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.
