Secondary plant metabolites are natural bioactive compounds which are an important income for the pharmaceutical, food, cosmetic, agriculture, and other sectors due to their health-promoting properties and prevention and treatment of some diseases. The secondary metabolites can be classified into three main groups: phenolic compounds, terpenoids, and nitrogen compounds. The secondary metabolism in plants is a mechanism of adaptation and evolution as a defense to harsh environmental factors that induce stress. According to the hormesis curve of each plant model, the stress can be divided into distress (bad stress that leads to damage and ultimately plant death) or eustress (good stress that leads to activation of secondary metabolism). The environmental factors can be divided into biotic and abiotic which can be artificially induced to activate plant defense responses leading to the production of secondary metabolites. Several approaches to this process called elicitation have been proposed in the last decades with different types of metabolism-inducing factors or elicitors. Novel elicitation using abiotic factors includes electromagnetic waves (including several wavelengths of the light spectra, and electric and magnetic fields), acoustic waves, nanostructures, volatile compounds, nutrient deprivation, and several metals and salt soil pollutants. In the same order, novel elicitation using biotic factors include new bacteria consortium, fungi, phytohormones, and miRNA solutions. In general, the purpose of elicitation is to interact with the biochemical routes in order to produce secondary metabolites in high quantities, usually with negative effects in biomass production or morphology, but increasing plant quality in terms of aroma, taste or color. The metabolic profile and general response of elicitation vary greatly depending on the plant model, the elicitor level or concentration, and the stimulation time. Considering these facts, this chapter clearly and concisely discusses the most current strategies of elicitation for the increase of secondary metabolites production in plants.

Role of Stress and Defense in Plant Secondary Metabolites Production

The Essential oils (EOs) are important bioactive secondary metabolites of aromatic plants, such as Basil, and find extensive utilization in pharmaceutical sector as therapeutic agents. A more eco-friendly industrial production of EOs requires new sustainable methods to improve yield and quality of these bioactive compounds in aromatic plants. Here, we evaluated the biostimulation effects of natural organic materials, such as humic substances (HS) extracted from a humified green compost made with artichoke biomasses. The molecular characteristics of HS from green compost were determined by 13C-CPMAS NMR spectroscopy and offline pyrolysis–gas chromatography/mass spectrometry and combined to the bioactivity of the EOs extracts to investigate the structural–activity relationship. Basil plants were grown under HS treatments at different concentrations and the yield, the chemical composition and the antioxidant and antimicrobial properties were determined for EOs extracted by steam distillation method from Basil leaves. HS showed a positive effect on total extraction yield of the most bioactive EO components, with a progressive enhancement of abundance for eugenol, eucalyptol and geranyl acetate with increasing HS concentrations. Humic biostimulation influenced the EOs antioxidant proprieties, as well as their antimicrobial activity against some common Gram-negative bacterial strains such as Klebsiella pneumoniae. The HS biostimulation of Basil plants was associated to their structural hydrophobic characteristics that may have favoured the interactions with the Basil rhizome and to specific bioactive molecular components, such as a lignin-derived phenols and polar saccharidic and aminoacidic compounds. Our results support the potential use of humic substances from green compost as promising effective biostimulants to improve the production and bioactive properties of EOs in aromatic plants.

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Humic substances from green compost increase bioactivity and antibacterial properties of essential oils in Basil leaves

Effects of potassium fulvic acid and potassium humate on microbial biodiversity in bulk soil and rhizosphere soil of Panax ginseng

Protected cultivation has a significant contribution in vegetable production. We investigated whether humic acid addition to soil and Piriformospora indica can improve the nutritional quality of greenhouse tomato. We conducted a pot experiment, in which the effects of P. indica inoculation, humic acid addition, and Cu spiking to soil (0, 120, 240, and 500 ppm Cu) were tested. Humic acid addition to soil spiked with 500 ppm Cu decreased the Cu concentration in the fruits of plants inoculated with P. indica from 0.65 to 0.40 mg 100 g Fw−1, which is still above the maximum allowed limits of Cu in tomato by World Health Organization (WHO). The lycopene and ascorbic acid content of tomato fruits were consistently improved by humic acid addition and P. indica inoculation. The antioxidant enzymes’ activity changed in response to humic acid addition, Cu spiking to soil, and P. indica inoculation. With increasing Cu level up to 240 ppm, the activity of superoxide dismutase (SOD) and peroxidase (POD) increased significantly. However, with spiking more Cu to soil, the activity of antioxidant enzymes reduced and the MDA content increased significantly. Addition of humic acid to soil and/or presence of P. indica increased the activity of antioxidant enzymes when the soil spiked with 500 ppm Cu. This study indicated that addition of P. indica and humic acid to the soil can enhance the nutritional quality of greenhouse tomato by reduction of Cu toxicity as a common pollutant in the greenhouse media and increasing the antioxidant content of fruits.

Contribution of Piriformospora indica on improving the nutritional quality of greenhouse tomato and its resistance against cu toxicity after humic acid addition to soil

Abstract Stimulating the production of secondary metabolites by elicitation seems to be an accepted tool in several in vitro cultures. In vivo application of chemical elicitation is, however, much less frequently discussed. The paper summarizes the experimental findings on the effects of jasmonic acid, methyl-jasmonate (MeJa) and salicylic acid (SA) treatments on medicinal and aromatic plants. Growth behaviour, yield, stress responses, essential oil (EO) and phenolic compounds are discussed. Exogenous application of MeJa or SA mostly by foliar spraying in concentrations between 10 −4 and 4 mM, induced various changes in the plants. In most cases, the elicitation enhanced the volatile accumulation, although the opposite reaction was observed too. Numerous in vivo trials also demonstrated an altered EO composition. Positive effects of treatments on the concentration of phenolic type compounds were described, while in some cases the components exhibited contradictory behavior or no significant changes. Unfortunately, phenolic composition was rarely determined. There is evidence for the dose dependency of the reactions to both elicitors, but systematic studies on this issue are missing. Some results indicate that lower concentrations in replication may be at least as effective as higher ones. Changes in secondary metabolite accumulation may be induced by the elicitors directly or through other physiological processes. The in vivo results should be considered in the complex system of plant ontogenesis, environmental conditions, timing of the treatment, type of the elicitor and concentration. Research results indicate a promising aspect of chemical elicitation in vivo , but further well-designed studies are needed for the individual species. 

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Stimulating secondary compound accumulation by elicitation: Is it a realistic tool in medicinal plants in vivo?

In function of the green revolution the indiscriminate use of agrochemicals and pesticides in agriculture has been also shown in the production of medicinal plants, resulting in the increase of productivity but with high residual contamination and low rates in the production of secondary metabolites responsible for the biological and pharmacological activity in vegetable drugs. In another hand, new techniques of elicitation has been applied to stimulate the medicinal plants production through the organic and agroecological management, contributing for the increase of performance, quality and production. In this context, it is aimed with this review to present such as the humic substances: fulvic acid, humic acid and efficient microorganisms which influence and help the ontogeny and the secondary metabolites production of medicinal plants. The reviewed articles show that the use of fulvic acid, humic acid and efficient microorganisms in the production of medicinal plants contributes on the increase of biosynthesis, secondary metabolites production such as coumarins, alkaloids, phenylpropanoids and essential oils, as well as the increase of nutrients absorption, growth and development of species.

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E. A. Marques

Papers

Humic Substances and Efficient Microorganisms: Elicitation of Medicinal Plants—A Review