Quantitative Structure-Activity Relationship of Humic-Like Biostimulants Derived From Agro-Industrial Byproducts and Energy Crops

A practical guide to the discovery of biomolecules with biostimulant activity

The growing demand for sustainable solutions in agriculture, which are critical for crop productivity and food quality in the face of climate change and the need to reduce agrochemical usage, has brought biostimulants into the spotlight as valuable tools for regenerative agriculture. With their diverse biological activities, biostimulants can contribute to crop growth, nutrient use efficiency, and abiotic stress resilience, as well as to the restoration of soil health. Biomolecules include humic substances, protein lysates, phenolics, and carbohydrates have undergone thorough investigation because of their demonstrated biostimulant activities. Here, we review the process of the discovery and development of extract-based biostimulants, and propose a practical step-by-step pipeline that starts with initial identification of biomolecules, followed by extraction and isolation, determination of bioactivity, identification of active compound(s), elucidation of mechanisms, formulation, and assessment of effectiveness. The different steps generate a roadmap that aims to expedite the transfer of interdisciplinary knowledge from laboratory-scale studies to pilot-scale production in practical scenarios that are aligned with the prevailing regulatory frameworks.

Enhancing Lettuce Yield through Innovative Foliar Spray of Biopolymers Derived from Municipal Biowastes

Municipal waste biomass could be valorized as an alternative feedstock to produce compounds beneficial for agricultural applications. The foliar spray application of biostimulants emerges as a promising and innovative technique due to its environmental safety and ability to enhance crop yields. In recent years, the exploitation of biopolymers obtained through alkaline hydrolysis of the solid anaerobic digestate from municipal biowastes has attracted researchers' interest. The aim of this study is to investigate the effects on lettuce growth of a product obtained through alkaline hydrolysis from municipal biowaste, Biopolymers (BPs), and of a derivate subjected to a further oxidation process, Biopolymers Oxidate (BPs OX). The effects of the treatments at various concentrations were evaluated by monitoring plant growth and observing the trends in the activities of the main enzymes involved in the nitrogen metabolic pathway of lettuce. Results suggest that the best treatments in terms of fresh weight were achieved by using BPs at 10 mg/L and BPs OX at 100 mg/L, increasing yield by around 28% and 34%, respectively. The innovative aspect of this work was to make easier for farmers the biopolymers application by testing a foliar spray methodology for BPs and BPs OX, which has never been tested before in any crop.

Role of the molecular structure of humified organic matter in rice plant response to environmental lead pollution

Humified organic matter has been shown to decrease Pb toxicity in plants. However, there are still gaps in our understanding of the mechanism by which this phenomenon occurs. In this study, we aimed to assess the ability of humic substances (HSs), humic acids (HAs), and fulvic acids (FAs) to enhance defense mechanisms in rice plants under lead (Pb)-stressed conditions. HS fractions were isolated from vermicompost using the chemical fractionation methodology established by the International Humic Substances Society. These fractions were characterized by solid-state NMR and FTIR. Chemometric analysis was used to compare humic structures and correlate them with bioactivity. Three treatments were tested to evaluate the protective effect of humic fractions on rice plants. The first experiment involved the application of humic fractions along with Pb. The second comprised pretreatment with humic fractions followed by subsequent exposure to Pb stress. The third experiment involved Pb stress and subsequent treatment with humic fractions. The root morphology and components of the antioxidative defense system were evaluated and quantified. The results showed that HS + Pb, HA + Pb, and FA + Pb treatment preserved root growth and reduced the levels of O2- and malondialdehyde (MDA) in the roots by up to 5% and 2%, respectively. Pretreatment of the plants with humic fractions promoted the maintenance of root growth and reduced the contents of O2-, H2O2, and MDA by up to 48%, 22%, and 20%, respectively. Combined application of humic fractions and Pb reduced the Pb content in plant tissues by up to 60%, while pretreatment reduced it by up to 80%. The protective capacity of humic fractions is related to the presence of peptides, lignin, and carbohydrate fragments in their molecular structures. These results suggest that products could be developed that can mitigate the adverse effects of heavy metals on agricultural crops.

Electrospun films incorporating humic substances of application interest in sustainable active food packaging

Sustainable active food packaging is essential to reduce the use of plastics, preserve food quality and minimize the environmental impact. Humic substances (HS) are rich in redox-active compounds, such as quinones, phenols, carboxyl, and hydroxyl moieties, making them functional additives for biopolymeric matrices, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Herein, composites made by incorporating different amounts of HS into PHBV were developed using the electrospinning technology and converted into homogeneous and continuous films by a thermal post-treatment to obtain a bioactive and biodegradable layer which could be part of a multilayer food packaging solution. The morphology, thermal, optical, mechanical, antioxidant and barrier properties of the resulting PHBV-based films have been evaluated, as well as the antifungal activity against Aspergillus flavus and Candida albicans and the antimicrobial properties against both Gram (+) and Gram (-) bacterial strains. HS show great potential as natural additives for biopolymer matrices, since they confer antioxidant, antimicrobial, and antifungal properties to the resulting materials. In addition, barrier, optical and mechanical properties highlighted that the obtained films are suitable for sustainable active packaging. Therefore, the electrospinning methodology is a promising and sustainable approach to give biowaste a new life through the development of multifunctional materials suitable in the active bio-packaging.

Lignin nanoparticles from hydrotropic fractionation of giant reed and eucalypt: Structural elucidation and antibacterial properties

A hydrotropic solution of maleic acid (MA) was exploited to fractionate giant reed (AD) and eucalypt (EUC). The pre-treatment was successful for AD, while it was unsatisfactory for EUC, likely due to unoptimized reaction conditions. Interestingly, lignin nanoparticles (LNP) were produced via spontaneous aggregation following spent liquor dilution. LNP were studied by a plethora of analytical techniques, such as thermogravimetry, electron microscopy, and Nuclear Magnetic Resonance spectroscopy (NMR). Notwithstanding LNP from both AD and EUC showed similar thermal behaviour and morphology, a greater content of aliphatic hydroxyl, carboxyl, guaiacyl and p-hydroxyphenyl moieties was reported for AD-LNP, whereas EUC-LNP had a larger amount of syringyl groups and a higher S/G ratio. Also, the 1H-DOSY NMR indicated the lower size of AD-LNP. Moreover, the LNP were found to negatively impact on the development of several human or plant pathogens, and their bioactivity was related to the occurrence of guaiacyl and p-hydroxyphenyl moieties and a lower the LNP size. We therefore found that MA delignification allows both to achieve high delignification efficiency and to obtain LNP with promising antibacterial effect. Such LNP may help counteracting the antibiotics resistance and sustain the quest for finding sustainable agrochemicals.

Waste milk humification product can be used as a slow release nano-fertilizer

The demand for milk has increased globally, accompanied by an increase in waste milk. Here, we provide an artificial humification technology to recycle waste milk into an agricultural nano-fertilizer. We use KOH-activated persulfate to convert waste milk into fulvic-like acid and humic-like acid. We mix the product with attapulgite to obtain a slow-release nano fulvic-like acid fertilizer. We apply this nano-fertilizer to chickweeds growing in pots, resulting in improved yield and root elongation. These results indicate that waste milk could be recycled for agricultural purposes, however, this nano-fertilizer needs to be tested further in field experiments.

Waste to Wealth Approach: Improved Antimicrobial Properties in Bioactive Hydrogels through Humic Substance-Gelatin Chemical Conjugation

Exploring opportunities for biowaste valorization, herein, humic substances (HS) were combined with gelatin, a hydrophilic biocompatible and bioavailable polymer, to obtain 3D hydrogels. Hybrid gels (Gel HS) were prepared at different HS contents, exploiting physical or chemical cross-linking, through 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide (EDC) chemistry, between HS and gelatin. Physicochemical features were assessed through rheological measurements, X-ray diffraction, attenuated total reflectance (ATR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscopy (SEM). ATR and NMR spectroscopies suggested the formation of an amide bond between HS and Gel via EDC chemistry. In addition, antioxidant and antimicrobial features toward both Gram(-) and Gram(+) strains were evaluated. HS confers great antioxidant and widespread antibiotic performance to the whole gel. Furthermore, the chemical cross-linking affects the viscoelastic behavior, crystalline structures, water uptake, and functional performance and produces a marked improvement of biocide action.

Effects of the Structure and Molecular Weight of Alkali-Oxygen Lignin Isolated from Rice Straw on the Growth of Maize Seedlings

The abundant and low-cost features of lignin in combination with its natural activities make it a fascinating biopolymer for valorization, especially, in agriculture as an active plant growth regulator. However, the structure-activity relationship of lignin in regulating plant growth and metabolism remains unclear. In this work, rice-straw-based low-molecular-weight (LWM, 1860 Da) and high-molecular-weight (HMW, 6840 Da) alkali-oxygen lignins are structurally and comparatively investigated to understand their effects on the growth and metabolism of maize seedlings. The results indicate that LMW lignin at 150 mg·L^-1 displays early growth stimulation in maize. Under the optimal concentration of LMW lignin (25 mg·L^-1), the growth of maize shoot is ∼83% higher than that of the control one. Furthermore, LMW lignin also has a positive effect on the upregulation of photosynthetic pigment, carbohydrate, and protein synthesis. In contrast, HMW lignin shows an overall inhibitory effect on the above-mentioned biochemical parameters. Based on the structural characterization, LMW lignin contains a higher syringyl/guaiacyl ratio (0.78) and carboxyl content (1.64 mmol·g^-1) than HMW lignin (0.43 and 1.27 mmol·g^-1, respectively), which demonstrates that methoxyl and carboxyl content of lignin may play a decisive role in seedling growth.

Humic substances from composted fennel residues control the inflammation induced by Helicobacter pylori infection in AGS cells

Helicobacter pylori (H. pylori) is a common human pathogen causing inflammation. Recent studies have suggested a sophisticated interplay between mitochondria, innate immunity and inflammatory response, thus proposing mitochondrial disfunction as the hallmark of severe inflammatory disorders. In this study, humic substances isolated from composted fennel residues (HS-FEN) were tested as potential therapeutical strategy to restore the mitochondrial physiology and control the inflammation associated with H. pylori infection. The molecular features of HS-FEN were characterized by infrared spectrometry, thermochemolysis-GC/MS, NMR spectroscopy, and high-performance size-exclusion chromatography (HPSEC), which revealed the presence of aromatic polyphenolic components arranged in a rather stable conformation. In vitro results showed antioxidant and anti-inflammatory properties of HS-FEN, that was found to increase the expression level of OPA-1 and SOD-2 genes and in AGS cells stimulated with H. pylori culture filtrate (Hpcf) and concomitantly decrease the expression level of Drp-1 gene and IL-12, IL-17 and G-CSF proteins. The hydrophobic features of HS, their conformational arrangement and large content of bioactive molecules may explain the beneficial effects of HS-FEN, that may potentially become an interesting source of anti-inflammatory agents capable to counteract or prevent the H. pylori-related inflammatory disorders.

Quantitative Structure-Activity Relationship, Ontology-Based Model of the Antioxidant and Cell Protective Activity of Peat Humic Acids

Peat humic acids are well known for their wide range of biological effects which can be attributed to the complex chemical structure of naturally occurring humic substances. One of the promising tools is an ontology-based quantitative analysis of the relationship between physical and chemical parameters describing a chemical structure of peat humic acids and their biological activity. This article demonstrates the feasibility of such an approach to estimate the antioxidant and cell protective properties of the peat humic acids. The structural parameters of the peat humic acids were studied by electronic, fluorescence, infrared, ¹³C-NMR spectroscopy, titrimetric analysis, elemental C,H,N, and O- analysis, and gel chromatography. Antioxidant and antiradical activities were assessed by physicochemical methods of analysis: electronic paramagnetic resonance, cathodic voltammetry, ABTS^•+ scavenging, assay of DPPH radical-scavenging activity, assay of superoxide radical-scavenging activity, iron chelating activity, and scavenging of hydroxyl radicals. Cytoprotective activity was evaluated by the neutral red-based cytotoxicity test in 3T3-L1 cell culture in a wide range of concentrations. Assessment of intracellular ROS production was carried out using a 2,7-dichlorodihydrofluoresceindiacetate (DCFDA) fluorescent probe. Intracellular ROS production was induced using two common prooxidants (tert-butyl hydroperoxide, Fe²⁺ ions). We suggested an ontology-based model for the antioxidant and cytoprotective activity of humic acids based on experimental data and numerical models. This model establishes the way to further research on the biological effects of humic acids and provides a useful tool for numerical simulation of these effects. Remarkable antioxidant and cell protective activity of humic acids makes them a promising natural source of new pharmaceutical substances that feature a wide range of biological effects.

A comparative study of tea waste derived humic-like substances with lignite-derived humic substances on chemical composition, spectroscopic properties and biological activity

Emerging demand for humic substances escalated the short supply of coal-related resources from which humic substances are extracted in large quantities for various applications. Production of humic-like substances from lignocellulosic waste materials similar in structural and functional properties to humic substances has gained interest recently. Tea waste is a by-product from tea manufacturing factories enriched in lignocellulose is used to extract two types of humic fractions. One fraction has purified humic-like acid (HLA), and the other has unpurified humic and fulvic acids called as humic-like substances (HLS). Elemental composition, spectroscopic (¹³C CPMAS NMR and FTIR) properties, and biological activity of tea waste derived humic-like substances (TWDHLS) were compared with commercially available humic acid (CHA) extracted from lignite. Elemental analysis and FTIR characterization showed slight differences between HLA and HLS, while NMR results revealed that both have similar carbon distribution and are abundant in cellulosic polysaccharides and lignin derivatives. The presence of more stable compounds in TWDHLS contribute to its recalcitrant nature. NMR spectra of CHA significantly varied with TWDHLS and were rich in aliphatic compounds. The biological activity of TWDHLS and CHA was studied at five different concentrations (0, 20, 40, 80, and 160 mg L^-1). The results show that soil application TWDHLS at 80 mg L^-1 concentration showed better results on the growth of tea nursery plants similar to CHA, contrasting to the variation in their structural properties. Our findings revealed that TWDHLS could be used not only as a potential plant biostimulant but also as a better substitute for humic substances.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

Valorization of lignins from energy crops and agro-industrial byproducts as antioxidant and antibacterial materials

BACKGROUND

Developing eco-friendly antioxidant and antimicrobial substances originating from biomass residues has recently attracted considerable interest. In this study, two lignosulfonates and various oxidized water-soluble lignins were investigated for their antioxidant properties, as assessed by ABTS, DPPH and Folin-Ciocalteu methods, and their antimicrobial activity against some bacterial strains responsible for human pathologies.

RESULTS

The lignosulfonates showed the largest antiradical/antimicrobial capacity, whereas the other substrates were less effective. The observed antioxidant/antibacterial properties were positively correlated with lignin aromatic/phenolic content. The positive correlation between antiradical and antimicrobial activities suggests that lignin scavenging capacity was also involved in its antibacterial activity. A greater antimicrobial performance was generally observed against Gram-positive bacterial strains, and it was attributed to the intrinsic larger susceptibility of Gram-positive bacteria to lignin phenols. A significant though lesser inhibitory activity was also found against Escherichia coli.

CONCLUSION

Our results confirmed the dependence of lignin antioxidant/antibacterial power on its extraction method and chemical structure, as well as on the type of bacterial strains. Identifying the relationship between lignin molecular composition and its antioxidant/antibacterial features represents an advance on the potential future use of renewable and eco-compatible lignin materials in nutraceutical, pharmaceutical and cosmetic sectors. © 2021 Society of Chemical Industry.

Extracellular laccase-activated humification of phenolic pollutants and its application in plant growth

Humification processes of phenolic pollutants may play a profound role in environment purification and plant growth. However, little literature is performed to explore exoenzyme-driven humification to polymerize 17β-estradiol (E2) and humic constituents (HCs), and the effects of their polymeric precipitates on plant growth are usually overlooked. Herein, E2 conversion and radish (Raphanus sativus L.) growth were systematically investigated under humification mediated by extracellular laccase (EL) of Trametes versicolor. Results disclosed that EL-assisted humification achieved a wonderful E2 conversion efficiency (>99%) within 2-h, but the presence of HCs such as humic acid (HA), vanillic acid (VA), and ferulic acid (FA) impeded E2 elimination significantly. Compared with HC-free, the kinetics constants declined by 2.84-, 5.72-, and 5.22-fold with HA, VA, and FA present, respectively. Intriguingly, three close-knit self/cross-linked precipitates (i.e., E2-HA, E2-VA, and E2-FA hybrid precipitates) in dark gray, dark brown, and deep yellow were created after a continuous humification by phenolic radical-initiated polymerization mechanisms. The formation of these humified precipitates was extremely effective on circumventing phytotoxicity caused by monomeric E2, VA, or FA. Furthermore, they acted as humic-like organic fertilizers, accelerating seed germination, root elongation, and enhancing NaCl-tolerance of radish through the combination of oxygen-contained functional components and auxin structural analogues with unstable and stubborn carbon skeletons. This is the first study reporting the pollution purification and plant growth promotion in EL-activated humification. Our findings frame valuable perspectives regarding the natural detoxification and carbon sequestration of phenolic pollutants and the application of their polymeric precipitates in global crop production.

A study on structural evolution of hybrid humic Acids-SiO2 nanostructures in pure water: Effects on physico-chemical and functional properties

Humic acids (HA) are considered a promising and inexpensive source for novel multifunctional materials for a huge range of applications. However, aggregation and degradation phenomena in aqueous environment prevent from their full exploitation. A valid strategy to address these issues relies on combining HA moieties at the molecular scale with an inorganic nanostructured component, leading to more stable hybrid nanomaterials with tunable functionalities. Indeed, chemical composition of HA can determine their interactions with the inorganic constituent in the hybrid nanoparticles and consequently affect their overall physico-chemical properties, including their stability and functional properties in aqueous environment. As a fundamental contribution to HA materials-based technology, this study aims at unveiling this aspect. To this purpose, SiO₂ nanoparticles have been chosen as a model platform and three different HAs extracted from composted biomasses, manure (HA_Man), artichoke residues (HA_Art) and coffee grounds (HA_Cof), were employed to synthetize hybrid HA-SiO₂ nanoparticles through in-situ sol-gel synthesis. Prepared samples were submitted to aging in water to assess their stability. Furthermore, antioxidant properties and physico-chemical properties of both as prepared and aged samples in aqueous environment were assessed through Scanning Electron Microscopy (SEM), N₂ physisorption, Simultaneous Thermogravimetric (TGA) and Differential Scanning Calorimetric (DSC) Analysis, Fourier Transform Infrared (FT-IR), Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR) spectroscopies. The experimental results highlighted that hybrid HA-SiO₂ nanostructures acted as dynamic systems which exhibit structural supramolecular reorganization during aging in aqueous environment with marked effects on physico-chemical and functional properties, including improved antioxidant activity. Obtained results enlighten a unique aspect of interactions between HA and inorganic nanoparticles that could be useful to predict their behavior in aqueous environment. Furthermore, the proposed approach traces a technological route for the exploitation of organic biowaste in the design of hybrid nanomaterials, providing a significant contribution to the development of waste to wealth strategies based on humic substances.

Extraction optimization and quality evaluation of humic acids from lignite using the cell-free filtrate of Penicillium ortum MJ51

Bio-solubilization of lignite is a promising technology to transform coal into humic acids (HAs) which are broadly used in agriculture. In this work, HAs were extracted from lignite using the cell-free filtrate (CFF) of Penicillium ortum MJ51. The extraction method was optimized using response surface methodology (RSM) based on the interactive effects of nitric acid concentrations, coal loading ratio, extraction temperature and time as input factors, and the absorbance of HAs at 450 nm wavelength as the output response. Under optimized conditions (lignite pretreated with 4.7 N HNO₃, coal loading ratio of 4.9%, temperature of 77.3 °C and time of 8.6 hours), the absorbance at 450 nm peaked at 70.28, and the concentration and extraction yield of HAs were 31.3 g L⁻¹ and 63.9%, respectively, which were dramatically higher than those observed for traditional biological methods (0.7 g L⁻¹ and 14.1%, respectively). The qualities of HAs produced under optimized conditions were evaluated and compared with those extracted by the conventional chemical method. The optimized process resulted in better HA quality indices, including lower molecular mass; higher nitrogen; less aromatic carbon; more aliphatic and carboxylic carbon; and higher bioactivity for promoting plant growth. Moreover, the anti-flocculation ability was improved, thereby supporting its applicability in agriculture. Extraction of HAs from lignite using the CFF of P. ortum MJ51 provides a novel technological approach for the efficient conversion of lignite to bio-active HAs.

Bio-solubilization of lignite is a promising technology to transform coal into humic acids (HAs) which are broadly used in agriculture.

Biological Activities and Emerging Roles of Lignin and Lignin-Based Products─A Review

Bioactive substances, displaying excellent biocompatibility, chemical stability, and processability, could be extensively applied in biomedicine and tissue engineering. In recent years, plant-based bioactive substances such as flavonoids, vitamins, terpenes, and lignin have received considerable attention due to their human health benefits and pharmaceutical/medical applications. Among them is lignin, an amorphous biomacromolecule mainly derived from the combinatorial radical coupling of three phenylpropane units (p-hydroxypenyl, guaiacyl, and syringyl) during lignification. Lignin possesses intrinsic bioactivities (antioxidative, antibacterial, anti-UV activities, etc.) against phytopathogens. Lignin also enhances the plant resistance (adaptability) against environmental stresses. The abundant structural features of lignin offer other significant bioactivities including antitumor and antivirus bioactivities, regulation of plant growth, and enzymatic hydrolysis of cellulose. This Review reports the latest research results on the bioactive potential of lignin and lignin-based substances in biomedicine, agriculture, and biomass conversion. Moreover, the interfacial reactions and bonding mechanisms of lignin with biotissue/cells and other constituents were also discussed, aiming at promoting the conversion or evolution of lignin from industrial wastes to value-added bioactive materials.

Formation of Humic-Like Substances during the Technological Process of Lignohumate® Synthesis as a Function of Time

The composition, structure, and biological activity of humic-like substances (HLS) synthesized in the process of lignosulfonate conversion for the production of the humic product Lignohumate® (LH) were examined. It is shown that during the hydrolytic-oxidative process, the transformation of raw material and accumulation of HLS occur. Data on the chemical (elemental content, functional groups, FTIR) and spectral (absorbance and fluorescence) parameters and biological activity (in phytotest) combined with PCA show that the LH samples can be divided into three groups, depending on the duration of synthesis: initial raw material (0-time sample); “young” HLS (15–30 min), and “mature” HLS in 45–120 min of treatment. During the first 30 min, reactions similar to the ones that occur during lignin humification in nature take place: depolymerization, oxidative carboxylation, and further polycondensation with the formation and accumulation of HLS. After 45–60 min, the share of HLS reaches a maximum, and its composition stabilizes. Biological activity reaches a maximum after 45–60 min of treatment, and at that stage, the further synthesis process can be stopped. Further processing (up to 2 h and more) does not provide any added value to the humic product.

Humic acids induce a eustress state via photosynthesis and nitrogen metabolism leading to a root growth improvement in rice plants

Chemical eustressors induce a eustress state "positive stress" increasing the resistance and improve the plant growth. The potentiality of humic acids (HA) to act as a eustressor has been scarcely explored. The present study aims to evaluate how HA with different structural characteristics induce differently, a eustress state in rice plants through the regulation of photosynthesis. The photosynthetic performance index showed an initial eustress state in plant by HA application characterized by reduction in photosynthesis followed by an increase in photosynthetic efficiency. The HA as a chemical eustressor triggering changes in plant metabolism indicate that the interaction of HA with root system induces a roots growth stimulus preceded by an initial positive stress. The eustress caused by HA is differentiated and is related to its chemical-physics characteristics. The HAVC, with a predominance of C_Alkyl-(O,N), C_Alkyl-di-O, C_Aromatic-O structures and greater polarity, stimulated the accumulation of N-NO₃^- and of soluble sugars in the sheath, increase carbohydrates content in the root and the root emission, resulting in higher total biomass production. The HASOIL, with a predominance of C_COOH-(H,R), C_Alkyl-O, C_Aromatic-H,R structures and greater hydrophobicity caused a decrease in N-NH₄⁺ and N-amine. The HARN, with a predominance of C_Alkyl-O, C_Alkyl-H,R, and CO, characterized by average polarity, caused an increase in photosynthetic pigment and N-NH₄⁺ content. These results are keys to understand that quality of soil organic matter is related to plant development and that HA are efficient proxies for elucidate its function in natural environments.

Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters

Humic substances (HS) are dominant components of soil organic matter and are recognized as natural, effective growth promoters to be used in sustainable agriculture. In recent years, many efforts have been made to get insights on the relationship between HS chemical structure and their biological activity in plants using combinatory approaches. Relevant results highlight the existence of key functional groups in HS that might trigger positive local and systemic physiological responses via a complex network of hormone-like signaling pathways. The biological activity of HS finely relies on their dosage, origin, molecular size, degree of hydrophobicity and aromaticity, and spatial distribution of hydrophilic and hydrophobic domains. The molecular size of HS also impacts their mode of action in plants, as low molecular size HS can enter the root cells and directly elicit intracellular signals, while high molecular size HS bind to external cell receptors to induce molecular responses. Main targets of HS in plants are nutrient transporters, plasma membrane H⁺-ATPases, hormone routes, genes/enzymes involved in nitrogen assimilation, cell division, and development. This review aims to give a detailed survey of the mechanisms associated to the growth regulatory functions of HS in view of their use in sustainable technologies.

Lignin Nanoparticles: A Promising Tool to Improve Maize Physiological, Biochemical, and Chemical Traits

Lignin, and its derivatives, are the subject of current research for the exciting properties shown by this biomass. Particularly attractive are lignin nanoparticles for their eco- and biocompatibility compared to other nanomaterials. In this context, the effect of nanostructured lignin microparticles (LNP), obtained from alkaline lignin by acid treatment, on maize plants was investigated. To this end, maize seeds were primed with LNP at five concentrations: 80 mg L-1 (T80), 312 mg L-1 (T312), 1250 mg L-1 (T1250), 5000 mg L-1 (T5000) and 20,000 mg L-1 (T20000). Concerning the dose applied, LNP prompted positive effects on the first stages of maize development (germination and radicle length). Furthermore, the study of plant growth, biochemical and chemical parameters on the developed plants indicated that concerning the dose applied. LNP stimulated beneficial effects on the seedlings (fresh weight and length of shoots and roots). Besides, specific treatments increased the content of chlorophyll (a and b), carotenoid, and anthocyanin. Finally, the soluble protein content showed a positive trend in response to specific dosages. These effects are significant, given the essential biological function performed by these biomolecules. In conclusion, this research indicates as the nanostructured lignin microparticles can be used, at appropriate dosages, to induce positive biological responses in maize. This beneficial action deserves attention as it candidates LNP for biostimulating a crop through seed priming.

A Beginner's Guide to Osmoprotection by Biostimulants

Water is indispensable for the life of any organism on Earth. Consequently, osmotic stress due to salinity and drought is the greatest threat to crop productivity. Ongoing climate change includes rising temperatures and less precipitation over large areas of the planet. This is leading to increased vulnerability to the drought conditions that habitually threaten food security in many countries. Such a scenario poses a daunting challenge for scientists: the search for innovative solutions to save water and cultivate under water deficit. A search for formulations including biostimulants capable of improving tolerance to this stress is a promising specific approach. This review updates the most recent state of the art in the field.

Phenol-rich fulvic acid as a water additive enhances growth, reduces stress, and stimulates the immune system of fish in aquaculture

Aquaculture has become imperative to cover the demands for dietary animal protein. Simultaneously, it has to overcome prejudices from excessive use of antibiotics and environmental impacts. Natural supplements are traditionally applied orally. In this study, we demonstrated another pathway: the gills. Humic substances are immunostimulants and a natural part of every aquatic ecosystem, making them ideal to be used as bath stimulants. Five and 50 mg C/L of a fulvic acid-rich humic substance was added for 28 days to the water of juvenile rainbow trout (Oncorhynchus mykiss). This fulvic acid is characterized by a high content of phenolic moieties with persistent free radicals and a high electron exchange capacity. The high concentration of the fulvic acid significantly increased growth and reduced the food conversion ratio and the response to a handling-stressor. Phagocytosis and potential killing activity of head kidney leukocytes were increased, as well as the total oxyradical scavenging capacity (TOSC) and lysozyme activity in the gills. In conclusion, immunostimulation via gills is possible with our fulvic acid, and the high phenolic content improved overall health and stress resistance of fish.

Soybean Plant Metabolism under Water Deficit and Xenobiotic and Antioxidant Agent Application

The aim was to evaluate the interactive effects on biochemistry and physiology of soybean plants exposed to simultaneous xenobiotic and water deficit stresses, and the possible attenuation of plant damage by an antioxidant agent. Soybean plants were submitted to eight different soil water potentials, in two experiments (first experiment: -0.96, -0.38, -0.07, -0.02 MPa, and second experiment: -3.09, -1.38, -0.69, -0.14 MPa), xenobiotic, and antioxidant agent applications. Was observed a reduction in water status, gas exchange, photosynthetic pigments, photosystem II quantum yield, and increased leaf temperature in plants under low water availability. Water deficit also induced oxidative stress by the increased production of reactive oxygen species, cellular and molecular damage, and induction of the antioxidant defense metabolism, reduction of gas exchange, water status, and photosynthetic efficiency. The xenobiotic application also caused changes, with deleterious effects more pronounced in low soil water availability, mainly the reactive oxygen species production, consequently the antioxidant activity, and the oxidative damages. This indicates different responses to the combination of stresses. Antioxidant enzyme activity was reduced by the application of the antioxidant agent. Principal Component Analysis showed a relation with the antioxidant agent and reactive oxygen species, which is probably due to signaling function, and with defense antioxidant system, mainly glutathione, represented by thiols.

Biostimulant Potential of Acetic Acid Under Drought Stress Is Confounded by pH-Dependent Root Growth Inhibition

Recent reports of acetic acid-induced drought tolerance and avoidance across a diverse range of plant species encourage consideration of this low-cost commodity organic acid as a biostimulant. These results are surprising as they contrast with earlier studies showing pH-dependent root growth inhibition at similar concentrations. We test the hypothesis that the concentration of the membrane permeable undissociated form of acetic acid (CH₃COOH) selectively inhibits maize root growth, and subsequently evaluate its impact on seedling water use and growth under deficit irrigation. We demonstrate conclusively for the first time that when germinating maize on filter paper, low pH exacerbates, and high pH mitigates, this inhibition of root growth in a predictable manner based on the dissociation constant of acetic acid. The buffering capacity of potting media can reduce this root damage through keeping the acetic acid primarily in the membrane impermeable dissociated form (CH₃COO^-) at near neutral pH, but peat substrates appear to offer some protection, even at low pH. While both deficit irrigation and acetic acid reduced water use and growth of maize seedlings outdoors, there was no significant interaction between the treatments. Twenty nine millimolar total acetic acid (CH₃COOH + CH₃COO^-) reduced transpiration, compared to lower and higher concentrations, but this did not specifically improve performance under reduced water availability, with parallel declines in shoot biomass leading to relatively consistent water use efficiency. Any acetic acid biostimulant claims under water stress should characterize its dissociation level, and exclude root damage as a primary cause.