Ethylene is a simple gaseous molecule that plays a significant role in the life cycle of plants. It functions as a plant hormone, influencing various developmental processes and responses to environmental signals. Understanding ethylene’s actions provides insight into how plants grow, mature, and adapt. It is involved in processes from a plant’s earliest stages to its final senescence.
The Basics of Ethylene
Ethylene, chemically known as ethene, is a colorless, flammable hydrocarbon gas. Plants naturally produce ethylene from the amino acid methionine. This production involves a two-step enzymatic pathway, converting methionine to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase, and then to ethylene by ACC oxidase.
Ethylene’s gaseous nature allows it to diffuse freely through plant tissues and into the surrounding atmosphere, acting as a signaling molecule. This unique property distinguishes it from other plant hormones, which are typically transported through vascular tissues. Its production occurs in virtually all parts of higher plants, with its synthesis regulated by various internal and external cues.
Ethylene’s Primary Role: Fruit Ripening
Ethylene is most widely recognized for its significant influence on fruit ripening, particularly in “climacteric” fruits. These fruits, such as bananas, apples, tomatoes, and avocados, continue to ripen even after harvest. Their ripening process is characterized by a significant increase in both respiration and ethylene production, where an initial amount of ethylene triggers the production of even more.
During this ethylene-induced ripening, fruits undergo biochemical transformations. Their color changes as chlorophyll degrades and other pigments, like carotenoids, become more prominent. The fruit’s firmness softens due to the breakdown of cell walls. Starch converts into sugars, increasing sweetness, and organic acid levels decrease. Complex volatile compounds are also released, contributing to the fruit’s characteristic flavor and aroma, a phenomenon readily observed when a ripe apple accelerates the ripening of nearby unripe fruits.
In contrast, “non-climacteric” fruits, like strawberries, grapes, and citrus, do not exhibit this ethylene burst or an increase in respiration after harvest. They must be harvested when fully ripe, as they will not ripen further once picked. While ethylene can still influence their degradation and senescence, it does not control the full ripening process as it does in climacteric fruits.
Beyond Ripening: Other Plant Processes
Beyond its prominent role in fruit ripening, ethylene also influences several other processes throughout a plant’s life. It plays a role in senescence, the natural aging and deterioration of plant parts. This includes the yellowing and wilting of leaves and the fading of flowers. Ethylene promotes the breakdown of chlorophyll and other cellular components as these organs reach the end of their functional lifespan.
Ethylene also regulates abscission, the shedding of plant organs such as leaves, flowers, and fruits. As auxin levels decline in a plant part, ethylene triggers the formation of an abscission layer at the base of the organ. This layer weakens, leading to the detachment of the senescing part. Ethylene also influences seed germination by helping to break dormancy and promoting the sprouting of bulbs and potatoes.
Plants also produce ethylene in response to various environmental stresses. When exposed to conditions like drought, flooding, wounding, or pathogen attacks, plants increase their ethylene production. This stress-induced ethylene can trigger defense mechanisms, such as producing defensive compounds or promoting leaf abscission to reduce water loss. In submerged plants like rice, ethylene can promote stem elongation, allowing the plant to “escape” from being underwater.
Managing Ethylene’s Influence
Humans have learned to manipulate ethylene’s effects to improve the quality and shelf life of agricultural produce. In commercial settings, controlled atmosphere (CA) storage is a widely used technique to inhibit ripening and extend the freshness of fruits like apples and apricots. This involves storing produce in environments with regulated levels of temperature, humidity, oxygen, and carbon dioxide, which suppress ethylene production and action. Conversely, ethylene generators are used to ripen climacteric fruits on demand, for example, by exposing bananas to a controlled concentration of ethylene gas to ensure uniform ripening before market.
Ethylene inhibitors, such as 1-methylcyclopropene (1-MCP), are also employed to prevent premature ripening or senescence. 1-MCP works by binding tightly to ethylene receptors in the fruit, effectively blocking its signaling pathway. This treatment can significantly extend the storage and display life of various fruits and ornamental plants, although it might impact the full development of aroma and flavor. Aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA) are other compounds that inhibit ethylene synthesis, allowing fruits to mature longer on the tree before harvest.
At home, consumers can apply simple strategies to manage ethylene’s influence on their produce. Storing climacteric fruits like apples or ripe bananas separately from non-climacteric fruits such as berries can help prevent premature degradation. Placing unripe climacteric fruits in a paper bag traps the ethylene they produce, concentrating the gas and accelerating their ripening. Conversely, refrigerating produce slows down ethylene production and its effects, extending freshness.