Phytotoxicity including autotoxicity and allelopathy is the immediate or indirect biochemical impact of one organism on the germination, growth, survival, and reproduction of other organisms or improvement of neighbouring plant species through the arrival of substances into the environment. This biological phenomenon effect might be either growth-enhancing (synergistic) or inhibiting (hostile), contingent upon the chemical substances delivered from donor plants and target species. Allelopathy has been viewed not just as a nature-accommodating way to control unwanted plant spices and biocidal products, but, additionally, a potential explanation for causing autotoxicity in yield. The application of chemical agents to reduce weed infestation may have negative consequences on human health as well as the environment. Plants with allelopathy activities derived from secondary metabolites could be an alternative strategy and have an expected function in sustainable weed biocontrol and boost global agricultural production and food security. Thus, protecting biodiversity, ensuring food safety, improving food, and nutrient quality, as well as crop production, are urgently needed as population and consumption are increasing. So, the objective of this study is to present recent advancements on phytotoxicity and allelopathic effect of plant extracts (sorghum, sunflower, rice, and corn), for sustainable food and crop production in agroecosystems.

1. Background

Any immediate or backhanded chemical impact of one plant on the germination, development, or improvement of adjoining plant species through the arrival of substances into nature is called phytotoxicity (allelopathy) [1, 2]. Active substances are released from various parts of plant species into the environment by different paths including (phenolic acids, terpenoids, and alkaloids) [39]. Several factors mark their toxicity, for example, flux rate, age, concentration, climatic and environmental conditions, and the metabolic state of the plant [9]. Their quality and amount vary according to the season, cultivar, age, and plant organ [9, 10]. Allelopathy assumes a significant job in natural as well as managed biological systems, particularly agroecosystems, for example, weed control, food and crop security, and yield refoundation, because of its antagonistic consequences for germination and seedling growth [11]. However, synthetic herbicides could successfully manage weeds, but can also bring about a few impairments to the human wellbeing and environment, and increment herbicide-safe weeds [9, 12]. Allelopathy has been taken into account not solely as a nature-friendly methodology for killing weeds but additionally as a possible explanation for resulting in autotoxicity in crop output [13]. Thusly, this has urged overall research to limit reliance on manufactured herbicides and to find new other options.

Yield maximization is required to meet human needs, as its populations are growing [6]. Weeds are unintended plants when growing with intended plants which usually compete with other plants for resources such as water, space, nutrients, and light and decrease the quality and quantity of crops [12, 14].

Plants that produce allelopathy properties are known to have a major part in sustainable weed control [9, 15]. Many plant species discharge allelochemicals into nature and have organically dynamic aggravates that stifle the development and improvement of other plants [9, 16]. Crop allelopathy has been reported as an ecofriendly, economical, and sustainable approach that could probably be utilized as a herbicide for weed control [9, 17]. Cereal crops such as sunflower [8, 14, 18, 19], sorghum [14, 18, 20, 21], and rice [22, 23] have been extensively evaluated to display allelopathic influence on other plants (crops and weeds). Corn (Zea mays L.) is the third significant food cereal crop that is commonly cultivated all over the world for human, animal feed, and industrial uses [24, 25].

It possesses many allelochemicals and, hence, has phytotoxic potential against itself [24], weeds [21], and other crops [24]. Because of the widely spaced corn rows, weeds are regarded as one of the major restricting factors for corn yield and affecting economic returns [24]. Therefore, the present review aims to present the potential application of allelopathy of grain crops on germination and seedling growth of other plant species as a potential natural herbicide.

2. Allelopathy

Allelopathy as a potential plan for controlling many weeds in agriculture is the subject of much research and was coined by Prof. Hans Molisch in 1937 [5, 8], derived from the Greek word ‘allelon’ signifying ‘one another’ and ‘pathos’ signifying ‘hardship’ [6, 26]. ‘Pathos’ likewise signifies ‘feeling’ or ‘sensitive’ and accordingly could be utilized to allude to both good (sympathetic) and negative (woeful) connections [6]. Rice [4] characterized allelopathy as the impact of one plant on different plants by means of the arrival of active agents into the climate. However, this is widely passable to indicate the stimulatory (beneficial) or/and inhibitory (detrimental) biochemical interaction between plants containing microbes [6, 27], but many ecologists prefer definitions including only negative effects in allelopathy [6]. Since the emergence of the term allelopathy, a study has been begun in various scopes. The International Allelopathy Society, in 1996, modified the meaning of this term as ‘‘Allelopathy alludes to any interaction including chemical compounds delivered by plants, microbes, viruses, and fungi that impact the development and improvement of biological and agricultural systems (except animals), including positive and adverse consequences” [5, 28]. A plant that has potential allelopathic activity is known as the “donor plant” while the plant influenced by allelopathic chemicals from the contributor plant is known as the “acceptor plant” [29].

Active substances with potent activity are potentially useful molecules in the area of pharmacology, agricultural engineering, and others. In the last ages, chemical interaction in the environment is advancing fast. Natural products isolated from medicinal plants and other crops have potential use not only in pharmacology, but in many areas especially as herbicides and pesticides [30, 31]. Active substances delivered from plants, forcing allelopathic impacts, are called allelochemicals or allelochemics [3, 6, 32]. They are categorised as secondary metabolites and often their functioning in the plant is not known, except a few of them which are known to have primary capacities (for example as intermediates of lignifications) or defend plants against an animal that feeds on plants and microorganisms [26, 33]. Chemicals can be produced by plants either above (leaves, stems, seeds, buds, fruits, pollens, and flowers) or below (roots, rhizomes, and stems) the ground, or both and resulting in allelopathic effects in various plant communities [1, 11, 3436]. These different plant parts produce various amounts of allelochemicals. The biosynthesis and excretion of these biomolecules are affected by biotic and abiotic factors, following a featured temporal and dynamic pattern [31, 3739].

Allelochemicals are often water-soluble substances [35], which can be released into the environment by diverse routes (Figure 1) such as root exudation, leaching from aboveground parts such as aerial plant parts, volatilization, and decomposition of the material [46, 9, 26, 40, 41]. These chemicals can pass or influence another plant straight by take-up of the influenced plant or in a roundabout way by impacts of the allelochemical on soil microorganisms that are either plant development promoters or that are pathogenic [5].

Phenolic compounds (Figure 2) are the most important and widespread allelochemicals among secondary plant metabolites, with numerous functions including allelopathy function. Other allelochemicals include alkaloids, terpenoids, flavonoids, carbohydrates, amino acids, steroids [11, 26, 33, 42], benzoxazinoids, cyanogenic compounds, cinnamic acid derivatives, and ethylene [6]. Their concentrations in plants vary according to plant parts and types of solvents [43, 44]. These allelochemicals have been extracted from more than 30 families of earthly and oceanic plants that have real or likely phytotoxicity [6]. Chon and Nelson [11] reported that fewer certain allelochemicals have been identified, although allelopathic interaction between plants has been identified for centuries. The effects of allelopathic chemicals released from plants are probably derived from a combination of several allelopathic compounds and may have effects on germination, growth, and development [33]. Laboratory experiments have shown the greater effect of combination solutions of allelochemicals than the same concentrations of single compounds when they are employed independently [33]. Additionally, they exhibited that a combination of some allelochemicals such as polyphenols, carbohydrates, and amino acids can have allelopathic properties, although concentrations of single compounds have much lower inhibitory levels [34]. There are a lot of factors that can affect the production of allelochemicals in plants such as biotic and abiotic factors, water and nutrient availability, light, pesticide treatment, and disease [33]. The choice of allelopathic plants is a decent and normally utilized methodology for the identification of plants with secondary metabolites [6, 17].

Researchers have suggested the important role of allelopathy in the assurance of supplement elements, soil substance qualities, mycorrhizae, microbial environment, plant variety, invasion, dominance, progression, and the peak of normal vegetation [4, 26].

3. Bioassays as a Means for the Study of Phytotoxicity

Bioassays as a useful tool for the study of phytotoxicity have been effectively used to recognize the biological property of various active agents (allelopathic/allelochemical effect) and synthetic compounds [1]. There are many designs for bioassays that have been proposed to assess interactions of phytochemicals on plants (phytotoxicity) and microorganisms (plant guard and anti-infection effect), microbial mixtures and microorganisms on plants (phytotoxicity/pathogenicity), and organisms on microorganisms (anti-infection action) [5]. Biological activity concerning allelopathic research usually includes seed germination, coleoptile development tests, entire seedling/plant tests, membrane impacts through the revelation of electrolyte spillage and ethane creation, photosynthetic effect (oxygen growth and chlorophyll creation), and others [5]. Biosynthetic gene clusters may also manage these allelopathic interactions. Late genomic investigations of weedy plants are giving an in-depth overview of the development of unwanted grasses and the systems of yield weed communications and are probably going to majorly affect weed control and plant breeding [45]. Because of the difficulty of the process of separating competitive compounds from allelopathic interactions under field conditions [5], the study of allelopathy based on biological assays have been intensively carried out in the laboratory or controlled conditions and in some case conducted bioassays under controlled conditions are crucial for understanding the phenomena of allelopathy [5, 46].

Various studies deciding on the hereditary systems related to crop-weed interactions have indicated that the phytotoxicity impacts are profoundly intricate. These have been classified into biological and physiological impacts, for example, hindrance of the division of a cell and elongation, antioxidant systems disturbance, rising cell film penetrability, and impacts of allelochemicals of microbes and the prompt ecology [4750]. The importance of laboratory, growth chamber, and greenhouse bioassays is clear to address this allelopathic action in nature [5, 46]. Additionally, for phytotoxicity of secondary metabolites, bioassays are also important during isolation, purification, and identification [46].

Although plant growth bioassays [51], bioassays based on pigment analysis [52], electrolyte leakage as a bioassay [52, 53], isolated systems as bioassay specimens [54], and bioassays to detect inhibition of photosynthesis [55] have been used in plants, the most frequent tests reported in the allelopathy study are perhaps seed germination bioassays. Seed germination is regarded to be the foremost significant stage, particularly beneath pressure conditions, in which some biochemical alterations occur and give the fundamental structure for the succeeding development and improvement [36]. However, the disadvantage of this process is that seed germination is probably not as touchy to the impacts of chemicals released as plant development or other plant forms [5].

Due to the fact that germination is perhaps not the main target, many researchers pay additional attention to the sprouting seedlings since they are the major target of most allelochemicals [5, 56]. The lengths of the radicle, hypocotyl, and coleoptile are frequently measured [5]. Allelochemicals have a number of apparent impacts on plant growth and development comprising seeds darkened and swollen; inhibited or slow germination rate; swelling or necrosis of root tips; decreased radicle (root) and coleoptile (shoot) expansion; discolouration and lack of root hairs; root axis curling; increased number of seminal roots; lowered reproductive capacity; and decreased dry weight accumulation [6]. These morphological effects are probably triggered by a range of specific actions on the receiver plants at the cellular or molecular level [6]. Kamal [57] illustrated that leaves contain higher amounts of allelochemicals than roots, but this can be reversible in some plants; occasionally stems are the primary principal source of toxicity, and leaves are often major sources of allelochemicals. Thus, the natural path could be employed to improve food production and lessen human and natural health effects, especially by diminishing our reliance on pesticides and manures [58].

4. Allelopathy and Weed Management

Future farming exploration should progressively incorporate natural, physiological, and anatomic strategies, to comprehend horticultural yields in situ and their collaboration with the environment just as living beings affecting their long haul wellbeing and efficiency (agrarian ecogenomics) [59]. Undesirable weeds that have grown close to crop types are known for fast adaptation and invasion, which can bring about a tremendous decrease in yearly plant yield around the world [45, 60, 61]. Currently, there are at least 512 herbicide-resistant weed species in the world [62].

A major problem of modern agriculture which causes a ten percent drop in agricultural output globally is weeds [11, 63]. Weeds are the attacker, competitive, upsetting, and unwanted components growing with intended plants and they pose multidimensional problems for plants in every cropping system, more importantly, a reduction in crop yields as a result of interference (competition) [12, 14]. Weeds react with agricultural crops on the different roads such as chemical reactions, rivalry for purses, and hereditary host-parasite interaction. These relationships occur during an environmental period, but they can result in a variety of alterations on a developmental timeline [45]. Nevertheless, the molecular mechanisms of this fast adaptation for the development of weedy plants in crop fields are yet unknown. Reference plant genomes have provided new insights into the developmental and physiological processes that plant species use to respond to their environments [64]. Allelopathy phenomenon has appeared to play an imperative part in diverse disciplines of agriculture and biology fields that might be used to control pests [8]. It is gaining popularity for weed management due to the concerns rising from chemical usage [3, 34, 65]. Allelochemicals are currently known to be future chemicals as a potential strategy for pest management that has no negative environmental consequences [65]. Sustainable agriculture is required to maintain the natural resources and allelopathy as an important tool in agriculture may serve as a fundamental for long-term agriculture in the future [8]. As a result, in order to be sustainable, future weed control approaches must limit the usage of herbicides and instead employ allelopathic methods and other weed management approaches [8, 66]. Present-day agribusiness is focused on the production and depends mostly on engineered inputs to address undesirable grasses [14, 67]. Irregular utilization of herbicides to combat unwanted herbs in recent decades has led to environmental and ecological issues to the planet and its inhabitants [8, 14] as follows:(1)Weeds are becoming more resistant to several essential herbicides, such as dinitroanilines and S-triazines, and herbicide-resistant crops, in contrast to pest-resistant crops, will cause farmers to rely more and more on herbicides, hence increasing herbicide use in farming.(2)Weed population shifts in those species that are associated with the crops infested, for example, Avena fatua in oat and sorghum and Oryza fatua in rice, and this has resulted in minor weeds becoming more dominant.(3)Increased environmental pollution and health risks especially from the pollution of surface and groundwater, for both human and cattle utilization; for example, the groundwater of the USA comprises a considerable amount of widely used herbicides alachlor and atrazine.(4)Potential exposure of future generations to toxic residues of herbicides due to its effects on the environment.

As a result of these reasons, growing concerns have been raised regarding the usage of herbicides to combat undesirable grasses [8, 14]. The present aim is to reduce reliance on traditional techniques and synthetic pesticides and to develop alternative weed management tactics [1, 14, 68]. Therefore, FAO Expert Consultation Group on ‘Weed Ecology and Management’ has recommended alternative strategies to remove or minimise the usage of herbicides to control weeds [8]. Weeds were managed using mechanical and cultural means before the invention of herbicides. Current knowledge of plant morphology, physiology, natural product chemistry, and inter- and intraplant relationships has demonstrated that allelochemicals, trap crops, and smothering crops probably could be used to control weed. Allelopathy has the potential to aid in weed management by inhibiting unwanted plant seed sprouting and seedling development [8].

A number of allelopathic methodologies have been suggested for potential weed repression: firstly, selection of cover crop residues (weeds that cover crops) and developing their varieties to suffocate key weeds within a specific region. Secondly, the use of alive rotating crops or their leftovers as mulches that inhibit the development of encompassing unbeneficial herbs. Finally, identifying allelochemicals with herbicidal action in plants or microorganisms [11, 69]. Hoagland et al. [5] argued that the effects of phenolic allelochemicals probably at very low concentrations are stimulatory, while in larger doses restrain functions within the accepting species. It ought to be referenced that allelochemicals must fulfil some conditions before they may be registered as herbicides: demonstrated phytotoxic activities somewhere in the range of 10−5 and 10−7 M, depicted synthetic structure, plants with a known method of action, known time of habitation in soil, conceivable poisonous on human wellbeing, and suitability of creation at the manufacturing level [70, 71].

5. The Importance of Crops and Their Potential Allelopathic Effects

Plant foods are one of humanity’s most important needs, and people and plants are presently harmoniously intertwined due to their common dependence on them for survival. Plants provide 80 percent of human nutrition; thus, seven billion people cannot be fed without agriculture. On the other hand, people guarantee the endurance of a portion of our significant crops, given that plant breeding and domestication have disposed their capacity to survive on their own [72]. There are approximately 420,000 plant species living on the planet [73, 74], in which probably around 2,500 species have been domesticated for cultivation [75]. Around 100 species account for 90 percent of the calories consumed by humans; other types function as springs of fundamental supplements, for example, proteins, nutrients, and minerals, and as medicinal prescriptions [72, 73]. Thus, taking the best strategy and alternative methods to increase productivity is essential to overcome the enemy of crops.

Chon and Nelson [11] and Jabran and Farooq [76] demonstrated that some species including crops, weeds, agroforestry trees, and fruit trees have been discovered to have allelopathic activity. Only crop plants, however, attracted the greatest attention for the research of probable allelopathy in nursery bioassays and field circumstances as well [76]. Various investigations have demonstrated that there are huge contrasts between crop cultivars in their capacity to stifle weeds and these distinctions have been clarified to some degree by methods for variable ability to emit synthetic substances influencing weed development, i.e., allelopathy [77]. Crops have been proposed to show allelopathic effects (Table 1, Figure 3), on other plants such as corn (Z. mays L.), sunflower (H. annuus), barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), beet (Beta vulgaris L.), oats (Avena sativa L.), rye (Secale cereal), and lupin (Lupinus lutens L.). Its extracts possess different allelopathic compounds as potential use of herbicides, which play a critical part in weed management [6, 65, 76], and have developed structural models for the manufacture of the herbicides [65], with the synchronous decrease of herbicide application as an appealing view for sustainable farming [84]. The foremost commonly detailed crops and other species with allelopathy activity incurred weeds, and treated crops are listed in Table 2. The allelopathic effects of corn parts (root and shoot), with two solvent extracts (water and ethanol), were studied against some indicator species (corn, Johnson grass, wheat, sunflower, and canary grass). Results showed that active substances in corn parts greatly prevented the growth of the tested species. This is probably due to the allelochemicals present in plant part tests that contribute to this effect [1].

5.1. Sorghum

Sorghum (Sorghum arundinaceum (Desv.) Stapf is an annual crop from the family Poaceae [85]. Sorghum may be used for a variety of purposes, including human nutrition (Africa, China, and India) and animal feed grain (America and Australia). Sorghum has recently gained popularity as a biofuel crop of the future due to its diverse use and adaptability to alterations in the state of the agricultural climate. For instance, in the United States, more than 30 percent of grain sorghum is currently utilized in the production of fuel ethanol, which results in a large amount of dried distiller grains with solubles as a byproduct. Consequently, the need for additional value-added vents for sorghum has become critical to maintaining the sorghum business’s financial viability [86]. Sorghum has additionally become another age bioenergy crop as a result of its wide flexibility to different agricultural climatic statuses and its capacity to limit inputs, for example, water as well as nitrogen, which might be essential to sorghum benefits as a bioenergy crop [87].

Sorghum is one of the foremost broadly studied plants with regard to its allelopathic potential [14]. Sorghum water extract’s effects on weed species (sprouting response and plantlet growth) were evaluated in various bioassays. Cheema and Khaliq [20] investigated the phytotoxicity impact of sorghum to combat undesirable herbs in watered wheat and the effect of concentration and frequency of sorgaab application in the semiarid region of Punjab. They found that the use of sorgaab spray lowered the dry weight of the weed up to 49% and improved wheat crop by 21%. Sorghum stalk incorporation into the soil at 2, 4, and 6 Mg ha−1 decreased unwanted plants by 42, 48, and 56%, correspondingly. There was not much difference between one, two, or three sorgaab sprays at 1 : 10 and three sprays at 1 : 20 ratio at 90 days.

Ayeni and Kayode [21] studied the allelopathic activity of powdered extracts of sorghum stem residues on the sprouting and development of Euphorbia heterophylla L. They found that sorghum extracts caused a reduction in the sprouting of E. heterophylla weed and the level of the reduction improved with increasing concentration. Similarly, Cheema et al. [18] conceded a study of the feasibility of allelopathic extracts of mature sorghum in field trials as a natural strategy to control weeds for the wheat crop. A concentration of 100, 50% water extracts, was sprayed 30 days after planting the wheat crop while control plots received no spray. They revealed that spraying sorghum 100% water extracts reduced weed up to 53% and it suppressed individual weed species as Chenopodium album 38%, Fumaria parviflora by 40%, Coronopus didymus by 62%, and Rumex dentatus 74%. The use of 100% sorghum extract enhanced wheat production by 14%. Sorgaab’s allelopathic action was studied in Eucalyptus camaldulensis Dehnh, Dalbergia sissoo Roxb., Acacia nilotica L. Willd. Ex Delile, Populus deltoides W. Bartram ex H. Marshall, and Nicotiana tabacum L. The results revealed that repeated applications of sorgaab reduced dry biomass by 62 percent [88].

Recent greenhouse research found that combining 150 g/mL sorgoleone with 7.5 mg/mL tartary buckwheat (Fagopyrum tataricum Gaertn.) showed more prominent development hindrance of broadleaf weeds than grass weeds instead of individual concentrates specifically [89]. In particular, the blend of these two molecules repressed the development of Aeschynomene indica L., Rumex japonicus Houtt., and Galium spurium L., by 90%, 96%, and 100%, individually, while the use of sorgoleone alone prompted the development concealment of these weeds species by 81%, 83%, and 75%, separately because of the mechanism of action of sorgoleone is directed towards the photosynthetic electron transport chain [90, 91]. From a structural point of view, sorgoleone is like plastoquinone (a lipid benzoquinone), bringing about a rivalry with the typical electron acceptor at the plastoquinone binding site on the D1 PSII protein, which can prevent plastoquinone A reoxidation by plastoquinone B [92, 93]. This mechanism is similar to that of atrazine and sorgoleone is, in this manner, a serious inhibitor that contends with atrazine for the plastoquinone-restricting domain [94]. In research bioassays, watery concentrates derived from above and underground of some sorghum crossbred were found to suppress soybean radicle improvement [95].

5.2. Sunflower

Sunflower (Helianthus annuus L.) is one of the world’s most important oilseed crops. Sunflower seeds can be ground into flour, used to decorate a variety of recipes, or eaten raw. It is an appropriate choice for animal feed due to its high nutritional value [96]. Sunflower is another crop plant being described to have a potent allelopathic property [14, 97] and also exhibits autotoxicity [8]. Nikneshana et al. [19] evaluated the allelopathic activity of sunflower on several crops and associated weeds. They concluded that their results show inhibitory effects of sunflower against some harmful plants such as Hordeum spontaneum and Lolium rigidum in wheat and Amaranthus retroflexus in safflower. In the laboratory, the phytotoxic effects of fresh and dried water extracts of sunflower root, shoot, and leaves on wheat and maize sprouting and plantlet growth were investigated. Results showed (15.21%) germination inhibited, reduced radical development (21.66%), plumule growth (28.44%), and dropped plantlets dry biomass (31.05%) in wheat. In the case of maize, a similar impact was seen. The dry extract was more effective phytotoxic than fresh, and the leaf of sunflower was more inhibitory than root sand shoot extracts effects [97]. Furthermore, Cheema et al. [18] studied the feasibility of allelopathic extracts of mature sunflower in field trials as a natural strategy to control weeds for the wheat crop. A concentration of 100, 50% water extracts, was sprayed 30 days after planting the wheat crop while control plots received no spray. They revealed that spraying sunflower 100% water extracts reduced weed up to 53% and it suppressed individual weed species such as Chenopodium album 26%, Fumaria parviflora by 33%, Coronopus didymus by 42%, and Rumex dentatus 73%. The use of sunflower extract enhanced wheat grain production by 7%. Sunflower is another cereal crop despite having inhibition effects on other plants; it also has autotoxicity. Nikneshana et al. [98] evaluated the allelopathic activity of sunflower on several crops and associated weeds. They concluded that their results show inhibitory effects of sunflower against some harmful herbs such as wild barley (Hordeum spontaneum) and ryegrass (Lolium rigidum) in wheat and redroot pigweed (Amaranthus retroflexus) in safflower. Among sunflower cultivars, Suncross-42 was discovered generally to have inhibitory action against germination and dry weight amassing of weeds, while Gulshan 98 was only active against weed root development [99]. Pannacci et al. [100] assessed the allelopathic potential of sunflower cultivars to combat Sinapis alba L., Lolium multiflorum Lam., wheat. Sunflower cultivars were extremely shown to prevent weed with little impact on wheat.

5.3. Rice

Rice is also the most stable food and has antiweed and anticrop properties [13]. Some rice assortments discharge chemicals which may influence significant undesirable plants, microorganisms, and pathogenic around the cultivated crops and soil attributes. These have been distinguished in rice root secretes and decompositions, which interact with ecological conditions as an environmentally beneficial agroproduction system [101]. Several natural molecules have been recognized as potential rice inhibitory chemicals [101, 102].

Albeit various investigations have been attempted, allelopathy among rice and barnyard grass is the most profoundly inspected in light of the fact that both the yield and weed genomes are promptly accessible. The mechanism of allelopathic interactions between a typical crop (rice) and weed (barnyard grass) has been determined. Rice reacts to barnyard grass stress by expanding the creation of potential allelopathic agents such as momilactones, phenolic acids, flavones, and their aglycones, hence increasing allelopathic action against target weeds in the rice-barnyard grass coculture framework [103, 104]. Nine rice varieties’ root exudates were tested against sunflower broomrape, wheat, rice, clover broomrape, and lettuce seeds. All rice species impeded the growth of wheat seedlings. Root exudates from two rice cultivars (I-Kung-Pao and Yliangyou 3218) aided lettuce germination. Autotoxicity was higher in two cultivars (Ganxin 203 and I-Kung-Pao) than in others. The greatest germination rates of clover broomrape and sunflower broomrape were induced by cultivars (I-Kung-Pao, Yongyou 13, Yongyou 15 and Zhongzao 22) [13]. The bioassay was carried out to investigate the biological activity of various portions of rice crops, as well as the hereditary and phenotypic characters, on Echinochloa crus-galli P. Beauv. var. oryzicola Ohwi. Duchungjong on E. crus-galli had a higher inhibitory effect (77.7%) than the inhibition by other varieties. The Damaging strain has a very high rate of germination inhibition produced by a leaves-plus-straw combination (95.9%). Daegudo had the greatest proportion of suppression (93.2%) by husk remains [22]. Kolahi et al. [23] evaluated the potential chemical activity of hull extracts of 13 rice cultivars, on seed sprouting and plantlet development of wild oat (A. ludoviciana), which had a significant reduction of wild oat. They came to the conclusion that rice husk extracts might be employed as a natural herbicide source. The allelopathic effect of rice (root, stem, leaf, and whole) and various rice extract concentrations (0%, 25%, 50%, and 100%) on maize germination and seedling development was investigated. The results showed that there was a considerable influence on corn germination percentage, radicle, and coleoptile weight and length [25].

The allelopathic potential of 44 rice cultivars was evaluated in laboratory, greenhouse, and field trials. Rice cultivars displayed remarkable variation in their ability to prevent barnyard grass growth and development. In the laboratory, Gin shun rice cultivar extracts showed the best action on the seedling growth and weight by 61%, while Kasarwala mundara cultivar extracts displayed the best action on seed germination, and speed by 23%, 46% respectively. In a greenhouse trial, the Philippine 2 cultivar showed the highest percentage of inhibition on seedling growth (57%), length (74%), and dry weight (74%). In the field trial, the Juma 10 cultivar showed a significant activity impact by decreasing weed tiller number, leaf area, leaf, stem, and dry weight (80%, 49%, 61%, 74%, and 68%) in any order. These outcomes propose that there are contrasts in the plant species for the chemical hindrance of barnyard grass [105]. Rice varieties’ allelopathic capacity will most certainly have a significant influence on paddy weed control if combined with cultural management methods and the administration of modest dosages of herbicides. As a result, if allelopathic rice is produced using integrated cultural management methods, it is possible to minimise pesticide use in paddies [106].

5.4. Corn

Zea mays L. also known as maize or corn is one of the most agriculturally known commercial plants [107], as well as the third most cultivated food crop after wheat and rice [108], belonging to the grass of the monocot family Gramineae (Poaceae) [109]. It is a member of cereal crops such as barley (Hordeum vulgare), rice (Oryza sativa), wheat (Triticum spp.), and sorghum (Sorghum bicolor) [109, 110]. The origin of maize is said to be from Southern Mexico. However, modern maize was derived from a wild ancestor called Teosinte (Euchlaena), but it has been suggested that modern maize was derived from a Mesoamerican maize variety called Chapalote [111]. It was domesticated by native peoples in Mesoamerica roughly 8000 years ago. Currently, maize planting areas broaden to the Americas, Europe, Africa, South Asia, and the Fareast [111] and are now cultivated globally [112].

It is not only a vital supply of oil for cooking, fuel, livestock, and poultry feed [113], but different goods like plastics, dye, shampoo, tiles, and wrapping materials are also formed from maize. Moreover, corn is one of the built-up demonstrated life forms for hereditary qualities inquire about; presently, it is one of the driving models for plant utilitarian genomics [109]. Corn plant has been utilized as a medicinal plant in treating various ailments due to being rich in phytochemicals [114, 115]. It is right now farmed on around 100 million hectares in 125 developing nations, and it is one of the top three crops in 75 of those nations [116]. Although approximately 78% of the world’s corn production is used to feed livestock, especially in industrialised countries, human consumption is steadily expanding in many emerging and developed nations, including Africa and Latin America [116]. Corn is used in the European Union (EU) as both a feed and a raw material for industrial goods. As a result, breeders in the US and the EU concentrate on agronomic features for use in the animal feed sector, as well as many industrial qualities such as starch, dextrose, glucose, fuel alcohol, and fructose corn syrup [117].

Corn may be yellow, orange-yellow, white, purple, mottled, red, sun red, or brown [116]. There are a number of corn types that have been used for food, fodder, and fuel purposes which are categorised according to the component, kernel features, and endosperm as follows [111, 116].

Flour corn Zea mays var. amylacea is mostly grown in the Andean area. Its endosperm is primarily made up of downy starch, making it simple to crush and turn into meals.

Popcorn Zea mays var. everta: kernels have a larger percentage of firm endosperm than any other maize kernel. It is cultivated on a limited level than other varieties, yet popped kernels are enjoyed as a snack item all throughout the world.

Dent corn Zea mays var. indentata is the most extensively farmed form for grain and silage animal feed, and it is the dominating kind farmed in the US. It is distinguished by solid endosperm on the edges and base of the kernel, with downy starch filling the remainder. It requires particular processing to be suitable for human consumption.

Flint corn Zea mays var. indurata: kernels are distinguished by a significant proportion of solid endosperm around a tiny downy center. It is cultivated mostly for food in Europe and Latin America.

Waxy corn Zea mays var. Certain: in comparison to the typical 70 percent amylopectin and 30 percent amylose starch composition, kernels contain totally amylopectin and no amylose starch. It is recommended in East Asia to utilize waxy corn for food and some industrial purposes; it provides starch comparable to tapioca.

Sweet corn Zea mays var. Saccharata and Zea mays var. rugosa are cultivated for its sweet corn and are frequently cooked and consumed as a vegetable. Ears are collected approximately 18–20 days after pollination when the kernel contains roughly 70% moisture. Because of one or more recessive mutations in the genes that prevent the sugar conversion to starch within the endosperm, sweet corn growth is significantly higher in sugar and lower in starch.

Corn may grow in a variety of agroecological conditions, although it prefers a warm temperature. At least the average daily temperature for adequate growth of corn is 20 °C with warm daytime (25–30 °C) and chilly nights. Temperatures above 40°C and below 8°C lead to severe detrimental conditions [112]. High temperature affects adversely kernel development and bulk and the accumulation of endosperm zein protein. Corn producers and consumers are expected to account for a major portion of the rising demand for cereals overall [113]. Therefore, to increase corn production, substantial changes are required in agronomic practices [113].

Past examinations demonstrated that corn has chemical agents that influence different plant species and eventually bring about a decrease in seed sprouting and production. Cyclic hydroxamic (Hx) acids such as 2,4-dihydroxy-7-methoxy-2H-1, 4-benzoxazin-3-one (DIMBOA), and 2, 4-dihydroxy-3H-1, 4-benzoxazin-3-one (DIBOA) are natural compounds, found in cereals, including such crops (corn and wheat). These are produced by corn plants, under stress conditions during seedlings and play a major role against pests and diseases, possessing allelopathy activity [76, 118, 119]. Benzoxazinoids (Bx) are natural phytotoxins that work as synthetic resistance mixes in a few species and the importance of concentrations of exudate of Bx for plant−plant interactions is as yet a disputable inquiry [84].

Neal et al. [119] found that DIMBOA is the main benzoxazinoids compound in corn root exudates in chromatographic analyses (Table 3) and other crops such as wheat, rye, and some dicotyledons [122].

Kato-Noguchi et al. [120] isolated three allelopathic agents (Figure 4) from corn germination at the early growth stage (seedling) by using acetone extract, which were 5-chloro-6-methoxy-2-benzoxazolinone (Cl-MBOA) and 6-methoxy-2-benzoxazolinone (MBOA), 2, 4-dihydroxy-3H-1, 4-benzoxazin-3-one (DIBOA). The same author later [120] evaluated the effects of new (Cl-MBOA) by comparison with its analogues MBOA and BOA on seedlings of some dicots and monocots plants. The author found the inhibitory effect of these allelochemicals on seed sprouting and the development of roots and shoots of crabgrass, cockscomb, lettuce, cress, timothy, and ryegrass. Cl-MBOA was more inhibitor to them followed by MBOA and BOA, respectively. They also concluded that corn germination possesses at least three allelochemicals which may have an impact on the development and germination of other plants [120].

The allelopathy effects of two corn cultivars (301 and 704) and some crops tested on sprouting and development of wheat plantlet. Wheat root number and coleoptile length were reduced as a result of the study’s findings [123]. Likewise, aqueous extracts of corn from (roots, stems, and leaves) were evaluated against germination of wild barley (Hordeum spontaneum) and found a significant reduction in the shoot at different concentrations. Corn extracts reduced the radicle length of wild barley seedlings (16–47%) as well as wild barley dry weight [35]. Ayeni and Kayode [21] studied the allelopathic properties of aqueous extracts of maize inflorescence residues on the sprouting and development of Euphorbia heterophylla L. The results showed that extracts from maize inflorescence caused a considerable reduction in the germination of E. heterophylla weed and the proportion of the reduction improved with increasing concentration of the extracts and especially inhibition level seems to be more evident on the coefficient of velocity (COV) [The speed of germination known as COV], number of leaves harvested, and relative growth rates (RGR). Another experiment was performed to study the biological activity of corn water extracts and root exudates against corn and peanut (Arachis hypogaea). Results showed the significant inhibitory effects of corn aqueous extracts (shoot and root) on seed germination of both corn and peanut. The shoot extracts were more inhibited than water extracts of root, but its influence on seedling development was the opposite [24].

Regarding germination, when compared to the control, aqueous extracts of the corn stalk and root completely suppressed seed germination of Phalaris minor and Sorghum halepense. Aqueous extracts of corn shoot inhibited seed sprouting in corn species, with inhibition of 82.2%, followed by sunflower (76.6%) as compared to control. The greatest biological property of corn root extracts was discovered against sunflower and little seed canary grass (100%) on sprouting in comparison to the control. Regarding seedling growth, aqueous extracts of corn shot and root suppressed seedling growth of all tested species except corn root and shoot length of little seed canary grass, Johnson grass, and sunflower, which was remarkably influenced by the administration of an aqueous extract of corn shoot. Concerning fresh weight, sunflower root is influenced fundamentally by utilization of shoot extracts followed by wheat, and corn individually contrasted with control and in examination with different plants. In addition, a watery concentrate of corn root likewise apparently influenced root new weight of both sunflower and wheat [1]. The study found that corn has even autotoxicity on itself despite having inhibition effects on other plants, and water extracts of corn root and shoot had a significant impact on seed germination, radicle and shoot elongation, and fresh weight of all plant species at different levels [1]. Phytotoxicity of ZnO nanoparticles (ZnO NPs) on sprouting and root length of corn and cucumber were studied. ZnO NPs had no effect on the germination of the targets plant but could (1,000 mg L−1) decrease the length of corn and cucumber roots by 17% and 51%, correspondingly [124].

6. Conclusions

It can be concluded that the response of target plants to biocidal allelochemicals by receptors could be a better pathway to overcome undesired plants and improve crop yields. Allelochemicals are typically classified as secondary molecules which are metabolically active in plants and microorganisms; its accumulations vary according to the stage of growth (time and season) and have both positive (stimulatory) effects and negative (inhibitory) effects on plant tests and exhibite significant herbicidal activities in seed germination and growth, by exuding chemicals. This observed suppression could be attributed to the allelochemical effects that are present in different plant parts and various quantities, which lead to a reduction not only in seed germination, even the yield is affected by the length of root and shoot (radicle and hypocotyl) which are two important parameters of seedling growth as well.

However, the required concentration of allelochemicals for seed germination to be inhibited or suppressed is probably higher than it to hinder seedling growth as a whole. Seeds in higher concentrations probably have a lower germination rate and need more time to germinate so the procedure of extracts is another factor to determine the effects. It is necessary to study phytochemicals and the relative abundance of allelochemicals on the plant spices to exert an inhibitory effect. It can be seen that the extracts from plant parts or its derived natural molecules have considerable potential to stop or suppress germination and seedling development of various weeds and plants that could be potentially used as natural herbicides to control weeds, which is ecofriendlier rather than chemical herbicides although target plant species have different responses to the phytotoxicity effects.

The mechanism of this phenomenon also should be elucidated by isolating allelochemicals and determining the chemicals and their toxicity in inhibiting or suppressing sprouting and plantlet growth of many undesirable weeds using different solvents and HPLC, to verify this influence under different conditions. More importantly, genetically modifying corn and other crops to produce potent allelopathy could be another possible strategy to suppress or resist weeds and other harmful plants instead of using chemical herbicides that induce increasing the crop yield. A greater discovery of this study is that farmers may soon employ natural herbicides to prevent the negative consequences of employing chemical substances and provide economic benefits.

Data Availability

All the data used to support the findings of the study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.