Allelopathic Potential of Tropical Plants—A Review

Abstract

The need to meet food demand becomes more urgent as it is forecasted to increase by 50% over the next century. Thus, agronomists promote sensible tools and approaches to eradicate factors that hamper crop production, mainly weeds. The constant use of chemical herbicides to control weeds leads to an increased risk of herbicide-resistant weed populations, environmental pollution, unsafe agricultural products, and negative effects on human health. These problems have caused an interest among researchers to replace synthetic herbicides with alternatives. The purpose of this review was to present the current knowledge base on allelopathic tropical plants and their potential for use in the development of natural product-based, environmentally friendly herbicides for sustainable agriculture, and to stimulate future discussion on this topic. The defence mechanisms of tropical plants have received particular attention because of their potential weed control ability as a natural pesticide that can prevent the overuse of synthetic pesticides. The ancient knowledge of the toxic properties of various tropical plants gives us a basis for creating a novel pest control approach. The synthesis of biopesticides based on allelochemicals opens up the possibility of utilizing natural compounds in crop protection and demonstrates the ability to deal with evolved pesticide resistance.

1. Introduction

Allelopathy is known as the effect of a plant on associated plants or microbes through the release of chemical compounds [1]. In 1937, Hans Molisch first introduced the word “allelopathy” [2]. The term allelopathy comes from two Greek words, “allelon”, which means “of each other”, and “pathos”, which means “to suffer”. Allelopathy means one plant’s direct or indirect effect, including micro-organisms, upon the other [3]. Allelopathy is a type of chemical interference between plants that may cause an increase or decrease in neighbouring plant growth [4]. According to Bahadur et al. [5], allelopathy is the injurious result of one organism upon others that influences the development of neighbouring plants through the production of phytotoxic compounds. Hence, in 1996, the international allelopathic society broadened its definition to include any process involving secondary metabolites produced by plants, microorganisms, bacteria and fungi that either stimulate or inhibit the growth and development of agriculture and biological systems [3].

Many plant species have been proven to perform allelopathic activities, which can affect the associated plant species [6]. They produce secondary metabolites and many of these substances have allelopathic potential known as allelochemicals, which have both stimulatory and inhibitory effects. More recently, there has been a focus on utilizing allelochemicals as sources of new herbicides and novel modes of action. Allelopathy is essentially important in agriculture for its potential as a bioherbicide that can limit the overuse of synthetic herbicides [7]. Stored grain and field crop pests have been reported to be controlled effectively by allelopathy [8,9]. Allelopathy has also been effective in affecting the growth of surrounding plants by root exudates and leachates. The common visible symptom of allelopathy includes germination inhibition, root and shoot length reduction, discolouration, lowering seed production for next-generation, etc. [10]. Many physiological and biochemical changes are also occurring in the plant by allelopathy, including reduction of photosynthesis, cell membrane damage, DNA mutation and denaturing of protein [11,12]. It is a very complicated defence mechanism of the plant.

The tropics are parts of Earth that lie between the Tropic of Cancer and the Tropic of Capricorn in latitude [13]. The equator and portions of North America, South America, Africa, Asia, and Australia are all considered to be in the tropics [14]. A third of the world’s population resides in the tropics, which make up 36% of the planet’s surface. The average temperature in the tropics is between 25 and 28 ℃. The amount of rainfall varies significantly from one tropical region to another. A portion of the Amazon Basin in South America, for example, receives about 274.32 cm of rain annually. There are drier climates in other tropical regions, for example, the Sahara Desert in northern Africa receives only two to ten centimetres of rainfall per year. The Plant/crop diversity is very high in the tropics [15]. The phrase “tropical” can refer to a wide range of flora, including palms, orchids, and others. In order to prevent evaporation and shield themselves from excessive heat and sunlight, the leaves of many tropical plants are waxy or glossy [16]. These plants also produce flowers that have colourful petals or leaves, which attract pollinators like butterflies to their blooms. They cover a broad spectrum, from little herbaceous plants to massive trees [17]. The ability of many tropical plant species to endure colder temperatures than others in their genus or family has led to their introduction to locations outside their natural range. These plants have specialized adaptations that allow them to thrive in hot settings without succumbing to heat stress or a shortage of water [18]. Numerous plant/crop species are in competition with one another in an area that maintains ideal growing circumstances throughout the year. These plants have been forced to evolve in order to gather the vital nutrients that are necessary for their continued existence in this environment [19]. Tropical environments are especially amenable to the evolution of allelopathic survival strategies because the generally consistent temperatures and mild frost-free winters generate a year-round growing season that is conducive to plant growth [20]. This permits many species to grow and compete for nutrients, sunshine, water, and other resources. Due to their benefits in such a competitive environment, these are the best geographic zones to find allelopathic species.

Food security will become a major issue when the world population increases, putting pressure on available cultivable land and the yields required. In the next decade, agriculture must sustainably increase food production from less land using natural resources more efficiently with less environmental impact to fulfil the needs of a growing population [21]. Crop production faces biotic and abiotic restrictions and socioeconomic and crop management challenges [22]. Weeds are the most important biotic constraint to agricultural production worldwide. Many reports have shown that weeds caused the highest potential yield loss (10–100%) to crops, followed by plant diseases caused by bacteria, fungi and viruses, and animal pests (insects, nematodes, rodents, birds, mites, etc.), which are of less significance [23,24,25].

Pest management in agro-ecosystems is very much pesticide-based [26]. The regular use of synthetic pesticides increases the resistance of pests against pesticides and ecological imbalance and environmental pollution. Synthetic pesticides can create the opportunity for other harmful organisms to cause disease by killing beneficial pathogens and organisms [27]. To achieve sustainable agriculture, therefore, the use of natural products in substitution of agrochemicals is crucial. Effective natural pesticides have been developed by scientists in an effort to reduce dependency on chemical weed management [28,29,30]. The allelopathic potential of tropical plants could help the development of natural pesticides for long-term, environmentally friendly pest control [31]. But little information is available regarding the allelopathic potential of tropical plants. Understanding these factors could greatly enhance agricultural management and drastically lessen its negative effects on crop output. Therefore, it is important to identify potential growth inhibitor compounds to develop a natural pesticide based on natural compounds derived from tropical plants. Herein, the allelopathic potential of plants with a focus on tropical regions, and their potential for application in sustainable agriculture, are reviewed. In this manuscript, we narrowed down to tropical plants allelopathy in sufficient details. The focus on tropical plants may allow for an alternative weed control approach.

2. Allelopathy in Sustainable Agriculture

Sustainable agriculture comprises “management procedures that work with natural processes to conserve all resources, minimize waste and environmental impact, prevent problems and promote agroecosystem resilience, self-regulation, evolution and sustained production for the nourishment and fulfilment of all.” The overuse of chemical fertilizers, pesticides, and other agricultural chemicals has a negative impact on the environment and threatens the physical–chemical properties of soil and water. In order to achieve the goal of sustainable agriculture, extensive research is needed and has been carried out on plant breeding, soil fertility and tillage, crop protection, and cropping systems [32,33].

In today’s farming, allelopathy offers enormous promise for enhancing yields, variety, ecosystem health, nutrient recycling, nutrient conservation, weed management, and pest control. In addition to this, allelopathic compounds exist as secondary metabolites in plants and are inherited from the parent species [7]. The prevalence of these chemicals varies among species and cultivars, allowing for the selection of the allelopathic feature in agriculture. Allelopathy can play a vital role in multiple cropping systems, crop rotations, and cover cropping [34]. Thus, biological management using naturally occurring allelopathic substances in agriculture is vital and useful.

The role of allelopathy in the agricultural field is undeniable. The allelopathic interaction of weeds and crops on other weeds can be used as a natural herbicide [35]. Weeds compete with cultivated crops for available resources. In recent years, allelopathic management has become popular to avoid laborious work and environmental pollution [36,37]. By employing this method, there will be less need for the use of herbicides. Allelopathic management of weeds can be utilized in different ways.

Allelopathic crops could be applied as green manure, cover crop and smoother crops to control weeds through creating desired manipulations in cropping patterns. They can be intercropped with major crops to control specific weed species. Allelopathic crops can be cultivated in agricultural fields to control the germination of particular weeds. Many agricultural plants like Sorghum bicolor (L.) Moench, Solanum lycopersicum L., Oryza sativa L., and Triticum aestivum L. can control weeds [38]. These allelopathic potential plants could offer an unfavourable environment for weeds to survive. Plants have gained more allelopathic potential in recent years through crop engineering and breeding [39]. These crop cultivars can be applied through mulching and intercropping with other plants, which have allelopathic effects for reducing the development of weeds. Yuba and Rasen, two cultivars of Medicago sativa L. have more allelopathic effects than common M. sativa [40]. Rondo, a cultivar of O. sativa, has a strong weed suppressive ability and high yielding potential [41]. Some genotypes of O. sativa have been introduced as having allelopathic potential on weeds. Crops with allelopathic potential could be applied to control weeds, reduce dependency on chemical herbicides, develop crop production, and reduce crop production expenses.

In sustainable weed management, phytotoxic extracts of the plant have a high impact as a bioherbicide, but many of its applications are not profitable for field conditions. The compounds become active by modifying their chemical structure towards the target plant. The activities of allelopathic compounds may be a higher or lower force on target plants [42]. The biodegradability character of allelopathic compounds makes it more favourable as bioherbicides [7,23,43]. The best-known examples of natural bioherbicides are phytotoxic water extracts of S. bicolor and Helianthus annuus L. They are named Sorgaab and Sunfaag, respectively, and control the weeds without crop yield losses [44,45]. S. bicolor water extract decreased the biomass of the weed Echinochloa crus-galli (L.) P. Beauv. by 40% and raised O. sativa production by 18% [46]. To control Cyperus rotundus L., the aqueous extracts of Brassica napus L., S. bicolor, and H. annuus were sprayed in a Gossypium hirsutum L. field with glyphosate [47]. The use of glyphosate in combination with the water extracts of Sorgaab and B. napus resulted in an increase in G. hirsutum seed yield of 15–21% and a reduced glyphosate dose (67–75%) required to control the C. rotundus weed (67–75%) [47]. Application of both extracts in cotton at 18 L ha⁻¹ each with glyphosate (767 g ha⁻¹) at 21 days after spray drastically reduced (93%) the competitive weed population of C. rotundus [47].

The residual activity of allelopathic plants can reduce the germination of weeds. Different plant parts of allelopathic plants fall and are mixed with soil. These plant parts release phytochemicals in the soil through decomposition. This residual activity of allelopathic plants can be used for controlling weeds. Echinochloa colona (L.) Link is a common weed in a rice field used as a tested plant to assess the effect of modified soil with H. annuus, S. bicolor and Brassica juncea (L.) Czern. residue. Modified soil with residues has a great negative impact on the biomass of E. colona [48].

3. Selected Tropical Species and Their Allelopathic Potential

Plants have created an incredible arsenal of chemical weaponry, which has enabled them to not only flourish in their natural habitats, but also successfully colonise and establish themselves in a wide variety of new situations. The development of allelochemicals by tropical plants has been exceptionally effective, possibly as a result of evolutionary adaptations to survive competition with other species in such favourable growth conditions. Many unknown chemical compounds are released into the environment from tropical plants, which could affect the growth and development of the associated plants [49]. The allelochemicals released by tropical plants and their allelopathic activity are listed in Table 1.

Ageratum conzyoides L. originated from Central and South America and the West Indies. It is the worst weed in China, a noxious weed in Angola and Malaysia [50,51]. The extract of A. conzyoides has a powerful phytotoxic effect on T. aestivum development. The leaves of A. conzyoides secrete phenolic compounds, which are known as the major allelochemicals. Ambrosia artemisilifolia L. was found in China, and its main origin is North America. It causes serious harm in crop fields, nurseries, orchards and tourist areas. Scientists found that the chemical compounds of A. artemisilifolia have a phytotoxic effect on the emergence and growth of associated plants [52]. Ambrosia trifida L. released allelochemicals through leaching to develop their competitiveness, which has a negative effect on the plants around them [53]. Asystasia gangetica L. was initially brought to Malaysia from India in the years 1876 and 1923 in the form of decorative plants [54]. A. gangetica is a common pest in oil palm [55]. It spreads rapidly in most plantations and smallholdings in Malaysia. It thrives well in nearly all soil types, especially to well-aerated deep soils, sandy beaches and peat soils.

Borreira alata (Aubl.) DC. is a species of Rubiaceae widespread in tropical areas. Aqueous extracts of Borreria species have allelopathic potential and showed inhibitory effects on two varieties of Brassica campestris L. [56]. Motmainna et al. [36] reported that the methanol extract of B. alata reduces the survival rate and seedlings growth of weedy rice. Plantation crops and pastures are under threat from the invasive weed Chromolaena odorata (L.) R.M.King & H.Rob.

Cleome rutidosperma DC. is a tropical plant, that contains some chemical pesticide components responsible for toxic and insecticidal properties and could serve as an alternative of synthetic pesticide [57,58]. In Malaysia, C. rutidosperma planted along field margins has the potential to be used as a part of an insect management programme by discouraging Plutella xylostella from laying their eggs on agricultural crops [57]. Cleome viscosa L. has shown strong insecticidal potential and may be useful in pest management [58]. This weed is mostly observed in Saccharum officinarum L. fields and reduces S. officinarum production by 50% or more [58]. C. odorata is native to Mexico, the West Indies and tropical South America. It has a large taproot and leaves that give off a strong odour when crushed. This weed is resistant to insect attack because of its oil, which possesses insect-repelling qualities. C. odorata leachates and extracts reduced crop production, and volatile chemicals were produced from the plant’s aerial parts. Vijayaraghavan et al. [59] confirmed that C. odorata is harmful and has an allelopathic effect on others. Decomposed residues of C. odorata were hazardous for up to six months, but after six months stimulated crop growth. According to the research, the primary allelochemicals in C. odorata include phenolics, amino acids and alkaloids [60]. Cyperus digitatus Roxb. is a herb found in swamps or seasonally flooded areas, ditches and river banks [61]. The whole plant is used in Pakistan as a skin anti-allergic [62].

Eupatorium adenophorum Spreng. (Asteraceae) originated in South America. The origin of this weed is the Americas. It grows on a large scale and releases hormones and toxins. The allelopathic potential of E. adenophorum can inhibit associated plants [63]. Eichhornia crassipes (Mart.) Solms (Water hyacinth) originated in South America [19]. E. crassipes is used for water purification because releasing the chemical compounds from the plant can reduce the development of algae and aquatic plants [64]. E. globules (Myrtaceae) is a fast-growing plant that originates in the Australian continent and the nearby islands. The fast growth rate and adaptability of Eucalyptus create an ecological problem. The ecological problem is the main factor of its strong allelopathic effect. In the forest, significant soil erosion and water loss are occurred by Eucalyptus sp. [65].

A sedge called Fimbristylis miliaceae (L.) Vahl. is the fifth and third most troublesome weeds in Muda and Besut, Malaysia, where rice is grown. The growth characteristics of O. sativa were shown to be negatively impacted by F. miliacea, as reported by Ismail and Siddique [66]. Many crops, including rice, are threatened by Ischaemum rugosum Salisb. which has been declared a noxious weed in at least 26 countries of the world [67]. Because of its high seed dormancy, this plant is responsible for significant losses in rice yield, ranging from 50–60% [68].

Lantana camara L. is a poisonous weed that originated in the Americas. It can be poisonous to both humans and weeds. Mishra [69] suggested that it has a potential allelopathic effect on the development of neighbouring plants. Lindernia crustacea (L.) F. Muell. is also a popular and useful ethnomedicinal plant that has been traditionally used throughout the world [70]. It is one of the most familiar plants used as medicinal plants in Indonesia and Malaysia. The tropical Asian plant Leptochloa chinensis (L.) Ness. has a wide distribution now, including in South and Southeast Asia, Australia, and Africa, and where it thrives in both flooded and upland habitats. With the switch from transplanting O. sativa to direct seeding, L. chinensis expanded widely in Malaysia [71].

Mikania micrantha Kunth originated in Central and South America and is harmful to crops and weeds. The amount of released allelochemicals is increased according to the growth of M. micrantha [72]. In South and Southeast Asia, it was ranked among the top 10 worst exotic weed species. Due to its quick growth rate, it poses a significant concern in plantations and may prevent neighbouring plants from flourishing by rapidly erecting a dense canopy over the host plant and blocking out most of the sunlight, and by the release of allelochemicals into the environment. Ismail and Chong [73] identified caffeic acid, P-hydroxybenzaldehyde, resorcinol and vanillic acid as the allelochemicals present in the leaf extracts of M. micrantha. Sesquiterpenoids were identified by Shao et al. [74], and these chemicals prevented the seeds of test species from germinating and seedlings from growing.

The invasive weed Parthenium hysterophorus L. is a native of America. It has now reached 50 nations in Asia, Oceania, and Africa that are considered tropical or subtropical. At the moment, this weed has spread to ten states in Malaysia. The state of Kedah has most of it [75]. The release of allelochemicals from this plant may prevent seedling development and the nutritional requirements of plants [76,77]. Motmainna et al. [78] reported the methanol extract of P. hysterophorus had morphological and physiological parameters of several weeds and crops. At a particular dose, phytotoxins that have a negative influence on plant development may have little or no inhibition in other plants [1].

Wedelia trilobata (L.) Hitchc., native to Africa, grows in high-density. Many experiments were conducted by using Wedelia. According to Azizan et al. [79], it has an allelopathic effect on associated plants. W. trilobata originates in tropical America. It has a negative effect on the production of cereal crops [80]. So, we can say that allelopathy is the main factor in inhibiting the development of neighbouring plants and their reproduction in surroundings. Xanthium indicum DC. is a highly useful medicinal plant that thrives in warmer climates such as Malaysia, China, Northern America, and Brazil, as well as other parts of India [81]. Chemical components such as sesquiterpene, phenol, polysterol and glycoside found throughout the plant of X. indicum can be used to demonstrate a variety of pharmacological effects [82].

Table 1 summarize different tropical plants with their allelochemicals and allelopathic activity to several plant species.

Table 1. Tropical plants with their allelopathic activity.

Tropical Plants	Native	Allelochemicals	Allelopathic Activities	Sensitive Plants	Reference
A. gangetica	Tropical Asia	Indole-3-carboxaldehyde and (6R,9S)-3-oxo-α-ionol	Cause 10% yield reduction	Cucumis sativus L.	[83,84]
Artemisia annua L.	China	Artemisinin	Inhibit growth and root enlargement	Ipomoea lacunose L., L. sativa, P. oleracea, A. retroflexux	[85]
Bidens pilosa L.	America	Polyacetylenes, flavonoids, phenolic acids, terpenes, and fatty acids,	Inhibit the growth	O. sativa, L. sativa, V. radiata, Z. mays, S. bicolor	[86]
Brachiaria mutica (Forssk.) Stapf	northern and central Africa	Tannin, saponin	Germination and growth suppression	Mimosa pudica L.	[87]
Brassica nigra (L.) K.Koch	North Africa, Asia	Sinigrin	Germination and radicle length inhibition	Avena fatua L.	[88]
C. rutidosperma	Tropical America	Gallic acid, quercetin	Yield reduction	S. officinarum	[57]
C. odorata	Central America	Fatty acids, phenols	Yield loss	Vigna radiata (L.) R.Wilczek, Solanum melongena L., Capsicum annuum L., Phaseolus vulgaris L.	[89]
Cyperus sp.	Tropical Asia	Chlorogenic acid, myricitrin, catechin, apigenin, quercetin, luteolin, chrysin, rutin	Reduces yield by 86% and 93%	O. sativa	[90,91]
Chrysanthemoides monilifera (L.) Norl.	Southern Africa	β-maaliene, α-isocomene, β-isocomene, δ-cadinene, 5-hydroxycalamenene, 5-methoxycalamenene	Reduced emergence and growth performances	Acacia mearnsii De Wild., Isotoma axillaris Lindl.	[92,93]
Conyza bonariensis (L.) Cronquist	South America	(4Z)-lachnophyllum lactone	Suppression of growth	Cuscuta campestris Yunck.	[94]
C. sativus	Southern Asia	Gallic acid, coumaric acid, p-hydroxybenzoic acid, caffeic acid, syringic acid	Inhibition of germination, radicle and hypocotyle length	E. crus-galli	[95]
Cymbopogon citratus (DC.) Stapf	Southeast Asia	Monoterpenes, sesquterpenes	Emergence and growth reduc-tion	E. crus-galli	[96,97]
Datura metel L.	America, Mexico	Scopolamine, hyoscyamine, atropine	Low emergence and growth	P. hysterophorus	[98,99]
Eucalyptus camaldulensis Dehnh.	Australia	p-coumaric, gallic, gentisic, p-hydroxybenzoic, syringic acid, vanillic acid, catechol	Suppression of germination and growth	P. oleracea	[100,101]
E. crassipes	South America	Loliolide	Inhibit emergence and growth	E. crus-galli	[102]
E. colona	Asia	Tricin	Inhibit germination and seedling growth	O. sativa, Glycine max (L.) Merr.	[103]
Eucalyptus globulus Labill	South-eastern Australia	Hyperoside, kaempferol 3-O-glucoside, shikimic-succinic acids	Inhibit germination, growth and physiological parameters	L. sativa, Agrostis stolonifera L.	[104]
F. miliaceae	South America	Hexanedioic acid dioctyl ester, di-n-octyl phthalate	Cause 44% and 96% yield loss	Ludwigia hyssopifolia (G.Don) Exell., Echinochloa colonum (L.) Link., Cyperus iria L., Paspalum dilatatum Poir.	[66,105]
L. chinensis	Tropical Asia	Alkaloids compounds, organic acids	Cause 10–35% yield loss	Ruellia tuberosa L., Brassica chinensis L., B. oleracea, E. crus-galli, Amaranthus viridis L., Z. mays	[106,107]
Leptospermum scoparium J.R.Forst. & G.Forst.	Australia, New Zealand	Leptospermone	Inhibit germination and growth	A. retroflexus, A. theophrasti, C. arvensis, Cannabis sativa L., Sesbania grandiflora (L.) Pers., D. sanguinalis, E. crus-galli	[108]
M. micrantha	South and Central America	Dihydromikanolide, deoxymikanolide, 2,3-epoxy-1-hydroxy-4,9-germacradiene-12,8:15,6-diolide.	Inhibit radicle and shoot length	R. sativus, Lolium perenne L., Trifolium repens L.	[74]
M. sativa	Asia	Salicylic acid, p-hydroxybenzoic acid	Growth suppression	Digitaria ciliaris (Retz.) Koeler, C. album, Amaranthus lividus L., Portulaca oleracea L., Commelina communis L.	[109]
P. hysterophorus	America	Caffeic acid, parthenin	Suppress germination and seedling growth	Eleusine indica (L.) Gaertn., Digitaria sanguinalis (L.) Scop	[110]
Peganum harmala L.	Morocco, Iran, Spain, Italy	Galllic acid, vanillic acid, syringic acid, cinnamic acid, caffeic acid, trans-ferulic acid, 3,4 hydroxybenzoic acid	Suppress germination and seedling growth	A. fatua, C. arvensis	[111]
Piper longum L.	Indo-Malaya region	Sarmentine	Inhibit growth	L. chinensis, Convolvulus arvensis L., Conyza canadensis (L.) Cronquist, Sinapis arvensis L.,	[112]
Sphenoclea zeylanica Gaertn.	Africa	Zeylanoxide A,B, epi-zeylanoxide A,B, secologanic acid, 7-epi-vogeloside, secologanoside, vogeloside	Inhibit seedlings growth	Lactuca sativa L., O. sativa	[113]
Sesbania virgata (Cav.) Pers.	Bolivia, Brazil, Argentina	Catechin	Inhibit growth	S. lycopersicum, R. sativus, L. sativa, and O. sativa	[114]
Solanum forskalii Dunal	Arabic nations	Vanillic acid, salicylic acid, protocatechuic acid	Inhibit germination	Z. mays, Brassica compestris L., T. aestivum	[115]
S. bicolor	Africa	Sorgoleone	Inhibit germination and growth	Coronopus didymus L., Phalaris minor Retz., C. rotundus, S. nigrum., A. retroflexus, Ambrosia artemisiifolia L., Cassia obtusifolia L.	[46]
Salvia leucophylla Greene	California	Camphor, 1,8-cineole, β-pinene, α-pinene, and camphene	Germination and growth reduction	B. campestris, Papaver rhoeas L.	[116,117]
Stylosanthes guianensis (Aubl.) Sw.	Central and South America	Phenolic acids, coumarin and long-chain fatty acids	Suppress germination and growth	Monochoria vaginalis (Burm.f.) C.Presl, E. crus-galli	[118]
Tagetes minuta L.	Southern South America	Alkaloid, saponin, flavonoid and terpenoid	Germination and growth inhibition	E. crus-galli, C. rotundus	[119]

Table 1. Tropical plants with their allelopathic activity.

Tropical Plants	Native	Allelochemicals	Allelopathic Activities	Sensitive Plants	Reference
A. gangetica	Tropical Asia	Indole-3-carboxaldehyde and (6R,9S)-3-oxo-α-ionol	Cause 10% yield reduction	Cucumis sativus L.	[83,84]
Artemisia annua L.	China	Artemisinin	Inhibit growth and root enlargement	Ipomoea lacunose L., L. sativa, P. oleracea, A. retroflexux	[85]
Bidens pilosa L.	America	Polyacetylenes, flavonoids, phenolic acids, terpenes, and fatty acids,	Inhibit the growth	O. sativa, L. sativa, V. radiata, Z. mays, S. bicolor	[86]
Brachiaria mutica (Forssk.) Stapf	northern and central Africa	Tannin, saponin	Germination and growth suppression	Mimosa pudica L.	[87]
Brassica nigra (L.) K.Koch	North Africa, Asia	Sinigrin	Germination and radicle length inhibition	Avena fatua L.	[88]
C. rutidosperma	Tropical America	Gallic acid, quercetin	Yield reduction	S. officinarum	[57]
C. odorata	Central America	Fatty acids, phenols	Yield loss	Vigna radiata (L.) R.Wilczek, Solanum melongena L., Capsicum annuum L., Phaseolus vulgaris L.	[89]
Cyperus sp.	Tropical Asia	Chlorogenic acid, myricitrin, catechin, apigenin, quercetin, luteolin, chrysin, rutin	Reduces yield by 86% and 93%	O. sativa	[90,91]
Chrysanthemoides monilifera (L.) Norl.	Southern Africa	β-maaliene, α-isocomene, β-isocomene, δ-cadinene, 5-hydroxycalamenene, 5-methoxycalamenene	Reduced emergence and growth performances	Acacia mearnsii De Wild., Isotoma axillaris Lindl.	[92,93]
Conyza bonariensis (L.) Cronquist	South America	(4Z)-lachnophyllum lactone	Suppression of growth	Cuscuta campestris Yunck.	[94]
C. sativus	Southern Asia	Gallic acid, coumaric acid, p-hydroxybenzoic acid, caffeic acid, syringic acid	Inhibition of germination, radicle and hypocotyle length	E. crus-galli	[95]
Cymbopogon citratus (DC.) Stapf	Southeast Asia	Monoterpenes, sesquterpenes	Emergence and growth reduc-tion	E. crus-galli	[96,97]
Datura metel L.	America, Mexico	Scopolamine, hyoscyamine, atropine	Low emergence and growth	P. hysterophorus	[98,99]
Eucalyptus camaldulensis Dehnh.	Australia	p-coumaric, gallic, gentisic, p-hydroxybenzoic, syringic acid, vanillic acid, catechol	Suppression of germination and growth	P. oleracea	[100,101]
E. crassipes	South America	Loliolide	Inhibit emergence and growth	E. crus-galli	[102]
E. colona	Asia	Tricin	Inhibit germination and seedling growth	O. sativa, Glycine max (L.) Merr.	[103]
Eucalyptus globulus Labill	South-eastern Australia	Hyperoside, kaempferol 3-O-glucoside, shikimic-succinic acids	Inhibit germination, growth and physiological parameters	L. sativa, Agrostis stolonifera L.	[104]
F. miliaceae	South America	Hexanedioic acid dioctyl ester, di-n-octyl phthalate	Cause 44% and 96% yield loss	Ludwigia hyssopifolia (G.Don) Exell., Echinochloa colonum (L.) Link., Cyperus iria L., Paspalum dilatatum Poir.	[66,105]
L. chinensis	Tropical Asia	Alkaloids compounds, organic acids	Cause 10–35% yield loss	Ruellia tuberosa L., Brassica chinensis L., B. oleracea, E. crus-galli, Amaranthus viridis L., Z. mays	[106,107]
Leptospermum scoparium J.R.Forst. & G.Forst.	Australia, New Zealand	Leptospermone	Inhibit germination and growth	A. retroflexus, A. theophrasti, C. arvensis, Cannabis sativa L., Sesbania grandiflora (L.) Pers., D. sanguinalis, E. crus-galli	[108]
M. micrantha	South and Central America	Dihydromikanolide, deoxymikanolide, 2,3-epoxy-1-hydroxy-4,9-germacradiene-12,8:15,6-diolide.	Inhibit radicle and shoot length	R. sativus, Lolium perenne L., Trifolium repens L.	[74]
M. sativa	Asia	Salicylic acid, p-hydroxybenzoic acid	Growth suppression	Digitaria ciliaris (Retz.) Koeler, C. album, Amaranthus lividus L., Portulaca oleracea L., Commelina communis L.	[109]
P. hysterophorus	America	Caffeic acid, parthenin	Suppress germination and seedling growth	Eleusine indica (L.) Gaertn., Digitaria sanguinalis (L.) Scop	[110]
Peganum harmala L.	Morocco, Iran, Spain, Italy	Galllic acid, vanillic acid, syringic acid, cinnamic acid, caffeic acid, trans-ferulic acid, 3,4 hydroxybenzoic acid	Suppress germination and seedling growth	A. fatua, C. arvensis	[111]
Piper longum L.	Indo-Malaya region	Sarmentine	Inhibit growth	L. chinensis, Convolvulus arvensis L., Conyza canadensis (L.) Cronquist, Sinapis arvensis L.,	[112]
Sphenoclea zeylanica Gaertn.	Africa	Zeylanoxide A,B, epi-zeylanoxide A,B, secologanic acid, 7-epi-vogeloside, secologanoside, vogeloside	Inhibit seedlings growth	Lactuca sativa L., O. sativa	[113]
Sesbania virgata (Cav.) Pers.	Bolivia, Brazil, Argentina	Catechin	Inhibit growth	S. lycopersicum, R. sativus, L. sativa, and O. sativa	[114]
Solanum forskalii Dunal	Arabic nations	Vanillic acid, salicylic acid, protocatechuic acid	Inhibit germination	Z. mays, Brassica compestris L., T. aestivum	[115]
S. bicolor	Africa	Sorgoleone	Inhibit germination and growth	Coronopus didymus L., Phalaris minor Retz., C. rotundus, S. nigrum., A. retroflexus, Ambrosia artemisiifolia L., Cassia obtusifolia L.	[46]
Salvia leucophylla Greene	California	Camphor, 1,8-cineole, β-pinene, α-pinene, and camphene	Germination and growth reduction	B. campestris, Papaver rhoeas L.	[116,117]
Stylosanthes guianensis (Aubl.) Sw.	Central and South America	Phenolic acids, coumarin and long-chain fatty acids	Suppress germination and growth	Monochoria vaginalis (Burm.f.) C.Presl, E. crus-galli	[118]
Tagetes minuta L.	Southern South America	Alkaloid, saponin, flavonoid and terpenoid	Germination and growth inhibition	E. crus-galli, C. rotundus	[119]

4. Bactericidal Efficacy of Tropical Plants

The effectiveness of existing antibiotics against a wide range of infectious diseases is of growing concern to scientists. Finding new medication alternatives for overcoming drug resistance has been the focus of many studies. Scientists have been interested in phytochemicals from plants for a long time because of their variety in structure, lack of negative side effects, and a higher level of public acceptance [120]. The majority of these phytochemicals are antimicrobial. Phytochemicals prevent bacteria that cause a number of infectious diseases by stopping efflux pumps and breaking down cell membranes [121]. Numerous studies on the antibacterial properties of tropical plants against various microorganisms have been published [122,123]. Murraya koenigii (L.) Spreng. is a small tree with dark grey bark found in Bangladesh, Malaysia, India, Sri Lanka, Nepal, and Burma. The plant has a long history of medicinal application, including treatment for a stimulant, diarrhoea, stomach problems, and insect bites [124]. According to the findings of Rahman and Gray’s study [125], the growth of Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Proteus vulgaris was inhibited by M. koenigii leaf that had been extracted using petroleum ether. In another study, leaf aqueous extract of M. koenigii was found to be effective on the growth of Aeromonas hydrophila, Citrobacter freundii, Shewanella putrefaciens, Vibrio alginolyticus, Vibrio harveyi and Vibrio vulnificus [123]. Pseuduvaria macrophylla (Oliv.) Merr., Acalypha wilkesiana Müll.Arg., and Duabanga grandiflora (DC.) Walp. were the most promising plants because of their broad-spectrum antibacterial characteristics [126]. Gram-negative bacteria such as E. coli and C. freundii were inhibited by all three plants [127]. Okigbo and Mmeka [127] found that leaf extracts of Vernonia amygdalina Delile and C. citatrus and Garcinia kola Heckel seed extract can control three pathogens, namely, Candida albicans, S. aureus, and E. coli.

5. Insecticidal Efficacy of Tropical Plants

The repeated use of synthetic insecticides harms non-targeted organisms, leaves residue, and breeds insect pest resistance. To combat these issues, researchers have been looking into eco-friendly solutions, such as plant-derived chemicals. P450, Glutathione-S-transferases, and esterase are examples of allelochemical defence mechanisms in insects [128]. These enzymes are normally concentrated in the midgut of the insect and allow for the quick removal of hazardous compounds that have been consumed. The endocrinologic balance of insects is disrupted by essential oils and their constituents [129]. It is possible that a combination of secondary metabolites, each with its own unique set of physical qualities, could be more effective at discouraging insects and herbivores for longer than any one chemical used alone. The tropical plant Azadirachta indica A. Juss. has potentially been effective against a wide variety of economically significant pests. Azadirachtin, which is thought to be more abundant in the kernel than in the leaves and other plant tissues, has been associated with pesticidal activity [130]. Products containing neem oil have been shown to be effective against a wide variety of medically and veterinary important pests, including mosquitoes, without harming beneficial insects like bees [131]. In particular, besides the experimental data, many formulations deriving from neem seeds showed antifeedancy, fecundity suppression, ovicidal and larvicidal activity, growth regulation and repellence against a great number, around 400, of different insects, also at low dosages In addition, a number of formulations developed from neem seeds showed antifeedancy, larvicidal activity, fecundity suppression, ovicidal, growth regulation, and repellence against a large number, over 400, of different insects [132,133,134]. Another potential allelopathic plant, Asimina tribola (L.) Dunal, possesses pesticidal, antitumor, and anti-feedant characteristics. Asimicin is known to have pesticidal activity against Calliphora vicina, blowfly larvae, spotted spider mite, mosquito larvae, melon aphid, and numerous additional agricultural pests [135]. Tripterygium forrestii Loes, Milletia pachycarpa Benth, and Rhododendron molle G. Don were potential insecticides against pentatomids, aphids, leaf-beetles, caterpillars, and plant lice [136].

6. Fungicidal Efficacy of Tropical Plants

Plant diseases, in particular fungal infections, are significant factors in the loss of agricultural crops around the world. Extracts from diverse species of plants have been proven to be effective against several phytopathogenic fungi without causing any undesirable side effects [137]. The antifungal compounds found in plant extracts were discussed in detail in several reports. Saponins are powerful antifungal substances that are found in high concentrations in healthy plants and help them overcome fungal infections [138]. Cineol, eugenol, galangin, galangol, pinen, camphor, and methylcinamate are some essential oils that may have antifungal properties. Several substances were identified and were thought to be responsible for the antifungal activity, namely 3-(4-hydroxyphenyl)-2(E)-propenoate isobutyric acid, butyric acid, valeric acid and caproic acid, tiliacorine, guaianolides, acetoxychavidol acetate [137]. The leaf extracts of tropical plants, namely, Ocimum gratissimum L., C. citratus and C. odorata and fruits of Xylopia aethiopica (Dunal) A.Rich. show promising control against Ustilaginoidea virens, Utilago maydis, Phizopus spp, and Curvularia lunata by 10–60% growth reduction [139]. Alabi et al. [140] reported that aqueous extracts of E. globulus and Ocimum gratissimum L. were found to be effective to control the seedling wilting of Vigna unguiculata (L.) Walp. induced by Sclerotium rolfsii. Tomato fruit rot, which is frequently observed, can be considerably decreased by the application of Cassia alata L., Alchornea cordifolia (Schumach. & Thonn.) Müll.Arg., and Moringa oleifera Lam. as postharvest agents [141].

7. Herbicidal Efficacy of Tropical Plants

The essential oils of forest trees, particularly pine species, could be suggested as alternative herbicides. Essential oils of Pinus radiata D. Don obtained by hydro distillation were composed of monoterpene hydrocarbons [142]. P. radiata belongs to the Pinaceae family that comprises about 250 species, which are divided into three subgenera, based on needle, seed and cone characteristics: Ducampopinus, Strobus and Pinus. Pinus is the largest genus of conifers occurring naturally [143]. The major components of this oil were determined as β-pinene (16.8–35.21%) and α-pinene (11.06–21.9%) [142]. P. radiata displayed a phytotoxic effect against germination and seedling growth of S. arvensis L., Phalaris canariensis L. and Trifolium campestre Schreb [142]. P. radiata oil was rich in monoterpenes, especially α, β-pinene and limonene, which are known for their phytotoxic effects. Pine species are known to possess a potent herbicidal activity [144]. Recently, we have demonstrated that Pinus pinea L. and Pinus pinea L. displayed inhibitory effects against germination and seedling growth of S. arvensis, Lolium rigidum Gaudin and Raphanus raphanistrum L. [145].

Citrus trees, which are part of the Rutaceae family, can be found all over the world, especially in tropical and subtropical areas [146]. Brazil, one of the top citrus-producing nations, has an average production of 20 to 25 tons per hectare [147]. Due to rising output in the food processing industry, more citrus residues are being generated (seeds). Since waste generated from consumption in nature is more likely to be decomposed by microorganisms, its disposal usually results in environmental pollution issues. Because of this, it is especially important to use these leftovers in a way that is efficient, cheap, and safe for the environment, since they could be useful and make profit [148].

The negative effect of C. citratus was confirmed on seed germination of B. pilosa and Bidens subalternans DC. by the use of its leaf aqueous extracts [149]. Both germination and germination rate were negatively affected by increasing doses of extract. Furthermore, cytogenetic investigations verified the cytotoxic and genotoxic effects of aqueous extracts from C. citratus leaves on root tip meristem cells of L. sativa [150]. L. sativa root cells have exhibited chromosome abnormalities and apoptosis after being exposed to the extracts [151]. The seed germination and seedling growth of E. crus-galli are adversely affected by the essential oil of C. citratus in field conditions [96].

B. napus can be used as green manure or as a cover crop to prevent weeds and protect the soil structure [151], however, it is still unclear how much of a role allelopathy has in this process. After conducting laboratory tests, Yasumoto et al. [152] observed that B. napus tissue, aqueous extracts, and root exudates had auto-toxic effects on seed germination and plant development. Additionally, they noticed a decline in H. annuus growth in the field after B. napus planting and recommended that root exudates were responsible. Similar to this, Jafariehyazdi et al. [153] reported that the aqueous extracts of various Brassica spp. inhibited the germination and growth of H. annuus

The full potential of allelopathic plant extracts for weed control can only be achieved if they are systematically incorporated into integrated weed management systems. Taking an example, WeedLock is a commercial bioherbicide obtained from plant extract and showed a promising weed control efficacy [29]. WeedLock is a ready-to-use bioherbicide developed from Solanum habrochaites S. Knapp & D.M. Spooner (wild tomato) plant extract and marketed locally in Malaysia by EntoGenex Industries Sdn. Bhd since 2017 [23]. Normal plant physiological and biochemical processes are interrupted after exposure to WeedLock, and this is linked to the generation of ROS (reactive oxygen species) [11]. Some commercial plant-based bioherbicides are listed in

Table 2. Plant-based marketed bioherbicides.

Biopesticides	Manufacture	Source of a.i.	Sensitive Weeds	References
Avenger Weed Killer	Avenger Organics, Gainesville, GA, USA	Citrus limon (L.) Osbeck	D. sanguinalis	[154]
Beloukha	Belchim Crop Protection, Londerzeel, Belgium	B. napus	Amaranthus retroflexus L.	[75]
BioWeed	Barmac PTY Ltd., Stapylton, Australia	P. radiata	Ochna serrulata Walp.	[155]
GreenMatch	Marrone Bio Innovations, Inc., Davis, CA, USA	Citrus sinensis (L.) Osbeck	Solanum nigrum L.	[154]
GreenMatch EX	Marrone Bio Innovations, Davis, CA, USA	C. citratus	Euphorbia spp.	[154]
WeedZap	JH Biotech Inc., Ventura, CA, USA	Cinnamomum verum J. Presl and Syzygium aromaticum (L.) Merr. & L.M.Perry	E. crus-galli	[154]
WeedLock	EntoGenex Industries, Kuala Lumpur, Malaysia	S. habrochaites	A. conyzoides	[11,29]
Weed Slayer	Agro Research International, Sorrento, FL, USA	S. aromaticum	E. crus-galli	[154]

.

8. Mechanism Underlying Allelopathy

The entire procedure of allelopathy activates through allelochemicals. Plant allelochemicals are discharged into the surroundings through plant parts, including leaves, fruits, seeds, flowers, stem, bark, and roots. Plant allelochemicals are composed of several compounds, namely, aldehydes, organic acids, ketones, lactones, fatty acids, quinines, amino acids, coumarins, flavonoids, tannins, phenolics, terpenoids, purines, alkaloids, and other compounds [156]. In the environment, allelochemicals are released through leaching, volatilization and exudation [157]. These processes are influenced by biotic and abiotic pressure, plant species, age and organs [23]. Targeted plants can be affected by allelochemicals either positively or negatively. The growth and development of targeted species can be enhanced or suppressed by the released allelochemicals (Figure 1). In positive allelopathy, allelochemicals modulate cell membranes, growth regulatory substances and enzymes to increase the growth and yield of the targeted plants [3]. Allelochemicals influence several growth hormones and enzymes to improve root capability to absorb mineral contents and water efficiently. Growth stimulation of root hairs and the development of roots, allowing them to absorb water and mineral contents efficiently, are examples of positive allelopathy [37]. This can be accomplished by altering membrane permeability and speeding up cell division in response to various enzymes and growth hormones. They were furthermore facilitating soil microorganisms that can improve the roots’ ability to absorb nutrients and water.

Allelochemicals from donor plants connect with vulnerable plants and create a negative effect on their functions and growth. This is called negative allelopathy. Negative allelopathy occurs when plants want to dominate other living organisms in a certain environment [53]. Allelochemicals affect the production, functions, contents, and activities of distinct enzymes in diverse ways. Previous research demonstrated chlorogenic acid, caffeic acid, and catechol to suppress the main enzyme—phosphorylase—involved in seed germination [158]. Allelochemicals influence plant growth by altering several stages of respiration, including electron transfer in mitochondria, oxidative phosphorylation, CO₂ production, and ATP enzyme activity. Allelochemicals can affect regulatory systems such as DNA intercalation, DNA polymerase I inhibition, and protein biosynthesis inhibition linked to decreased plant development. After being exposed to allelochemicals, recipient plants may create reactive oxygen species (ROS) and modify antioxidant enzyme activity in the contact area. Eventually, rapid protein degradation and apoptosis or necrosis occurred from this oxidative imbalance [3]. Allelochemicals change cell permeability and membrane function when exposed at sufficient concentrations, which may result in cellular content leakage and, as a result, cell death via apoptosis and necrosis. Allelochemicals can regularly influence the activity of Na+/K+ pumps involved in ion absorption across the plasma membrane in living roots, inhibiting the growth of target plants.

8.1. Changes in the Structure of Cells

Allelochemicals affect the structure and shape of plant cells. Cheng and Cheng [3] investigated how C. arvensis and Nepeta meyeri Benth. release allelochemicals that can change the profile of polymorphic DNA of receiver plants with random amplification. Citral is an essential oil extracted from C. citratus, disturbing microtubules in Arabidopsis thaliana (L.) Heynh. and T. aestivum roots [159]. The cortical microtubules were not affected as strongly as mitotic microtubules. It has been reported by Graña et al. [160] that in A. thaliana seedlings, the structure of cells is strongly affected by citral with the thickening cell wall. Moreover, root hair development and intercellular activity are reduced significantly by citral.

8.2. Increases in the Antioxidant System

Allelopathic activities have a high impact on reactive oxygen species (ROS) and redox state balance in the cell. When allelochemicals are exposed to the recipient plant, it produces ROS [11,12]. Superoxide dismutase, peroxidase, and ascorbic acid peroxidase are the antioxidant enzymes in which allelochemicals modify the activity to defend against oxidative stress [161]. In the root development process of Phaseolus aureus Roxb., the activity of proteases, polyphenol oxidases (PPOs) and peroxidase are modified by caffeic acid and also endogenous phenolics contents decrease in hypocotyl cutting [162]. Interaction of allelopathy significantly affects ROS production and antioxidant capabilities [163].

8.3. Increase in Cell Membrane Permeability

Allelochemicals increase not only antioxidant enzyme activity, but also stimulate free radicals levels. As a result, lipid peroxidation is greater in the membrane, causing inhibition of the scavenging effect on activated oxygen and injuring the complete membrane systems [164]. The essential oil extracted from C. citratus inhibits the activity of lipid peroxidation and electrolyte leakage to injure the cell membrane of E. crus-galli [96]. Another study indicated that pyrogallic acid stimulates ROS in Microcystis aeruginosa [165].

8.4. Plant Growth Regulators System

Allelochemicals modify plant growth regulators and several phytohormones to inhibit their germination, growth and development. Phenolic allelochemicals can induce the activity of Indole acetic acid (IAA) and reduce the interaction between POD with IAA and also stimulate hormones by bounding gibberellic acid (GA) or IAA [166]. DTD [4, 7-dimethyl-1-(propan-2-ylidene)-1,4,4a,8a-tetrahydronapthalene-2,6(1H,7H)-dione] and HHO [6-hydroxy 5-isopropyl-3,8-dimethyl-4a,5,6,7,8,8a-hexahydronapthalen-2(1H)-one] are the allelochemicals extracted from Ageratina adenophora (Spreng.) R.M.King & H.Rob. [167]. IAA contents in the O. sativa roots are decreased by HHO or DTD application. Soltys et al. [168] investigated that in S. lycopersicum roots, auxin and ethylene hormone imbalances are induced by cyanamide.

8.5. Functions and Activities of Various Enzymes

Metabolism of phenylalanine is regulated by gallic acids, caffeic acid and phenols through restraining cinnamic acid-4-hydroxylase and phenylalanine ammonia-lyase activities. Shao et al. [169] observed that Chrysanthemum indicum L. and its rhizospheric soil aquatic extract decreased nitrate reductase and dehydrogenase activities of root and not only decreased soluble protein and sugar content but also reduced the growth of root in same species. Cheng et al. [170] investigated the enzyme activity and polypeptide accumulation of glutamine synthetase of Spirodela polyrhiza (L.) Schleid. by diethyl phthalate. The inhabitation of glutamine synthetase in the assimilation of nitrogen makes diethyl phthalate toxic towards S. polyrhiza.

8.6. Influence on Respiration

Allelochemicals influence plant growth by affecting CO₂ generation, oxidative phosphorylation, ATP enzyme activity, and electron transfer in mitochondria. Juglone is an allelopathic compound that disrupts the root oxygen uptake of soybean and corn by entering the mitochondria of root cells [3]. In Z. mays and soybean, hypocotyls and radicle respiration are affected significantly by camphor, alpha-pinene and other monoterpenes. The primary function of alpha-pinene is to inhibit electron transfer, oxidative phosphorylation, ATP production and damage energy metabolism in mitochondria [171]. The separation of mitochondria is caused by camphor.

8.7. Effect on Plant Photosynthesis

Many allelochemicals affect photosynthesis by increasing the decomposition rate of photosynthetic pigments. The reductions of ATP synthesis, enzyme activity, and electron transfer occur by reducing pigment contents, resulting in photosynthesis inhibition [172]. Wang et al. [173] demonstrated that allelochemicals affect the PSII and influence its function to inhibit photosynthesis. Chlorophyll a, b and carotenoids are inhibited significantly by the methanol extract of P. hysterophorus [12,28,78]. The essential oil hinders photosynthetic metabolism by affecting alpha-amylase activity in seeds [78].

8.8. Influence on Water and Nutrient Uptake

Absorption of essential nutrients through roots can affect allelochemicals as well as increase water stress in the plant. In the plasma membrane, they can restrain the movement of Na+/K+—ATPase, which is involved in the absorption and transportation process. It has been reported by Barros de Morais et al. [174] that the residues of H. annus reduced plant growth, translocation and accumulation of nutrients in R. sativus. The concentration of allelochemicals and their classification is strongly connected with the uptake of ions. Dibutyl phthalate at low concentration increases N absorption, but P and K absorption is decreased. On the other hand, N, P, and K absorption inhibit at high concentrations [175]. According to Mohammadkhani and Servati [176], a small amount of diphenylamine increases N and K absorption and reduces P absorption by S. lycopersicum roots.

9. Limitations and Future Prospective

We have gone over the beneficiary effects of allelopathy and employing allelochemicals and/or natural compounds as potential biopesticides. However, there are some problems associated with the development of natural products such as biopesticides.

Most allelochemicals that enter the environment are made up of more than one substance, and the amounts can vary depending on the situation. In some situations, an allelochemical may not be allelopathic on its own, but it may be more allelopathic when combined with other allelochemicals [177]. Both the plant’s internal processes and external conditions interact to determine the nature and quantity of allelochemicals secreted [177]. Different genotypes and cultivars have different allelopathic effects [178]. Even among plants that share a habitat or are closely related taxonomically, there is no guarantee that they will exude the same amount or quality of allelochemicals or exhibit the same allelopathic effects [179,180].

Natural compounds have a short half-life in the environment, which is beneficial in terms of environmental toxicity, but a pesticide must last long enough to have the intended effect. Nevertheless, some natural pesticides do not last long enough to be effective [181]. Due to their structural complexity, many allelochemicals are too costly to be seriously evaluated for use as agricultural chemicals. For instance, the potential herbicide cyclic tetrapeptide toxin is also quite costly. Many researchers failed to identify promising allelochemicals with simpler structures and they found it too expensive to use as a pesticide.

Allelochemicals are known as environmentally friendly because they are produced naturally. In contrast, some allelochemicals were found to be toxic to humans and mammals (e.g., fumonisins, aflatoxin, ricin, AAL-toxin) [178]. This feature of some allelochemicals has made them less desirable for the creation of pesticides for the control of the pest. This aspect of some natural phytotoxins has decreased interest in them for the development of herbicides for weed management.

The ancient knowledge of the toxic properties of various allelopathic plants gives us a basis for creating a novel weed control approach. The development of bioherbicides based on allelochemicals opens up the possibility of utilizing natural compounds in plant protection and demonstrates the ability to deal with evolved herbicide resistance. Future study is needed to identify and isolate the potential compounds with formulation techniques. The level and extent of the phytotoxic effect of tropical plants at different growth stages in agronomic and environmental conditions need to investigate. New bioherbicides from the active compound of tropical plants need to formulate. To detect and identify the gene expression under allelopathic conditions, a molecular assay is required. The efficacy of tropical plants as pests and disease control agents needs to be evaluated.

10. Conclusions

Improved management of the world’s agricultural resources is required to assure a sufficient food supply in light of a rising global population while reducing the industry’s negative effects on the environment. Pests are viewed as a threat from an agronomic perspective, having major implications for agricultural productivity and yield losses. There are various strategies used to manage pests. Therefore, it is essential to incorporate several pest management techniques into a holistic strategy. In sustainable weed management, phytotoxic extracts of the plant have a high impact as biopesticide, but many of its applications are not profitable for field conditions. The compounds become active by modifying their chemical structure toward the target species. The activities of allelopathic compounds may be a higher or lower force on target species. The biodegradability character of allelopathic compounds makes it more favourable as biopesticides. The allelochemicals isolated from various tropical plants have bio-pesticidal properties. The ancient knowledge of the toxic properties of allelopathic tropical plants gives us a basis for creating a novel pest control approach. The pesticidal properties of tropical plants open up the possibility of utilizing natural compounds in plant protection. With the new tools of molecular genetics, proteomics, metabolomics profiling, modern and sophisticated methods of chemistry and biochemistry, we may create new compounds, based on the structure of potential natural pesticidal compounds from tropical plants to develop selective and eco-friendly pesticides.