Ethylene is a naturally occurring gaseous plant hormone that influences various stages of a plant’s life cycle. It plays a significant role in processes such as fruit ripening, the aging of leaves and flowers (senescence), and the shedding of plant parts (abscission). Understanding how this hormone functions is important for managing plant growth and development, particularly in agricultural settings.
Understanding Ethylene Production Triggers
Ethylene production in plants can be stimulated by both internal developmental cues and external environmental stressors. Ethylene synthesis increases naturally at specific developmental stages, including seed germination, the ripening of certain fruits (climacteric fruits like bananas and tomatoes), and the senescence of flowers and leaves.
Plants also increase ethylene production in response to various environmental stresses. Physical damage, such as wounding from cuts or bruises, triggers a rapid increase in ethylene, which activates defense and wound healing. Environmental challenges like drought, flooding, and extreme temperatures can also induce ethylene formation, enabling adaptation and survival. Attacks by pathogens also elevate ethylene levels, aiding defense. Other plant hormones, such as auxin, can indirectly influence ethylene production.
Conditions and Substances That Do Not Stimulate Ethylene
Several conditions and specific chemical compounds can prevent an increase in ethylene production or block its effects in plants. Low oxygen levels, for instance, significantly inhibit ethylene synthesis. This is because the enzyme ACC oxidase (ACO), which converts ACC to ethylene, requires oxygen for its activity. Under hypoxic or anoxic conditions, ACO activity is reduced, leading to lower ethylene production.
Low temperatures also slow down metabolic processes, including ethylene biosynthesis enzymes. Cold conditions can inhibit the activities of both ACC synthase (ACS) and ACC oxidase (ACO), leading to ACC accumulation but reduced conversion to ethylene.
Specific chemical inhibitors are employed to prevent ethylene synthesis or block its perception. Aminoethoxyvinylglycine (AVG) directly inhibits ACC synthase (ACS), an enzyme responsible for a rate-limiting step in ethylene production. By blocking ACS, AVG prevents ACC formation, reducing overall ethylene levels.
Another approach involves blocking ethylene receptors, which are proteins that bind ethylene and initiate its signaling pathway. Silver thiosulfate (STS) works by binding to these receptors, preventing the plant from sensing ethylene and inhibiting its effects. Similarly, 1-Methylcyclopropene (1-MCP) is a gaseous compound that binds tightly to ethylene receptors, effectively blocking ethylene’s action and delaying ripening or senescence. High carbon dioxide concentrations can also inhibit ethylene’s effects by competing for binding sites on its receptors.
Practical Applications of Ethylene Control
Controlling ethylene production and perception has practical applications in agriculture and horticulture, particularly in post-harvest management. Manipulating ethylene levels extends the shelf life of fresh produce and cut flowers. For example, storing fruits and vegetables under low oxygen or cold temperatures reduces ethylene production and sensitivity, slowing spoilage. Chemical inhibitors like 1-MCP are widely used to delay ripening and senescence in fruits such as apples and avocados, preserving their quality during storage and transport.
Conversely, understanding ethylene’s role also enables its controlled application. Ethylene gas is deliberately introduced into ripening rooms to promote uniform ripening of climacteric fruits like bananas and mangoes. This controlled exposure ensures fruits reach optimal ripeness for market. Ethylene can also induce flowering in certain crops, such as pineapples, leading to synchronized harvests.