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.

Algae and salt stress in rice: physiological and biochemical mechanisms

Seaweed extracts (such as those obtained from Ascophyllum nodosum or Ecklonia maxima) are used as biostimulants in rice grown under salt stress, and their possible usefulness is linked to the bioactive compounds usually described in them: betaines, polyphenols and substances that act like phytohormones (cytokinins, auxins and abscisic acid). These compounds are thought to take part in processes related to ionic homeostasis, that is, the way the plant handles excess sodium and keeps a suitable balance with potassium in its tissues. That balance is a key physiological parameter for cell viability, which is why it is one of the first things looked at when salt tolerance is assessed. In addition, osmotic stress generates reactive oxygen species, which the plant neutralises with its own antioxidant systems, including enzymes such as superoxide dismutase. The working hypothesis is that certain biostimulants may help the plant cope better with that load, but the response depends on the product, the variety, the timing of application and the severity of the salinity, and it should not be taken for granted.

From a biochemical point of view, one of the most frequently cited mechanisms is osmotic adjustment, which involves the accumulation of compatible osmolytes such as proline and glycine betaine. These compounds help the plant maintain a more favourable leaf water potential, cell turgor and stomatal opening when soil water is harder to access because of the salts. Along the same lines, the use of microalgae (for example Chlorella vulgaris and Spirulina platensis) added to irrigation water or applied ahead of the period of greatest stress has been explored, with the idea of preparing the plant metabolically to face ionic stress. These explanations are best read as plausible mechanisms rather than guarantees: an improvement in a physiological parameter, such as photosynthetic rate or proline content, does not automatically translate into higher yield in the field, and the rice variety has a strong influence on how the plant responds.

When it comes to agronomic performance, the sensible approach is to treat these applications as a complement to baseline salinity management, not a substitute for it. Irrigation water quality, drainage and salt leaching, the choice of tolerant varieties and a balanced nitrogen fertilisation plan remain the pillars. If a seaweed extract is tested, it is prudent to do so on a plot with an untreated control, under the same soil and irrigation conditions, and to compare the variables that matter to the grower: grain yield, grain quality (amylose content, proportion of broken grains) and nitrogen use efficiency. Reducing conventional nitrogen fertilisation on the expectation of a biostimulant effect is not advisable without technical advice and without having verified the response on the farm itself; nor should a seaweed extract be understood as a replacement for base fertilisers.

For orderly practical implementation, application is usually planned around two key moments in the rice cycle: the seedling stage, a phase in which rice tends to be sensitive to salt, and the start of panicle formation, another particularly sensitive phase. In flood irrigation systems, the addition of live microalgae (Chlorella spp.) to the irrigation water has also been explored, although this route requires control of water quality, compatibility with other inputs and management of the water layer. It is essential to adjust the programme to the real salinity of the soil and water: measure electrical conductivity before deciding, do not assume that a higher dose compensates for severe salinity, and record every application so you can later assess what worked. It is best to avoid applying outside the right stage, overdosing and skipping an untreated control for comparison. Doses, dilutions and intervals must always follow the product label and the recommendations of a technician who knows the plot.

Related articles

• Ecological agricultural biostimulants
• Certified organic fertilizers
• Natural bioprotectants for crops

References

• Sustainable agroecology — FAO