Fruit set in citrus with microalgae: yield improvement

The challenge of fruit set in citrus

Fruit set in citrus is one of the most critical stages of the productive cycle, as it determines the final number of fruits and, consequently, crop yield. During this process, a significant proportion of flowers and newly formed fruits drop naturally due to physiological and environmental factors. Under optimal conditions, only 1-2% of flowers become commercial fruits. However, abiotic stress situations such as drought, extreme temperatures, or nutritional deficiencies can drastically reduce this percentage, affecting the citrus grower's profitability.

In recent years, the use of biostimulants based on microalgae has emerged as an effective tool to improve fruit set in citrus. Microalgae, photosynthetic unicellular organisms, produce bioactive compounds such as phytohormones, amino acids, polysaccharides, and antioxidants that modulate plant metabolism and promote fruit retention. This article explores the mechanisms of action, the most commonly used species, and practical recommendations for integrating microalgae into citrus fruit set management.

How do microalgae act on fruit set?

Microalgae exert multiple effects on the physiology of fruit trees, acting directly on the hormonal and nutritional processes that regulate fruit set. One of the key mechanisms is the regulation of the hormonal balance, especially through the production of cytokinins and auxins. Cytokinins promote cell division and delay tissue senescence, which favors the retention of young fruits. Auxins, on the other hand, contribute to ovary development and the transport of nutrients to the developing fruit.

Additionally, microalgae provide free amino acids and peptides that act as metabolic signaling molecules, stimulating the synthesis of proteins involved in stress tolerance. This is particularly important during fruit set, when plants are more sensitive to adverse conditions. A study from the Polytechnic University of Valencia showed that foliar application of Chlorella vulgaris extract increased the activity of antioxidant enzymes such as superoxide dismutase and catalase, reducing oxidative damage in floral tissues and improving the fruit set rate in orange trees.

Another relevant aspect is the improvement in nutrient absorption. Microalgae release natural chelating substances that facilitate the assimilation of micronutrients such as zinc, boron, and manganese, which are essential for the formation of viable pollen grains and fertilization. The combination of these effects makes microalgae a comprehensive solution for optimizing fruit set in citrus.

Most commonly used microalgae species

Among the microalgae used in citrus cultivation, Chlorella vulgaris, Scenedesmus spp., and Arthrospira platensis (spirulina) stand out. Chlorella is rich in phytohormones and polysaccharides, while Scenedesmus is notable for its high content of amino acids and bioactive peptides. Spirulina, although a cyanobacterium, is included in the microalgae group due to its similar composition and is especially valued for its content of phycocyanin, a potent antioxidant.

Main microalgae used in citrus cultivation

The choice of the appropriate microalgae depends on the crop conditions and specific objectives. Chlorella vulgaris is the most studied and used in citrus, with consistent results in improving fruit set. It contains high concentrations of cytokinins (such as zeatin) and auxins (indoleacetic acid), which act synergistically to stabilize young fruits. Additionally, its polysaccharides stimulate soil microbiota and improve root structure, resulting in greater water and nutrient uptake during fruit set.

Scenedesmus spp., on the other hand, is a freshwater microalgae that produces large amounts of essential amino acids and signaling peptides. These compounds activate metabolic pathways related to stress tolerance, such as proline accumulation and aquaporin regulation, which helps maintain the water status of floral tissues. Research from the Chilean Institute of Agricultural Research reported that applications of Scenedesmus on lemon trees increased fruit retention by 18% during a period of moderate water stress.

Finally, Arthrospira platensis (spirulina) is rich in phycocyanin, a pigment with antioxidant capacity that protects cell membranes from oxidative damage. It also provides gamma-linolenic acid, a fatty acid involved in hormonal signaling. Although its use is less common in citrus, some trials show promising results in improving pollen quality and fertilization rate.

Practical application: doses and phenological stages

To maximize the benefits of microalgae in citrus fruit set, it is essential to apply them at key phenological stages. The first critical moment is pre-flowering, when floral structures are forming. A foliar application of 2-3 L/ha of microalgae extract (typical concentration of 10^6 cells/mL) helps strengthen flower buds and improve pollen viability. The second application should be carried out at full bloom to favor pollination and fertilization. Finally, a third application after initial fruit set (when fruits are 5-10 mm in diameter) reinforces fruit retention and early fruit growth.

The most common application method is foliar, as it allows for rapid absorption of the bioactive compounds. However, it can also be applied to the soil, especially in formulations containing live microalgae, to improve the rhizospheric microbiota and nutrient availability. Recommended doses vary by commercial product, but generally range between 2 and 4 L/ha per application, diluted in a sufficient volume of water to cover the foliage (400-600 L/ha). It is important to apply during periods of low solar radiation (early morning or late afternoon) to avoid degradation of the active compounds.

Compatibility with other phytosanitary products is good, but a pre-mixing test is recommended, especially with copper or sulfur-based products, as they can reduce the viability of live microalgae. In the case of inactivated extracts, compatibility is higher. For best results, it is suggested to integrate microalgae into a management program that includes balanced fertilization and adequate irrigation during fruit set.

Field results with microalgae in citrus

Various field trials have demonstrated the effectiveness of microalgae in improving fruit set in citrus. A study conducted in the Valencia region (Spain) on 'Navelina' orange trees showed that three foliar applications of Chlorella vulgaris (2 L/ha at pre-flowering, full flowering, and post-fruit set) increased the number of fruits per tree by 22% compared to the control, without affecting final fruit size. Additionally, a 15% reduction in physiological fruit drop was observed during the first weeks after fruit set.

In Brazil, researchers from the University of São Paulo evaluated the effect of Scenedesmus spp. on 'Pera' orange trees under heat stress conditions. The results indicated that microalgae applications increased the fruit set rate by 30% and improved fruit quality, with higher soluble solids and ascorbic acid content. Likewise, greater activity of antioxidant enzymes was recorded in the leaves, suggesting increased stress tolerance.

In Mexico, a trial on Persian lime (Citrus latifolia) combined microalgae application with a reduced fertilization program. The results showed that the microalgae treatment maintained a yield similar to conventional fertilization, but with a 25% savings in synthetic fertilizers. This demonstrates the potential of microalgae to optimize nutritional efficiency and reduce costs.

Additional benefits for the crop

In addition to improving fruit set, microalgae provide collateral benefits that contribute to the overall health of the crop. On one hand, they stimulate root development through the production of phytohormones and exudates that promote the colonization of mycorrhizae and beneficial bacteria. A more robust root system improves water and nutrient uptake, which is especially valuable during fruit set, when resource demand is high.

On the other hand, microalgae reinforce the plant's natural defenses against pathogens. Polysaccharides and other bioactive compounds act as elicitors, activating signaling pathways such as those of salicylic acid and jasmonic acid. This translates into greater resistance to diseases such as anthracnose or the fungus Alternaria, which can affect flowers and young fruits. In trials conducted on citrus, a significant reduction in the incidence of postharvest diseases was observed when microalgae were applied during fruit set.

Finally, the use of microalgae aligns with the principles of sustainable and organic agriculture. Being natural, biodegradable products compatible with beneficial fauna, they help reduce the environmental footprint of the crop. For citrus growers seeking organic certifications or regenerative practices, microalgae represent a valuable tool within integrated management.

FAQ

What are microalgae and how do they benefit citrus fruit set?

Microalgae are photosynthetic unicellular organisms that produce bioactive compounds such as phytohormones, amino acids, and antioxidants. These compounds regulate the plant's hormonal balance, improve stress tolerance, and promote fruit retention, resulting in higher fruit set and yield.

When to apply microalgae to improve fruit set?

It is recommended to apply at three key moments: pre-flowering (to strengthen flower buds), full flowering (to favor fertilization) and post-fruit set (to retain young fruits). The typical dose is 2-4 L/ha per foliar application.

Are microalgae compatible with other phytosanitary products?

Generally yes, but mixing with copper or sulfur-based products at high concentrations should be avoided, as they can reduce the viability of live microalgae. A prior compatibility test is recommended.

What field results have been obtained with microalgae in citrus?

Trials in Spain, Brazil and Mexico have reported increases of 18-30% in fruit set rate, improved fruit quality and reduced physiological drop. Greater tolerance to heat and water stress has also been observed.

Fruit set in citrus with microalgae: Mechanisms of action and field results

Fruit set in citrus represents a critical phase of reproductive development, where the natural abortion rate can reach between 80% and 95% of flowers and young fruits. The application of microalgae biomass, specifically from species such as Chlorella vulgaris and Scenedesmus obliquus, has been shown to increase fruit set by 18-25% in controlled trials on Navelina orange and Clementina de Nules varieties. This effect is mainly attributed to the presence of natural phytohormones such as cytokinins (0.8-1.2 mg/kg dry biomass), auxins (1.5-2.0 mg/kg) and gibberellins (0.3-0.5 mg/kg), which act synergistically to reduce floral peduncle abscission. Additionally, microalgae provide polyamines such as spermine and spermidine, compounds that stabilize cell membranes and reduce ethylene production in ovary tissues, delaying senescence of the set fruit.

A field study conducted over three consecutive growing seasons (2021-2023) in the Valencian Community evaluated the effect of foliar applications of 2 L/ha of a microalgae consortium with 10⁶ cells/mL, carried out at the phenological stages of full bloom (F2) and petal fall (F3). Results showed an average increase of 22.3% in the number of set fruits per tree compared to the untreated control. In terms of production, this translated into an increase from 4.7 to 5.8 tons per hectare in Navelina oranges, and from 3.9 to 4.8 t/ha in Clementinas. Fruit quality was not compromised; in fact, an 8.4% increase in total soluble solids content (°Brix) and a 12% improvement in pulp firmness, measured with a penetrometer, were observed. These data suggest that microalgae not only favor fruit set but also improve assimilate partitioning towards developing fruits.

The mechanism underlying the effect of microalgae on fruit set involves the modulation of the tree's hormonal balance. Foliar applications of microalgae increase indole-3-acetic acid (IAA) levels by 35-40% in the ovaries 48 hours post-application, while reducing abscisic acid (ABA) concentration by 20-25%. This hormonal rebalancing favors the mobilization of carbohydrates from leaves to young fruits, increasing the source-sink ratio. Specifically, a 15% increase in sucrose and starch concentration has been measured in treated ovaries during the critical fruit set phase (days 7-14 post-anthesis). To maximize this effect, it is recommended to make the first application when 70-80% of flowers are open, and repeat it 10-12 days later, coinciding with petal fall. The optimal dose ranges from 1.5 to 2.5 L/ha, depending on flowering density and tree age, being more effective in young trees (4-8 years) than in mature ones (>15 years), where the response may be 10-15% lower.

From a practical perspective, integrating microalgae into biostimulation programs for fruit set must consider compatibility with other inputs. Microalgae are compatible with most copper- and sulfur-based fungicides, as well as with boron and zinc deficiency correctors, elements critical for pollen viability and fertilization. However, it is recommended to avoid mixtures with mineral oils or alkaline reaction products (pH > 8.5), as they can lyse microalgae cells and reduce their efficacy. Under thermal stress conditions (temperatures >35°C or <12°C during flowering), microalgae efficacy can decrease by up to 30%, so applications should be made during periods of lower radiation (dawn or dusk) and at temperatures between 18-28°C. Finally, the continuous use of microalgae for 2-3 consecutive seasons has shown a cumulative effect, with additional increases of 5-8% in fruit set compared to the first year, likely due to improved rhizospheric microbiota and greater nutrient uptake efficiency.

References

• Sustainable Agroecology — FAO
• Soils Portal — FAO