Roles of Ethylene and Abscisic Acid in Plant Development

Plants don’t leave growth, ripening, or survival to chance. They rely on a sophisticated internal signaling system – plant hormones – to coordinate every stage of their life cycle. Among the most consequential of these are ethylene and abscisic acid (ABA). Though they work in very different ways, together they govern critical processes from fruit ripening and leaf shedding to drought survival and seed dormancy. For horticulturists, understanding these two hormones is not just academic – it directly shapes how crops are grown, harvested, and managed after harvest.

What is ethylene and why does it matter? 🔗

Ethylene is a gaseous plant hormone with a uniquely simple molecular structure – it is the only known plant hormone that exists as a gas at room temperature. Despite its simplicity, it acts at trace concentrations and influences a wide range of developmental and stress-related processes throughout the plant’s life, including fruit ripening, flower wilting, and the shedding of leaves and fruit. It is produced in virtually all plant tissues – leaves, stems, roots, flowers, fruits, and seeds – and its synthesis is triggered by both developmental cues and environmental stress.

Ethylene and fruit ripening 🔗

Ethylene’s most commercially significant role is in fruit ripening. As fruits mature, they begin producing ethylene, which then initiates a cascade of biochemical changes – softening of the flesh, conversion of starch to sugars, reduction of acids, development of aroma compounds, and changes in skin color. Fruit ripening results in changes to firmness, sugar content, acid levels, aroma, and color, all driven largely by this single gaseous signal.

Fruits are broadly categorized into two groups based on their ripening behavior. Climacteric fruits – including apples, peaches, bananas, and tomatoes – show a substantial rise in ethylene production and respiration rate during ripening. This self-amplifying cycle means that once ethylene production starts, it accelerates, allowing these fruits to continue ripening even after harvest. Non-climacteric fruits such as grapes and strawberries do not show this pattern and must be harvested at peak ripeness since their ripening does not continue post-harvest.

Leaf abscission and senescence 🔗

Beyond ripening, ethylene also drives leaf abscission – the natural shedding of leaves, flowers, and fruit. It does this by stimulating the production of cell wall-degrading enzymes in the abscission zone, a specialized layer of cells at the base of leaf stalks and fruit pedicels. This same mechanism is relevant in ornamental horticulture: cut flowers and potted plants exposed to ethylene during shipping and handling undergo accelerated floral senescence and abscission, causing significant economic losses for the floriculture industry.

Ethylene and stress responses 🔗

Ethylene is also produced in response to mechanical damage, pathogen attack, flooding, and temperature extremes. In waterlogged or submerged conditions – common in rice cultivation – ethylene promotes rapid stem elongation, helping the plant grow above the water surface. This escape response illustrates how ethylene acts not just as a developmental signal but also as a survival mechanism under environmental stress.

How ethylene is used in horticultural practice 🔗

The ability to manipulate ethylene levels has made it one of the most practically useful hormones in commercial horticulture. Ethylene management takes two main directions: promoting its action where ripening or abscission is desired, and inhibiting it where prolonged shelf life or delayed maturity is the goal.

Ethephon: the ethylene-releasing agent 🔗

Ethephon (2-chloroethylphosphonic acid) is a plant growth regulator that releases ethylene after being absorbed into plant tissue. It is applied to growing plants to regulate flowering, induce abscission, and hasten ripening in many horticultural crops, including apples, citrus, olives, and pineapple. For example, ethephon is used to synchronize the ripening of early-season market apples and to assist mechanical harvesting by loosening fruit attachment. In macadamia nut production, ethephon can cause up to a 15-fold increase in nut abscission within four weeks compared to untreated trees, significantly reducing harvest costs.

Inhibiting ethylene to extend shelf life 🔗

On the other side, controlling ethylene is essential for post-harvest management. 1-Methylcyclopropene (1-MCP), marketed as Harvista™, works by binding to ethylene receptors in the fruit, effectively blocking the fruit from responding to ethylene and delaying ripening. AVG (aminoethoxyvinylglycine), sold as ReTain®, inhibits ethylene biosynthesis itself, slowing down color development and softening. Both compounds are widely used to extend storage windows and manage harvest timing in orchards. Commercial ripening rooms, meanwhile, use catalytic generators to produce controlled concentrations of ethylene gas – typically 500 to 2,000 ppm for 24 to 48 hours – to uniformly ripen bananas, citrus, and other imported fruits before they reach retail.

Abscisic acid: the stress hormone 🔗

Abscisic acid (ABA) functions as the plant’s primary stress hormone. It is an isoprenoid compound synthesized mainly in the chloroplasts and other plastids, and its levels rise sharply when plants encounter drought, salinity, cold, or other adverse conditions. While ethylene generally promotes maturation and senescence, ABA’s primary role is to slow things down – conserving resources, enforcing dormancy, and keeping the plant alive under stress. ABA regulates a wide range of physiological events including seed dormancy, seedling development, and the limitation of abiotic stress responses such as drought, salinity, cold, and heat.

ABA and stomatal closure 🔗

One of ABA’s most immediate and vital functions is controlling stomatal closure. Stomata are the tiny pores on leaf surfaces that regulate gas exchange and water loss. When soil moisture drops, ABA accumulates in guard cells, triggering the closure of stomata. ABA reduces transpirational water loss by closing stomata while simultaneously defending against microbial entry through those same pores. This dual protective role makes stomatal regulation one of ABA’s most critical contributions to plant survival. Importantly, this ABA-driven closure overrides even the signal from blue light – which normally promotes stomatal opening – meaning drought stress takes priority over photosynthetic efficiency when survival is at stake.

ABA and seed dormancy 🔗

ABA is also the key regulator of seed dormancy. During seed development, ABA promotes the synthesis of storage proteins and lipids while blocking premature germination. As ABA gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. This ABA-gibberellin balance is tightly regulated: a dynamic balance between ABA synthesis and catabolism versus gibberellin activity controls whether a seed remains dormant or transitions to germination. In horticulture, this is directly relevant to managing seed lots, ensuring uniform germination, and planning nursery operations.

ABA and broader stress tolerance 🔗

ABA levels increase significantly under drought and salinity stress, stimulating stomatal closure, gene expression changes, and a range of adaptive physiological responses. These responses include the accumulation of osmoprotectants – compounds that maintain cell turgor under dehydration – and the activation of antioxidant enzymes that protect cells from oxidative damage. ABA also plays a role in bud dormancy, converting the apical meristem into a protective dormant bud before winter, ensuring that an unseasonably warm spell does not trigger premature growth. In horticultural crops exposed to seasonal cold, ABA’s role in maintaining bud dormancy is critical to preventing frost damage and ensuring normal spring development.

The contrasting but complementary roles of ethylene and ABA 🔗

While ethylene and ABA are often described as having opposing functions, they interact in important ways. In non-climacteric fruits such as grapes and strawberries, ABA rather than ethylene is the primary hormone controlling ripening, promoting color development and sugar accumulation. In climacteric fruits, ABA has been shown to positively regulate ACC synthase – the enzyme that produces ethylene’s precursor – meaning ABA can actually stimulate ethylene biosynthesis and thus accelerate ripening in some contexts. This cross-talk highlights that these hormones do not operate in strict isolation but as part of a complex, interconnected signaling network.

From a horticultural management perspective, the contrast between the two is practically useful. Ethylene drives processes that need to happen – ripening, abscission, color development – and can be deliberately applied or inhibited to control timing and quality. ABA, on the other hand, helps crops endure – surviving drought, resisting premature germination, and entering winter dormancy safely. ABA triggers physiological processes including bud dormancy, seed germination, stomatal closure, and the regulation of stress-responsive gene expression, all of which have direct relevance to managing crop cycles and improving resilience under climate variability.

Practical applications in horticultural crop management 🔗

The knowledge of ethylene and ABA signaling has translated into concrete crop management strategies. Post-harvest ripening rooms that use controlled ethylene exposure are standard in banana and citrus supply chains. Cold storage facilities are managed to minimize ethylene accumulation – keeping ethylene-sensitive crops like cut flowers and leafy vegetables separate from ethylene-producing fruits. Applied knowledge of ethylene can be used to enhance fruit quality, control pre-harvest fruit drop, and extend shelf life through both chemical and environmental management approaches.

On the ABA side, exogenous ABA applications are being explored to improve drought tolerance in vegetable and fruit crops, particularly under water-stressed field conditions. Understanding ABA-mediated dormancy is central to seed priming techniques used in nursery production, where growers manipulate germination timing to achieve uniform seedling emergence. As climate variability increases pressure on water availability and temperature stability, the practical relevance of ABA management in horticulture is only growing.

What do you think? Given that ethylene can both improve fruit quality and cause significant post-harvest losses depending on how it is managed, how should small-scale growers without access to commercial ripening rooms approach ethylene control? And as drought becomes a more frequent challenge in agriculture, could targeted ABA-based treatments become a standard part of crop management – or does their complexity make that unlikely?

References

1. https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/04:_Plant_Physiology_and_Regulation/4.04:_Hormones/4.4.05:_Ethylene
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