Aromatic Plants: Alternatives for Management of Crop Pathogens and Ideal Candidates for Phytoremediation of Contaminated Land

Abstract

Crop diseases due to fungal pathogens cause significant resulting economic losses in agriculture. For management of crop diseases, farmers use synthetic pesticides. However, the frequent application of these chemicals leads to accumulation in soil and therefore presenting pollution problems. Essential oils (EOs) sourced from aromatic plants are safer alternatives and are effective against a variety of crops pathogens. In addition to their role as the sources of EOs, aromatic plants are gaining much attention in rehabilitation strategies. In phytoremediation processes, suitable plants species are used to clean-up polluted sites. Mining activities and electricity generation processes have resulted in significant amounts of tailings and coal fly ash. Mine tailings and coal fly ash are disposed in dumpsites, converting productive lands to unusable waste sites. These solid waste materials contain toxic metals and therefore posing serious risks to the health of the environment. Aromatic plants can be cultivated in contaminated sites and therefore be used for restoration of polluted lands. The EOs can be sourced from these aromatic plants as they are free from metal-toxicity and can therefore be used to generate revenues. This review highlights the role of aromatic plants in the control of crops pathogens and also their application in phytoremediation processes.

1. Introduction

Plant diseases caused by infectious pathogens are a global concern to agriculture, significantly impacting on food security and human health [1]. To control disease outbreaks, synthetic pesticides are applied at regular intervals, but their use is ineffective and associated with health risks to humans and the environment. The use of pesticides leads to build-up of toxicants, which should be replaced by biodegradable alternatives. Pathogens readily acquire resistance to fungicides, making them ineffective crop protection agents. Aromatic plants produce essential oils that are effective for prevention and protection of crops against infectious diseases in the field as well as during storage. In addition to their potential as biopesticides, aromatic plants have gained a lot of interest in phytoremediation strategies. Mining activities produce mine waste, while generation of electricity from coal combustion results in production of fly ash and these solid waste materials are disposed in ash and tailings dams [2]. Harsh conditions prevailing on fly ash and mine tailings dams include unfavorable pH, very low levels of nutrients required for plant growth, and unacceptable concentrations of toxic metals [3]. All these converting valuable lands to unproductive waste disposal sites. Aromatic plants have demonstrated potential in remediation of areas contaminated with toxic levels of heavy metals [4]. These species are high value crops as they produce EOs that are free from metal contamination and therefore can be used simultaneously for restoration strategies and as sources of valuable products.

2. Crop diseases and excessive use of agrochemicals

The infestation of crops leads to a reduction in yield, increased production costs and may even be harmful to human and animal health. Pesticides are effective in decreasing pathogen load. Synthetic pesticides are applied at regular intervals throughout the growing season of the crop. Their use has significantly increased agricultural yields and contributed to improved food quality. However, using synthetic chemicals for crop protection is associated with health risks to humans and the environment. Fungal and bacterial pathogens readily acquire resistance to bactericides and fungicides, making them ineffective crop protection agents. For pesticides to be effective, there is a need to use higher doses or their substitution with new and sometimes, highly toxic products. In many cases the pathogen will become resistant to the pesticide used because of the single compound nature of most pesticides. Global warming and accompanying climate changes have resulted in increased resistance of several bacterial and fungal pathogens [5].

The excessive use and misuse of agrochemicals has led to increased bioaccumulation of toxic metals in soils and water and eventual toxicity to humans via food intake and the environment. Pesticides can remain in soil for long after they are applied, continuing to harm the ecosystem. How the pesticide is bound by soil components, how readily it is degraded and environmental conditions determine how long it can remain in soil [6]. Unacceptable levels of organochlorine pesticides (OCPs) residues and potentially toxic metals (Pb, Cr, Zn, Cu, and Fe) were reported in beans and cowpea [7]. The OCPs are volatile, stable and can be bound to the soil components and persist in air, negatively impacting the health of humans, animals and the environment [8]. Furthermore, in several countries, the level of pesticide residues detected in the medicinal plants were above the permissible limits as prescribed by the World Health Organization [9, 10]. Soil contains an abundance of biologically diverse organisms that are essential for agricultural sustainability and the use of pesticides have contributed to their decline [11]. Azole fungicides are extensively used for control of fungal diseases, but their over-use of azole has resulted to contamination of air, soil and crops, mainly because of their lipophilic characteristic [12]. The long-term use of copper-based pesticides has led to build-up in soil worldwide and presenting a potential public health problem due to Cu entering the food chain [13, 14]. Public pressure is also increasing to reduce the use of synthetic chemical products [15, 16]. Furthermore, the European Union has prohibited the use of contaminant plant protection products since 2020. These drawbacks emphasize the discovery of sustainable and environment-friendly pathogen control practices to manage diseases and ensure the safety of consumers.

3. Plants-based products as alternatives for control of crops pathogens

Crops diseases remain as one of the greatest threats to the sustainable development of society, leading to significant agricultural loss and costs incurred on awareness and the development of management strategies. Recently attention has been given to organic farming and food safety owing to the many challenges of using synthetic chemicals and consequently protection of consumers health. A natural solution to this problem could be the use of essential oils that have been shown to demonstrate antibacterial, fungicidal, herbicidal, nematocidal, acaricidal and insecticidal, properties. EOs are mixtures of volatile bioactive compounds that are obtained from various plant parts. They are characterized by a mixture of secondary metabolites including alcohols, phenolics, aldehydes, ketones, terpenoids and other secondary metabolites contributing synergistically or by additive effect to treat infectious diseases [15, 17].

Fungal pathogens are the major causes of economic losses in agriculture worldwide. The fungal species including Aspergillus, Fusarium, Penicillium, Phytophthora and Botrytis are among the pathogens that contribute significantly to agricultural losses as they can cause decay, accelerated ripening, and production of mycotoxins [18, 19]. The Aspergillus species have contributed to significant losses and the most reported species include Aspergillus fumigatus, Aspergillus flavus and Aspergillus niger which produce aflatoxins that exhibit carcinogenic and mutagenic properties, therefore affecting human health [20, 21]. Azoles are the most widely used for control of diseases caused by Aspergillus genus, but studies demonstrated an increasing prevalence of azole-resistant strains such as A. fumigatus and these problems are associated with higher clinical burdens and mortality rates [22, 23]. Aspergillus fumigatus is a mold found in soil, compost and releases volatile spores onto air, continuously breathed by humans and could be the cause of one of the most common fungal ailments [24, 25]. Fusarium species cause significant agricultural losses in crops including potato, pea, bean, wheat, corn, cabbage, cucumber and rice worldwide [26, 27]. These pathogens are found in soil, plant, air and aquatic environment causing diseases in humans, animals and plants [28]. Fusarium pathogens produce mycotoxins that affect the quality of crop produces and threatening human health [29]. Fusarium species infect both plants and humans and are resistant toward antifungal agents including amphotericin B, itraconazole, fluconazole and echinocandins and therefore continues to be a problem for patients with compromised immune systems [30]. Botrytis cinerea causes pre- and postharvest decay of various crops such as strawberries and this pathogen is resistant to fungicides including benzimidazole and dicarboximide [31, 32]. Phytophthora infestans causes late blight diseases in potato and tomato crops worldwide, affecting the economy and the quality and quantity of the crop [33]. Development of resistance was reported for Phytophthora infestans against fluazinam which was attributed to the widespread use of the fungicide [34]. Penicillium spp. are the most important cause of postharvest decays of fruits and vegetables, causing blue and green mold [12, 35, 36]. These fungal pathogens contribute to losses in crops such as apple, pear, and citrus fruits and contribute to mycotoxin accumulation in processed fruit products [37, 38]. Penicillium species are resistant toward fungicide resistance including thiabendazole, guazatine, imazalil and propiconazole [39, 40].

4. Effectiveness of EOs in the protection against plant pathogens

A variety of plants pathogens that affect agricultural produce include viruses, bacteria, fungi, nematode and parasitic plant. EOs are increasingly recognized as potential pesticides in agriculture. EOs can be used as lures for detecting and monitoring insects. Coriandrum sativum and Nerium indicum EOs are strong attractants to Cyrtorhinus lividipennis, a rice planthopper. Insecticidal activity of EOs is connected to a decrease in acetylcholinesterase activity [19]. There are approximately 350 bacteria known to be phytopathogenic, such as Proteobacteria, Actinobacteria and Firmicite [19]. Nematodes are a very destructive group of plant pathogens and the mode of action of EOs against nematodes include GABA, acetylcholinesterase inhibition and octopamine synapses [16]. Approximately 30% of all crop diseases are as a result of infections by phytopathogenic fungi that can produce toxins and carcinogenic substances [19]. EOs diminish the influence of fungal infections by acting on cell walls, cell wall alterations and modifications to gene expression. Bioactivity depends on the composition of the oil, the functional groups present in the major compounds as well as their synergistic effects. In most cases, the oils are more effective as antimicrobial and anti-insecticidal agents than the major components on their own, indicating the important synergistic contribution of minor compounds in bioactivity.

The EOs of thyme and manuka demonstrated strong fungistatic activity against Aspergillus niger, Fusarium culmorum, Phytophthora cactorum, demonstrating complete inhibition [41]. Cymbopogon schoenanthus, Lippia multifliora and Ocimum americanum EOs were found to reduce the contamination rates of Colletotrichum dematium and Fusarium spp., Cladosporium sp. and Macrophomina phaseolina [42]. The EO of Salvia sclarea and Salvia dolomitica demonstrated fungicidal activity against Aspergillus, Penicillium, Trichoderma viride and Fusarium species [43]. An added benefit from the application of EO was revealed from a study on Tuta absoluta, a tomato pinworm. Treatment with extracts of Achillea millefolium and Achillea sativum, reduced the number of infested leaves, which was accompanied by induced release of herbivory plant volatiles as the plant defense mechanism [44]. Numerous studies report the wide use of Mentha species for management of plant pathogens and insect pests due to the action of alcohols, phenolics, aldehydes, ketones, terpenoids and other secondary metabolites [45, 46]. The combination of EOs has also demonstrated synergistic effect. For example, the combination of tea tree oil and mint had improved activity against Aspergillus niger when mixed together [47]. The combination of more than one EO contributes to the inability of pathogens to develop resistance against the EOs. EOs are promising biocontrol agents because several bioactive compounds contribute to the activity of the oil therefore overcoming pathogen resistance. These plant-based products offer advantages including easy degradation, wide spectrum biological activities, cost-effective, renewable in nature, and demonstrate low toxicity [48]. Cultivation of bioactive EO producing plants is a sustainable method to obtain natural products for purpose as biopesticides. Most EOs produced can be used in a variety of applications from industrial to agriculture and in many cases the plants can be cultivated using environmentally-friendly techniques increasing their use as natural products [49]. Table 1 displays a few EOs with their major compounds and the variety of crops fungal pathogens against which they are effective which were reported from the year 2015. The aromatic plants selected are belonging to families of species that are promising in phytoremediation processes.

Essential oil	Major compounds	Pathogens	References
Mentha longifolia	Menthone (48%); eucalyptol (21.6%)	Aspergillus flavus, Aspergillus niger, Fusarium culmorum	[50]
Salvia dolomitica	1,8-cineole (18.9%) and β-caryophyllene (13.1%)	Aspergillus niger, Aspergillus flavus, Trichothecium roseum	[45]
Mentha arvensis	Menthol (69.2%), Menthone (19.9%)	Alternaria alternata	[51]
Rosmarinus officinalis	1,8-Cineole (53.48%); α-Pinene (15.65%)	Fusarium verticillioides	[52]
Helianthus annuus	α-Pinene (50.65%)	Aspergillus niger, Candida albicans, Cryptococcus neoformans	[53]
Salvia officinalis	1,8-cineole, α-thujone and camphor, β-caryophyllene, borneol, viridiflorol,; α-pinene and camphene	Botrytis cinerea, Penicillium expansum, Rhizopus stolonifer	[54]
Cinnamomum zeylanicum	cinnamaldehyde (52.4%), benzaldehyde (12.31%),	Aspergillus niger, Colletotrichum acutatum	[55]
Cymbopogon flexousus	Citral (81.84%)	Colletotrichum acutatum, Colletotrichum gloeosporioides	[56, 57]
Origanum vulgare	Carvacrol (89.98%), β-caryophyllene (3.34%)	Botrytis cinerea	[58]

Table 1.

Aromatic plants with potential in management of crop diseases.

5. Disadvantages of essential oils and their formulation

The potential of the use of EOs in organic food production is evident. Although there is considerable evidence of their efficacy, their application is still lacking. Farmers are still reluctant or have not welcomed the use of these products owing to a variety of disadvantages. They are limited due to their high volatility and low stability, low water solubility, strong influence on organoleptic properties, composition variability and phytotoxic effects [58, 59]. The pre-harvest use is further limited because they are very sensitive to light and elevated temperatures will cause oxidation and eventual biodegradation. To overcome the limitations, product formulations are being investigated in pesticides applications. These involve a combination of an active ingredient and inactive materials that act as additives that involve specialized processing of the product to improve its biological qualities, durability, and stability [60]. The formulation for pesticides is performed in two ways, as liquids including aqueous and dispersions and solids including wettable powders and water-dispersible granules or as controlled-release systems [61]. Factors that are considered in formulations include the mode of application, the crop and agricultural practices [62]. The application of nanotechnology for formulation of new products, using polymer-based nanocapsules, or encapsulation with metallic nanoparticles could result in increased stability and efficacy of EOs and therefore reduce the required dosage for application. In encapsulation formulations, EOs are trapped in the carrier matrix in which the release of bioactive components is controlled [63]. Nanoencapsulation is based on encapsulating EOs in materials including lipid nanomaterials, polymeric nanoparticles and clay nanomaterials resulting in improved characteristics [64, 65, 66]. The antimicrobial activity of the essential oil-based nanoemulsions is much stronger than in free EOs due to increased surface area which influence the transport of Eos [42]. Recently, a study on the powdered formulations of EOs demonstrated promising results for the control of corn pathogens and evaluating the efficacy of bio-fungicide formulations. Powdered formulations of Cymbopogon giganteus and Eucalyptus camaldulensis EOs were effective against Aspergillus flavus, Aspergillus parasiticus, Aspergillus niger, Rhizopus and Fusarium spp. at concentration ranging between 0.5 and 1.0% dosages highlighting the possibility of EOs in the management of corn diseases [43].

6. Environmental pollution

Mining, industrial and agricultural activities have resulted to significant environmental pollution and land degradation. Overuse of pesticides pollution has resulted to environmental pollution and an increased risk of harmful effects on the ecosystem, and entering into the food chain. The residual concentration of pesticides in agricultural soils is often above the permissible limits by the regulations and therefore the challenge is to reduce these chemicals from the soils [67, 68].

Mining activities generates a big amount of solid mine waste, which contributes enormously to environmental pollution due to the release of potentially toxic elements [69]. In 2022, mine tailings generated worldwide were estimated to be more than 14 billion metric tons, with potential to contaminate soil, water and air [70]. Mine tailings remain in dams/open ponds without further treatment after extraction of valuable minerals and negatively impacting the environment [71]. Abandoned mine tailings damage local land resources and pose severe environmental pollution. Toxic elements including lead, chromium, arsenic, nickel, copper, and cobalt occur in the tailings can enter into the food chain and impact the health of humans and animals [72, 73]. Toxic levels of metal can negatively impact the quality of soil, pollute ground and surface as well as agricultural produce. These poses high risk of food chain contamination and consequently health problems [74]. The influence of heavy metals toxicity effects in living organisms it through their interference in metabolism processes and possibly cause mutagenesis by accumulating in the bodies [75]. They can also be endocrine disruptors, mutagenic, teratogenic while others can cause neurological conditions among children and infants [76].

Generation of electricity has resulted in enormous generation of solid waste which is an important environmental hazard. Solid waste is generated from the combustion of coal in the coal-fired power plants (C-FPPs) and Coal Fly Ash (CFA) is one of the major by-products produced by C-FPPs, which is one of the major global concerns [77]. Its disposal causes significant environmental and economic problems, requiring large quantities of energy, water and land [78]. The disposal of CFA in landfills and ash ponds is the primary management technique, but the fine particles of CFA are dispersed by wind into the atmosphere and remain suspended for a long period of time resulting to air pollution. The fly ash disposal in ash dumps constructed near the power stations could also lead to water and soil contamination through leaching or seepage of pollutants into ground and surface water [79]. CFA disposal sites are hazardous due to the presence of toxic metals such As, Se, Cr, Cd, Pb and Hg and these are continuously being discharged into the environment due to improper disposal of fly ash [80, 81]. These pollution problems can be addressed through phytoremediation strategies that utilize plants species including shrubs, trees and grasses.

7. Phytoremediation: a sustainable approach for addressing environmental pollution

Phytoremediation techniques are cost-effective, environmentally-friendly and effective in the removal of pollutants from pollutes sites through the use of plants [82]. Phytoremediation is the utilization of suitable plant species at polluted areas to remove/reduce the toxic levels of pollutants [83]. Plants reduce the pollutants on the contaminated site by taking up toxicants through their roots and translocating or transforming them into less toxic forms [84]. Phytoremediation is able to remediate polluted areas due to the variety of mechanisms plants species may use to either remove or detoxify contaminant. Strategies include phytodegradation, phytostabilization, phytovolatilization, rhizofiltration, and phytoextraction [85, 86]. The purpose of these approaches differs such as containment, remediation, stabilization, leaching of contaminants, and detoxification [87]. Phytostabilization involves the use of plants that can reduce the movement of pollutants through accumulation by roots [88]. In phytoextraction, the pollutants are transferred to the harvestable plant parts [10]. Phytodegradation involve the breakdown of organic pollutants into less toxic or non-toxic forms through the production of degradation enzymes [89]. The use of plants in restoration processes is a more sustainable and feasible approach as it restores vegetation and prevent erosion, therefore improving the chemical properties of soil overtime.

Ideal species for phytoremediation strategies should have high biomass-producing capabilities, tolerant to toxic effects of metals and contaminants, easy to cultivate, have high absorption capacity. An added advantage includes high value economic crop, with no or low risk of contamination in use of end. Phytoremediation technologies are very appropriate for restoration of soils contaminated with pesticides, mining and industrial activities [90, 91]. Various conventional treatment methods are being used for clean-up of contaminated sites but they are often ineffective, expensive and technically difficult [92]. Chemical and physical techniques also showed to be expensive and not sustainable to the environment. Contaminated sites are remodeled using phytoremediation as a sustainable strategy to lower the pollution load. Plants can help clean up many types of pollutants including metals, radionuclides, and organic pollutants [explosives, nutrients, chlorinated solvents, surfactants, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), insecticides/pesticides, and various hydrocarbons] [93].

8. Suitability of aromatic plants in phytoremediation process

Recently, aromatic plants have gained interest in phytoremediation strategies and are being explored for their phytoremediation potential and also use of their biomass to extract essential oils that are added-value products. The use of essential oil producing aromatic plants from the Poaceae, Lamiaceae, Asteraceae, and Geraniaceae families can be used for phytoremediation of heavy metal contaminated sites. These plants can act as phytostabilisers, hyper accumulators, bio-monitors, and metallophytes. It has been observed that heavy metal stress enhances the essential oil percentage of certain aromatic crops but that the metals are not present in the essential oils produced [94]. Aromatic grasses received much attention due to their ability to accumulate high biomass and are therefore cultivated for high value essential oil such as Citronella (Cymbopogon winterianus Jowitt ex Bor), Lemon grass (Cymbopogon flexuosus (Nees ex Steud.) Watson), Vetiver (Chrysopogon zizanioides (L.) Nash), and Palmarosa (Cymbopogon martinii (Roxb.) Watson) [4, 95]. Grasses provide vegetation cover in a reasonable short period of time and therefore are ideal candidates for restoration of polluted lands. Cymbopogon winterianus Jowitt (citronella) and Cymbopogon flexuosus (lemon grass) can tolerate soils polluted due to mining activities and organic amendment assist in establishment of these species [96]. Cymbopogon martinii can accumulate of toxic concentrations of Cd, Cr, Pb, Ni and can be used for phytoremediation of sewage sludge [97].

Other families including Ocimum, Mentha, Lavender, Salvia, Rosemary and Chamomile have also demonstrated potential in phytoremediation technology. Lavandula angustifolia L. has phytoremediation potential for soils polluted with Cu and Pb and the inoculation of arbuscular mycorrhizal fungi results to improved plant growth and essential oil yield [98]. Ocimum basilicum L. has phytostabilization potential and can therefore be used to remediate soils polluted with Cd, Co, Cr, Cu, Ni, Pb, and Zn [99]. Mentha piperita is a metal tolerant aromatic plant that has the capacity to stabilize As, Cd, Ni, and Pb at the root level demonstrating its phytostabilization potential [100]. Chamomile (Matricaria chamomilla L) has high tolerance to toxic elements and is a suitable candidate for phytoremediation of soils contaminated with As, Cd, Pb and Zn [101]. Sage (Salvia sp. family—Lamiaceae) was found to be an effective hyperaccumulator crop against a variety of heavy metals [102]. Salvia verbenaca was evaluated for its potential in the phytorestoration of mine tailings moderately contaminated with copper [103]. In the study, increased accumulation of metals was reported for plants grown in medium treated with compost. Studies to evaluate the potential of scented geraniums, Pelargonium roseum, to uptake and accumulate heavy metals nickel (Ni), cadmium (Cd), or lead (Pb) demonstrated that this species is a hyperaccumulator and is effective in phytoremediation strategies [104]. Salvia sclarea L. has potential for remediation of heavily contaminated sites and is classified as Pb hyperaccumulator, and Cd and Zn accumulators [105]. The levels of Pb, Cu and Cd in the essential oil of Salvia sclarea L. grown on heavily polluted soils were lower than the accepted maximum permissible concentrations. Cultivation of Salvia sclarea in polluted sites increase the abundance of plant-growth promoting rhizobacteria significantly over time, thereby significantly impacting microbial communities in soils [106]. Pot trials evaluated the efficiency of Lantana camara in the phytoremediation of Cd, Co, and Pb and the results showed an increased accumulation of metals in the plant parts [107]. Development of vegetation cover on polluted sites is significant because these will prevent air pollution and dispersal of pollutants by wind, reduce mobility and toxicity of heavy metals and improve soil health. The revegetation of ash disposal sites and mine tailings is the best strategy for immobilization of pollutants, thus preventing erosion and leaching of toxic metals into the ecosystem. Aromatic plants have demonstrated potential in remediation of polluted soils and therefore should be explored for other contaminated areas such as coal fly ash dams that are sparsely reported in literature. Some aromatic species that displayed potential for phytoremediation processes are listed in Table 2.

Plant	Mechanism	Elements	Medium	Comments	References
Mentha longifolia	Phytostabilization	Pb, Cd, Cu, Mn, Ni, Zn, Co	Contaminated wetlands	The wild mint showed seasonal fluctuations in accumulation potential with elements being highest in the plant roots	[108]
Salvia sclarea	Phytoextraction	Zn	Contaminated soils	The plant is a Zn accumulator and the tolerance mechanism employed upon exposure to excess Zn include increase in the nutrient uptake, leaf pigment and levels of phenolic compounds	[109]
Thymus daenensis Celak.	Phytoextraction	Pb, Cd	Contaminated soils	The plant is able to absorb significant amounts of Pb and Cd	[110]
Helianthus annus L	Phytoextraction	Pb and Cd	Pb and Cd contaminated soils	The plant was effective in the uptake of Cd and Pb, and can remediate soils polluted with both Pb and Cd	[111]
Alyssum murale L.	Phytoextraction	Ni	Industrial Ni-contaminated soil (heavy clay, sand, organic muck)	Maximum Ni extraction was achieved in A. murale grown in unfertilized clay soil accompanied by higher irrigation rate	[112]
Rosmarinus officinalis L.	Phytoextraction	Cd, Zn, Cu, Ni, Cr and Pb	Contaminated soils	The average translocation of metals from soil to root of was found to be in the order of Ni > Pb > Cd > Zn > Cr > Cu	[113]
Matricaria chamomilla L.	Phytoextraction	Cd	Hoagland hydroponic solution	Chamomile is tolerant to Cd stress and is considered a metal excluder	[114]
Vetiver	Phytostabilization	Fe, Mn, Zn, Cu, Pb, Ni, Cr and Al	Contaminated iron ore mine-soil	The plant is an excellent candidate for remediation and restoration of iron-ore mine spoil dumps	[115, 116]
Citronella (Cymbopogon winterianus)	Phytostabilization	Cd	Contaminated soils	The plant can tolerate 50 and 100 mg/kg treatments of cadmium and the growth was inhibited at 200 mg/kg ultimately leading to the plant death	[117]
Chamomile (Matricaria spp.)	Phytoextraction, phytostabilization	As, Pb, Zn, Cd	Contaminated soils	As, Pb, and Zn were retained in roots, Cd showed good ability to translocate to the shoots	[118]
Mentha pipertia (L.)		Zn, Pb, Cd	Contaminated soils	The plant can grow in contaminated soils treated with sewage sludge	[118]

Table 2.

Aromatic plants with phytoremediation potential.

9. Risk assessment of EOs sourced from aromatic plants grown on polluted soils

The use of edible crops for bioremediation strategies is not feasible because the heavy metals can enter into food chain through absorption by crops, and consequently consumption by human or animals. Aromatic crops hold a superior position over food crops for phytoremediation purposes as their use is associated with minimum risk of food chain contamination. Several authors reported that aromatic crops could be grown on heavy metal contaminated sites without causing any significant risks of metal transfer to by-products and alterations in essential oil composition [33, 119]. The essential oil of Salvia officinalis cultivated in soils contaminated with metals was found to be free from hazardous heavy metals [120]. These authors reported that heavy metals in EOs extracted from aromatic crops grown on heavy metal contaminated soil were well within the critical limits as specified by FSSA [121]. Essential oils extracted from aromatic plants grown in polluted sites can be used for non-edible products such as soap and detergents manufacturing, cosmetics and perfumery and as insects repellents, therefore minimizing food chain contamination [33]. Hydro-distillation process for essential oil extraction results to less contamination of essential oil by heavy metals [122, 123]. Despite many studies on the use of indigenous species in phytoremediation processes, there remain the knowledge gap with regards to the exploitation of aromatic plants, which present an advantage of producing essential oils that are free from metal toxicity. Aromatic plant resources are very abundant, and they can be used on large scale. These plants offer a novel option for their use in phytoremediation of heavy metal contaminated sites. Extraction of metal-free essential oil will be beneficial to the economy of any nation by exporting these natural products [124].

Essential oil composition is influenced by environmental factors such as; climatic changes, geographical origin, and seasonal factors and consequently influencing the biological properties of the EOs. The composition and biological properties of essential oils of Helichrsyum splendidum was influenced by different environmental conditions. Author [125] compared antifungal properties of the EOs sourced from different geographic locations and the bioactivities was influenced by varying levels of major constituents in the respective oils against crops pathogens. Oils characterized with high levels of germacrene d and spathulenol were more active as compared to oils characterized by δ-cadinene and α-cadinol, highlighting that activity is closely linked to the chemical composition of the EOs. Plants grown in polluted soils and fly ash may produce essential oils that may have enhanced production of essential oils and therefore could have improved activity. Many aromatic plants remain unexplored for its potential in phyto-strategies and biological effects. Poplars are tree species that have favorable characteristics for phytoremediation strategies that include quick establishment, fast growth, large biomass accumulation, extensive and deep root systems, high rates of transpiration, ease asexual propagation, grow effortlessly on marginal lands, are not edible and can live for long [126]. Such plants should be investigated more for both their phytoremediation potential and biological properties. Piper aduncum has noteworthy activity against Colletotrichum musae that causes post-harvest banana fruit rot disease. This is a fast-growing shrub that thrive in poor soils, dominate degraded forests and abandoned lands [127, 128]. These characteristics make this species ideal for phytoremediation species. As a biopesticide, these can be cultivated in abandoned/contaminated lands followed by extraction of EOs, therefore turning unproductive lands to commercially viable ones.

10. Future prospects

Aromatic plants are important sources of safer and effective agrochemicals and therefore more attention should be given toward their cultivation, preservation, and sustainable development. Mining activities have resulted in significant damages to land and phytoremediation strategies are able to solve problems simultaneous by providing vegetative covers in polluted sites and species to be selected based on their potential for generating revenues. Many aromatic plants are under explored and there is a need to conduct more studies on phytoremediation of these species, with emphasis to those essential oil producing plants. There is an increasing demand of essential oil and aromatic plants can be grown on contaminated sites, especially those that are moderately polluted such as fly ash deposits and as such there is no secondary pollution due to metal toxicity in the essential oils. The cultivation of aromatic crops at heavy metal contaminated sites has often been suggested as a profitable and feasible option. The benefits of using aromatic plants for phytoremediation purpose can be categorized under two main headings as, environmental and economic beneficial. Being high value economic crops, monetary benefits can also be obtained by growing them in contaminated areas.

11. Conclusion

EOs extracted from aromatic plants are more effective for protection and prevention of crops against infectious diseases in the field as well as during storage. EOs are a substantial by-product of aromatic crops that are generally used for non-edible purposes such as the production of soaps and detergents, insect repellents, cosmetics, and scents, and they might be considered a viable option for decreasing food chain contamination. In addition to their potential as biopesticides, aromatic plants have gained a lot of interest in phytoremediation strategies due to their potential for generating income. The use of EOs as biofungicides will help to reduce the chemical fungicides application doses, avoid pathogen resistance and solve problems simultaneously through cultivating of these species in previously unproductive lands. Problem solving strategies should be approached in a manner that support sustainable development of regions, especially in developing countries. Aromatic plants are high value economic crops emerging as candidates for remediation of contaminated sites as they produce essential oils that are free from metal toxicity. Essential oils sourced from plants growing in harsh conditions can lead to production of essential oils with improved activity. Aromatic plants can be cultivated from previously unproductive lands and agricultural soils that are polluted by use of synthetic pesticides at lower cost and therefore reducing costs for farmers. This is an ecologically sustainable method, protecting the environment and restoring the soil structure and providing vegetative cover thereby preventing dispersion of pollutants.

Acknowledgments

The authors would like to thank Tshwane University of Technology for providing the necessary resources.

Conflict of interest

The authors declare no conflict of interest.