Seaweed extracts have emerged as potent biostimulants in sustainable agriculture, with the aim of boosting plant productivity and resilience to environmental stresses. Seaweed extraction encompasses both conventional and green extraction techniques, such as acid or alkaline hydrolysis, enzyme-assisted extraction, microwave-assisted extraction, and ultrasound extraction. Seaweed extracts regulate hormonal balance, control nutrient transporters, accelerate photosynthesis, and improve stress responses through complex signalling processes and gene expression modifications. Seaweed extracts are rich in bioactives, including polysaccharides, proteins, polyunsaturated fatty acids, vitamins, pigments, polyphenols, minerals, and plant growth hormones. Interestingly, the hydrophilic qualities and chelating abilities of the polysaccharides found in seaweed make them useful in many sectors. The hydrophilic characteristics were instigated by the presence of OH, COOH, CONH, and SO3H in the polymer structure. Moreover, the indirect effects of seaweed are to enhance soil health by encouraging beneficial microbial populations and nutrient cycling. This review offers fresh insights into seaweed production and its diverse impacts on soil-plant ecosystems, emphasizing their transformative potential in evolving sustainable agricultural practices.
Introduction
Seaweed extracts are one of the most promising biostimulants. The fastest-growing biostimulant business is represented by seaweed extract. Unlike terrestrial animals, seaweeds produce various stress-related chemicals that are important for their survival. All seaweeds at some stages of their life cycle are unicellular, such as zygotes or spores, and may be temporarily planktonic. It is reported thatin coastal and marine environments, seaweeds are multicellular macroscopic organisms’ rich in polysaccharides, enzymes, polyunsaturated bioactive peptides, and fatty acids. Many seaweeds exhibit activities that promote plant growth, which has led to their continued and widespread use as organic fertilizers and manure in horticulture and agriculture.
Origin and overview of species used for extract production
Seaweeds and macroalgae are thought to comprise roughly 10,000 species. They were primarily divided into three groups: Phaeophyta (brown), Rhodophyta (red), and Chlorophyta (green). They were classified based on their pigmentation. Approximately 2,000 species of brown seaweed make up the second most abundant group; they are found in temperate zones on rocky shorelines, where their biomass is the highest. Among the brown seaweeds, Ascophyllum nodosum, Macrocystis pyrifera, Ecklonia maxima, and Durvillea potatorum are the most frequently used commercially by extraction industries and other commonly used seaweed species are listed in Table1. It is stated that brown seaweed extracts are commonly utilized in horticulture crops as they promote plant development and improve crop tolerance to abiotic challenges such as severe temperatures, flooding, drought, nutrient deficiencies, and salinity.In the case of red algae, which is the largest group of marine macroalgae found in the deep sea. Majority of green seaweed live in freshwater. Blue-green algae are widespread on rocky and temperate sandy shores.
| Brown algae (Phaeophyta) | Ascophyllum nodosum, Fucus vesiculosus, Ecklonia maxima, Durvillea protatorum, Sargassum spp, Durvillea antarctica, Laminaria digitata, Fucus spiralis |
| Red algae (Rhodophyta) | Macrocycstis pyrifera, Porphyra perforate, Kappaphycus alvarezii, Gracilaria edulis, Gracilaria dura, Hypnea pannosa, Gracilaria corticata |
| Green algae (Chlorophyta) | Ulva lactuca, Enteromorpha prolifera, Ulva armoricana, Halimeda opuntia |
Chemical composition of seaweed extract
Several bioactive compounds in the seaweed extracts were shown in Figure 1.
Proximate composition
Seaweeds provide critical amino acids such as alanine, glutamic acid, proline, glycine, arginine, and aspartic acid. A total of 52.27% and 53.89% of the amino acids were found in the two red seaweeds, Hypnea pannosa and Hypnea musciformis, respectively, which together contained all the necessary amino acids. However, the brown seaweed Sargassum ilicifolium had the highest concentration of crude protein compared to the green seaweeds Ulva lactuca. According to a prior study, Phaeophyta seaweed species had a greater protein content than Chlorophyta and Rhodophyta seaweed species. Gracilaria corticata has the lowest protein content in red algae. Another recent study found that the green alga, Halimeda opuntia had the ultimate ash level, whereas Ulva lactuca had the lowest. Both green seaweed (Chaetomorpha antennina) and brown seaweed (Padina antillarum) contain excessive concentrations of ash. Furthermore, the green seaweed Caulerpa chemnitzia had the highest dietary fiber content compared to Acanthophora spicifera, Ulva rigida, Ulva intestinalis (Chlorophyta), and Sargassum wightii (Phaeophyta). Thus, brown seaweed species contain the most protein, and dietary fiber and ash content were more prominent in the seaweed species of Chlorophyta.
According to Vijay et al., more minerals are found in brown seaweeds (Laminaria , Sargassum, and Undaria) than in red seaweeds (Porphyra). Similarly, many brown seaweeds have C: N ratios over 15-20, whereas some green seaweeds (e.g., Ulva) have a low ratio of 10 or lower. In addition, Phaeophyta have abundant water-soluble vitamins, such as riboflavin, thiamine, and nicotinic acid.
Ultimate composition
Rhodophyta is primarily composed of phycoerythrin, which gives it a reddish or purplish color. It is observed that only chlorophyll a is present in red seaweeds, and since light is only absorbed in the visible spectrum’s red and blue areas, there is an absorption gap in the center. It is found that to close this gap and enhance light processing, red seaweeds develop phycobilisomes (PBS) in their thylakoid membrane. Brown algae color from a pigment called fucoxanthin, which is one of the most prevalent carotenoids (comprising approximately 10% of all carotenoids produced in nature). Furthermore, the primary cause of the characteristic green hue in Chlorophyta is that it contains the same amounts of chlorophyll a and b as higher plants.
Hydrocolloids, which are usually produced from seaweeds and are mostly extracted from certain species and genera of brown and red seaweeds, are known as alginate, agar, and carrageenan. Agarose can be produced with good gelling characteristics, and its gelling and melting points fluctuate between 30 and 40°C and 80-90°C, respectively, depending on the concentration, molecular weight, and density of the side group. Agarose is widely used in the textile industry, bakery products, and gel electrophoresis. Moreover, the gelling point of 30-50°C and melting point of 50-70°C of carrageenan make it an excellent thickening, emulsifying, and gelling agent. It is widely used in food and beverages, the paint industry, and as a suspending agent in antacids. Similarly, alginate is a substance with good gelling features that is composed of repeating units of 阝-1,4- linked d-mannuronic acid (M) and l-guluronic acid (G) in different ratios and generates gels by chemical or physical bonding. The hydrophilic characteristics were caused by OH, COOH, CONH, and SO3H. Thus, we can deduce that the hydrogel expands in water because water molecules become trapped inside the pores, changing the hydrogen bonding. Additionally, bacteria release different hydrogels and organic debris into soil via enzymatic breakdown.
Brown seaweeds contain phlorotannins, which are potent plant elicitors that stimulate a variety of metabolic pathways, including the phenylpropanoid/ shikimate pathway, which is involved in the production of phenolic compounds. In brown seaweed, polyuronoides (fucoidans and alginates) are also present, and their hydrophilic properties, gelling, and chelating powers make them valuable constituents in the food processing, agriculture, and pharmaceutical sectors. Laminarins are bioelicitors of plant defense responses against fungal and bacterial pathogens. Green seaweedsare known to produce the complex, acidic polysaccharide ulvan, which is used in cosmetics and medical industries. Likewise, agar and carrageenan are found in significant amounts in red seaweeds such as Gracilaria and Kappaphycus. These industrial polysaccharides are frequently used in antifouling, antimalarial, and antibiotic applications in the culinary, pharmaceutical, textile, paint, and biotechnological sectors.
Diterpenes and sesquiterpenes such as dictyols, zonarols, and dolabellane are abundant in brown algae belonging to the genera Dictyota and Dictyopteris. These compounds have a diversity of intriguing biological activities, encompassing cytotoxic, antifungal, antiviral, and antibacterial properties. However, the Laurencia genus of red algae produces secondary metabolites, primarily terpen oids. Moreover, sesquiterpenes (caulerpenynes) from the green alga Caulerpaprolifera have cytotoxic and antimicrobial properties.
Nutrient uptake and assimilation
Numerous vital nutrients such as micronutrients, trace elements, and growth-promoting hormones are abundant in seaweed extracts. This may aid in improving the nutrition of plants, thereby promoting their growth and development. For example, Ulva linza contains a variety of nutrients such as Na+, K+, Ca2+, Cl”, and Mg2+. These findings imply that green seaweed may be studied in the future for use in the food, cosmetic, pharmaceutical, or dietary supplement industries. According to study, adding 225 L ha–1 Saccharina japonica extract to loamy sand results in increase in total nitrogen and soil organic carbon. Application of Laminaria and Ascophyllum nodosum extracts in maize showed that the leaves could significantly absorb more Zn, B, S, Fe, Mo, Cu, Mg, Ca, and Mn than the controls. Seaweed extracts provide nutrients that are easily absorbed by leaves via the stomata and hydrophilic pores in the cuticle.
The activation of enzymes involved in lignin production and photosynthesis is copper dependent. Lignin strengthens cell walls and provides structural integrity to plants. In particular, applying brown seaweed extract topically to grapevine leaves makes them more susceptible to Cu absorption. Similarly, several enzymes involved in photosynthesis, respiration, and nitrogen metabolism are also stimulated, seaweed extract @ 3 % showed improved biochemical and physiological characteristics in banana, viz., potassium (363.38 mg/100 g), calcium (5.63 mg/100 g), starch (14.30%), reducing sugars (11.15%), total carbs (29.57 g), Total sugar (18.95%), carotene (36.65 µg/g), and non-reducing sugars (7.41%). Likewise, in tomatoes seaweed extract concentration @10% attained higher concentrations of vital minerals such as potassium (563.21 mg/100 g), zinc (0.86 mg/100 g), calcium(152.28 mg/100 g), iron (15.32 mg/100 g), and magnesium (41.44 mg/100 g) than the control. Thus, the application of seaweed extract showed increased nutrient uptake in plants and some examples are listed in Table 2.
| Seaweed species | Crop | Mode of application | Observation |
| Ascophyllum nodusum | Grape | Foliar spray | Increased uptake of Cu, Ca2+, and K+ |
| Rapeseed | In nutrient solution | Increasing concentration of Cu, Mg, and Mn in the entire plant | |
| Ecklonia maxima | Tomato | Foliar spray | Increased fruit Ca concentration |
| Mustard | Root application | Increased P&K level | |
| Ascophyllum nodusum and Laminaria spp. | Maize | Foliar spray | Increased absorption of Zn, Fe, S, Mg, Ca, B, Cu, Mo and Mn |
| Kappaphycus alvarezii and Gracilaria edulis | Maize | Foliar spray | Enhanced N, P, K uptake |
Methods of preparation of seaweed extracts
Seaweeds must be processed into liquid extracts or soluble powders because they decompose slowly when dried and can initially hinder plant growth by producing poisonous sulfhydryl chemicals that can linger for up to 15 weeks. The biological activity of these extracts strongly depends on the extraction method and raw material, which can be acid extraction, alkali extraction, or further technology such as microwave, ultrasound and enzyme assisted extraction. The bioactive compounds which can be extracted from seaweeds using different methods are tabulated in Table 3.
| Seaweed species | Extraction process | Bioactive compound | Extraction yield(% of bioactive compound) |
| Undaria pinnatifida | Enzyme assisted extraction | Fucoxanthin | 94% |
| Kelp | Enzymatic hydrolysis | Alginic acid | 20.16 g/l of seaweed |
| Polyphenol | 205.56 mg/l of seaweed | ||
| Oligosaccharide | 4.00 g/l of seaweed | ||
| Mannitol | 12.10 g/l of seaweed | ||
| Free amino acid | 5.00 g/l of seaweed | ||
| Sargassum cymosum | Alkaline extraction | Sodium alginate | 45.79% |
| Undaria pinnatifida | Microwave assisted extraction | Fucoidan | 12.3% |
| Ecklonia maxima | Sub critical water extraction | Phenolics (Phloroglucinol) | 26.02 ± 0.05 mg/g of seaweed |
| Acid extraction | Sodium alginate | 58.7% | |
Kappaphycus alvarezii | Ultrasound-microwave assisted extraction | Polysaccharides | 61.25 ± 0.70% |
| Saccharina lactisima | Solvent extraction | Protein | 10.6% |
| Ascophyllum nodosum | Alkaline sub critical water extraction | Crude | 80.52 % |
| Alginate | 38.25 % | ||
| Fucoidan | 39.38 % |
Acid hydrolysis
In the process of acid hydrolysis, freshly chopped seaweed (Ascophyllum) biomass was exposed to sulfuric or hydrochloric acid at 40-50°C for 30 min. Acid hydrolysis has been reported to promote polysaccharide depolymerization and eliminate complex phenolic compounds. For instance, the green seaweed Valoniopsispachynema could provide a large quantity of total sugar (38.5 ± 0.2 mg/g) by hydrolysis with diluted 3% sulphuricacid, which greatly increased algal biomass and lipid production. In addition, someone achieved the maximum sugar extraction (153.00 ± 5.96 mg/g biomass) using 3.5% w/v HNO3 for 10 g dried Sargassum cinereum biomass.
Alkaline hydrolysis
In Alkaline hydrolysis, the biomass of A. nodosum is removed in KOH or NaOH solutions at “relatively low” temperatures ranging from 70 to 100°C. For instance, a maximum yield of 51.84% sodium alginate was obtained via alkaline extraction at 80°C for 195 min of extraction time, demonstrating the viability and cost- effectiveness of the process.
Microwaveassisted extraction (MAE)
In microwave-assisted extraction, microwave radiation is used to heat a slurry prepared from dried algal biomass in either water or a solvent supported by microwaves, to extract bioactive chemicals. The microwave heating mechanism relies on the ionic conduction of bioactive chemicals produced from seaweed into the solvent along with dipole polarization. Someone found that the maximum yield of ulvan (41.9%) from Ulva pertusa, which had significant antioxidant properties, was obtained in vitro when an MAE of 600 Wof microwave radiation was applied for 43.63 min, with a water-to-raw-material proportion of 55.45 ml/g and a pH of 6.57.
Ultrasound assisted extraction (UAE)
In UAE, ultrasonic waves travel through liquid, solid, and gaseous media at high frequencies (over 20 kHz) by rarefactions, which causes the longest gap between wave particles and compression, which causes the lowest space between wave particles. Under ultrasonic waves, the extraction solvent cavitates, allowing the release of bioactive compounds from a range of seaweed biomass. For instance, the yields achieved using UAE ranged from 20.4-36.9%, whereas those obtained with traditional extraction were between 10.5% and 19.3%. The extraction yield increased from 1.5- fold to 2.5-fold under UAE conditions (35 kHz, 50% ethanol, and 30 min). Another study reported that under ideal ultrasound settings (15 min and buffer solvent), the maximum phycoerythrin yield (33.85 mg/g) for Padina cruentum was achieved. Else state that, using NaOH as a solvent, UAE achieved a 3.4% protein yield (w/w) from Neoporphyra haitanensis after 20 min of ultrasound treatment at ambient temperature.
Enzyme assisted extraction (EAE)
Digestive enzymes such as carbohydrases and proteases extract bioactive compounds from seaweeds (Sargassum fulvellum, Ecklonia cava, Sargassum horneri, and Sargassum thunbergii). These enzymes can transform water-insoluble compounds in seaweed into water-soluble compounds. Someone reported that this technique is important for the synthesis of seaweed gel with an average yield of 28.65% because it can also produce soluble materials that are insoluble in water. Enzymatic hydrolysis, specifically utilizing the “Miura” enzymatic cocktail (glycoside, β-glucanase, hydrolase, xylanase, and cellulase), demonstrated enhanced outcomes in the extraction of soluble proteins, carbohydrates, phycocyanin, and phycoerythrin, with increases of approximately 890%, 65%, 510%, 195%, respectively, in comparison to the optimal alkaline hydrolysis extraction (2.5% Sodium carbonate and 80°C).
Alkaline extraction at high pressure is the most popular extraction method. The advantage of this approach is its prominent level of extractability and mild polysaccharide degradation into oligomers, which are the physiologically active components of seaweed extracts.
Mechanism of action: Molecular and metabolic pathway
Seaweed extracts regulate hormonal balance, control nutrient transporters, accelerate photosynthesis, and improve stress responses via complicated signaling processes and gene expression modifications. Seaweed extracts also improve carbon and nitrogen metabolism by activating defense systems and modifying metabolite profiles, such as TCA cycle metabolites and secondary metabolites, which modulate plant metabolic pathways. For example, the application of Ascophyllum nodosum extract in Arabidopsis downregulates the degradation of chlorophyll genes (AtCLH1 and AtCLH2) and upregulates drought-responsive genes in soybean, which prevents cold stress and drought stress. In addition, upregulation was associated with transcription genes (COR78/RD29A and DREB1A), which regulate cold stress tolerance by cryoprotecting chloroplasts. Someone found that in Arabidopsis thaliana, treatment of extremely sulphated λ-carrageenan (35% sulphation) was linked with a higher activity of oxalate oxidase and expression of Jasmonic acid signalling linked genes, such as, plant defense (PDF1.2), pathogenesis related (PR-3), and alleneoxide synthase (AOS).
Someone reported that S. vulgare extract decreasedthe infection level of Alternaria solani, Xanthomonas campestri sp. Vesicatoria in Sweet pepper and tomato, thereby increasing defense enzyme activity, including peroxidase, ammonia-lyase, β-1,3-glucanase, phenylalanine, polyphenol oxidase, and also upregulating the PR1- a, ETR-1, and PinII genes linked to ET and Jasmonic acid mediated signaling. Furthermore, the mechanism by which Na+ is bound to roots is due to an increase in the concentration of Na+ in the soil solution, which stimulates passive Na+ transport via a group of non-selective cation channels in root cells, which is carried by mass flow via the xylem to the shoots. Consequently, the loss of the Na+ transporter HKT1 gene in Arabidopsis causes hypersensitivity to salt in mutant lines, resulting in elevated Na+ levels in the leaves. Despite this, the pretreatment of Arabidopsis with Ascophyllum nodosum extract (Super fifty) resulted in considerable upregulation of genes in hormonal signaling, precise photosynthesis, and growth under both oxidative stress and control conditions. Recently, someone suggested that use of a seaweed extract under water stress conditions resulted in a substantial rise in the content of syringic acid, pelargonidin-3-rutinoside, cyanidin-3-glucoside, chlorogenic acid, rutin, p-coumaric acid, epicatechin, and pelargonidin- 3-glucoside while, quercetin, catechin, sinapic acid, caffeic acid, epigallocatechin gallate, and ellagic acid were dramatically reduced.
Influence of seaweed extract on plant growth and development under biotic stress
The Ascophyllum nodosum extract provides elicitor chemicals that encourage increased enzyme activity to alleviate stress, thereby strengthening the plant’s defense system against biotic stressors. In Ascophyllum nodulosum extract treatment, betaines may support healthy plant growth by acting as stress- induced osmoprotectors in plant cells. Seaweed extracts promote early flowering and boost fruit development in various agricultural plants, including tomatoes and peppers. Seaweed extracts trigger plant defense mechanisms against devastating bacterial, viral, and fungal infections, shielding crops from diseases that may result in significant financial losses.
The impact of powdery mildew, late blight triggered by Phytophthora infestants, and gray mold caused by Botrytis cinerea decreased by 37%, 36%, and 80%, respectively, after foliar application of Sargassum fusiforme extract on tomato plants. Someone found that the methanolic extracts of Cystoseria myrica and S. cinereum exhibited the greatest antifungal activity by increasing the total free amino acid content and antioxidant enzyme activities of Fusarium oxysporum infected roots and shoots. Else identified that the application of Sargassum horneri extract increases the photosynthetic ability of tomatoes and boosts tomato yield by 4.6 to 6.9%. In addition, the application of Sargassum tenerrimum extract on Macrophomina treated tomato plants showed a considerable increase inindole acetic acid and kinetin levels throughout the vegetative and generative phases. In the investigation, it is reported that, the extract of Gelidium serrulatum evoked salicylic acid and jasmonate signalling and reduced the bacterial and fungal diseases. The impact of different species of seaweed extract on plants against plant pathogens are listed in Table 4.
| Crop | Seaweed species | Disease | Causal organism | Findings |
| Arabidiopsis | Ascophyllum nodusum | Bacterial disease | Pseudomonas syringae and Xanthomonas. campestris | Expression of defense -relate genes is stimulated |
| Strawberry | Ascophyllum nodusum | Powdery mildew | Podosphaera aphanis | Defence enzymes and secondary metabolites expression is increased |
| Grape | Ascophyllum nodusum | Bunch rot | Botyrtis cinerea | Defence-related gene expression is increased |
| Wheat | Ascophyllum nodusum | Septoria tritici blotch | Zymoseptori atritici | Enhanced expression of antioxidant metabolism, octadecanoid-based pathways, phenylpropanoid and defense-related PR protein, |
| Tomato | Sargassum tenerrimum | Charcoal rot | Macrophomina phaseolina | Stimulate the defence-related hormones and genes |
| Sesame | Sargassum tenerrimum | Charcoal rot | Macrophomina phaseolina | Stimulate physio-bioche- mical pathways |
| Potato | Ascophyllum nodosum and Laminaria digitata | Potato cyst-forming nematode | Globodera rostochiensis | Hatching of juvenile nematode is delayed and reproductive capacity is reduced |
Consequences of seaweed extract on plant growth and development under abiotic stress
Seaweed extract treatment may promote plant development, yield, and quality by promoting the buildup of secondary metabolites such as simple sugars, abscisic acid, proline, alcohols, and antioxidant chemicalsand/or stimulating metabolic pathways, thereby minimizing the detrimental impacts of salinization and their impact on some of the plant under abiotic stress (Table 5). Hormone pathways also mediate various aspects of plant growth and development, including as nutrient uptake, and the molecular and physiological reactions to abiotic stress. Plants exposed to salt and drought that are treated with seaweed extracts show increased proline and sugar (glucose, fructose, and sucrose). In addition, seaweed extract-treated crops showed enhanced growth and survival in drought and salinized environments, recovering from the detrimental impacts of frost and heat. Moreover, Seaweed extracts diminish abiotic stressors by boosting the antioxidant capacity of enzymes that scavenge reactive oxygen species (ROS), such as catalases, superoxide dismutase, peroxidases, and phenolic antioxidants. When crops were subjected to abiotic stress, seaweed extracts increased the expression of genes that respond to stress, including Na+/K+ transporters and late embryogenesis abundant (LEA) proteins like dehydrins and aquaporins. Genes that respond to drought, including LEA4-5 , LEA4-6, and LEA7, were also strongly upregulated after treatment with a biostimulant based on Ascophyllum nodusum. Therefore, we can find that seaweed bioactive compound and mechanism helps plant to grow under stress conditions.
| Stress | Crop | Seaweed species | Findings |
| Freezing stress | Arabidopsis | Ascophyllum nodusum | Lipophilic compound preserves membrane integrity and modifies the expression of genes that are sensitive to freezing stress, improving freezing tolerance. |
| Salt Stress | Turf grass | Ascophyllum nodusum | Enhanced the growth by maintaining the K+/Na+ ratio higher. |
| Arabidopsis | Ascophyllum nodusum | Decreased oxidative damage caused by salinity by promoting glutathione S transferase expression. | |
| Avocado | Ascophyllum nodusum | Reduced the impact of salt stress on avocado growth and yield by increasing nutrient absorption. | |
| Canola | Gelidium crinale, Cystoseira spp., and Ulva lactuca | Alleviated the growth inhibition caused by NaCl by dramatically lowering the inhibition of total carbohydrate accumulation, chlorophyll a and b. | |
| Tomato | Padina gymnospora | Under salt stress, seaweed-treated plants shown a 3.4% decrease in production, whereas untreated plants exhibited a 28.7% decrease as seaweed compared to treated plants. | |
| Drought Stress | Tomato | Ascophyllum nodusum | Increased resilience to drought stress by regulation of dehydrin expression. |
| Phaseolus vulgaris | Ascophyllum nodusum | Enhanced resistance to the stress of drought through an impact on proline metabolism. | |
| Tomato | Ascophyllum nodusum | Fruit produce was enhanced by 225% and 271% at 50% field cap acity by soil drenching and foliar spray at 5 ml/l, respectively. | |
| Maize | Ecklonia maxima | Plant metabolism was altered by foliar treatment, which also increased plant diameter and height while lessening drought stress. | |
| Sugarcane | Ascophyllum nodusum | Increased stalk yield by 3.08 Mg/ha and sugar yield by 3.4 Mg/ha . | |
| Heat stress | Spinach | Ascophyllum nodusum | High germination%, seedling vigor, germination rate, antioxidant metabolism, and low malondialdehyde, hydrogen peroxide. |
Conclusion
Seaweed based biostimulants hold great promise for sustainable agriculture due to their rich composition of nutrients, plant hormones, and bioactive compounds like polysaccharides, polyphenols, and antioxidants. These extracts enhance plant growth, improve nutrient uptake, and boost tolerance to both biotic and abiotic stresses such as drought, salinity, and temperature extremes. They also strengthen plant immunity by activating defense-related genes and metabolic pathways. Furthermore, seaweed applications improve soil health by promoting beneficial microbial activity and nutrient cycling. Modern extraction techniques-such as alkaline, enzymatic, microwave, and ultrasound methods-help preserve the functional compounds and increase the effectiveness of these biostimulants. Different seaweed types (brown, red, and green) offer unique benefits, and their application has proven successful in crops like maize, tomato, sugarcane, and grape. In addition to improving crop yield and quality, seaweed extracts offer an eco-friendly alternative to chemical fertilizers and pesticides. As agriculture faces challenges from climate change and resource limitations, seaweed biostimulants present a viable solution.
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