Among the deleterious pests of crops, weeds rank the first as it causes severe loss of crop yields, which is even more than the combined losses caused by plant pathogens and insect pests worldwide. Around 33% - 53% crop produce is damaged if weeds are not controlled in the crop fields   . Even after adopting conventional weed control techniques, 13% - 30% of crop produce is lost from the farmers’ fields   .
Since the inception of first commercial herbicide in 1940s farmers are using thousand tons of herbicide every year to control weeds across the globe. Excessive use of synthetic herbicide lead to a tremendous environmental hazards    including development of herbicide resistant weed biotypes . Therefore, for the sake of pollution free earth ecosystem and sustainability in crop productivity, conventional agriculture has to be improved by limiting the use of synthetic agrochemical including chemical herbicides. Researchers around the world are therefore now searching for new natural plant products to develop biodegradable and cost effective herbicides for sustainable weed management.
The impact of plants upon neighboring plants or their associated micro-flora and/or macro-flora by the production of allelochemicals is known as allelopathy . Allelopathic substances are released into the surrounding environment (atmosphere or rhizosphere) through a number of ways  , and upon release may suppress the germination, growth and establishment of neighboring plants, even the donor species itself either directly by affecting their physiological properties   , or indirectly by modifying the rhizosphere soil properties through influencing the microbial biomass carbon and microbial community   . Thus, allelopathic substances play a vital role in regulating the structure of plant communities  , and thus could be used as lead for biodegradable herbicide production  .
Medicinal plants have a great history as a source of potential therapeutic agents, which have been incorporated into modern medicine   . There are about 422,000 flowering plant species in the world, out of which 52,885 plant species are considered medicinal plants . Of these, the biological activity of only about 6% has been screened, with 15% having been phytochemically evaluated   . On the other hand, there are about 400,000 secondary metabolites in plants  , of which only a few have been examined for their biological or herbicidal activities . The biological or herbicidal potentialities of rest of the compounds are still unexplored. The present study reviews the present status and future prospects of allelopathic medicinal plants in controlling major crop weeds for sustainable agricultural production.
2. Allelopathic Activities of Medicinal Plants
Medicinal plants are considered as an important source of secondary metabolites that usually have a number of biological functions. It has been reported that medicinal plants species possess strong allelopathic potential    . Many researchers around the world are now showing their keen interest on medicinal plants for searching new natural plant products,  - . Islam and Kato-Noguchi  stated two reasons for this increasing interest: 1) the easier screening process of phytotoxic plants from medicinal plants and 2) the possibility to have more bioactive compounds in medicinal plants than other plants. Fujii et al.  carried out the first comprehensive studies of allelopathic medicinal plants. They evaluated 387 Japanese medicinal plants and observed that a considerable number of those have growth inhibitory potential   . Azizi et al.  examined 56 aromatic medicinal plants of 22 families from Iran for their allelopathic potential and found 51 species inhibited the seedling growth of lettuce. Gilani et al.  also evaluated 81 medicinal plant species of Pakistan and reported 78 as allelopathic potential inhibiting the root growth of lettuce. Mardani et al.  recently evaluated the allelopathic potential of 83 Iranian medicinal plants and observed more than 80% root growth suppression of lettuce by Peganum harmala, Berberis vulgaris, Artemisia aucheri and Ferulago angulata. Amini et al.  investigated the allelopathic potential of 68 medicinal and wild plant species belong to 19 plant families grown in Iran. Among the examined plant species, stigma and style of Crocus sativus, leaves of Artemisia kopetdaghensis, Mentha piperita, Zhumeria majdae, Frulago subvelutina, flowers bud of Eugenia caryophyllata, flower of Perovskia abrotanoides, fruits of Melia azedarach and Ruta graveolen had the strongest inhibitory effects on lettuce seedling growth. Islam et al.  on the other hand, examined the allelopathic properties of 55 tropical medicinal plant species of Bangladesh representing 32 different families. All the medicinal plant species except Garcinia mangostana (Clusiaceae) and Schleichera oleosa (Sapindaceae) showed allelopathic potential. Moreover, among the plant species, Citrus aurantifolia (Rotaceae), Moringa oleifera (Moringaceae), Annona muricata (Annonaceae), Aegle marmelos (Rutaceae), Cinnamomum tamala (Lauraceae) and Azadirachta indica (Meliaceae) completely (100%) inhibited the growth of Raphanus. sativus. From two different studies Piyatida and Kato-Noguchi  and Suwitchayanon et al.  evaluated 25 medicinal plants of Thailand and reported that the extract obtained from Cymbopogon nardus, Piper retrofractum, Piper sarmentosum, Rhinacanthus nasutus and Tinospora tuberculata possessed the highest allelopathic potential against the seedling growth of lettuce. Qasem  examined 30 medicinal plant species of Jordan for their herbicidal activities against Amaranthus retroflexus and Chenopodium murale in laboratory and glasshouse conditions. In laboratory study, the aqueous extracts of Alhagi maurorum, Capparis spinosa, Citrullus colocynthis, Lavandula officinalis, Origanum syriacum, Rhus coriaria. Ricinus communis, Rosmarinus officinalis and Teucrium polium either prevented seed germination or reduced seedling growth. Whereas, in glasshouse experiment addition of 2 g kg−1 dried shoot material of L. officinalis or R. officinalis to the potted soil mixture was highly toxic to weed germination and growth, and addition of 16 g kg−1 of C. spinosa, R. coriaria, and T. polium severely reduced weed growth. Many other researchers for example, Khanh et al.  , Azizi and Fujii  , Khan et al.  , Anjum et al.  , Laosinwattana et al.  , Islam and Kato-noguchi  Islam and Kato-Noguchi  , Khan et al.  , Islam and Kato-Noguchi  , Islam et al.  , Islam and Kato-Noguchi  , Itani et al.  , Baličević et al.  , Qasem  and Algandaby and El-Darier  worked with several medicinal plants species and observed that most of the species possessed strong allelopathic properties and inhibited the growth of test plant species with different inhibition values.
3. Scope of Using Natural Plant Products in Weed Management
Since the inception of agriculture, different types of natural plant products have been used to control pests mainly insect and disease organisms. More evidences have been found in the world literature about the use of natural product as fungicides, insecticides, and other pesticides than as herbicides . Although natural product based herbicides for weed control strategies have not yet been as successful as like other pesticides, however, there have some notable success stories . Nowadays, the use of allelopathic medicinal plants has been suggested as a viable option for alternative weed management in sustainable agriculture   . Yang and Tang  , described two ways of plant utilization for weed management: 1) the active compounds are isolated, identified and chemically synthesized; if possible, these compounds or their active analogs are produced by the chemical industry, 2) plant tissue or a crude product of the plant, such as aqueous or organic extract is used directly. The last one is more labor intensive, but is often economically and ecologically sounds, and does not require sophisticated technology, and thus is suitable for the farmers in developing countries and also for organic farming.
Duke et al.  proposed three approaches to select the sources of natural products for the discovery of potential herbicides: 1) to obtain pure compounds from other scientists, 2) to obtain biological material that has not previously been studied, and 3) to use ethnobiological and/or chemical ecology clues. However, ethnobotanical knowledge showed better performance only in case of pharmaceutical, rodenticide and insecticide discovery, but not for phytotoxins. In this backdrop chemical ecology provides some important clues. They suggested two chemical ecology approaches to identify phytotoxic plants: a) allelopathic activity has been the most common approach; an example of this strategy is the discovery of sorgoleone from allelopathic sorghum species   , b) the second approach is to examine compounds on that plant species have sequestered to avoid autotoxicity, for examples, artemisinin and hypericin .
4. Prospects and Status of Allelopathic Plants in Weed Management
The allelopathic plants have been suggested as a viable option for alternative weed management under sustainable agriculture     . These plants could be exploited in weed management through a number of processes. For example, use them as cover/smother crops    , rotational/companion crops  , application of their extracts     , incorporation of their residues or their different parts as mulch    , application of allelopathic substances as natural herbicides   , use of allelopathic plant extracts with lower herbicide doses    or development of allelopathic crop cultivars through breeding program  .
Substantial number of reports has been documented in the literature about the successful use of allelopathic plants, their extracts/residues, or the allelopathic substances into the crop fields as a substitute of synthetic herbicides to control weeds. Mushtaq et al.  opined that herbicide use could be reduced by 75% through integration with sorghum and sunflower extracts without compromising yield and net benefits for cost-effective and eco-friendly control of wild oat and canary grass in wheat. Xuan et al.  found that alfalfa pellets at 1 - 2 t・ha−1 completely inhibited the emergence of Dopatrium junceum, Lindernia pyxidaria and Elatine triandra, and also reduced the number and dry weight of emerged Echinochloa oryzicola. Hong et al.  worked with ten allelopathic higher plants (Ageratum conyzoides, Bidens pilosa, Blechnum orientale, Eupatorium canabium, Euphorbia hirta, Galactia pendula, Leucana glauca, Melia azedarach, Morus alba, and Tephrosia candida) and revealed that all the species at 2 t・ha−1 significantly reduced paddy weed growth and promoted the rice growth and yield. Among those species, B. pilosa and T. candida showed the highest potential to reduce more than 80% weeds and to increase rice yields by up to 20%. Many other examples are found in the literatures that are potential to control weeds  . From another field experiment in Thailand, Laosinwattana et al.  reported that the powders of the Aglaia odorata leaves inhibited the emergence and growth of Digitaria adscendens, Trianthema portulacastrum, and Amaranthus gracilis at a dosage of 1 t・ha−1. A bioherbicide PORGANICTM was then developed from the leaf extracts of this plants, which had significant inhibitory effects on Echinochloa crusgalli and Sphenoclea zeylanica at a dosage of 10 kg・ha−1 .
Xuan et al.  demonstrated that allelopathic plant extracts performed better than synthetic herbicides to control weeds. They observed that Japanese alfalfa variety (Rasen) inhibited 80% total weed biomass and promoted 81% rice yield when compared with the control (without any weed and fertilizer management). In contrast, herbicide treatment suppressed 75% paddy weeds but increased rice yield by only 10%, and those for hand weeding were about 70% and 25%, respectively.
As stated earlier, an increasing number of weeds are now gaining resistant to several synthetic herbicides. Therefore, it is much difficult to control them through synthetic herbicides. Allelopathic plants showed promising results in controlling those weeds. In Japan, Rotala indica showed strong resistance to some common herbicides. Application of alfalfa and rice byproducts at the rate of 1 - 2 t・ha−1 could completely inhibit the emergence of this noxious weed, and A. conyzoides, B. pilosa, G. pendula, L. glauca and Piper methysticum inhibited significantly R. indica at the same dose .
Xuan et al.  also reported that 70% - 80% weed reduction could be attained through the incorporation of 1 - 2 t・ha−1 of strong allelopathic plants. Although the inhibitory activity of allelopathic plant biomass is dose-dependent, they suggested not exceeding the limit above 2 t・ha−1. Otherwise, it will be laborious and costly. The most important obstacle to use plant biomass is that it needs water for decomposition. Therefore, it is very difficult to apply plant biomass in highland areas where water crisis exists. To overcome this problem many researchers currently focuses on allelopathic substances rather than the plant itself. They stated that if allelopathic substances with stronger activity on weeds are deployed as a tool for new natural herbicides development, the burden from plant residue application could be minimized.
Sorgoleone, an isolated allelochemicals exudate from Sorghum root hair, has been characterized as a potent bio-herbicide. It is phytotoxic to broadleaf and grass weeds at concentrations lower than 10 µM  . Post-emergent foliar application of Sorgoleone, at a similar concentration to labelled field rates of Atrazine (0.6 kg a.i. ha−1), inhibited growth of most of the small-seeded broadleaf weeds . Pre-emergence soil applications were also toxic to certain small-seeded weed species . The allelochemicals of tomato e.g., tomatine and tomatidine also reduce the growth of weeds, pathogenic fungi at a satisfactory level .
Several types of allelochemicals for examples, phenolics, terpenoids, alkaloids, coumarins, tannins, flavonoids, steroids and quinines are involved in the phytotoxic activities of the allelopathic plants  . Recently, a number of compounds extracted from higher plants, such as cineole, benzoxazinones, quinolinic acid and leptospermones, have been commercially applied in crop fields to control weeds such as Benzoxazinones and Quinolinic acid by BASF, Germany; Cineole as Cinmethylene by Shell, USA; Letospermones as Triketones by Zeneca, Letospermones as Mesotrione by Syngenta AG    . A brief description of the natural products (plant extracts or essential oils) commercially used in the crop fields is summarized in Table 1.
Moreover, these herbicides are not quite enough to control a vast number of weeds, and also not effective to all weeds. Therefore, searching of new natural plant products effective for weed control is very crucial. Isolation and identification of new natural plant products might lead to the development of new natural herbicides.
5. Challenge for Medicinal Plant Allelopathy in Weed Management
Although selection and/or identification of allelopathic plants from medicinals plants are much easier than other group of plants, isolation and identification of
Table 1. Examples of commercial products containing natural products used for weed management in organic agriculture    .
strong allelopathic substance is laborious, time consuming and need very sophisticated equipment’s. Hence, to date very few works have been conducted to isolate and characterize the allelopathic substances from allelopathic medicinal plants. A list of allelopathic substances isolated from different medicinal plants is given in Table 2. These substances showed strong phytotoxic potential against different target plant species under laboratory condition. However, their phytotoxic potential under field condition not yet reported. It is well known that even a substance showing strong phytotoxic activity on target plants in laboratory experiments, may not perform satisfactorily in the field conditions due to the influence of several soil factors like soil pH, organic carbon, organic matter and available nitrogen    . Therefore, more emphasis should be
Table 2. Isolated compound of different medicinal plants and their phytotoxicity against test plant species.
given on evaluating the bioactivity of allelopathic substances or the allelopathic plant extracts in both laboratory and field condition. If the extracts or the isolated compound shows strong activity in both conditions only then it could be recommend for new natural herbicide development.
From the above discussion, it is clear that there is immense prospect of allelopathic medicinal plants for the isolation and identification of allelochemicals and applies them as a tool for new bio-herbicide development. On the other hand, issues such as environmental pollution, unsafe agricultural products, human health concerns, may be dealt appropriately if allelopathic medicinal plants or bio-herbicides developed from their allelochemicals are use in the crop fields for weed management instead of synthetic herbicides. Undoubtedly, these natural products based crop production system will continue to attract increasing attention and, ultimately, will play a major role in agricultural productivity in a most sustainable way. However, before implication of allelochemicals as bio-herbicide the following points should be considered:
1) both laboratory and field experiments should be conducted exclusively, to study its interaction with various physical, chemical and biological properties of soil.
2) the physiological and ecological mechanisms of allelopathy for example, the movement of allelochemicals, its fate in soil, mode of action, selectivity etc. should be studied.
3) the impact of use of allelochemicals from agronomic and environmental point of view should be brought under consideration. However, allelochemicals are not always safe for the human and animals. There are some allelochemicals that are toxic to human beings and are carcinogenic, for example, AAL-toxin and fumonisin are toxic to mammalian cells . Some other allelochemicals are reported to cause dermatitis .
If the identified allelochemicals satisfy the above three criteria then it could be used for the development of natural herbicides. Moreover, except very few limitation, natural product based agriculture is the only way to achieve sustainable crop production for the pollution free green earth ecosystems. Allelopathic medicinal plants could provide the clues to the researchers of that arena and still there is a lot of vacant space for the scientists to work with allelopathic medicinal plants. Therefore, in future more and more emphasis should be given by the researchers to isolate and identify the new allelopathic substances from the medicinal plants, examine their bioactivity under laboratory and field conditions, if satisfy both then recommend them to use as a tool for new natural herbicides development for the evergreen world.