The ethylene paradox: how repeated harvesting triggers a programme of premature senescence — and how to counter it.
Multi-harvest crops are widely grown in both protected and open-field vegetable production: cucumber, indeterminate tomato, pepper, courgette, snap bean. Their biological potential allows fruiting over an extended period — in theory, from early spring right through to the first autumn frosts. In practice, though, agronomists and growers often see a sharp drop in productivity and accelerated wear on the leaf canopy after the third or fourth harvest wave. This is commonly written off as a natural developmental cycle, but plant physiology points to a different, human-driven cause: senescence induced by the harvest process itself.
Ethylene’s dual nature: from ripening to decline
The plant hormone ethylene (C₂H₄) plays a central role in this process. In agronomic practice it is best known as the ripening hormone, driving tissue softening, colour change and sugar accumulation in fruit — without it, no marketable produce would be possible.
But ethylene has a second, equally important physiological function: it is the key senescence hormone. In leaf tissue, ethylene activates the expression of genes responsible for:
chlorophyll degradation (leaf blade yellowing);
protein and nucleic acid hydrolysis;
formation of the abscission layer at the petiole, leading to leaf drop.
In a typical annual plant’s ontogeny, ethylene synthesis peaks at the final stage of the life cycle, after seed formation is complete. In multi-harvest crops, however, this timeline is disrupted by human intervention.
The mechanism of induced exhaustion
Every act of picking a fruit is a significant mechanical and hormonal stress event for the plant. A growing fruit (especially its seed cavity) is an active donor of auxins and cytokinins — hormones that suppress leaf senescence and stimulate nutrient flow. By removing the fruit, we take away that “rejuvenating” signal.
Furthermore, the point where the fruit stalk was attached becomes a wound surface. In response to the injury, cells in that zone synthesise stress ethylene. Because harvesting happens repeatedly and regularly, ethylene concentration in the plant’s tissues and intercellular space steadily rises, in a self-catalysing loop (ethylene stimulates the synthesis of more ethylene).
Once C₂H₄ concentration reaches a threshold, the senescence cascade is triggered. The plant “reads” the high ethylene level as a signal that its reproductive task is complete and starts mobilising resources from the leaves into the remaining fruit, while preparing to shed its vegetative mass. Visually this shows up as yellowing of the lower and middle leaf tiers, arrested apical meristem growth, and a sharp reduction in the size of new fruit set.
Control tools: senescence blockers
To extend the productive period and keep the leaf canopy photosynthetically active, substances classified as ethylene biosynthesis inhibitors or receptor blockers are used.
The key, best-studied compound in this group is aminoethoxyvinylglycine (AVG) and its commercial analogues. AVG works by reversibly inhibiting the enzyme ACC synthase, which catalyses the key step converting S-adenosylmethionine into the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The product does not eliminate ethylene synthesis entirely — that would halt fruit ripening — it merely dampens the amplitude of stress-related ethylene spikes, preventing the physiological baseline from crossing into a toxic concentration.
Another approach uses cobalt salts or aminooxyacetic acid, but their phytotoxicity limits their use. In organic production, seaweed extracts rich in polyamines (spermidine, putrescine) — natural ethylene antagonists — are used instead.
Synergy with mineral nutrition: preventing leaf wear
It is important to understand that ethylene blockers only lift the hormonal veto on continued vegetative growth. They cancel the “strike camp” programme, but they do not supply the plant with building material for new fruiting waves. That is why anti-ethylene products are essentially useless without a parallel adjustment of the nutrition programme.
Sound feeding during a multi-harvest cycle needs to tackle two tasks:
Offsetting nutrient removal. Every tonne of cucumber or tomato fruit removes a significant amount of potassium, calcium and micronutrients. Potassium deficiency in particular can, on its own, trigger marginal leaf scorch and premature leaf senescence.
Sustaining nitrogen metabolism in ageing tissue. As leaf yellowing begins, cytokinin concentration synthesised by the roots falls within the leaf. Applying nitrogen through the root at this point is often ineffective, since the root system is itself subject to senescence. The solution is foliar feeding with chelated amino acids and peptides. These deliver nitrogen directly to the leaf blade, bypassing the root, and serve as a substrate for chlorophyll synthesis.
Conclusion
Premature senescence in multi-harvest crops is therefore not an inevitable fate but a manageable physiological process. Combining anti-ethylene therapy (relieving hormonal stress) with targeted foliar nutrition (restoring plastic resources) shifts the productivity curve further out along the timeline.
An effective approach is to alternate applications every 10 days of products such as Foliart Calcium Aminochelate and Foliart VSG-30-Ca at 1 L/ha. With the right programme, harvesting of marketable fruit can continue from early spring deep into autumn, maximising the genetic potential of the variety or hybrid.
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