Salinity resistance in rice with algae

Introduction

Rice cultivation (Oryza sativa) is essential for global food security, but its productivity is severely affected by soil and irrigation water salinity. It is estimated that over 800 million hectares of agricultural land worldwide are affected by salts, and rice, classified as moderately sensitive, suffers yield reductions that can exceed 50% under severe salt stress conditions. In regions such as the Mediterranean, the Ebro Delta, or the Guadalquivir marshes, saltwater intrusion and sodium chloride accumulation in the soil profile are growing problems exacerbated by climate change and aquifer overexploitation.

Facing this challenge, biostimulants based on unicellular algae emerge as an effective and sustainable tool. Microalgae, such as Scenedesmus and Chlorella, produce bioactive compounds —phytohormones, compatible osmolytes, polysaccharides, and antioxidants— that activate the plant's natural defenses and improve its ability to tolerate abiotic stress. This article provides an in-depth analysis of the mechanisms by which algae confer salinity tolerance in rice, recommended doses, application timings, and agronomic results documented in field trials.

Salinity tolerance mechanisms in rice

Salinity affects rice through two main components: osmotic stress and ionic stress. Osmotic stress occurs due to the reduction in soil water potential, hindering water uptake by roots. Ionic stress, on the other hand, results from the accumulation of Na+ and Cl- ions in tissues, which alter cellular metabolism, inhibit key enzymes, and cause toxicity. Additionally, excess sodium displaces potassium, calcium, and magnesium, inducing nutritional imbalances.

Rice possesses natural tolerance mechanisms that include sodium exclusion at the root level, vacuolar compartmentation, synthesis of osmolytes such as proline and glycine betaine, and activation of enzymatic antioxidant systems (superoxide dismutase, catalase, ascorbate peroxidase). However, when salinity exceeds certain thresholds — soil electrical conductivity >3 dS/m — these mechanisms become insufficient, and external support is required to maintain productivity.

Recent research has shown that the application of microalgae-based biostimulants can significantly enhance these endogenous mechanisms. For example, field trials have demonstrated that treatment with Scenedesmus increases the activity of antioxidant enzymes by 30-40% in rice plants subjected to salt stress, reducing lipid peroxidation and cellular damage. Furthermore, the phytohormones present in algae — such as cytokinins and auxins — stimulate root growth, improving soil exploration and water and nutrient uptake.

Unicellular algae as biostimulants against salt stress

Biochemical composition and mechanisms of action

Microalgae are unicellular photosynthetic organisms that produce a wide range of bioactive compounds. In the context of salinity resistance in rice, the following groups stand out:

• Phytohormones: auxins, gibberellins, cytokinins, abscisic acid, and brassinosteroids. They regulate growth, stomatal opening, and the expression of stress tolerance genes.
• Compatible osmolytes: proline, glycine betaine, trehalose. They help maintain cellular osmotic balance without interfering with metabolism.
• Exopolysaccharides (EPS): improve soil aggregation, moisture retention, and rhizospheric microbial activity.
• Antioxidants: carotenoids, polyphenols, superoxide dismutase. They neutralize reactive oxygen species generated by salt stress.
• Essential nutrients: nitrogen, phosphorus, potassium, magnesium, iron, zinc. Algae act as slow-release biofertilizers.

The application of Scenedesmus biomass to the soil or foliarly triggers a cascade of signals in the plant. For example, cytokinins promote cell division in root meristems, while abscisic acid regulates stomatal closure to reduce water loss. Additionally, compatible osmolytes accumulate in the cytoplasm, counteracting osmotic stress and protecting protein structures.

Available scientific evidence

Agronomic studies have evaluated the effect of Scenedesmus on rice under controlled salinity conditions. In a representative trial, rice plants treated with 1 g/L of Scenedesmus biomass showed a 25% increase in root length and an 18% increase in shoot biomass compared to the saline control. Proline concentration in leaves increased by 40%, and superoxide dismutase activity was 35% higher. These results indicate a clear improvement in stress tolerance.

Another study demonstrated that the application of Chlorella vulgaris on rice reduced sodium uptake by 20% and increased the K+/Na+ ratio by 30%, improving the nutritional status of the plants. Furthermore, grain yield increased by 15% compared to the saline treatment without algae. These findings support the use of microalgae as a viable strategy to mitigate the effects of salinity in rice cultivation.

Application of algae in rice: doses and phenological stages

Application method

Microalgae can be applied both to the soil (root zone) and foliarly. The root application method is especially effective for improving soil structure and microbial activity, while the foliar method allows for rapid absorption of bioactive compounds. In rice, a combination of both is recommended to maximize benefits.

• Soil application: incorporate the algae biomass into the irrigation water or directly into the soil before sowing. Dose: 2-5 kg/ha of dry biomass, depending on the product concentration.
• Foliar application: spray onto the leaves during early crop stages. Dose: 1-2 L/ha of algae suspension (concentration 1 g/L).

Key phenological stages

Rice is most sensitive to salinity during the seedling and flowering stages. Therefore, applications should coincide with these critical periods:

• Seedling stage (15-20 days after sowing): the first foliar application helps establish a robust root system and activate early tolerance mechanisms.
• Tillering stage (30-40 days): a second application (root or foliar) reinforces the plant's ability to maintain vegetative growth under saline stress.
• Flowering stage (60-70 days): a third foliar application protects the reproductive organs and improves grain set, a time when salinity can cause sterility and yield losses.

Exact doses must be adjusted according to the concentration of the commercial product and the specific soil and climate conditions. A soil electrical conductivity analysis is recommended to determine the severity of the saline problem and adjust the application program.

Agronomic results and benefits

Improved yield and grain quality

Field trials conducted under moderate salinity conditions (EC 4-6 dS/m) have reported yield increases of 12-20% in rice treated with microalgae compared to untreated controls. The improvement is attributed to a higher photosynthesis rate, better grain filling, and reduced floral sterility. Additionally, grain quality, measured by the percentage of whole grains and amylose content, is maintained or improved, which is relevant for commercialization.

Additional benefits

• Reduced oxidative stress: algae decrease the accumulation of malondialdehyde (MDA) by 25-30%, an indicator of oxidative damage.
• Improved nutrient uptake: the increase in the K+/Na+ ratio and the greater availability of micronutrients such as zinc and iron contribute to more balanced nutrition.
• Sustainability: microalgae are a renewable resource, cultivated in controlled systems, that generate no toxic waste and integrate perfectly into organic agriculture.
• Compatibility with other inputs: they can be combined with organic fertilizers and other biostimulants, enhancing synergistic effects.

The use of unicellular algae therefore represents a technically and economically viable solution for farmers facing salinity problems in rice cultivation. The investment in biostimulants is recovered through increased production and reduced losses, while improving long-term soil health.

FAQ

How do algae help improve salinity resistance in rice?

Unicellular algae produce bioactive compounds such as phytohormones, osmolytes, and antioxidants that activate the plant's defense mechanisms. These compounds improve the plant's ability to maintain water balance, protect cell membranes, and detoxify reactive oxygen species generated by salt stress.

What is the recommended dose of algae for rice under saline conditions?

It is recommended to apply between 2 and 5 kg/ha of dry algae biomass to the soil, or 1-2 L/ha of foliar suspension (concentration 1 g/L). Applications should be made at key stages: seedling, tillering, and flowering, adjusting according to the severity of salinity.

Are algae compatible with organic fertilizers?

Yes, microalgae are fully compatible with organic fertilizers and other biostimulants. In fact, their combination can generate synergies that further improve stress tolerance and crop nutrition. It is recommended to consult with a technical advisor to design an integrated program.

What type of algae is most effective for salinity resistance in rice?

The most studied species are Scenedesmus and Chlorella. Both have shown positive effects on salinity tolerance, although Scenedesmus stands out for its high production of osmolytes and polysaccharides. The choice depends on the availability of the commercial product and the specific conditions of the crop.

Physiological and biochemical mechanisms of algae in mitigating salt stress in rice

The application of seaweed extracts (mainly Ascophyllum nodosum and Ecklonia maxima) in rice crops subjected to salt stress has demonstrated significant increases in tolerance, with improvements of up to 35-40% in germination rate under conditions of 100 mM NaCl. This effect is attributed to the presence of bioactive compounds such as betaines, polyphenols, and phytohormones (cytokinins, auxins, and abscisic acid) that modulate gene expression related to ionic homeostasis. Field studies in saline soils of the Mekong Delta (Vietnam) reported that foliar application of 2 L/ha of seaweed extract at the tillering stage reduced sodium concentration in leaf tissues by 28% and increased the K+/Na+ ratio by 42%, a critical parameter for cell viability. Additionally, a 22% increase in the activity of the enzyme superoxide dismutase (SOD) was observed, suggesting a greater capacity to detoxify reactive oxygen species generated by osmotic stress.

From a biochemical perspective, seaweeds act on the biosynthesis of compatible osmolytes such as proline and glycine betaine. In controlled trials with the rice variety IR64 (susceptible to salinity), the application of a microalgae consortium (Chlorella vulgaris and Spirulina platensis) at a rate of 5 g/L in irrigation water for 10 days prior to salt stress (EC of 6 dS/m) increased proline accumulation by 67% compared to the untreated control. This increase in osmolytes allowed maintaining a leaf water potential of -1.2 MPa versus -1.8 MPa in stressed plants without treatment, improving cell turgor and stomatal opening. As a result, the net photosynthetic rate remained at 18.5 µmol CO2/m²/s, 31% higher than untreated plants under the same salinity conditions. These data indicate that seaweeds not only act as direct biostimulants but also metabolically prepare the plant to face ionic stress.

In terms of agronomic performance, results from experimental plots in the Punjab region (India) with salt-affected soils (ECe of 4.8 dS/m) showed that the combined application of seaweed extract (1.5% v/v) plus a 20% reduction in conventional nitrogen fertilization achieved a grain yield of 4.2 t/ha, compared to 2.9 t/ha in the saline control without treatment. This represents a 44.8% improvement in productivity. Grain quality analysis revealed that the amylose content remained at 22.3% (within the optimal range for human consumption), while the control showed a reduction to 18.7%, associated with a higher percentage of broken grains. Additionally, nitrogen use efficiency (NUE) improved by 38%, increasing from 32.5 kg grain/kg N applied in the control to 44.9 kg/kg in the seaweed treatment. These data suggest that seaweeds can partially replace synthetic fertilizers without compromising productivity under saline stress conditions.

For effective practical implementation, it is recommended to apply seaweed extracts at two key stages of the rice cycle: first, at the seedling stage (10-14 days after sowing) at a dose of 1.5-2 L/ha via foliar application, and second, at the beginning of panicle formation (approximately 45-50 days after transplanting) at a dose of 2-3 L/ha. In flood irrigation systems, the incorporation of live microalgae (Chlorella spp.) at a density of 10⁵ cells/mL in the irrigation water for 5-7 consecutive days has shown promising results, reducing the electrical conductivity of the water by 15% due to the active uptake of Na+ and Cl- ions. It is crucial to adjust the dose according to the soil salt concentration: for moderate levels (EC of 3-5 dS/m), a dose of 2 L/ha of seaweed extract is recommended, while for severe levels (EC > 6 dS/m), it can be increased up to 4 L/ha, always

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References

• Sustainable agroecology — FAO