness following storage.

　　1-MCP, unlike ReTain, is a gas and is applied postharvest. 1-MCP will temporarily render apple fruit insensitive to ethylene, thereby arresting ripening and slowing senescence. Both compounds are approved for food use. ReTain was registered for food application in 1997, and 1-MCP was registered in July of 2002.

　　ReTain reduces ethylene synthesis, but does not affect ethylene sensitivity. 1-MCP inhibits the fruit’s ethylene sensitivity and thereby inhibits ethylene’s ability to induce the formation of additional ethylene (termed autocatalytic ethylene), which is responsible for the rapid and large rise in ethylene associated with ripening. Thus both treatments inhibit ethylene formation, but through entirely different mechanisms

　　ReTain has proven to be an effective and consistent compound for the control of drop and for harvest management of McIntosh. However, there has been some difficulty in obtaining consistent responses from1-MCP applications to McIntosh. Studies from New York and some preliminary studies in Michigan have revealed variable responsiveness to this new growth regulator. A published study by Dr. Christopher Watkins from Cornell suggested that the high levels of ethylene that accumulate in ripening McIntosh fruit may be partially responsible for the reduced efficacy of 1-MCP on this variety. The rapid ripening and softening of this variety may also affect the capacity of 1-MCP to exert its influence.

　　　With that in mind, in 2002 we began a project to see if we could suppress ethylene accumulation in McIntosh by applying ReTain prior to harvest, thereby rendering the fruit more responsive to 1-MCP. This study was an outgrowth and continuation of a study funded by the Michigan Apple Research Committee and the Michigan State Horticulture Society evaluating the impact of ethylene manipulation using ReTain, 1-MCP, and Ethrel on Gala, Jonathan and Honeycrisp apple fruit. It was determined that Ethrel enhanced color formation, but also advanced ripening slightly. Insofar as the mode of action of Ethrel is the short-term enhanced release of ethylene, the data were consistent with what is already known about this material.

　　　In 2002, additional funding from Valent Biosciences, the producers of ReTain, enabled the work on McIntosh to be completed and to be continued this year.
In August of 2002, we applied ReTain at full label rate approximately four weeks prior to anticipated harvest to a block of Redmax McIntosh located at the Joe Klein’s orchard in the Fruit Ridge area north of Grand Rapids in Michigan. Klein has been a long-term cooperator with research and extension staff from Michigan State University (MSU) and, in this case, made trees available for the project.

　　　Fruit treated with ReTain and untreated control fruit (UTC) were harvested twice weekly beginning about two weeks prior to the anticipated harvest date and continuing for about six weeks. On each harvest date, hundreds of fruit were pulled at random from the block of trees and brought to MSU’s campus for maturity analysis and storage. Half the ReTain-treated and UTC fruit received a post-harvest application of 1 ppm 1-MCP overnight. Fruit were then placed in refrigerated air storage at 32°F. The storage duration varied from one month to six months. Similar treatments were also given to Cortland fruit, but those data are not presented here.

　Results

　　　The application of ReTain delayed ripening as expected. ReTain suppressed ethylene formation markedly (Fig. 1). UTC fruit underwent their normal increase in internal ethylene content (IEC) beginning Sept. 23, marking the onset of fruit ripening. However, no increase in IEC was detected in ReTain-treated fruit even at the conclusion of the study Oct. 7. The increase in the starch index, indicating loss of starch as it is converted to sugars, was delayed by about 10 days by ReTain. Additionally, the softening of fruit was delayed by about 10 days as well.

　　　After one month of storage in air, the effectiveness of the ReTain and the 1-MCP treatments was evident based on the internal ethylene content of the fruit. The degree to which these chemical treatments can influence ethylene production was phenomenal, resulting in a 5000-fold difference between the most effective treatment (ReTain plus 1-MCP) and the untreated controls. It was interesting to see that the ethylene levels of ReTain-treated fruit remained constant across all harvest dates, but the ethylene levels of the 1-MCP treated fruit increased after the September 23 onset of ripening. The progressive loss of control of ethylene production by 1-MCP is very likely a result of the fruit becoming less responsive to 1-MCP as they begin to ripen so that 1-MCP is less able to slow or halt ripening.

　　　Recall, however, that 1-MCP does not inhibit ethylene production as ReTain does; 1-MCP inhibits ethylene action. The distinction is important in helping us understand what may be going on inside the fruit. The ReTain-treated fruit, for instance, though they may produce lower amounts of ethylene than control fruits, may be ripening at a similar rate. As it turns out, the 1 ppm ethylene found in the ReTain-treated fruit after ones month storage, while much less than the 500 ppm ethylene in the control fruit, is still sufficient to drive ripening at nearly the same rate as the high levels in the control fruit. On the other hand, the much lower level of ethylene produced by the 1-MCP treated fruit is indicative of the whole ripening process being inhibited – including the synthesis of ethylene. When 1-MCP was combined with ReTain, the ripening process was inhibited doubly in McIntosh. The ReTain reduced the capacity of the fruit to synthesize ethylene and the 1-MCP treatment that followed inhibited the ripening process, which again, impacts ethylene synthesis, resulting in extremely low levels of ethylene for the fruit harvested prior to October 7.

　　A measure of the capacity of the two treatments and their combination to retard ripening can be gathered from the firmness data after one month of air storage. Notice that the untreated control fruit softened the most rapidly and that the degree of softening increased for the later harvests of more mature fruit. The ReTain-treated fruit, despite having 1/500th the amount of ethylene in their tissues, were only about one pound firmer. However, the fruit receiving 1-MCP were two to three pounds firmer than the ReTain-only treatment for the first several harvest dates. For those fruit harvested after September 23, the degree of softening was much greater and the difference between the 1-MCP and ReTain-treated was reduced considerably, again emphasizing the impact of harvest maturity on 1-MCP effectiveness. Firmest of all were the fruit receiving 1-MCP and ReTain, with fruit maintaining the firmness at harvest for all except the last harvest date, which was still three pounds firmer than all other treatments.

　　　After three months storage in air at 32°F, fruit had softened considerably relative to the one-month fruit. The treatments yielding fruit with greater than 12 pounds firmness; note that the data are for fruit held an additional seven days at room temperature.

　　　For control fruit, only two harvest dates had fruit above the 12 lb. threshold. Fruit from the ReTain and 1-MCP treatments fared somewhat better, having three and five harvest dates, respectively, with 12 lb. or better fruit. The combined treatment of 1-MCP and ReTain, however, performed exceptionally well, with fruit harvested as late as October 7 maintaining an acceptable level of firmness. Furthermore, while we don’t show the data, we also found that fruit from the combination treatment harvested as late as September 29 were still firmer than 12 lb. even after six months storage in air. It is interesting note that, as for one-month storage, fruit harvested after the initiation of natural ripening (September 23) as measured by internal ethylene levels, were softer than 12 lb. for all but the combination treatment. This again emphasizes the importance of harvesting and applying storage treatments in a timely manner.

　　　We were impressed by the effectiveness of the combination treatment, and so currently are repeating this study to see if the data are consistent across years so that we can develop a recommendation regarding the use of the combination of ReTain and 1-MCP on McIntosh.

　　　Our interpretation of the current season’s data is that the ReTain suppressed ethylene sufficiently that it enabled the 1-MCP to be effective for a much longer period into the harvest season. As a result, the combination provided marked benefits in terms of firmness retention well beyond what was obtainable from either treatment alone. We are hopeful that these two new technologies, both valuable in their own right, together will result in better quality for the consumer and improved prices for the apple industry.
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