Ethylene and the Regulation of Fruit Ripening
What is Ethylene?
Fruit ripening is the set of processes that occur from the later stages of growth and development until the fruit is ready to be consumed. Fruit ripening results in changes in fruit quality characteristics. The firmness of the fruit flesh typically softens, the sugar content rises, and acid levels are reduced. Aroma volatiles are released, and the true flavor of the fruit develops. The color of fruit typically darkens, the skin and flesh soften, and the green background color fades.
Ethylene is a gaseous plant hormone that plays an important role in inducing the ripening process for many fruits, together with other hormones and signals. An unripe fruit generally has low levels of ethylene. As the fruit matures, ethylene is produced as a signal to induce fruit ripening. Ethylene production continues to increase after harvest, thus decreasing fruit shelf-life, storability capacity, and increasing its susceptibility to pathogen attacks. Thus, monitoring and managing ethylene production rates is of crucial importance so fruit does not become over-ripe on the tree or during postharvest storage, which will render it unmarketable and decrease profitability.
Fruit Ripening Patterns
Fruits are generally divided in two categories: climacteric and non-climacteric fruits. In general terms, climacteric fruits can ripen after harvest, whereas non-climacteric fruits cannot ripen after harvest. Climacteric fruit ripening is characterized by an increased rate of respiration, and then a burst of ethylene biosynthesis during ripening (Figure 1). The production of ethylene in climacteric fruits is also known as autocatalytic, which means an initial concentration of ethylene causes an increase in production of ethylene. This means once ethylene production starts, the fruit naturally increases the amount of signal made accelerating ripening. Some examples of climacteric fruits include peaches, bananas, apples, and avocados (Figure 2).
Non-climacteric fruits have a different ripening pattern. They do not have a peak of ethylene production or respiration during ripening (Figure 1); thus, they need to be harvested when they are fully ripe. Some examples of nonclimacteric fruits include cherries, grapes, strawberries, and blueberries (Figure 2).
Understanding the ripening pattern of the fruits you grow is very important for developing management strategies during development, determining their optimum harvest date, as well as designing postharvest storage practices.
How is ethylene measured?
Ethylene is a gas and is measured in the laboratory using a gas chromatograph, an analytical instrument that can measure different components in a gaseous sample. Ethylene production can be measured non-destructively by placing a single, or multiple fruits, inside a sealed container. After a determined amount of time, the gas inside the container is withdrawn with a syringe and analyzed with the gas chromatograph. For apples, a needle can be injected in the core cavity of the fruit and a gas sample is withdrawn for ethylene measurement. The amount of ethylene that a fruit produces can be used to determine the ripeness of a fruit and its storability potential.
Preharvest and Postharvest Ethylene Regulators
Ethylene production can be manipulated in different ways in preharvest and postharvest. For example, three preharvest plant growth regulators that manage and alter ethylene production, or perception, to influence ripening include Ethephon, ReTain®, and HarvistaTM.
Ethephon (Ethrel®, Bayer Crop Science; MotivateTM, Fine Americas; Ethephon 2, Arysta LifeScience North America, LLC), is an ethylene-releasing chemical. This can be applied as a preharvest growth regulator to promote fruit ripening. This would be used to accelerate the ripening process. However, Ethephon also accelerates fruit abscission, and may negatively impact fruit storability.
ReTain® (active ingredient: Aminoethoxyvinylglycine (AVG), Valent USA) inhibits the production of ethylene. This would be used to decelerate, or slow down, the ripening process. This also leads to delay in color development, which may have an impact on fruit quality and consumer opinion.
HarvistaTM (active ingredient: 1-Methylcyclopropene (1- MCP), AgroFresh) binds to ethylene receptors in the fruit. This blocks the fruit from “seeing” the ethylene, mimicking a low amount of perceived ethylene. This prevents the response to ethylene in the fruit, therefore, delaying ripening.
ReTain® and Harvista® both delay the fruit ripening process, which allows for keeping fruit on the tree for a longer time and preventing fruit drop. Additionally, the delay in fruit ripening allows for consistent maturity throughout the orchard. This helps to manage fruit harvest windows.
Regarding postharvest ethylene management, 1-MCP (SmartFreshTM) is a synthetic chemical that has a very similar structure to ethylene. It blocks perception of ethylene by the fruit, thus delaying post-harvest ripening. This can help maintain fruit quality, a high fruit firmness, and extend storage capacity, but can also have negative effects such as enhancement of physiological disorders during storage.
This article appears on April 15, 2021, Volume 12, Issue 1 of the Vegetable and Fruit News