Seaweeds contain several organic compounds in their structural composition with potential applications in agriculture. Some of these compounds have effects on the physiological and biochemical metabolism of seeds, seedlings, and plants (Battacharyya et al. 2015; Castellanos-Barriga et al. 2017; Ali et al. 2021). The use of algae extracts on plants and their impacts have been widely studied in agriculture; however, there is not complete knowledge of their potential effects on agronomic plants (Rayorath et al. 2008; Hernández-Herrera et al. 2016; Castellanos-Barriga et al. 2017; Gupta et al. 2022). In this way, our studies contribute to knowledge regarding the chemical characterization of two algae-based products and their effects on seed germination and seedling growth, including modifications in their biochemistry.
Laminaria and Ulva liquid extracts exhibit several chemical compounds, including sugars, organic acids, and phytohormones, that have the potential to affect plants and induce metabolic changes. Mannitol, maltitol, and sucrose are sugars found in algae (Kuwada et al. 2005; Battacharyya et al. 2015; Torres-Benítez et al. 2022). Mannitol is an osmo-protector that plays a vital role in osmoregulation (Kuwada et al. 2005). External application of mannitol has been shown to increase antioxidant enzyme activity in wheat and protect the roots against lipid peroxidation under stressful conditions (Seckin et al. 2009). Therefore, this substance can affect roots. Furthermore, mannitol residue can be present in the terminal reducing of laminarins (Vera et al. 2011), which confer the property of inducing resistance against plant pathogens (Vera et al. 2011; Ali et al. 2021).
Sucrose appears to be a nutrient that can increase lateral root formation and primary root elongation (Xu et al. 2010; Roycewicz and Malamy 2012). In our research, seedling quality and seed germination from seeds soaked with sucrose (0.1, 0.2 or 0.4 g/L) did not exhibit a difference compared to the control treatments (data not shown). Thus, the sugars present in seaweeds can stimulate the growth of seedlings and roots, while also potentially promoting a resistance to abiotic and biotic stress.
Organic acids are present in several seaweeds and play important roles in their germ cell formation (He et al. 2019). In plants, the addition of malic acid to nutrient solutions was able to protect Al-sensitive wheat seedlings from phytotoxic Al concentrations, particularly influencing root apices (terminal 3–5 mm of the root) (Delhaize et al. 1993). It is suggested that the chemical compounds present in LLE and ULE can have an effect on roots and provide resistance against abiotic stress. In contrast, some organic acids, such as edetic acid, 2-isopropylmalic acid and mesaconic acid, have not been described in the literature regarding their effects on seeds or plants. Therefore, further research is necessary to better understand their potential effects on plants. Additionally, organic acids can be considered an emerging category of biostimulants (Gupta et al. 2022), making them interesting chemical compounds for new tests and research on plants.
Germicidin (Li et al. 2013) and sphingosine (Veerman et al. 2010), found in two liquid seaweed extracts, such as hexadecasphinganine and phytosphingosine, have demonstrated antimicrobial efficacy, suggesting potential effects on plants or microorganisms. Thus, conducting in vitro tests against common bean pathogens may provide valuable information about the antimicrobial potential against phytopathogens of interest. Besides that, sphingosines can exhibit bioactive properties (Abeytunga et al. 2008; Veerman et al. 2010). Amino acids in seaweed biostimulants, such as 2-Amino-1,3,4,5-eicosanetetrol and guanine present in both liquid extract, can function as bioactive and elicitor components in plants (Ali et al. 2021; Gupta et al. 2022).
Gibberellin is an important phytohormone produced by higher plants, also found in seaweeds (Rayorath et al. 2008; Battacharyya et al. 2015; Ali et al. 2021; Gupta et al. 2022), responsible for promoting the growth and elongation of cells in plants (Rayorath et al. 2008; Hedden and Sponsel 2015). However, high concentrations can have inhibitory and deregulatory effects on growth properties (Hedden and Sponsel 2015). Nevertheless, the levels of Gibberellin A9 in ULE might promote seed germination, as well as the growth of roots and shoots in common beans. Although not detected, low gibberellin contents have been reported in Laminaria spp liquid extract (Ertani et al. 2018), suggesting a possible presence and effect on plants of this hormone in LLE. Laminarin is another compound found in Laminaria spp. that, upon contact with seeds, causes significant alterations in metabolism (Wu et al. 2016).
Seed germination is the initial step for successful seedling establishment and achieving optimal agronomic yield (Gupta et al. 2022) and liquid extracts derived from L. japonica or U. prolifera (LLE or ULE) appear to impact these processes. Additionally, this process affects crop production and determines the success or failure of future yields, beginning with imbibition and breaking dormancy seed (Rayorath et al. 2008; Gupta et al. 2022). Furthermore, seed germination involves hydrolysis of stored reserves, activation of enzymes, and emergence of the radicle and plumule (Rayorath et al. 2008; Gupta et al. 2022).
Seeds soaked with LLE or ULE exhibit initially enhanced germination parameters and root development. The enhancement of these parameters is likely attributed to the synergistic interactions among a group of molecules present in algae (Hernández-Herrera et al. 2014; Battacharyya et al. 2015; Gupta et al. 2022). Exogenous gibberellin has been shown to modify and enhance the germination and amylase activity of wheat seeds (Wang et al. 2016). This important phytohormone, along with sugars (such as mannitol and sucrose) and organic acids (such as malic acid) present in seaweeds, may strongly influence seed germination and root growth. Additionally, algae contain a rich array of vitamins and minerals that have the potential to nourish the seedling and positively influence these developmental processes (Hernández-Herrera et al. 2014; Battacharyya et al. 2015; Ali et al. 2021).
Soaking seeds with LLA or ULE leads to alterations in the biochemical metabolism of common bean seedlings. Modifications in the number of amino acids and proteins observed in seedlings from seeds treated with LLE or ULE may be involved in nitrogen metabolism (Engel et al. 2023) and protein synthesis (Baud et al. 2002). Engel and coauthors (2023) observed that treatments of soybean seeds with the brown seaweed Ascophyllum nodosum modified gene expression related to nitrogen metabolism, in addition to increasing seed production. These end-of-cycle results (R5) corroborate our findings, indicating a significant alteration in nitrogen metabolism increasing plant productivity.
The α-amylase is a key enzyme for seed germination and seedling development, playing a critical role in the hydrolysis of starch (Rayorath et al. 2008), exhibiting high activity in roots from seeds soaked with LLE and ULE. During these processes, polysaccharides are broken down into simple sugars such as glucose and fructose through hydrolysis, resulting in energy production and a continuous supply of carbon skeletons for the biosynthesis of primary cellular components (Rayorath et al. 2008). Significant changes in carbohydrate content may be linked to readily available energy resources (Baud et al. 2002) crucial for seedling development, resulting in increased length and fresh weight of seedlings. Furthermore, the synthesis and secretion of α-amylase in the aleurone layer might be substantially induced by gibberellic acid in plants (Rayorath et al. 2008; Hedden and Sponsel 2015), which also is present in ULE. Furthermore, even in the absence of gibberellin, other compounds present in L. japonica may increase alpha-amylase activity in plants, as observed in seed imbibition with Ascophyllum nodosum (Rayorath et al. 2008). Quantitative and qualitative research provides new insights to better understand the metabolic changes in plants that received treatments with algae.