Potential Allelopathic Candidates for Land Use and Possible Sustainable Weed Management in South Asian Ecosystem

: Weed management is one of the signiﬁcant challenges of ﬁeld crops since weeds pose a remarkable threat to crop productivity in South Asian countries, including Bangladesh. Allelopathy, a phenomenon whereby secondary metabolites produced and released by one plant species inﬂuence the growth and development of other species can be exploited in sustainable management. The focus of this study was to evaluate potential allelopathic plant species which can be further explored as alternatives to synthetic herbicides or incorporated as part of integrated weed management in sustainable agriculture. Two hundred ﬁfty-two plant samples from 70 families were collected from Bangladesh and evaluated with the sandwich bioassay. Thirty-one percent of the samples showed signiﬁcant allelopathic potential on lettuce radicle elongation. Among the species that showed substantial inhibition, more than 7% of the samples showed higher inhibition (HI) and 25% showed moderate inhibition (MI) on lettuce radicle. Fruit pulps of Couroupita guianensis (95.4%) , fruits of Phyllanthus emblica (95.4%) , and Acacia concinna (95.4%) showed the highest inhibition on lettuce radicle elongation. In contrast, the leaf of Bombax insigne had growth promoting activity by stimulating radicle (23%) and hypocotyl (80%) elongation of lettuce seedlings. This result suggested that the species with signiﬁcant plant growth inhibitory potential may play a vital role as an alternative to the increasing use of synthetic herbicides for sustainable weed management in agricultural land.


Introduction
Bangladesh, along with the other agriculture-dominated developing countries in the world, is addressing alarming problems of crop production because of the excessive application of agro-chemicals in agricultural land. Additionally, weeds, diseases, and pests are also critical issues of crop cultivation that negatively affect crop productivity. The consequence is the misapplication of several classified hazardous, persistent organic pollutants (POPs), pesticides, fungicides and weedicides [1], which has resulted in the potential contamination of water, soil, and food threatening environmental safety [2,3]. the environmental factors may be relatively manipulated to hold some bioassay characteristics (like physical, chemical, and biological) on field condition. This bioassay methodology has been explored in the identification of several allelopathic plants. It may draw attention to the effects of natural herbicides on farming to promote sustainable agriculture practices.
Given this context, the present study attempted to identify (a) the potential allelopathic species from Bangladesh (b) and examine the prospect of significant allelochemical isolating candidates as a future scope. In this study, a total of 252 plant samples from Bangladesh were evaluated to select promising allelopathic species through sandwich bioassay.

Study Area
The plant samples were collected from the South Asian country, Bangladesh, situated between 24 • 00 north latitude and 90 • 00 east longitude. The major neighboring country is India all around and in the southwest region, Myanmar. The capital city, Dhaka, is situated at 23 • 51 north latitude and 90 • 24 east longitude. Chittagong is another major city in a hilly area of Bangladesh located at 22 • 21 north and 91 • 50 east coordination. The Tropic of Cancer line passes through the country and touches Dhaka, Khulna and Chittagong divisions, and thus the climate of the territory becomes mainly subtropical. Though it is a sub-tropical country, the area accomplishes with the diverse plant species. Two hundred fifty-two samples were collected from different places of the country ( Figure 1). The sandwich bioassay method adopted in this study is an efficient tool for screening the allelopathic effect of leachate under laboratory conditions on a large scale. Besides, the dynamics of the environmental factors may be relatively manipulated to hold some bioassay characteristics (like physical, chemical, and biological) on field condition. This bioassay methodology has been explored in the identification of several allelopathic plants. It may draw attention to the effects of natural herbicides on farming to promote sustainable agriculture practices.
Given this context, the present study attempted to identify (a) the potential allelopathic species from Bangladesh (b) and examine the prospect of significant allelochemical isolating candidates as a future scope. In this study, a total of 252 plant samples from Bangladesh were evaluated to select promising allelopathic species through sandwich bioassay.

Study Area
The plant samples were collected from the South Asian country, Bangladesh, situated between 24°00′ north latitude and 90°00′ east longitude. The major neighboring country is India all around and in the southwest region, Myanmar. The capital city, Dhaka, is situated at 23°51′ north latitude and 90°24′ east longitude. Chittagong is another major city in a hilly area of Bangladesh located at 22°21′ north and 91°50′ east coordination. The Tropic of Cancer line passes through the country and touches Dhaka, Khulna and Chittagong divisions, and thus the climate of the territory becomes mainly subtropical. Though it is a sub-tropical country, the area accomplishes with the diverse plant species. Two hundred fifty-two samples were collected from different places of the country ( Figure 1)  The sample plant parts were freshly collected from the spots to evaluate the allelopathic potentiality of the different plant species. The samples were packed in separate paper bags and dried in the sun for 72 h. Then each sample was kept in an air-tight zip-lock paper bag separately for further use. A thick, unscented tissue was locked inside each bag to absorb the unwanted moisture trapped inside. These samples were sent to the Department of Biological Production Science, Laboratory of International Agro-Biological Resources and Allelopathy, Tokyo University of Agriculture and Technology, Japan for conducting the study. The majority of the plant samples were collected from the National Botanic Garden, Mirpur, Dhaka (DNBG), 44%, followed by Sher-e-Bangla Agriculture University, Dhaka-(SAU) and Chittagong University, Chittagong-(CU), 19%, Local market, Dhaka-LM, 7%, Bangladesh Agriculture University, Mymensingh-(BAU), 6%. Jahangirnagar University, Savar, Dhaka-(JU), 5%.

Sandwich Method
The sandwich method [35] was adopted to determine the allelopathy due to leachates from the selected plant samples in the laboratory condition ( Figure 2). All the plant samples were screened out by this method using a six-well plastic dish. The dimension of each well was 36 mm × 18 mm. Various amounts of dry plant materials (10 mg and 50 mg) and 10 mL of 0.75% autoclaved agar was set as a sandwich pattern. The amount of (10 or 50) mg per well (10 cm 2 ) considered based on the estimation of average fallen leaves about 3-5 tons per ha per year [35]. Only the autoclaved agar without plant material in multiple dishes was used as a control. Five lettuce seeds (Lactuca sativa var. legesse, Takii Seed Co., Ltd., Kyoto, Japan) used as receptor plants, were placed at the top of the agar layer. Dishes were appropriately sealed by cellophane tape to avoid external contamination or gaseous interaction. The dishes were wrapped in aluminum foils to prohibit the light interaction and incubated for three days at 25 °C in the incubator (NTS Model MI-25S). Each of the experiments was replicated at least three times.  [35], first step to the fourth step, condition with 0.75% w/v autoclaved agar for sandwich layers, oven dried plant materials (10 mg and 50 mg), five lettuce seeds (Lactuca sativa var. legesse, Takii Seed Co., Ltd., Kyoto, Japan) placed vertically.

Analytical Study
The experimental design of this study was set up as a completely randomized design (CRD) with nine replications. The statistical analysis was done by the evaluation of means (M), standard deviations (SD), and standard deviations variances (SDV) using Microsoft Office program 2016. The criterion of the standard deviation variance (SDV) estimated the range of significant effects of the species. Criteria indices: * = M + 0.5 (SD), ** = M + 1 (SD), *** = M + 1.5 (SD), **** = M + 2 (SD) indicate the radicle and hypocotyl inhibition rate.
The allelopathic potentiality was evaluated by comparing the differences of the inhibitions of radicles and hypocotyls of the test plants (Lactuca sativa) grown on agar sandwich without dried plant samples and the treatments with dried plant samples. Placing lettuce seeds at the top agar layer Figure 2. Sandwich method [35], first step to the fourth step, condition with 0.75% w/v autoclaved agar for sandwich layers, oven dried plant materials (10 mg and 50 mg), five lettuce seeds (Lactuca sativa var. legesse, Takii Seed Co., Ltd., Kyoto, Japan) placed vertically.

Analytical Study
The experimental design of this study was set up as a completely randomized design (CRD) with nine replications. The statistical analysis was done by the evaluation of means (M), standard deviations (SD), and standard deviations variances (SDV) using Microsoft Office program 2016. The criterion of the standard deviation variance (SDV) estimated the range of significant effects of the species. Criteria indices: * = M + 0.5 (SD), ** = M + 1 (SD), *** = M + 1.5 (SD), **** = M + 2 (SD) indicate the radicle and hypocotyl inhibition rate.
The allelopathic potentiality was evaluated by comparing the differences of the inhibitions of radicles and hypocotyls of the test plants (Lactuca sativa) grown on agar sandwich without dried plant samples and the treatments with dried plant samples using Equations (1) and (2)

Allelopathic Effect: Inhibition Diversity among the Plant Species
The inhibition diversity was evaluated based on the allelopathic effect of the leachates of the different plant parts on lettuce seedlings. Inhibition evaluation was done for all the 252 plant samples (Table 1) according to different plant species under different plant families. The inhibition percentage range on lettuce radicles and hypocotyls was −23.1 to 95.4 % and −150 to 80.0%, respectively, when treated with 10 mg. Whereas the range became 0.92 to 100%, −121 to 100% by the treatment of 50 mg dry plant sample. The inhibition of lettuce radicle varied more than the hypocotyls. Among the 252 plant samples, 81 showed potential radicle inhibition under 10 mg dry plant sample treatment evaluated by standard deviation variance (SDV). Among samples that showed potential inhibition of lettuce radicle, more than 17 samples (6% of total samples) showed higher inhibitory activity (HIA) that ranged from 76.9 to 95.4%., 63 samples (25% of total samples) showed moderate inhibitory activity (MIA) with 53.6-76.5% inhibition range. One hundred seventy-two samples (67% of total samples) showed lower inhibition activity (LIA) by the evaluation of the sandwich method. Among all evaluated plant samples, the highest plant species were examined from the family of Fabaceae (18 species), Asteraceae (17 species), Acanthaceae and Apocynaceae (12 species), Euphorbiaceae and Lamiaceae (10 species), Rutaceae (9 species), Malvaceae (7 species), Rubiaceae (7 species), and some other following species under different families also evaluated. Among the 252 plant samples the fruit pulp of Couroupita guianensis (Lecythidaceae), fruit of Phyllanthus emblica (Phyllanthaceae), and fruit (pod) of Acacia concinna (Fabaceae) showed most robust radicle inhibition value of 95.4% with 10 mg dry sample. Whereas under 50 mg treatment these three species showed ±98% inhibition state. The correlation between the inhibition percentages of radicle and hypocotyl of the samples revealed that the allelochemicals inhibition affects radicles more than the hypocotyls for both 10 mg and 50 mg dried plant matters.

Optimal Inhibition Effect by Allelopathic Plants
The allelopathic evaluation of the plant samples was done by the different parts of the plants like leaf, stem, flower, bark, peel, fruits, root, and seed. The composition of plant parts among the samples were leaves 78%, followed by stems 3%, flower 3%, fruit peel 3%, roots 2%, seed 4% and fruits 5%. In the present study, it was revealed that among the plant parts, fruit part showed the most vigorous inhibitory activities on the test plant, compared to other tissues. In this experiment, the radicle inhibition area was modeled by the assembling evaluation of all plant samples under 10 mg dry plant material leaching treatment. Invariably, the modeling area was representing a selection of the (higher inhibitory activity) HIA, (moderate inhibitory activity) MIA, (lower inhibitory activity) LIA and non-inhibitory activity (NIA) plant samples that emulated the radicle inhibition area (high, medium, low), and the appendage elongation area (Figure 3). Under the consideration of 10 mg of dry plant samples, the inhibition area was spread out from 0 to 95.4%. The fruit pulp of Couroupita guianensis showed the highest inhibitory effect 95.4%, followed by the fruit of Acacia concinna 95.4%, and fruit of Piper longum 91.3% of inhibition status on lettuce radicle. Although, some plant sample evaluations revealed that rather than inhibition, secondary metabolites leaching stimulated the radicle elongation of lettuce seedlings (0 to −23.1%). Bombax insigne caused 23.1% stimulation of lettuce radicle, followed by a leaf of Geodorum densiflorum (16.9%) and leaves of Alstonia macrophylla (6%). The observation of maximum inhibition status can be used to evaluate the higher potentiality of allelopathic plant species. The rate of the inhibition is different from species to species. Inhibitions by the allelochemicals are exhibited by relatively different patterns of changes, such as, the radicle color, the shape of the root, necrosis, darkened and swollen seeds, curling of root axis and reduction of the size [37]. The fruit pulp of Couroupita guianensis, under the lower concentration (10 mg) of treatment, caused discoloration of lettuce sprouts root. Additionally, maturation and elongation areas turned brown with curling orientation (Figure 4). The fruit of Phyllanthus emblica exhibited dark brown or black and thin root pattern, and curling formation ( Figure 5). This study found that lower (10 mg dry sample) concentration treatment indicated incomplete radicle inhibition. However, increasing the concentration showed further inhibition. Couroupita guianensis fruit pulp and Phyllanthus emblica fruit showed inhibition of  The observation of maximum inhibition status can be used to evaluate the higher potentiality of allelopathic plant species. The rate of the inhibition is different from species to species. Inhibitions by the allelochemicals are exhibited by relatively different patterns of changes, such as, the radicle color, the shape of the root, necrosis, darkened and swollen seeds, curling of root axis and reduction of the size [37]. The fruit pulp of Couroupita guianensis, under the lower concentration (10 mg) of treatment, caused discoloration of lettuce sprouts root. Additionally, maturation and elongation areas turned brown with curling orientation (Figure 4). The observation of maximum inhibition status can be used to evaluate the higher potentiality of allelopathic plant species. The rate of the inhibition is different from species to species. Inhibitions by the allelochemicals are exhibited by relatively different patterns of changes, such as, the radicle color, the shape of the root, necrosis, darkened and swollen seeds, curling of root axis and reduction of the size [37]. The fruit pulp of Couroupita guianensis, under the lower concentration (10 mg) of treatment, caused discoloration of lettuce sprouts root. Additionally, maturation and elongation areas turned brown with curling orientation (Figure 4).  The fruit of Phyllanthus emblica exhibited dark brown or black and thin root pattern, and curling formation ( Figure 5). This study found that lower (10 mg dry sample) concentration treatment indicated incomplete radicle inhibition. However, increasing the concentration showed further inhibition. Couroupita guianensis fruit pulp and Phyllanthus emblica fruit showed inhibition of radicle of approximately 99% and around 97%, respectively, by 50 mg dry sample, whereas Cinnamomum verrum (bark) showed complete suppression by 50 mg of the dry plant sample treatment on lettuce seedlings.

Selective Species: Maximal Status of Inhibition
The top eight inhibitory samples, namely, Couroupita guianensis (Fruit pulp), Phyllanthus emblica (Fruit), Cinnamomum verrum (Bark), Acacia concinna (Fruit), Piper longum (Fruit), Aegle marmelos (Leaf), Duranta repens (Leaf), and Chrysophyllum cainito (Leaf) were selected to check the correlations of inhibition and the weight of dry plant matter. The sandwich experiment further proceeded with an increased amount of plant samples. The amount of plant sample was extended from 10 mg and increased to 30, 50, 70 and 100 mg. Each experiment was replicated five times. The percentage of inhibition of lettuce radicle and hypocotyl was measured for each amount of dry plant sample used in the sandwich method. The correlation between the inhibition percentages of radicle and hypocotyl of the eight plants species revealed that the inhibition of radicles was more than the inhibition of hypocotyls. The correlation coefficient (0.605) suggested that the dry plant samples caused more inhibition on radicle rather than the hypocotyls, which indicates that the radicle is more susceptible to plant leachates.

Selective Species: Maximal Status of Inhibition
The top eight inhibitory samples, namely, Couroupita guianensis (Fruit pulp), Phyllanthus emblica (Fruit), Cinnamomum verrum (Bark), Acacia concinna (Fruit), Piper longum (Fruit), Aegle marmelos (Leaf), Duranta repens (Leaf), and Chrysophyllum cainito (Leaf) were selected to check the correlations of inhibition and the weight of dry plant matter. The sandwich experiment further proceeded with an increased amount of plant samples. The amount of plant sample was extended from 10 mg and increased to 30, 50, 70 and 100 mg. Each experiment was replicated five times. The percentage of inhibition of lettuce radicle and hypocotyl was measured for each amount of dry plant sample used in the sandwich method. The correlation between the inhibition percentages of radicle and hypocotyl of the eight plants species revealed that the inhibition of radicles was more than the inhibition of hypocotyls. The correlation coefficient (0.605) suggested that the dry plant samples caused more inhibition on radicle rather than the hypocotyls, which indicates that the radicle is more susceptible to plant leachates.

Discussion
From 233 plant species, 252 Bangladeshi plant samples were collected and studied Among the plant samples, the fruits of Couroupita guianensis, Phyllanthus emblica, and Acacia concinna showed the radicle inhibition value of 95.4% with 10 mg plant dry matter. Here, we would introduce the evaluated highly bioactive potential of allelopathic plant species, which have not been reported yet as allelopathic species. Two new plants with high allelopathic potential observed were the fruit pulp of C. guianensis and fruits of P. emblica, which with minimum dry quantity (10 mg) affected the radicle and hypocotyl growth, respectively. In this study evaluated another firm inhibitory specimen Acacia concinna fruits (pods), a tropical southern Asia medicinal plant already reported as a growth inhibitor on test plants (dicotyledonous and monocotyledonous plants) [38]. Therefore, with the approach of natural or ecological interaction of vegetation, weed management can be possible by direct or indirect involvement of the allelochemicals through suppression of the radicle development, curling, discoloration, delayed, slowing, stunted root formation [18,36,37]. Although leaves and roots are the most common sources of allelochemicals, it is also noticeable in different parts of the plant, such as stems, buds, fruits, peels, bark, and seeds [39]. However, the amount of allelochemicals varies from one tissue to another [40].
A member of Lecythidaceae family, Couroupita guianensis is native to the tropics of the northern part of South America and the West Indies, especially Amazon rainforest. It has always been a botanically fascinating plant due to the unique shape of the flowers and fruits, and it is widely spread as an ornamental plant in tropical and subtropical countries in the world [41]. It is a fastgrowing deciduous tree. The fruit pulp of the plant is one of the high potential candidates for this study. All parts of this plant (leaves, fruit, flowers, stems, roots, and seeds) have been reported to have medicinal value due to its properties, such as anti-inflammatory [42], allelopathic [43], wound healing [44], antimicrobial [44,45] and antioxidant [46,47]. Typically, the plant leaves appear to be the most consistent source of chemicals involved in phytotoxicity, while fewer and less potent Correlation coefficient higher than 5% (*), correlation coefficient higher than 1% (**).

Discussion
From 233 plant species, 252 Bangladeshi plant samples were collected and studied Among the plant samples, the fruits of Couroupita guianensis, Phyllanthus emblica, and Acacia concinna showed the radicle inhibition value of 95.4% with 10 mg plant dry matter. Here, we would introduce the evaluated highly bioactive potential of allelopathic plant species, which have not been reported yet as allelopathic species. Two new plants with high allelopathic potential observed were the fruit pulp of C. guianensis and fruits of P. emblica, which with minimum dry quantity (10 mg) affected the radicle and hypocotyl growth, respectively. In this study evaluated another firm inhibitory specimen Acacia concinna fruits (pods), a tropical southern Asia medicinal plant already reported as a growth inhibitor on test plants (dicotyledonous and monocotyledonous plants) [38]. Therefore, with the approach of natural or ecological interaction of vegetation, weed management can be possible by direct or indirect involvement of the allelochemicals through suppression of the radicle development, curling, discoloration, delayed, slowing, stunted root formation [18,36,37]. Although leaves and roots are the most common sources of allelochemicals, it is also noticeable in different parts of the plant, such as stems, buds, fruits, peels, bark, and seeds [39]. However, the amount of allelochemicals varies from one tissue to another [40].
A member of Lecythidaceae family, Couroupita guianensis is native to the tropics of the northern part of South America and the West Indies, especially Amazon rainforest. It has always been a botanically fascinating plant due to the unique shape of the flowers and fruits, and it is widely spread as an ornamental plant in tropical and subtropical countries in the world [41]. It is a fast-growing deciduous tree. The fruit pulp of the plant is one of the high potential candidates for this study. All parts of this plant (leaves, fruit, flowers, stems, roots, and seeds) have been reported to have medicinal value due to its properties, such as anti-inflammatory [42], allelopathic [43], wound healing [44], antimicrobial [44,45] and antioxidant [46,47]. Typically, the plant leaves appear to be the most consistent source of chemicals involved in phytotoxicity, while fewer and less potent toxins occur in roots [48]. The aqueous methanol extracts of C. guianensis leaves significantly inhibited germination and growth of dicotyledonous plants and monocotyledonous weeds [43]. The fruit pulp of the plant has not been studied yet as a potential plant growth inhibitor in the context of weed management. It contains several chemical constituents, such as indigo, indirubin, tryptanthrin, isatin, triterpenes, phenolic compounds, couroupitine, stigmasterol, and other essential oils [49,50]. However, there is no detailed research information about these chemicals as plant growth inhibitors. Conversely, sterols and their derivatives promote and maintain growth and development in plants [51]. In our further study, we are pursuing to identify the effect of these candidates chemical as plant growth inhibitors and their applications as new allelochemical on plants. Further studies on these candidates may identify the specific bioactive allelochemicals from the fruit pulps of C. guianensis, that may be a potential alternative to the synthetic herbicidal compounds. Nonetheless, depending on different types of soils, morphologies, and physiologies, allelochemicals may show a different mode of action, compositions, and developments among the plants of the same genus, even species [52,53]. This report revealed for the first time that Couroupita guianensis fruit pulps have significant allelopathic potential in weed management by releasing bioactive secondary metabolites.
Another promising allelopathic candidate in this study was the fruit of Phyllanthus emblica with 95.4% inhibition value. This plant is widely distributed in subtropical and tropical areas, and it contains ample amounts of superoxide dismutase and vitamin C [54]. This plant is famous as a traditional medicinal plant, widely used in Chinese herbal medicine and Indian Ayurvedic medicine [55]. The fruits, leaves, and bark of P. emblica are rich in polyphenol (primarily, gallic acid and ellagic acid) [56]. P. emblica fruit is used in Ayurveda and also clinical medicine. P. emblica fruit has been reported as an agent with antimicrobial [57,58], anti-inflammatory [59,60], and anticancerous properties [61,62]. It has been reported that it contains phenolic compounds that enthrall intense antioxidant activity and can protect the cells from the oxidative damage caused by free radicals [63]. However, P. emblica fruit contains a high amount of polyphenols (hydrolyzable tannins like Emblicanin A and Emblicanin B). Besides, excessive oxidation of phenol causes high toxicity and changing root color [64]. This study revealed that allelochemicals released from P. emblica reduced radicles growth and changed the color of the root of lettuce seedlings. However, the adverse effect of phytotoxins on the growth of a weed species at specific concentrations might cause less or no growth inhibition on other species [65]. Moreover, the interaction of bioactive allelochemicals disrupts physiological processes and hormonal balances of plants that obstruct the growth of other organisms [66,67].
However, another report identified, the relative germination ratio of root length of Indian chickpea inhibited when of aqueous leaf extract of Phyllanthus emblica increased with time and concentration [68]. Similarly, Japanese medicinal plant Emblica pectinate leaves exhibited 93% radical and 74% hypocotyl inhibition on lettuce plant [69]. However, P. emblica fruit was not revealed as potential allelopathic plant species. This study reported that Phyllanthus emblica fruit is another new significant allelopathic candidate in terms of sustainable weed management.
The total area of inhibition study of the entire species revealed that Bombax insigne appeared with growth-promoting potentiality, promoting approximately 23% elongation under the screening by the sandwich method. The plant is locally known as Shalmali (Sanskrit), Shimul-Tula (Bengali), Didu (Andaman) attaining an extended height up to 36 m with widespread branches [70]. Bombax insigne is therapeutically relevant containing phytochemicals like alkaloids, flavonoids, phenols, triterpenoids, saponins, and cardiac sterols [71]. Considerably, all the species might that have an impact on the promotion of the plant growth exhibit the adverse effect. Growth promotion or successions studied by Booth and Mania [72] on North American grasslands and abandoned Japanese field, resulted in allelopathy as growth promotor can stimulate the growth of pioneer species on pasture [73]. This study is considering plant growth inhibitor rather than growth promotion to focus on sustainability factors in weed management. However, growth promoting allelopathic species and releasing secondary metabolites may create an opportunity for growth promoting allelopathic studies.
According to sample screening and evaluations, potential eight species were further studied to examine the correlation of the relative concentrations (10-100 mg) of the plant material leaches and the increased suppression effect of the hypocotyl and radicle, respectively. However, it found that the inhibitory activity depended on the concentration of chemical extraction [74,75]. A higher level of active allelochemicals leachates may stop or reduce the growth of lettuce seedlings. More than 50 mg leached of dry fruit pulp of C. guianensis, suppressed nearly 100% of the length of the radicle of lettuce seedling. Consequently, the effect of allelochemicals resulted in higher inhibition of lettuce radicles than hypocotyls. Aslani et al. observed the inhibitory impact depending on the concentration of allelopathic plant extracts [76,77]. The high concentration of Coffea arabica fruit crude dry matter is an allelopathic inhibitor for hypocotyls and rootlets of L. sativa [78]. Barbosa reported that 10,000 µgmL −1 concentrations of sesquiterpenes in Brazilian pepper were found to inhibit the radicle growth of cucumber and lettuce by 50.5-84.5% and 88.6-92.4%, respectively [79]. Consequently, this study exhibited that the effectiveness of allelochemicals imposed higher inhibition on lettuce radicles than hypocotyls. Another report by Fujii et al. also indicated that L-DOPA had either no or very little influence on the growth of hypocotyl of lettuce [80].
In contrast, some allelopathic metabolites in higher concentrations inhibit growth but stimulate growth at lower levels. Several scientists previously evidenced this phenomenon. Sujeeun and Thomas named it as a rescue effect [21,[81][82][83]. Sujeeun and Thomas [21] speculated the consistency of physiochemical sorption of allelopathic compounds by the media used. Moreover, the precipitation or altered mobility of allelochemical related to pH changes can also be a reason behind this immobilization.
The results of this research indicate the evolution of new allelopathy species to be used to its most significant advantage in natural herbicide discovery and development. Furthermore, it is necessary to identify the specific bioactive allelochemicals and to understand the composition and the mode of action of promoting allelochemicals.

Conclusions
Evaluation of allelopathic species and discovery of natural herbicides can be a significant advantage for land use and sustainable weed management in the agroecosystem. Identification of those unknown allelopathic plants from Bangladesh might provide the opportunity for new natural herbicide development. This research was undertaken to evaluate the allelopathic potentialities of plants. Furthermore, understanding the mode of action and the prospect of an individual or combined allelopathic compounds is necessary. Consequently, to fulfill the demand for safe food for the vast populated and developing country, a salient natural alternative is necessary rather than the implementation of chemical pesticides, weedicides, and fungicides in the field. This study gives evidence that the fruit pulps of Couroupita guianensis and the fruit of Phyllanthus emblica are new potential allelopathic candidates due to releasing bioactive secondary metabolites that may promote sustainable weed management in agriculture.