Humic acid effect on catalase activity and the generation of reactive oxygen species in corn (Zea mays)

Ameliorative effects of humic acid and L-tryptophan on enzyme activity, mineral content, biochemical properties, and plant growth of spinach cultivated in saline conditions

Salinity poses a significant abiotic stress that limits plant productivity, thereby posing a serious threat to agricultural sustainability and worldwide food security. Techniques that can overcome this problem are needed. Recent focus has been placed on employing organic substances like humic acid (HA) and amino acids, including L-tryptophan (L-TRP), to mitigate the negative effects of salt stress on cultivated plants. Accordingly, in this research, the impact of foliar applications of HA and L-TRP, both separately and combined, on the growth parameters and biochemical properties of spinach subjected to salt stress was investigated. In the present study, eight treatments (1. control, 2. salt (NaCl), 3. HA, 4. L-TRP, 5. HA + NaCl, 6. L-TRP + NaCl, 7. HA + L-TRP, and 8. HA + L-TRP + NaCl) were investigated. The study showed that salt stress markedly reduced several growth properties in spinach, including plant height, number of leaves, leaf dimensions, and both fresh and dry weight. Additionally, it significantly lowered contents of chlorophyll (a, b, and total), carotenoid, polyphenol, lutein, anthocyanin, polyphenol oxidase, glycine betaine, relative water content, and the antioxidant enzyme activities (ascorbate peroxidase, catalase, peroxidase, and superoxide dismutase). On the other hand, significant increases were observed in sodium, chlorine, potassium, sulfur, zinc, nickel, proline, malondialdehyde, and hydrogen peroxide levels of spinach with salinity. Individual and combined applications of HA and L-TRP positively influenced plant growth, relative water content, activities of antioxidant enzyme, chlorophyll, and mineral contents of spinach under both normal and saline conditions. In conclusion, the combined use of HA and L-TRP under salt stress conditions is promising in mitigating the negative impacts of salinity and can be suggested as an effective alternative approach for cultivating spinach in saline environments.

Biochar combined with humic acid improves the soil environment and regulate microbial communities in apple replant soil

Apple replant disease (ARD) negatively affects plant growth and reduces yields in replanted orchards. In this study, biochar and humic acid were applied to apple replant soil. We aimed to investigate whether biochar and humic acid could promote plant growth and alleviate apple replant disease by reducing the growth of harmful soil microorganisms, changing soil microbial community structure, and improving the soil environment. This experiment included five treatments: apple replant soil (CK), apple replant soil with methyl bromide fumigation (FM), replant soil with biochar addition (2 %), replant soil with humic acid addition (1.5 ‰), and replant soil with biochar combined with humic acid. Seedling biomass, the activity of antioxidant enzymes in the leaves and roots, and soil environmental variables were measured. Microbial community composition and structure were analyzed using ITS gene sequencing. Biochar and humic acid significantly reduced the abundance of Fusarium and promoted the recovery of replant soil microbial communities. Biochar and humic acid also increased the soil enzymes activity (urease, invertase, neutral phosphatase, and catalase), the plant height, fresh weight, dry weight, the activity of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase), and root indexes of apple seedlings increased in replant soil. In sum, We can use biochar combined with humic acid to alleviate apple replant disease.

Do Vermicompost Applications Improve Growth Performance, Pharmaceutically Important Alkaloids, Phenolic Content, Free Radical Scavenging Potency and Defense Enzyme Activities in Summer Snowflake (Leucojum aestivum L.)?

Leucojum aestivum L. contains galanthamine and lycorine, which are two pharmaceutically valuable alkaloids. Vermicompost (VC), an organic waste product created by earthworms enhances soil quality and can improve the medicinal quality of the plant that is crucial to the pharmaceutical industry. The aim of this study was to determine the effects of four different VC concentrations (5 %, 10 %, 25 %, and 50 %) on L. aestivum growth parameters, alkaloid levels (galanthamine and lycorine), total phenol-flavonoid content, free radical scavenging potential, and defense enzyme activities (SOD and CAT) compared to control (no VC). The width, length, and fresh weight of the leaves were improved by 10 % VC treatment. The highest total phenolic content was found in the bulbs and leaves treated with 50 % VC. HPLC-DAD analysis of alkaloids showed that 10 % and 50 % VC treatments contained the most galanthamine in the bulb and leaf extracts, respectively. The application of 25 % VC was the most efficient in terms of lycorine content in both extracts. CAT activity was elevated at 10 %, 25 %, and 50 % VC. Based on the growth performance and galanthamine content of the bulbs and leaves, it can be concluded that a 10 % VC application was the most effective in the cultivation of L. aestivum.

Humic Acid Modulates Ionic Homeostasis, Osmolytes Content, and Antioxidant Defense to Improve Salt Tolerance in Rice

The sensitivity of rice plants to salinity is a major challenge for rice growth and productivity in the salt-affected lands. Priming rice seeds in biostimulants with stress-alleviating potential is an effective strategy to improve salinity tolerance in rice. However, the mechanisms of action of these compounds are not fully understood. Herein, the impact of priming rice seeds (cv. Giza 179) with 100 mg/L of humic acid on growth and its underlaying physiological processes under increased magnitudes of salinity (EC = 0.55, 3.40, 6.77, 8.00 mS/cm) during the critical reproductive stage was investigated. Our results indicated that salinity significantly reduced Giza 179 growth indices, which were associated with the accumulation of toxic levels of Na⁺ in shoots and roots, a reduction in the K⁺ and K⁺/Na⁺ ratio in shoots and roots, induced buildup of malondialdehyde, electrolyte leakage, and an accumulation of total soluble sugars, sucrose, proline, and enzymic and non-enzymic antioxidants. Humic acid application significantly increased growth of the Giza 179 plants under non-saline conditions. It also substantially enhanced growth of the salinity-stressed Giza 179 plants even at 8.00 mS/cm. Such humic acid ameliorating effects were associated with maintaining ionic homeostasis, appropriate osmolytes content, and an efficient antioxidant defense system. Our results highlight the potential role of humic acid in enhancing salt tolerance in Giza 179.

The biological and biochemical composition of wheat (Triticum aestivum) as affected by the bio and organic fertilizers

Microorganisms and organic compounds (humic and fulvic acid) offer viable alternatives to insecticides and mineral fertilizers. Even though many studies have shown the effects of biofertilizers and organic substances separately, little information is available on plant responses to the combined application of these bio-stimulants, even though these biological inputs have a high potential for simultaneous action. A two-year (2020/21-2021/22) field experiment was conducted to investigate the impact of organic and biofertilizers application on the growth, yield, and biochemical attributes of wheat (cv. Misr-1). Pre-planting, wheat seeds were inoculated with two biofertilizers including Mycorrhizae, and Azotobacter, and their combination (MIX), and control (un-inoculation) were considered the main plot factor. The subplot factor contained the foliar sprays of humic acid, fulvic acid, and control (no spray). The results revealed that the seed inoculation with mycorrhizae and azotobacter in combination with foliar-applied humic acid markedly (p ≤ 0.05) affected the growth, yield, and seed biochemical composition of wheat. Combination of mycorrhiza and azotobacter significantly (p ≤ 0.05) increased) plant height (100 cm), crop growth rate (18.69 g), number of spikelets per spike (22), biological yield (13.4 ton ha-1), grain yield (5.56 ton ha-1), straw yield (8.21 ton ha-1),), nitrogen (2.07%), phosphorous (0.91%), potassium (1.64%), protein content (12.76%), starch (51.81%), and gluten content (30.90%) compared to control. Among organic fertilizers, humic acid caused the maximum increase in plant height (93 cm), crop growth rate ( 15 g day-1 m-2),1000 grain weight (51 g), biological yield ( 11ton ha-1), grain yield (4.5 ton ha-1), protein content (11%), chlorophyll content (46 SPAD), and gluten (29.45%) as compared to all other treatments. The foliar application of humic acid combined with the mycorrhizae or azotobacter seed inoculation was efficient to induce wheat vegetative growth development, as well as yield and its components.

Humic Acid Modified by Being Incorporated Into Phosphate Fertilizer Increases Its Potency in Stimulating Maize Growth and Nutrient Absorption

Humic acid-enhanced phosphate fertilizer (HAP) is widely applied in Chinese agriculture due to its high efficiency. Although the structural composition and physicochemical properties of humic acid (HA) are significantly altered during HAP production, a clear understanding of the mechanisms underlying the biological effects of HA extracted from HAP fertilizer (PHA) on plant growth is still lacking. In the current study, we extracted PHA from HAP and assessed its effects on the dry biomass, phosphorus (P) and nitrogen (N) uptake, and P absorption rate of maize seedlings when supplied at different concentrations (2.5, 5, 10, and 25 mg C L^-1) in the hydroponic culture. The root vigor, root plasma membrane H⁺-ATPase activity, and root nitrate reductase activity were also determined as the representative indicators of the root capacity for nutrient absorption, and used to clarify the mechanism by which PHA affects the maize growth and nutrient absorption. The results showed that the dry biomass, phosphorus uptake, nitrogen uptake, and average phosphorus absorption rates were significantly higher by 14.7-27.9%, 9.6-35.1%, 17.9-22.4%, and 22.1-31.0%, respectively, in plants treated with 2.5-5 mg C L^-1 PHA compared to untreated controls. Application of 10-25 mg C L^-1 raw HA resulted in similar stimulatory effects on plant growth and nutrient absorption. However, higher levels of PHA (10-25 mg C L^-1) negatively impacted these indicators of plant growth. Furthermore, low PHA or high raw HA concentrations similarly improved root vigor and root plasma membrane H⁺-ATPase and nitrate reductase (NR) activities. These results indicate that lower concentrations of PHA can stimulate maize seedling growth and nutrient absorption to an extent that is comparable to the effect of higher concentrations of raw HA. Thus, the proportion of HA incorporated into HAP could be lower than the theoretical amount estimated through assays evaluating the biological effects of raw HA.

Impact of Exogenously Sprayed Antioxidants on Physio-Biochemical, Agronomic, and Quality Parameters of Potato in Salt-Affected Soil

Salinity is one of the harsh environmental stresses that destructively impact potato growth and production, particularly in arid regions. Exogenously applied safe-efficient materials is a vital approach for ameliorating plant growth, productivity, and quality under salinity stress. This study aimed at investigating the impact of foliar spray using folic acid (FA), ascorbic acid (AA), and salicylic acid (SA) at different concentrations (100, 150, or 200 mg/L) on plant growth, physiochemical ingredients, antioxidant defense system, tuber yield, and quality of potato (Solanum tuberosum L cv. Spunta) grown in salt-affected soil (EC = 7.14 dS/m) during two growing seasons. The exogenously applied antioxidant materials (FA, AA, and SA) significantly enhanced growth attributes (plant height, shoot fresh and dry weight, and leaves area), photosynthetic pigments (chlorophyll a and b and carotenoids), gas exchange (net photosynthetic rate, Pn; transpiration rate, Tr; and stomatal conductance, gs), nutrient content (N, P, and K), K+/ Na+ ratio, nonenzymatic antioxidant compounds (proline and soluble sugar content), enzymatic antioxidants (catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), and ascorbate peroxidase (APX)) tuber yield traits, and tuber quality (dry matter, protein, starch percentage, total carbohydrates, and sugars percentage) compared with untreated plants in both seasons. Otherwise, exogenous application significantly decreased Na+ and Cl- compared to the untreated control under salt stress conditions. Among the assessed treatments, the applied foliar of AA at a rate of 200 mg/L was more effective in promoting salt tolerance, which can be employed in reducing the losses caused by salinity stress in potato grown in salt-affected soils.

Bioactivity of Humic Acids Extracted From Shale Ore: Molecular Characterization and Structure-Activity Relationship With Tomato Plant Yield Under Nutritional Stress

The increasing demands for biostimulants in the agricultural market over the last years have posed the problem of regulating this product category by requiring the industry to make available the information about efficacy and safety, including the explanation of mode of action and the definition of bioactive constituents. In the present study, we tested the biostimulant proprieties of a sedimentary shale ore-extracted humic acid (HA) on Micro Tom tomato plants under increasing nutritional stress and investigated the correlation with the chemical features of HA by means of ultra-high resolution FT-ICR MS, FT-ATR, and ¹³C-NMR. Humic acid application proved effective in alleviating the nutritional stress by improving nutrient use efficiency, with results comparable to the control treatment supplied with higher NPK nutrition. Increased yield (up to +19%) and fruit quality (in the range +10-24%), higher ascorbic acid content and a better root growth were the main parameters affected by HA application. Molecular-level characterization identified the possible chemical drivers of bioactivity, and included flavonoids, quinones, and alkaloids among the most represented molecules, some of which exhibiting antioxidant, pro-oxidant, and antimicrobial activity. The redox effect was discussed as a determinant of the delicate homeostasis balance, capable of triggering plant defense response and eventually inducing a protective priming effect on the plants.

Can biostimulants be used to mitigate the effect of anthropogenic climate change on agriculture? It is time to respond

Anthropogenic climate change, namely climate alterations induced by human activities, is causing some issues to agricultural systems for their vulnerability to extreme events. Forecasts predict a global population increase in the near years that will exacerbate this situation, elevating the global demand for food. It will pose severe concerns in terms of natural resource usage and availability. Agriculture is one of the anthropogenic activities that will be more affected in the future. Climate extremes menace to affect the quantity and quality of crop production severely. Drought, water and soil salinity are considered among the most problematic factors that anthropogenic climate change will increase. This complex and worrying scenario requires the urgent implementation of sustainable measures which are capable of improving crop yield and quality, fostering the robustness and resilience of cropping systems. Among the more current methodology, the use of natural plant biostimulants (PBs) has been proposed to improve plant resistance to abiotic environmental stresses. The advantage of using these substances is due to their effectiveness in improving crop productivity and quality. Therefore, in this review, the most recent researches dealing with the use of natural PBs for improving plant resistance to drought and salinity, in an anthropogenic climate change scenario, have been reported and critically discussed.

Effect of water soluble humic acid applied to potato foliage on plant growth, photosynthesis characteristics and fresh tuber yield under different water deficits

Water scarcity is the main limiting factor in agricultural crop production in arid and semi-arid areas in northern China. Humic acid could improve the plant resistance to mitigate the abiotic drought damages, which is a potential strategy to improve the crop production in these regions. An experiment to investigate the effect of water soluble humic acid on plant growth, photosynthesis characteristics and fresh tuber yield of potato under different water deficits was carried out under greenhouse conditions in 2014 and 2015. Treatments included foliar application of fresh water (FW), humic acid diluted with water 500 times (HA) and control (CK), and the water deficits included 45%, 60% and 75% of the field water holding capacity. The HA treatment showed highly significant (P ≤ 0.01) effect on dry biomass, root/shoot ratio and photosynthesis parameters, improved the dry biomass above ground (DM-AG) by 14.12–36.63%, 11.62–36.26% and 7.85–20.85% over the whole growing season at water deficits of 45%, 60% and 75% of the field water holding capacity respectively in 2014 and 2015; decreased the root/shoot (R/S) ratio in the early growing season and increased the R/S ratio in the later growing season; showed an improved effect on leaf soil plant analysis development (SPAD), photosynthesis rate (Pn) and stomatal conductance (Gs) and decreased transpiration rate (Tr) and intercellular CO₂ concentration (Ci) compared with the control. HA usually showed a better effect on photosynthesis parameters in 60% of the field water holding capacity than 45% and 75% except on Pn. Compared with control, HA increased fresh tuber yield by 34.47–63.48%, 35.95–37.28% and 23.37–27.15% at 45%, 60% and 75% of the field water holding capacity respectively. HA enhanced the potato plant growth, and improved photosynthesis parameters and fresh tuber yield under different water deficits under green house conditions, and represents an opportunity to improve crop production and sustainability of agriculture in arid and semiarid regions.

Interaction between Humic Substances and Plant Hormones for Phosphorous Acquisition

Phosphorus (P) deficiency is a major constraint in highly weathered tropical soils. Although phosphorous rock reserves may last for several hundred years, there exists an urgent need to research efficient P management for sustainable agriculture. Plant hormones play an important role in regulating plant growth, development, and reproduction. Humic substances (HS) are not only considered an essential component of soil organic carbon (SOC), but also well known as a biostimulant which can perform phytohormone-like activities to induce nutrient uptake. This review paper presents an overview of the scientific outputs in the relationship between HS and plant hormones. Special attention will be paid to the interaction between HS and plant hormones for nutrient uptake under P-deficient conditions.

Effects of humic acid and plant growth-promoting rhizobacteria (PGPR) on induced resistance of canola to Brevicoryne brassicae L

The cabbage aphid, Brevicoryne brassicae L. (Hem: Aphididae), is an important pest of canola that can considerably limit profitable crop production either through direct feeding or via transmission of plant pathogenic viruses. One of the most effective approaches of pest control is the use of biostimulants. In this study, the effects of humic acid, plant growth-promoting rhizobacteria (PGPR), and integrated application of both compounds were investigated on life table parameters of B. brassicae, and the tolerance of canola to this pest. B. brassicae reared on plants treated with these compounds had the lower longevity, fecundity, and reproductive period compared with control treatment. The intrinsic rate of natural increase (r) and finite rate of increase (λ) were lowest on PGPR treatment (0.181 ± 0.004 day-1 and 1.198 ± 0.004 day-1, respectively) and highest on control (0.202 ± 0.005 day-1 and 1.224 ± 0.006 day-1, respectively). The net reproductive rate (R0) under treatments of humic acid, PGPR and humic acid + PGPR was lower than control. There was no significant difference in generation time (T) of B. brassicae among the tested treatments. In the tolerance test, plants treated with PGPR alone or in integrated with humic acid had the highest tolerance against B. brassicae. The highest values of total phenol, flavonoids, and glucosinolates were observed in treatments of PGPR and humic acid + PGPR. Basing on the antibiosis and tolerance analyses in this study, we concluded that canola plants treated with PGPR are more resistant to B. brassicae. These findings could be useful for integrated pest management of B. brassicae in canola fields.

Protein Profiling of Arabidopsis Roots Treated With Humic Substances: Insights Into the Metabolic and Interactome Networks

Background and Aim: Humic substances (HSs) influence the chemical and physical properties of the soil, and are also known to affect plant physiology and nutrient uptake. This study aimed to elucidate plant metabolic pathways and physiological processes influenced by HS activity. Methods: Arabidopsis roots were treated with HS for 8 h. Quantitative mass spectrometry-based proteomics analysis of root proteins was performed using the iTRAQ (Isobaric Tag for Relative and Absolute Quantification) technique. Out of 902 protein families identified and quantified for HS treated vs. untreated roots, 92 proteins had different relative content. Bioinformatic tools such as STRING, KEGG, IIS and Cytoscape were used to interpret the biological function, pathway analysis and visualization of network amongst the identified proteins. Results: From this analysis it was possible to evaluate that all of the identified proteins were functionally classified into several categories, mainly redox homeostasis, response to inorganic substances, energy metabolism, protein synthesis, cell trafficking, and division. Conclusion: In the present study an overview of the metabolic pathways most modified by HS biological activity is provided. Activation of enzymes of the glycolytic pathway and up regulation of ribosomal protein indicated a stimulation in energy metabolism and protein synthesis. Regulation of the enzymes involved in redox homeostasis suggest a pivotal role of reactive oxygen species in the signaling and modulation of HS-induced responses.

The humic acid-induced changes in the water status, chlorophyll fluorescence and antioxidant defense systems of wheat leaves with cadmium stress

The using of bio-stimulant in plants grown under stress conditions for enhancing nutrition efficiency and crop quality traits is an effective approach. One of the bio-stimulants, humus material, is defined as humic acid (HA). HA application as a promotion of plant growth to plants grown in the heavy metals-contaminated soils has promised hope in terms of effects on plants but the its limiting effect is the application dose. Therefore, the wheat seedlings were grown in hydroponic culture for 21 d and the various concentrations of humic acid (HA; 750 or 1500 mg L^-1) were treated alone or in combination with cadmium (Cd) stress (100 or 200 μM) for 7 d. The results showed that after Cd stress treatment, water content (RWC), osmotic potential (Ψ_Π) and chlorophyll fluorescence parameters decreased and proline content (Pro) increased for 7 d. In spite of activated peroxidase (POX) and ascorbate peroxidase (APX), stress induced the toxic levels of hydrogen peroxide (H₂O₂) accumulation. Cd stress triggered lipid peroxidation (TBARS content). HA application successfully eliminated the negative effects of stress on RWC, Ψ_Π and photosynthetic parameters. In the presence of HA under stress, the increased activation of superoxide dismutase (SOD), catalase (CAT) and NADPH-oxidase (NOX) enzymes and ascorbate, glutathione and GSH/GSSG ratio observed. Only 750 mg L^-1 HA under stress conditions induced the activities of monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), and dehydroascorbate (DHA) content. After the combined application of HA and Cd stress, the low contents of H₂O₂ and TBARS maintained in wheat leaves. Hence, HA successfully eliminated the toxicity of Cd stress by modulating the water status, photosynthetic apparatus and antioxidant activity in wheat leaves.

Element uptake and physiological responses of Lactuca sativa upon co-exposures to tourmaline and dissolved humic acids

Element migration and physiological response in Lactuca sativa upon co-exposure to tourmaline (T) and dissolved humic acids (DHAs) were investigated. Different fractions of DHA1 and DHA4 and three different doses of T were introduced into Hoagland's solution. The results indicated that T enhanced the contents of elements such as N and C, Si and Al in the roots and shoots. The correlation between TF values of Si and Al (R2 = 0.7387) was higher than that of Si and Mn (R2 = 0.4961) without the presence of DHAs. However, both DHA1 and DHA4 increased the correlation between Si and Mn, but decreased the one between Si and Al. CAT activities in T treatments were positively correlated to the contents of N and Al in the shoots, whose R 2 was 0.9994 and 0.9897, respectively. In the co-exposure of DHAs and tourmaline, DHA4 exhibited more impacts on element uptake, CAT activities, as well as ABA contents in comparison with the presence of DHA1, regardless of the T exposure doses. These results suggested that DHAs have effects on mineral element behaviors and physiological response in Lactuca sativa upon exposure to tourmaline for the first time, which had great use in guiding soil remediation.

Alleviation of iron toxicity in Schinus terebinthifolius Raddi (Anacardiaceae) by humic substances

One of the industrial pillars of Espírito Santo state, South East of Brazil, is iron-mining products processing. This activity brings to a high level of coastal pollution due to deposition of iron particulate on fragile ecosystems as mangroves and restinga. Schinus therebinthifolius (aroeira) is a widespread restinga species. This work tested iron toxicity alleviation by vermicompost humic substances (HS) added to aroeira seedlings in hydroponic conditions. Catalase, peroxidase, and ascorbate peroxidase are antioxidant enzymes that work as reactive oxygen species (ROS) scavengers: they increase their activity as an answer to ROS concentration rise that is the consequence of metal accumulation or humic substance stimulation. S. terebinthifolius seedlings treated with HS and Fe augmented their antioxidant enzyme activities significantly less than seedlings treated separately with HS and Fe; their significantly lower Fe accumulation and the slight increase of root and leaf area confirm the biostimulating effect of HS and their role in blocking Fe excess outside the roots. The use of HS can be useful for the recovery of areas contaminated by heavy metals.

Humic Substances: Determining Potential Molecular Regulatory Processes in Plants

Humic substances (HSs) have considerable effects on soil fertility and crop productivity owing to their unique physiochemical and biochemical properties, and play a vital role in establishing biotic and abiotic interactions within the plant rhizosphere. A comprehensive understanding of the mode of action and tissue distribution of HS is, however, required, as this knowledge could be useful for devising advanced rhizospheric management practices. These substances trigger various molecular processes in plant cells, and can strengthen the plant's tolerance to various kinds of abiotic stresses. HS manifest their effects in cells through genetic, post-transcriptional, and post-translational modifications of signaling entities that trigger different molecular, biochemical, and physiological processes. Understanding of such fundamental mechanisms will provide a better perspective for defining the cues and signaling crosstalk of HS that mediate various metabolic and hormonal networks operating in plant systems. Various regulatory activities and distribution strategies of HS have been discussed in this review.

Effects of humic acids from landfill leachate on plants: An integrated approach using chemical, biochemical and cytogenetic analysis

Biological process treatment of landfill leachate produces a significant amount of sludge, characterized by high levels of organic matter from which humic acids are known to activate several enzymes of energy metabolism, stimulating plant growth. This study aimed to characterize humic acids extracted from landfill sludge and assess the effects on plants exposed to different concentrations (0.5, 1, 2 and 4 mM C L^-1) by chemical and biological analysis, to elucidate the influence of such organic material and minimize potential risks of using sludge in natura. Landfill humic acids showed high carbon and nitrogen levels, which may represent an important source of nutrients for plants. Biochemical analysis demonstrated an increase of enzyme activity, especially H⁺-ATPase in 2 mM C L^-1 landfill humic acid. Additionally, cytogenetic alterations were observed in meristematic and F₁ cells, through nuclear abnormalities and micronuclei. Multivariate statistical analysis provided integration of physical, chemical and biological data. Despite all the nutritional benefits of humic acids and their activation of plant antioxidant systems, the observed biological effects showed concerning levels of mutagenicity.

Possible developments for ex situ phytoremediation of contaminated sediments, in tropical and subtropical regions - Review

The growing problem of remediation of contaminated sediments dredged from harbor channels needs to be resolved by a cost effective and sustainable technology. Phytoremediation, by ex situ remediation plants, seems to have the potential to replace traditional methods in case of moderately contaminated sediments. On the other side, the need to mix sediments with soil and/or sand to allow an easier establishment of most employed species causes an increase of the volume of the processed substrate up to 30%. Moreover the majority of phytoremediating species are natives of temperate climate belt. Mangroves, with a special focus on the genus Avicennia - a salt secreting species - should represent an effective alternative in terms of adaptation to salty, anoxic sediments and an opportunity to develop ex situ phytoremediation plants in tropical and subtropical regions. The use of humic acid to increase root development, cell antioxidant activity and the potential attenuation of the "heavy metals exclusion strategy" to increase phytoextraction potentials of mangroves will be reviewed.

Biostimulants in Plant Science: A Global Perspective

This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as "a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds." The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a "mode of action" for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

Biostimulants in Plant Science: A Global Perspective

This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as "a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds." The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a "mode of action" for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

Involvement of Hormone- and ROS-Signaling Pathways in the Beneficial Action of Humic Substances on Plants Growing under Normal and Stressing Conditions

The importance of soil humus in soil fertility has been well established many years ago. However, the knowledge about the whole mechanisms by which humic molecules in the rhizosphere improve plant growth remains partial and rather fragmentary. In this review we discuss the relationships between two main signaling pathway families that are affected by humic substances within the plant: one directly related to hormonal action and the other related to reactive oxygen species (ROS). In this sense, our aims are to try the integration of all these events in a more comprehensive model and underline some points in the model that remain unclear and deserve further research.

Effect of synthetic and natural water-absorbing soil amendments on photosynthesis characteristics and tuber nutritional quality of potato in a semi-arid region

BACKGROUND

The effect of water-absorbing soil amendments on photosynthesis characteristics and tuber nutritional quality was investigated in a field experiment in a semi-arid region in northern China in 2010-2012. Treatments included two synthetic water-absorbing amendments, potassium polyacrylate (PAA) and polyacrylamide (PAM), and one natural amendment, humic acid (HA), both as single amendments and compound amendments (HA combined with PAA or PAM), and a no amendment control.

RESULTS

Soil amendments had a highly significant effect (P ≤ 0.01) on photosynthesis characteristics, dry biomass, crop root/shoot (R/S) ratio and tuber nutritional quality. They improved both dry biomass above ground and dry biomass underground in the whole growing season by 4.6-31.2 and 1.1-83.1% respectively in all three years. Crop R/S ratio was reduced in the early growing season by 2.0-29.4% and increased in the later growing season by 2.3-32.6%. Soil amendments improved leaf soil plant analysis development value, net photosynthesis rate, stomatal conductance and transpiration rate by 1.4-17.0, 5.1-45.9, 2.4-90.6 and 2.0-22.6% respectively and reduced intercellular CO2 concentration by 2.1-19.5% in all three years.

CONCLUSION

Amendment treatment with PAM + HA always had the greatest effect on photosynthesis characteristics and tuber nutritional quality among all amendment treatments and thus merits further research.

Effects of humic acids derived from lignite and cattle manure on antioxidant enzymatic activities of barley root

To investigate the effects of humic acids (HAs) on the ability of plants to defend themselves against oxidative stress, barley was hydroponically cultured in the absence and presence of HAs, and the antioxidant enzymatic activities (catalase, superoxide dismutase, ascorbate and glutathione peroxidases) of root tissue were evaluated. Auxin-like structures in HAs, which were extracted from an oxidation product of lignite (LHA) and compost derived from cattle manure (CHA), were identified by pyrolysis-gas chromatography/mass spectrometry (GC/MS) with tetramethylammonium hydroxide. The LHA, which had the lower molecular weight, was more effective in promoting the growth of barley root than CHA. However, the amounts of auxin-like structures in the CHA were much higher than those for LHA. The antioxidant enzymatic activities were initially decreased in the presence of LHA and CHA at the first day after refreshing the culture solution, but were significantly increased on the second day. The CHA sample, which contained relatively high levels of phenolic acids that contained auxin-like structures, was effective in increasing four types of enzymatic activities, while the activities of catalase and ascorbate peroxidase were increased in the presence of LHA, which contains naphthalene derivatives. These results indicate that using HAs as a supplement can be effective in enhancing antioxidation enzymatic activities, while the appearance of the effects is retarded because of the decomposition and release of auxin-like compounds from HAs by organic acids from the plant roots.

Change in ATP-binding cassette B1/19, glutamine synthetase and alcohol dehydrogenase gene expression during root elongation in Betula pendula Roth and Alnus glutinosa L. Gaertn in response to leachate and leonardite humic substances

Humic substances (HS) are complex and heterogeneous compounds of humified organic matter resulting from the chemical and microbiological decomposition of organic residues. HS have a positive effect on plant growth and development by improving soil structure and fertility. They have long been recognized as plant growth-promoting substances, particularly with regard to influencing nutrient uptake, root growth and architecture. The biochemical and molecular mechanisms through which HS influence plant physiology are not well understood. This study evaluated the bioactivity of landfill leachate and leonardite HS on alder (Alnus glutinosa L. Gaertn) and birch (Betula pendula Roth) during root elongation in vitro. Changes in root development were studied in relation to auxin, carbon and nitrogen metabolisms, as well as to the stress adaptive response. The cDNA fragments of putative genes encoding two ATP-binding cassette (ABC) transporters (ABCB1 and ABCB19) belonging to the B subfamily of plant ABC auxin transporters were cloned and sequenced. Molecular data indicate that HS and their humic acid (HA) fractions induce root growth by influencing polar auxin transport (PAT), as illustrated by the modulation of the ABCB transporter transcript levels (ABCB1 and ABCB19). There were also changes in alcohol dehydrogenase (ADH) and glutamine synthetase (GS) gene transcript levels in response to HS exposure. These findings confirmed that humic matter affects plant growth and development through various metabolic pathways, including hormonal, carbon and nitrogen metabolisms and stress response or signalization.

Humic acids: Structural properties and multiple functionalities for novel technological developments

Humic acids (HAs) are macromolecules that comprise humic substances (HS), which are organic matter distributed in terrestrial soil, natural water, and sediment. HAs differ from the other HS fractions (fulvic acid and humins) in that they are soluble in alkaline media, partially soluble in water, and insoluble in acidic media. Due to their amphiphilic character, HAs form micelle-like structures in neutral to acidic conditions, which are useful in agriculture, pollution remediation, medicine and pharmaceuticals. HAs have undefined compositions that vary according to the origin, process of obtainment, and functional groups present in their structures, such as quinones, phenols, and carboxylic acids. Quinones are responsible for the formation of reactive oxygen species (ROS) in HAs, which are useful for wound healing and have fungicidal/bactericidal properties. Phenols and carboxylic acids deprotonate in neutral and alkaline media and are responsible for various other functions, such as the antioxidant and anti-inflammatory properties of HAs. In particular, the presence of phenolic groups in HAs provides antioxidant properties due to their free radical scavenging capacity. This paper describes the main multifunctionalities of HAs associated with their structures and properties, focusing on human health applications, and we note perspectives that may lead to novel technological developments. To the best of our knowledge, this is the first review to address this topic from this approach.