Fruit quality in avocado with biostimulants

Introduction

Avocado fruit quality is a determining factor for crop profitability, especially in export markets where high standards of firmness, oil content, size, and postharvest life are required. In recent years, the use of biostimulants has emerged as an effective tool to improve avocado fruit quality without resorting to synthetic inputs. These products, composed of natural substances such as amino acids, seaweed extracts, humic and fulvic acids, and beneficial microorganisms, act on key physiological processes that determine the final fruit quality.

In producing regions such as Mexico, Colombia, Peru, and Chile, where Hass avocado is the main export cultivar, growers face challenges including climate variability, soils with nutritional limitations, and disease pressure. Biostimulants offer a sustainable solution to optimize fruit physiology, improving soluble solids accumulation, firmness, and stress resistance during ripening. This article explores in depth how biostimulants can be integrated into avocado agronomic management to enhance fruit quality, supported by scientific evidence and field experiences.

What are biostimulants and how do they act on avocado?

Biostimulants are products containing substances and/or microorganisms whose function, when applied to plants or soil, is to stimulate natural processes to improve nutrient uptake, abiotic stress tolerance, and crop quality. In the case of avocado, the most studied biostimulants include seaweed extracts (such as Ascophyllum nodosum), free amino acids, humic and fulvic acids, and plant growth-promoting rhizobacteria (PGPR).

The mechanism of action of these compounds is diverse. For example, seaweed extracts contain phytohormones such as auxins, cytokinins, and gibberellins that regulate cell division, fruit set, and fruit development. Amino acids, such as proline and glycine betaine, act as osmoprotectants and nutrient chelators, improving the mobilization of calcium and potassium toward the fruit. Fulvic acids increase the availability of micronutrients in the rhizosphere and promote the formation of organomineral complexes that facilitate root uptake.

In field trials conducted in Hass avocado orchards in Michoacán, Mexico, foliar application of a biostimulant based on seaweed extracts and amino acids during the fruit filling stage increased firmness by 12% and oil content by 8% compared to the control. These results demonstrate the potential of biostimulants to modulate fruit physiology and improve its intrinsic quality.

Main fruit quality parameters in avocado

Avocado quality is assessed through multiple parameters that determine its market acceptance and shelf life. Among the most important are firmness, oil content, dry matter, size, skin color, and the absence of defects. Firmness is directly related to resistance during transport and controlled ripening; a firm fruit has lower susceptibility to mechanical damage and rot. Oil content, which in Hass must exceed 12% for harvest, is an indicator of physiological maturity and organoleptic quality.

Dry matter is another critical parameter, as it correlates with pulp yield and nutrient concentration. In avocado, a dry matter content between 21% and 26% is considered optimal for harvest. Size, measured by the individual fruit weight, influences commercial value, with sizes 40-60 being the most in demand for export. Finally, postharvest life depends on the respiration rate and ethylene production, processes that can be modulated by biostimulants.

Recent research has shown that applying biostimulants during fruit development can significantly improve these parameters. For example, a study published in the Journal of Agricultural Science reported that applying a biostimulant based on fulvic acids and potassium to avocado increased dry matter by 5% and reduced weight loss during storage by 15%. These findings underscore the importance of integrating biostimulation into the crop management program.

Biostimulants that improve firmness and postharvest life

Avocado firmness is one of the most valued attributes by marketers, as it determines the ripening window and shelf life at the retail level. Biostimulants containing calcium, boron, and silicon, either in chelated form or as part of natural extracts, are particularly effective at improving firmness. Calcium is a structural element of the cell wall, and its deficiency leads to soft fruits susceptible to rot. Amino acid-based biostimulants facilitate calcium translocation to the fruit, while seaweed extracts provide polysaccharides that strengthen the pectin matrix.

In a trial conducted in the Quillota region of Chile, the effect of a biostimulant composed of seaweed and silicon on the firmness of Hass avocado was evaluated. Results showed that treated fruits exhibited 18% greater firmness after 14 days of storage at 5°C, compared to the control. Additionally, the incidence of anthracnose was reduced by 30%, suggesting an indirect effect on pathogen resistance. Postharvest life was extended by an average of 5 days, representing a significant logistical advantage.

Another promising group of biostimulants are those based on beneficial microorganisms, such as Bacillus subtilis and Trichoderma harzianum. These microorganisms colonize the rhizosphere and produce metabolites that induce systemic resistance in the plant, reducing the incidence of postharvest diseases. Additionally, they improve the absorption of phosphorus and potassium, key nutrients for fruit quality. The combination of microbial biostimulants with seaweed extracts can enhance effects on firmness and shelf life.

Effect of biostimulants on oil content and dry matter

Oil content is the main indicator of maturity in avocado and is directly related to sensory quality. Biostimulants that promote lipid accumulation act by stimulating photosynthesis, fatty acid synthesis, and the mobilization of carbohydrates to the fruit. Seaweed extracts, rich in alginates and mannitol, have been shown to increase the activity of the enzyme acetyl-CoA carboxylase, key in the fatty acid synthesis pathway.

In a field study in the department of Quindío, Colombia, a biostimulant based on amino acids and micronutrients (zinc and manganese) was applied to Hass avocado during the fruit filling stage. The results indicated a 10% increase in oil content and a 7% increase in dry matter, compared to the control. Treated fruits also showed more uniform coloration and a lower incidence of internal spots. These effects are attributed to improved photosynthetic efficiency and greater translocation of photoassimilates to the fruit.

Dry matter, in turn, increases when biostimulants improve the absorption of potassium and phosphorus. Potassium regulates stomatal opening and cell turgor, while phosphorus is essential for ATP synthesis and membrane formation. Fulvic acids, by chelating these nutrients, facilitate their absorption and transport. In trials with avocado in Peru, the application of fulvic acids via irrigation increased dry matter by 6% and improved the pulp-to-seed ratio.

Integrated management: nutrition, irrigation, and biostimulation

To maximize fruit quality, biostimulation must be integrated with proper nutrition and irrigation management. Avocado is a crop demanding in potassium, calcium, and magnesium, nutrients that directly influence firmness and oil content. A balanced fertilization program, supplemented with biostimulants, can correct deficiencies and optimize the partitioning of assimilates to the fruit. For example, the application of biostimulants rich in calcium and boron during flowering and fruit set improves fruit retention and initial quality.

Irrigation also plays a crucial role, as water stress during fruit filling reduces oil and dry matter accumulation. Biostimulants containing humic and fulvic acids improve the soil's water retention capacity and water use efficiency. In regions with water deficits, such as the coast of Peru, the combination of drip irrigation and biostimulants has been shown to maintain fruit quality even under stress conditions.

Finally, it is important to consider the timing of application. Foliar applications of biostimulants during the rapid fruit growth phase (3-4 months after flowering) are most effective for influencing quality. Applications via irrigation, on the other hand, should be carried out throughout the cycle to maintain a constant supply of nutrients and bioactive compounds. Integrating these practices makes it possible to obtain high-quality fruits with a longer postharvest life and better market acceptance.

Frequently Asked Questions

When should biostimulants be applied to improve avocado quality?

The optimal application time depends on the objective. To improve firmness and postharvest life, it is recommended to apply biostimulants during the fruit filling phase, approximately 3-4 months after flowering. Applications at flowering and fruit set are also beneficial for increasing fruit retention and size.

Which biostimulant is best for increasing oil content in avocado?

Biostimulants based on seaweed extracts, especially Ascophyllum nodosum, have shown the best results for increasing oil content. Those containing amino acids and micronutrients such as zinc and manganese, which participate in fatty acid synthesis, are also effective.

Can biostimulants replace conventional fertilizers?

No, biostimulants do not replace fertilizers; rather, they complement them. Their function is to optimize nutrient uptake and utilization, improve stress tolerance, and stimulate physiological processes. For complete nutrition, it is necessary to maintain a balanced fertilization program.

How are biostimulants applied to avocado?

Biostimulants can be applied via foliar spray or to the soil (fertigation). Foliar application is more effective for correcting specific deficiencies and stimulating processes in the aerial part, while soil application improves the rhizosphere and nutrient absorption. The dosage and frequency depend on the product and crop conditions.

Impact of biostimulants on postharvest quality parameters of avocado

The strategic application of biostimulants during the phenological cycle of avocado (Persea americana Mill.) has shown quantifiable improvements in critical fruit quality attributes. Controlled trials on 'Hass' varieties under Mediterranean and subtropical climate conditions report increases of 12-18% in dry matter content at harvest, a key indicator of physiological maturity and industrial yield. This increase is directly correlated with a higher concentration of oils and total soluble solids, parameters that determine sensory acceptance and commercial value of the fruit. Specifically, foliar application of seaweed extracts (Ascophyllum nodosum) at doses of 2-3 L/ha during pre-flowering and fruit set has been shown to raise oleic acid content by 8% to 14%, improving the lipid profile and oxidative stability of the fruit during cold storage.

In terms of firmness and shelf life, biostimulants based on amino acids and plant-derived peptides have been shown to reduce the softening rate by 20-25% during the first 21 days of storage at 5°C. This is due to the enzymatic modulation of polygalacturonase and pectin methylesterase, key enzymes in cell wall degradation. Field data collected from commercial orchards in Michoacán indicate that applying 1.5 kg/ha of a complex of glycine, proline, and glutamic acid at three stages (flowering, fruit set, and fruit filling) increases initial firmness by 15% and delays the onset of chilling injury by 30%. As a practical recommendation, it is suggested to integrate these treatments with regulated deficit irrigation (RDI) management to enhance the synthesis of compatible osmolytes such as proline, improving tolerance to water stress and final fruit quality.

The incidence of physiological disorders such as black pulp and internal browning has also been significantly reduced with the use of biostimulants rich in phenolic compounds and polysaccharides. Field studies in avocado plantations in Chile and Peru report a 35-40% decrease in the manifestation of these disorders when 4 L/ha of a chitosan and propolis-based formulation is applied immediately postharvest. Additionally, the total antioxidant capacity of the fruit, measured by the DPPH and ORAC methods, increases by 22% to 28% with the application of biostimulants containing flavonoids and organic acids (such as salicylic acid and jasmonic acid). This improvement in the antioxidant profile not only protects the visual quality of the fruit but also extends the marketing window in export markets, reducing losses due to rejection at destination.

For effective practical implementation, it is recommended to establish an application program based on monitoring the fruit's water potential and degree-day accumulation. The optimal dose of biostimulants should be adjusted according to orchard age and production history: for young trees (3-5 years), a dose of 1.5 L/ha of a consortium of beneficial microorganisms (Bacillus spp. and Trichoderma spp.) combined with 2 kg/ha of humic acids improves fruit size by 10% and ripening uniformity. In mature orchards (>10 years), the application of 3 L/ha of a seaweed extract enriched with micronutrients (Zn, B, Mo) at postharvest and pre-flowering increases first-quality fruit yield by 18-22%. Integrating these practices with soil and foliar analysis every 60 days allows for dose adjustments and maximizes the cost-benefit ratio, achieving returns on investment exceeding 300% in premium markets.

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