Keywords

1 Introduction

The pervasive inputs of synthetic chemical fertilizers and other chemicals such as pesticides and herbicides into the agricultural system have reduced productivity and nutritional quality of crops and also caused negative impacts on the environment (Kramer et al. 2006). In recent years, biostimulants are receiving much attention in sustainable agriculture as a means of boosting crop growth and productivity in eco-friendly and cost-effective ways and reducing the negative impacts of chemical synthetic fertilizers and displayed positive influence on plant growth and productivity, and soil sustainability (Craigie 2011; Singh et al. 2011a, b). Seaweeds are plant-like organisms with simple internal structures that largely inhabit coastal areas. Seaweeds include the members of red, brown, and green marine algae. The use of natural seaweed as fertilizers has allowed the partial replacement of synthetic chemical fertilizers and they have great potential in improving soil physiochemical properties and have substantial contribution in increasing crop growth and productivity (Li et al. 2017; Renuka et al. 2018; Thilagar et al. 2016; Zodape et al. 2010). They have the immense ability in the mobilization of inorganic and organic acids, nutrient mineralization, and production of biologically active compound which can be utilized for applications for plant improvement making them as a suitable option for biofertilizers (Gayathri et al. 2015; Hernández-Carlos and Gamboa-Angulo 2011; Jäger et al. 2010; Prasanna et al. 2016). Seaweed extracts are non-toxic, non-polluting, biodegradable, and non-hazardous to the environment, humans and animals, and have become popular agents as metabolic enhancers.

The products of seaweed extracts can be used as liquid extracts in the agricultural system and applied in the form of a soil drench, foliar spray, and powder or granular form (Thirumaran et al. 2009). A number of studies revealed that a wide range of seaweed extract derived from several species such as Ascophyllum nodosum, Kappaphycus alvarezii, Sargassum muticum, Ecklonia maxima, Durvillaea pota-torum, Ulva lactuca, Caulerpa sertularioides, Padina gymnospora, Sargassum liebmannii, Sargassum johnstonii are useful as biofertilizers (Hernández-Herrera et al. 2014). Among marine algae, the extracts of brown marine algae are mainly used in agriculture practices because of their growth-stimulating effects on plants and for their alleviating the negative impacts of abiotic and biotic stresses (Guinan et al. 2012; Sharma et al. 2014). The chemical constituents of seaweed extract include complex polysaccharide, fatty acids, vitamins, phytohormones, and mineral nutrients (Lafarga et al. 2020). Besides, they have a prominent role in maintaining the productivity of terrestrial and aquatic ecosystems through nitrogen fixation and photosynthesis thereby enhancing the accessibility of nutrients by cycling and transformations (Moroney and Ynalvez 2009). Seaweed can be regarded as a potential source of biofertilizers in fresh or dried form and it is a good source of macro and microelements for plant nutrition and it also enhances the biochemical constituents like lipids, proteins, carbohydrates ash, dietary fiber, phenol etc in the plant. This technology can be implemented for organic farming for sustainable agriculture which is a better solution for an eco-friendly approach.

2 Utilization of Seaweeds as Metabolic Enhancers in Agriculture Field

Seaweeds are valuable resource for plant development as they have higher contents of amino acids, mineral substances, vitamins, and phytohormones (Stirk and Van Staden 1997). The extracts of brown algae are widely used in agriculture and they have been shown to enhance the productivity of agricultural and horticultural plants including tomato, potato, beet, legumes, citrus, grasses, rice, wheat, maize, sugarcane, broccoli, spinach, cabbage, carrot, pepper, cucumber etc (Battacharyya et al. 2015; Renuka et al. 2018). The regulator of the plant growth may vary from the fertilizers in many ways such as they alter and manage the cell division, control of shoot and root elongation and initiation of flowering and other metabolic functions. Whereas, fertilizers supply the nutrients required for normal plant growth and development (Allen et al. 2001). Moreover, cytokinin is considered as the most important plant growth regulator in seaweed. The trace elements present in marine algae act as enzyme activators and play important roles in plant nutrition and physiology (Senn 1987). The application of seaweed extract in plants increased the antioxidant metabolites (for example α-tocopherol, carotene, and ascorbic acid) as well as antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) activities (Allen et al. 2001). The impacts of seaweed extract on plant growth are summarized in Table 25.1.

Table 25.1 Seaweeds extract as plant promoting agents and biofertilizers
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3 Amelioration of Abiotic and Biotic Stresses Through Seaweeds

Abiotic and biotic stresses hamper growth and productivity of crops worldwide. In order to boost the agricultural production for the ever-increasing population chemical synthetic fertilizers are used. These synthetic chemical fertilizers posed a serious threat to the health of animals, plants, humans as well as for the entire biosphere (Damalas and Koutroubas 2016). The arable land is continuously shrinking due to the adverse effects of climate change and urbanization. The effects of climatic changes resulted into increased atmospheric CO2, temperature, nutrient imbalances (such as deficiency and toxicity of mineral) and soil salinization that affected the plant growth, productivity and quality of crops (Anderson et al. 2011; dos Reis et al. 2012; Matesanz et al. 2010). To meet the food demands for growing population, world food production must double by the year 2050 (Qin et al. 2011; Voss-Fels and Snowdon 2016). Therefore, an alternative approach is required to reduce the input of synthetic chemical fertilizers. In this context, plant biostimulant such as seaweeds are a new class of crop input into agricultural practices can reduce the application rate of synthetic chemical fertilizers and pesticides and thereby enhancing the quality and productivity of crops (Calvo et al. 2014; Van Oosten et al. 2017; Yakhin et al. 2017).

The pivotal role of seaweed extracts in enhancing plant growth as well as in increasing plant tolerance to abiotic (drought, salinity, nutrient deficiency, flooding, temperature stress) and biotic (diseases and insects) stresses has been reported (Jayaraman et al. 2011; Sangha et al. 2010). Among seaweeds, brown seaweeds (Ascophyllum nodosum, Fucus vesiculosus, F. serratus) are rich in phenolic compounds (Balboa et al. 2013; Keyrouz et al. 2011). Phenolic compounds are secondary metabolites which defend the cell and cellular components from the stressed induced damages and also chelate the metal ions (Wang et al. 2009). The phenolic compounds such as catechol (dihydroxy benzene) or galloyl (trihydroxy benzene), phloroglucinol, eckol and dieck showed strong chelating activities which make brown algae to effectively scavenge free radical (single oxygen, hydroxyl, superoxide, alkoxyl and peroxy radicals) and antioxidant activity (Andjelković et al. 2006). Study reported by Shibata et al. (2002). demonstrated that marine algae derived phlorotannis was highly efficient antioxidant as compared to the ascorbic acid, phlorofucofuroeckol, catechin, resveratrol, epigallocatechin eallate, α-tocopherol . The extracts of seaweed contain a number of plant hormone such as such as auxins, gibberellins, cytokinin, abscisic acid and brassinosteroids (Stirk et al. 2014). Osmolytes such as mannitol, an important protective compound in response to abiotic stresses are found in many brown algae such as Laminaria digitata, L. hyperborea, L. saccharina, Halidrys siliquosa, Fucus serratus, F. spiralis, F. vesiculosus, Ascophyllum nodosum, Alaria esculenta, Pilayella littoralis and Ectocarpus siliculosus. The chelating activity of mannitol revealed that seaweed can release the inaccessible elements to the soils (Reed et al. 1985).

Previous studies by several researchers revealed that seaweed extracts such as Lessonia nigrescens, Grateloupia filicina, Kappaphycus alvarezii, Gracilaria dura improved salinity and drought tolerance in several crops such as wheat, passion fruit, Arabidopsis, avocado and rice (Elansary et al. 2017; Di Stasio et al. 2018; Jithesh et al. 2018; Liu et al. 2019; Zou et al. 2019). Under salinity stress, both Rygex® and Super Fifty® the commercial extracts of A. nodosum triggered that accumulation of essential amino acids, antioxidant and minerals (Di Stasio et al. 2018). The extract of A. nodosum application mitigates the negative impacts of salinity stress in avocado and turfgrass by enhancing nutrient uptake (Elansary et al. 2017; Bonomelli et al. 2018).

Abiotic stress involves hyperosmotic and ionic imbalances leading to an oxidative stress generated from the overproduction of ROS (reactive oxygen species) and the antioxidant mechanisms (Debnath et al. 2011). ROS [includes the singlet oxygen (1O2), superoxide (O2−), hydroxyl (OH−), hydrogen peroxide (H2O2)] are to known to impair biomolecule like proteins, DNA and lipids (Das and Roychoudhury 2014; Mittler 2002). To mitigate such damaging impacts, plant resort to several natural defensive mechanisms. ROS production controlled by the enzymatic and non-enzymatic antioxidants such as SOD (superoxide dismutase), GPX (guaiacol peroxidase), CAT (catalase), APX (ascorbate peroxidase), DHAR (dehydroascorbate reductase), GR (glutathione reductase), MDHAR (monodehydroascorbate reductase), AsA (ascorbic acid), tocopherol, glutathione and phenolic compounds (Gruszka et al. 2018; Yadav et al. 2019). The study of Zou et al. (Zou et al. 2019) demonstrated that potential of Lessonia nigrescens derived polysaccharides in the alleviation of salt stress on wheat and found that polysaccharides application significantly increased the root/shoot length and fresh/dry matters of wheat. They also noticed that polysaccharides application maintained osmotic status of salt stressed wheat seedlings by increasing the proline and sugar content as well as regulating the ratio of Na+/K+. A study of Liu et al. (Liu et al. 2019) observed that the impact of polysaccharides from Grateloupia filicina augmented rice seed development and mitigate the salinity-induced damage. A study by Jayaraman et al. (Jayaraman et al. 2011) examined the impacts of Stimplex™, a commercial extract from A. nodosum on some common cucumber fungal pathogen such as Alternaria cucumerinum, Fusarium oxysporum, Didymella applanata. They found that Stimplex™ application resulted in a significant increase in activities of various defense related enzymes including peroxidase, polyphenol oxidase, lipoxygenase, chitinase, phenylalanine ammonia lyase, glucanase and β-1,3-glucanase. In addition, treated plants also displayed a higher level of phenolics content and reduced the disease incidence of all tested pathogens. Similarly, the study of Guinan et al. (2012) revealed that extracts of Super Fifty® and Ecoelicitor® were found to be more effective in imparting tolerance to Sclerotinia sclerotiorum and Alternaria brassicae. The abiotic and biotic stress-responsive genes are elicited by seaweeds are enumerated in Table 25.2.

Table 25.2 Effect of seaweed extracts on gene expression of plants under abiotic and biotic stress conditions
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4 Nutritional and Bioactive Potential of Seaweed

A number of studies revealed that macroalgae have huge potential due to their bioactive compounds endowed with a wide range of activities such as antioxidant, anti-inflammatory, anti-tumor and antimicrobial (Rocha et al. 2018; Trifan et al. 2019). Among macroalgae, phaeophyceae members accumulate a variety of compounds including sterols, lipids, proteins, terpenoids, vitamins, phlorotannins with a range of biological activities (Balboa et al. 2013; Jiménez-Escrig et al. 2012). Seaweeds are the macroscopic, multicellular marine algae that form a vital part of the marine coastal ecosystem. Approximately, there are 9000 species of macrolage classified into three groups based on their pigmentation such as Chlorophyta (greem algae), Phaeophyta (brown algae), and Rhodophyta (red algae). Pheophyta is the second most abundant group comprising about 2000 species. This group of microalgae commonly used in agriculture (Blunden and Gordon 1986; Khan et al. 2009). Brown algae such as Fucus spp., Ascophyllum nodosum, Sargassum spp., Turbinaria spp., and Laminaria spp. are used as biofertilizers in agricultural practices (Hong et al. 2007). For centuries seaweeds are used as source of organic matter and fertilizer nutrients. Around 15 million metric tonnes of seaweed products are produced annually (FAO 2006), a substantial portion of which is used for nutrient supplements and as biostimulants to increased plant growth and productivity.

A number of products such as Agri-Gro Ultra, Tasco®, Acadian®, Alg-A-Mic, Bio-GenesisTM High TideTM, Biovita, Espoma, Soluble Seaweed Extract, Guarantee®, Kelp Meal, Synergy, Kelpro, Kelprosoil, Maxicrop, Nitrozime and Stimplex® are formulated from Ascophyllum nodosum seaweed and used in agriculture practices as a plant growth stimulant. Kelpak, Seasol®, Profert® and AgroKelp products are obtained from the Ecklonia maxima, Durvillea potatorum, Durvillea antarctica and Macrocystis pyrifera seaweed, respectively also these are used in agriculture as plant growth stimulant (Khan et al. 2009). In addition, application of seaweed extracts on plants promotes the seed germination and its establishment, imparts tolerance to abiotic and biotic stresses, enhanced crop performance and productivity, and also enhanced shelf-life of postharvest products (Norrie and Keathley 2006).

Seaweed extracts has been shown to have a positive impact on plant nutrient uptake (Table 25.3). Turan and Köse (Turan and Köse 2004) found that foliar application of commercial seaweed extract enhanced the micro- and macronutrient content of grapevine leaves. Study of Rathore et al. (Rathore et al. 2009) revealed that application of Kappaphycus alvarezii increased N, P, K, and S content of soybean under rainfed conditions. Seaweed extracts improve plant via improvement of soil structure and micronutrient solubility in the soil, as well as influencing the plant physiology through changes in root morphology and increased root colonization by AM fungi (Halpern et al. 2015). The polysaccharides such as fucoidans and alginates are found in brown seaweeds and these polysaccharides are bound with metallic ions in soil to produce a gel that helps in the absorption of water and form an aggregate structure which assists the plant to grow a vigorous root architecture which in turn enhance the nutrient availability (Khan et al. 2009). Some of the studies also reported that the organic molecules in seaweed extracts can chelate the micronutrients and make them more accessible to plants. Spinelli et al. (Spinelli et al. 2010) showed that commercial extract of A. nodosum improved the solubility of micronutrients and that could be used to replace some of the standard iron chelates such as sequestrene.

Table 25.3 Effects of seaweed extracts on plant nutrient uptake
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5 Conclusion

Leveraging seaweeds as potential biostimulants have been shown to increase the plant growth, improved physiological and molecular machinery, and provides tolerance to abiotic and biotic stresses and better nutrient uptake in several crops and suggesting their usefulness in reducing chemical fertilizer application in agricultural practices without negatively affecting crop productivity. Although, commercial seaweeds extracts are readily available in the marketplace, they have not substantially reduced chemical fertilizers use in conventional agriculture. Before biostimulants can effectively use in agriculture system, it is important to find the most promising ones for those particular environmental conditions and the mechanism how they are best applied.