Control of fruit abscission in table grapes with biostimulants

Introduction

Fruit abscission in table grapes is one of the most critical problems faced by growers, as it can reduce yield by more than 30% in susceptible varieties. This physiological process, which involves the premature drop of newly set or developing berries, is regulated by a complex hormonal, nutritional, and environmental interaction. In recent years, the use of biostimulants has emerged as an effective tool to mitigate this phenomenon, improving fruit retention and final harvest quality.

In this technical article, aimed at agronomists, technical advisors, and table grape growers in Latin America, the physiological causes of abscission, triggering factors, and agronomic strategies based on biostimulation and nutrition that allow for sustainable control are analyzed. Data from field trials and practical recommendations for integrating these solutions into management programs are presented.

What is fruit abscission and why does it occur in table grapes?

Fruit abscission is a natural process of berry detachment that occurs at different phenological stages of the crop. In table grapes, the most significant losses are observed during fruit set and early fruit development, when the plant decides to abort those that do not have a sufficient supply of nutrients, water, or phytohormones. This self-regulation mechanism allows the vine to concentrate its resources on a smaller number of berries, but under stress or imbalance conditions, abscission can be excessive and economically damaging.

From an anatomical point of view, abscission occurs in the separation zone of the pedicel, where a layer of specialized cells forms, weakens, and breaks. This process is controlled by the action of hydrolytic enzymes such as cellulase and polygalacturonase, whose activity is regulated by the phytohormones auxin, ethylene, and abscisic acid. When auxin levels in the fruit decrease or ethylene levels increase, the abscission cascade is activated.

In table grapes, varieties such as Red Globe, Thompson Seedless, and Sugraone exhibit different degrees of susceptibility. For example, Red Globe is particularly sensitive to abscission during the fruit set phase, while Thompson Seedless may lose berries at later stages due to water or nutritional stress. Understanding varietal response is key to designing specific management strategies.

Factors triggering abscission in table grapes

Water stress

Water deficit during flowering and fruit set is one of the main factors inducing abscission. Lack of water reduces photosynthesis and carbohydrate production, limiting energy availability for fruit development. Additionally, water stress promotes the synthesis of abscisic acid, which accelerates the formation of the abscission layer. In field trials conducted in the Piura region of Peru, it was observed that controlled deficit irrigation during fruit set reduced berry retention by 25% in the Red Globe variety.

Hormonal imbalance

The relationship between auxins, gibberellins, cytokinins, and ethylene is decisive for fruit retention. A low concentration of auxins in the ovary after pollination prevents fruit development and triggers abscission. On the other hand, an excess of ethylene, whether due to stress or external applications, promotes berry drop. The application of biostimulants containing natural auxins or compounds that modulate ethylene metabolism can help maintain hormonal balance.

Nutritional deficiencies

Boron and calcium are two critical nutrients for fruit retention in table grapes. Boron participates in pollen germination and pollen tube growth, while calcium is essential for cell wall stability and pedicel integrity. Boron deficiency during flowering reduces fruit set, and calcium deficiency during fruit development increases susceptibility to abscission. Foliar analyses conducted in Mexican vineyards show that boron levels below 25 ppm and calcium levels below 0.8% are associated with higher rates of berry drop.

Adverse climatic conditions

Extreme temperatures, both high and low, affect pollen viability and stigma receptivity, reducing fertilization and fruit set. In table grapes, temperatures above 35°C during flowering can cause flower abortion and premature abscission. Likewise, late frosts or heavy rains during flowering wash away pollen and hinder pollination. In regions such as the Ica Valley in Peru, recurrent heatwaves have increased losses due to abscission in recent years.

Physiological mechanisms involved in abscission

Fruit abscission in table grapes is an active process regulated by hormonal and enzymatic signals. When the fruit does not receive enough photoassimilates or nutrients, auxin synthesis in the developing seeds is reduced. This decrease in auxin in the pedicel leads to increased sensitivity to ethylene, which activates the expression of genes encoding hydrolytic enzymes such as cellulase and polygalacturonase. These enzymes degrade the cell wall in the abscission zone, weakening the attachment between the fruit and the cluster.

Abscisic acid (ABA) also plays an important role, especially under water stress. ABA promotes senescence and abscission by inducing ethylene production. On the other hand, gibberellins and cytokinins can delay abscission by maintaining meristematic activity and cell division in the young fruit. Exogenous application of gibberellins is a common practice in table grapes to improve fruit set, but its effect depends on timing and concentration.

Recent research has shown that biostimulants based on seaweed extracts, such as Ascophyllum nodosum, can modulate the expression of genes related to abscission. These extracts contain natural phytohormones such as auxins, cytokinins, and betaines, which help maintain hormonal balance and reduce the activity of hydrolytic enzymes. Field trials in Chile reported an 18% reduction in berry abscission in table grapes treated with a seaweed biostimulant compared to the control.

Agronomic strategies for abscission control

Irrigation management

Maintaining an adequate water supply during flowering and fruit set is essential. Severe water deficits during these stages should be avoided. Drip irrigation with moisture sensors allows adjusting frequency and volume according to crop demand. In table grapes, a leaf water potential of -0.6 to -0.8 MPa during fruit set is considered optimal to minimize abscission.

Balanced fertilization

Fertilization must ensure adequate levels of boron, calcium, and zinc. Boron can be applied foliarly at doses of 200-400 g/ha during pre-flowering and full bloom. Calcium is applied in the form of chelates or calcium nitrate, at doses of 2-4 kg/ha per application, from fruit set to veraison. Zinc is also important for auxin synthesis, and its deficiency is associated with lower fruit set.

Use of biostimulants

Biostimulants based on seaweed extracts, amino acids, and beneficial microorganisms have shown promising results. For example, the application of a biostimulant containing cytokinins and betaines can increase fruit retention by improving carbohydrate partitioning and stress tolerance. In a trial conducted in the San Felipe Valley, Mexico, the application of a seaweed biostimulant at a dose of 1 L/ha during flowering and fruit set increased berry retention by 22%.

Use of biostimulants in fruit retention

Biostimulants are products containing substances and/or microorganisms whose function is to stimulate natural plant processes to improve nutrient uptake, water use efficiency, and stress tolerance. In the context of fruit abscission in table grapes, biostimulants act at the hormonal and metabolic level, favoring berry retention.

Seaweed extracts, especially from Ascophyllum nodosum, are rich in auxins, cytokinins, gibberellins, and betaines. These natural phytohormones help maintain adequate auxin levels in the young fruit, delaying senescence and abscission. Additionally, betaines act as osmoprotectants, reducing the impact of water and saline stress.

Amino acids such as proline and glycine betaine also contribute to the stability of cell membranes and the regulation of osmotic potential. Foliar application of amino acids at doses of 0.5-1 L/ha during flowering and fruit set can improve fruit retention by reducing oxidative stress.

Microbial biostimulants, such as plant growth-promoting rhizobacteria (PGPR), can enhance nutrient availability and phytohormone production. Strains of Bacillus and Pseudomonas have shown the ability to produce auxins and cytokinins, which favor fruit set and retention. In a study on table grapes in Brazil, inoculation with Bacillus subtilis increased berry retention by 15%.

It is important to note that the efficacy of biostimulants depends on the timing of application, dosage, and environmental conditions. Applications are recommended during critical stages such as pre-flowering, full bloom, and fruit set, with doses adjusted according to crop status and manufacturer recommendations.

Nutritional management to reduce abscission

Balanced nutrition is the foundation for minimizing fruit abscission. In addition to boron and calcium, other nutrients play a relevant role:

• Nitrogen: Excess nitrogen during flowering can promote vegetative growth at the expense of fruit set. It is recommended to apply nitrogen moderately and in split doses, avoiding high rates during pre-flowering.
• Phosphorus: It is essential for ATP formation and the energy required for cell division in the fruit. Adequate phosphorus levels favor fruit set.
• Potassium: It participates in carbohydrate translocation and water potential regulation. Potassium deficiency during fruit development can increase abscission.
• Magnesium: It is a component of chlorophyll and an enzyme activator. Its deficiency reduces photosynthesis and carbohydrate production.

Foliar analysis is recommended during pre-flowering and fruit set to adjust fertilization. Critical levels for table grapes are: N 2.2-2.8%, P 0.15-0.25%, K 1.2-1.8%, Ca 0.8-1.2%, Mg 0.2-0.4%, B 25-40 ppm, Zn 15-30 ppm.

Practical considerations for table grape growers

To implement an effective abscission control program, the following aspects should be considered:

• Identify the most susceptible varieties and adjust management accordingly.
• Monitor weather conditions and anticipate stress events (heatwave, frost).
• Apply biostimulants at critical phenological stages: pre-flowering, full bloom, and fruit set.
• Maintain a balanced fertilization program based on soil and foliar analysis.
• Avoid excess nitrogen and ensure adequate levels of boron and calcium.
• Use drip irrigation with sensors to avoid water deficits.
• Evaluate biostimulant response using control plots and fruit retention measurements.

Integrating biostimulants with good agronomic practices can significantly reduce losses due to abscission, improving crop profitability. It is recommended to work with technical advisors and input suppliers to design specific programs for each production condition.

FAQ

1. What is fruit abscission in table grapes? It is the premature drop of berries before harvest, caused by hormonal, nutritional, and environmental factors. It mainly occurs during fruit set and early fruit development, and can significantly reduce yield.
2. How do biostimulants help control abscission? Biostimulants provide natural phytohormones such as auxins and cytokinins, which maintain hormonal balance and delay the formation of the abscission layer. They also improve stress tolerance and carbohydrate partitioning to the fruits.
3. What are the most important nutrients for reducing abscission? Boron and calcium are critical. Boron promotes pollination and fruit set, while calcium strengthens the cell walls of the pedicel. Zinc, potassium, and phosphorus are also important.
4. When should biostimulants be applied? The most effective applications are made at pre-flowering, full bloom, and fruit set. It is important to follow the manufacturer's recommendations and adjust according to crop conditions.
5. Can abscission be controlled only with irrigation and fertilization? Proper irrigation and fertilization management is essential, but under stress conditions or in susceptible varieties, biostimulants offer an additional benefit by modulating hormones and improving stress resistance.

References

• Sustainable Agroecology — FAO
• Soils Portal — FAO