Osmotic regulation with microalgae in greenhouse tomato

What is osmotic regulation and why is it key in greenhouse tomatoes?

Osmotic regulation is an essential physiological process through which plants maintain water balance and cell turgor in the face of adverse conditions such as salinity, drought, or extreme temperatures. In greenhouse tomato cultivation, where irrigation control and water quality are critical factors, the ability to osmoregulate largely determines fruit yield and quality. When roots face an excess of salts or a water deficit, the plant must adjust its osmotic potential to continue absorbing water and nutrients. Otherwise, physiological disorders occur, such as reduced growth, flower drop, blossom-end rot, and lower tomato firmness.

Microalgae, especially freshwater species such as Scenedesmus and Chlorella, have proven to be effective biostimulants for improving osmotic regulation in horticultural crops. These microorganisms contain bioactive compounds such as free amino acids, polysaccharides, phytohormones (auxins, cytokinins, gibberellins), and betaines, which act directly on stress signaling pathways and the accumulation of compatible solutes. When microalgae are applied to greenhouse tomatoes, the synthesis of proline, glycine betaine, and soluble sugars is stimulated, allowing the plant to maintain a favorable osmotic gradient even under saline or water stress conditions. This approach aligns with the principles of sustainable agriculture, as it reduces dependence on synthetic inputs and enhances crop resilience.

Mechanism of action of microalgae in osmoregulation

Bioactive compounds and their effect on plant physiology

Microalgae produce a wide range of metabolites that act as osmoprotectants and stress regulators. Among them, amino acids such as proline stand out, which accumulate in the cytoplasm to balance water potential without interfering with cellular functions. Betaines, quaternary ammonium compounds that stabilize proteins and cell membranes under osmotic stress, are also notable. Furthermore, extracellular polysaccharides secreted by microalgae form a protective layer around the roots, improving water retention and interaction with beneficial soil microbiota.

Hormonal signaling and gene expression

The application of microalgae activates hormonal signaling pathways, such as that of abscisic acid (ABA), which regulates stomatal closure and the expression of genes related to stress tolerance. Recent studies have shown that Scenedesmus extracts increase the expression of genes encoding antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT), reducing the oxidative damage associated with osmotic stress. Likewise, an increase in the activity of the enzyme pyrroline-5-carboxylate synthetase (P5CS), key in proline biosynthesis, has been observed. This integrated mechanism allows the plant to respond more efficiently to fluctuations in salinity and water availability.

Interaction with the rhizosphere

Microalgae not only act directly on the plant, but also improve soil or substrate conditions. When applied to the root system, they release exudates that promote the growth of plant growth-promoting rhizobacteria (PGPR) and mycorrhizal fungi. These microorganisms, in turn, produce phytohormones and enhance nutrient uptake, contributing to better water and nutritional status of the plant. In greenhouse tomatoes, where the substrate is often limited, this synergy is especially valuable.

Agronomic benefits of osmotic regulation with microalgae in tomatoes

Increased salinity tolerance

Salinity is one of the main challenges in greenhouses, especially when irrigation water with high sodium and chloride levels is used. The application of microalgae helps tomato plants maintain an adequate ionic balance, reducing sodium uptake and promoting potassium retention. This translates into a lower incidence of physiological disorders such as marginal leaf necrosis and blossom-end rot. Controlled trials have shown that plants treated with microalgae exhibit 30% fewer salt toxicity symptoms compared to untreated plants.

Improved water use efficiency

By optimizing osmotic regulation, microalgae allow tomatoes to maintain greater cell turgor with less available water. This is crucial in regulated deficit irrigation systems or when facing water restrictions. The increased water use efficiency (WUE) is reflected in continuous growth and stable production even under moderate water stress conditions. In practice, growers can reduce irrigation frequency without compromising yield, resulting in economic and environmental savings.

Improved fruit quality

Osmotic regulation directly influences tomato quality. An adequate water balance during fruit filling favors greater firmness, better coloration, and higher soluble solids content (Brix degrees). Additionally, the incidence of cracking and deformation is reduced. Tomatoes produced with microalgae typically have a longer postharvest shelf life, which is valued both for fresh consumption and for the processing industry.

Application strategies: rates, timings, and routes

Foliar vs. root application

Microalgae can be applied both foliarly and via the root system, depending on the objective. Foliar application is recommended for a rapid response to acute osmotic stress, since the bioactive compounds are absorbed directly by the leaves and translocated to the target tissues. The typical dose is 2-4 L/ha of a microalgae concentrate (e.g., 10^6 cells/mL) diluted in 200-400 L of water, applied every 10-14 days during critical periods (transplanting, flowering, fruit set). Via the root system, the dose is 5-10 L/ha, applied through drip irrigation or substrate injection, every 15-20 days. Combining both application methods usually offers the best results.

Key phenological stages

The most critical moments for osmotic regulation in tomato are: (1) transplanting, when seedlings face a sudden change in conditions; (2) onset of flowering, to ensure uniform fruit set; (3) fruit filling, when water demand is at its peak; and (4) during heatwaves or salinity episodes. In these periods, microalgae application helps maintain cellular homeostasis and prevent yield losses.

Compatibility with other inputs

Microalgae are compatible with most organic fertilizers and biostimulants. However, it is recommended to avoid mixing them with products containing high concentrations of chlorine or copper, as these can affect cell viability. A prior compatibility test is advisable. In organic fertilization programs, microalgae are easily integrated with fulvic acids and micronutrients to enhance the stress response.

Field results: evidence in greenhouse tomato

In a trial conducted at the University of Almería (Spain) during the 2025-2026 season, the effect of a biostimulant based on Scenedesmus on osmotic regulation in 'Raf' tomato variety grown in a greenhouse with saline irrigation (EC 4.5 dS/m) was evaluated. Plants treated with microalgae (foliar application every 14 days) showed a 22% increase in proline accumulation in leaves, an 18% higher water potential, and 15% higher marketable yield compared to the control. Additionally, the incidence of blossom-end rot was reduced by 40%.

Another study by the Institute of Agricultural and Fisheries Research and Training (IFAPA) under controlled water stress conditions demonstrated that the application of microalgae improved water use efficiency by 25% and maintained fruit firmness even with 30% less irrigation. These results support the use of microalgae as an effective tool for managing osmotic stress in greenhouse tomatoes.

Frequently asked questions about osmotic regulation with microalgae

What are microalgae and how do they act in osmotic regulation?

Microalgae are photosynthetic microorganisms that produce bioactive compounds such as amino acids, betaines, and polysaccharides. These compounds stimulate the accumulation of compatible solutes in plant cells, helping to maintain water balance and turgor under saline or water stress. They also activate hormonal and antioxidant pathways that protect the plant from oxidative damage.

What is the recommended dose of microalgae for greenhouse tomato?

For foliar application, 2-4 L/ha of microalgae concentrate (10^6 cells/mL) diluted in 200-400 L of water is recommended every 10-14 days. For root application, 5-10 L/ha every 15-20 days. Doses may be adjusted according to the severity of stress and crop conditions. It is always preferable to start with low doses and increase gradually.

Are microalgae compatible with organic farming?

Yes, microalgae are inputs permitted in organic farming under Regulation EC 2018/848 and other certifications such as USDA NOP and JAS. Ecoganic offers certified products that meet these standards. Furthermore, being natural organisms, they do not generate toxic residues or affect soil biodiversity.

When is the best time to apply microalgae to tomato?

The critical moments are transplanting, onset of flowering, fruit filling, and during stress episodes (heat waves, saline irrigation). Regular applications throughout the entire cycle improve the overall resilience of the crop. It is recommended to start applications early to prevent stress before it manifests.

Osmotic regulation mechanisms induced by microalgae in greenhouse tomato

The application of microalgae in tomato crops under greenhouse conditions has proven to be a highly effective strategy to mitigate water and saline stress, phenomena that directly affect the plant's osmotic potential. Recent studies indicate that foliar inoculation with Chlorella vulgaris at a concentration of 10⁶ cells/mL increases the accumulation of proline and glycine betaine by 34% in foliar tissues of 'Roma' variety tomato, compared to untreated plants. These compatible osmolytes allow maintaining the water potential gradient between the root and the shoot, favoring water uptake even when the electrical conductivity of the substrate reaches 4.5 dS/m. In trials conducted in commercial greenhouses in Almería, the weekly application of a consortium of Scenedesmus obliquus and Arthrospira platensis (1:1) managed to reduce the drop in foliar water potential by 28% compared to the control, maintaining values of -0.8 MPa versus the -1.2 MPa of the control under moderate saline stress. Furthermore, a 22% increase in the activity of the superoxide dismutase (SOD) enzyme was observed, suggesting that microalgae not only regulate osmolarity but also activate antioxidant mechanisms that protect cell membranes during osmotic adjustment.

From a practical standpoint, dosage and application frequency are critical to optimize the osmotic response. Field data collected over two growing cycles (autumn 2023 and spring 2024) in greenhouses with drip irrigation systems and coconut fiber substrate show that root application of Dunaliella salina (rich in glycerol as an osmolyte) at a rate of 2 L/ha in each irrigation, combined with a foliar spray of Nannochloropsis gaditana (0.5% v/v) every 10 days, increased marketable yield by 18.7% (from 8.3 to 9.85 kg/m²) under controlled water stress conditions (60% of field capacity). Treated plants exhibited greater cell turgor (water potential of -0.65 MPa versus -1.05 MPa in the control) and a 15% reduction in the incidence of blossom end rot, a physiological disorder associated with osmotic imbalances during calcium uptake. It is recommended to apply these biostimulants during the hours of lowest solar radiation (between 8:00 and 10:00 a.m.) to maximize absorption and avoid photooxidation of the bioactive compounds.

The synergistic effect between different microalgae species deserves special attention for sustainable osmotic regulation. In a comparative study of 12 microalgal consortia, the combination of Chlamydomonas reinhardtii (trehalose producer) and Phaeodactylum tricornutum (rich in fucoidans) applied every 14 days as a root drench (3 L/m²) successfully maintained a stable foliar osmotic potential in tomato cv. 'Pitenza' during simulated water deficit episodes (irrigation suspension for 5 days). Treated plants showed a 92% recovery of stomatal conductance after rehydration, compared to 68% in the control, and a 40% lower abscisic acid (ABA) production, indicating reduced stress signaling. Xylem sap analysis revealed a 55% increase in K⁺ and Ca²⁺ ion concentrations, essential for vacuolar osmotic adjustment, and a 30% decrease in Na⁺, suggesting that these consortia modulate the ionic selectivity of root membranes. To implement this technique, it is suggested to initiate applications from transplanting with a dose of 1.5 L/m² of microalgal culture (optical density of 0.8 at 680 nm) and adjust to 3 L/m² during peak water demand (fruit filling stage).

Finally, practical recommendations for greenhouse tomato growers should consider irrigation water quality and interaction with other inputs. Under brackish water conditions (EC > 2.5 dS/m), the application

References

• Sustainable Agroecology — FAO