The family Leguminosae represents the third large plant family with 750 genera and more than 18,000 species having unusual flower structure, podded fruits, and the ability of 88% of the species examined to date to form nodules with rhizobia [1] . In the context of human importance, it is the second largest family after Graminiae. Grain and forage legume account for 27% of the world’s primary crop production, with grain legumes alone contributing 33% of the dietary protein nitrogen (N) needs of Humans [2] . Legume seeds contain 20% - 50% protein which creates an increasing interest for the utilization of proteins from different legumes. The applications in the food trade are almost limited to protein from legume like soybean Glycine max, whereas other legume plants are less used. Other major grain legumes are pea (Pisumsativum), broad bean (Viciafaba), lentil (Lens culinaris), common bean (Phaseolus vulgaris), Lupinus spp., and chickpea (Cicerarietinum) are generally intercropped with cereals to enhance crop yield, increase nitrogenuse efficiency, and reduce weed infestation and the occurrence of plant disease [3] - [6] .

Allelopathy is a phenomenon involving either direct or indirect and either beneficial or adverse effects of a plant (including microorganisms) on another plant through the release of chemicals in the environment [7] - [10] . Among the legume crops many are cultivated as monoculture system that reduces the crop yield with a great extent. The production of legumes declines in replanting conditions due to autotoxicity, a form of intraspecific allelopathy that occurs when a plant species releases chemical substances that inhibit or delay germination and growth of the same plant species [11] - [13] . The phytochemicals of the legume crops are well documented. Plants always produce a complex mixture of secondary metabolites which usually consist of members from different groups, such as polyphenol, terpenoids [14] - [16] . Among the secondary metabolites, some have allelopathic effects in some legumes such as in P. sativum [17] , velvet bean (Mucunapruriens) [18] , G. max [19] - [21] , and C. arietinum [22] .

These allelopathic chemical affect the legume plant growth and development and finally yield [20] . Baziramakenga et al. [23] studied the effects of benzoic acid and trans-cinnamic acid on growth of G. max (L.) grown in nutrient solution. The two allelochemicals reduced root and shoot dry biomass of soybean. He found that treated plants with fewer lateral roots and tended to grow more horizontally compared to the untreated plants. Lateral roots were stunted and less flexible. Asao et al. [24] mentioned sweet pea (Lathyrusoduratus L.) shoot fresh weight and dry weight greatly reduced by the release of autotoxic malonic acid, benzoic acids p-hydro- benzoic acids and vanillic acids from its roots into the rhizosphere. Assaduzzaman and Asao [25] investigated the autotoxicity of P. sativum, P. vulgaris, and V. faba in hydroponics either with or without activated charcoal (AC) addition. Growth and yield of the three beans were significantly reduced when grown in the culture solution without AC addition. In P. sativum plants the number of pods, pod fresh mass, number of seeds, and seed fresh mass were reduced when plants were grown in non-renewed culture solution without AC. The number of pods plant⁻¹ and fresh mass of pods⁻¹ plant in P. vulgaris, as well as pod number in V. faba, were decreased significantly to 49% - 67% without AC addition. Chung and Miller [26] found that alfalfa (Medicago sativa L.) aqueous extracts inhibit its seed germination and seedling growth, and that autotoxicity is concentration dependent and due to water-soluble toxic substances. Batish et al. [27] investigated that caffeic acid significantly suppressed the mung bean (Phaseolus aureus) root growth. Salama et al. [28] investigated the the inhibitory effects of M. sativa L. residues on seed germination, nutrient uptake and growth of V. faba L. grown in clay and sandy soil. In that research the extract of M. sativa strongly inhibited the seed germination index and the plumule and radicle lengths. In field experiments, residues and extracts of P. sativum plants suppressed the growth and population of several plant species [29] - [33] .

For the successful identification of different allelochemicals, right extraction and isolation method is predominantly necessary. During autotoxicity plants releases chemicals/allelochemicals to its rhizosphere [13] through various mechanisms. These include root exudation, crop residue decomposition, leachation, volatilization, and pollen spreading in some plants [7] [34] - [38] . So extraction of allelochemicals largely depends on the plant parts which are related to release of allelochemicals or which contain the allelochemicals such as soil or air. Many researchers identified the allelochemicals from legume crops through different techniques and methods using different plant parts and releasing areas. Han et al. [39] identified allelochemicals of G. max from rhizosphere soil extract by gas chromatography-mass spectrometry (GC-MS). Tomita-Yokotani et al. [40] identified allelochemicals from the volatile compounds of M. pruriens L. Chung et al. [41] identified the allelochemicals in M. sativa from its alfalfa leaves. In this study chemical separation procedures involved a methanol extract of fresh M. sativa leaves, treatment with activated charcoal, microcrystalline cellulose thin-layer chromatography (MCTLC), and finally separation by column chromatography. The various fractions were examined further by high-per- formance liquid chromatography (HPLC). Preliminary identification by HPLC was confirmed with GC-MS. Finally it was found that suggested that chlorogenic acid is involved in M. sativa autotoxicity. Kato-Noguchi [17] identified the allelochemicals of P. sativum L. by methanol extract of its shoots. Chemical structure of this compound was determined by high-resolution MS, IR and ¹H NMR spectral data. Asaduzzaman and Asao [25] identified the allelochemicals of P. vulgaris, and V. faba by the GC-MS analysis of root exudates by methanol adsorbed in AC form the hydroponic nutrient solution.

During allelopathy plant release chemicals into the surrounding environment termed as allelochemicals. Accumulated allelochemicals create detrimental effects on the growth of next season crop plants [7] . In agro ecosystem legume crops release a great number of allelochemicals. Assaduzzaman and Asao [25] identified benzoic, salicylic, and malonic acidsin the root exudates of P. vulgaris and lactic, benzoic, p-hydroxybenzoic, vanillic, adipic, succinic, malic, glycolic, and p-hydroxyphenylacetic acids in V. faba. In another study, pisatin in the P. sativum shoot was identified by Kato-Noguchi [13] . Asao et al. [24] found malonic acid, benzoic acids p-hy- drobenzoic acids and vanillic acids in L. oduratus roots extracts. Kato-Noguchi [42] isolated cis, trans-xan- thoxin and trans, trans-xanthoxin from the leaves of Puerariathunbergiana through the aqueous methanol extract. Miller [43] reported that various toxic compound potentially involved in autotoxicity are localized in the seed coat, fresh leaves, stems, crowns, dry hay, old roots, and soil residues of M. sativa. Abdul-Rahmanandand Habib [44] in their research found caffeic, chlorogenic, isochlorogenic, p-coumaric, p-hydroxybenzoic, and ferulic acids in M. sativa root exudates and residues. Chung et al. [41] also separated and quantified the allelopathic compounds chlorogenic and salicylic acid from fresh M. sativa, leaves and they suggested that chlorogenic acid is involved in alfalfa autotoxicity. Han et al. [39] extracted organic acids, alcohol, acetone, aldehyde, naphthalene phenyl and furan hydrocarbon from G. max rhizosphere many of which were reported as allelochemicals. Fujii [45] isolated amino acid, L-DOPA from hairy vetch (Vicia villosa) and cyanamide in M. pruriens.

In general allelochemical plays multiple roles on the physiology of crop plants. It can inhibit plant growth, alter mineral uptake, cause stomatal closure and induce water stress, influence respiration, affect photosynthesis and protein synthesis impair hormone balance and alter enzyme activities. Many researchers have done on the mechanism of allelochemicals in the legume crops [46] - [48] . Cruz et al. [49] investigated the effect of allelochemicals on the P. vulgaris root where root tip cells were extruded together and that cell organization was disordered with little cell differentiation Batish et al. [27] reported that the activity proteases in hypocotyl cuttings of P. aureus decreased significantly when to the caffeic acid was treated as allelochemical. Baziramakenga et al. [23] studied the effects of benzoic acid and trans-cinnamic acid on growth, mineral composition, and chlorophyll content of soybean G. max grown in nutrient solution. These allelochemical showed negative effects on G. max by altering ion uptake and transport, and by reducing chlorophyll content. In his research he found that soybean plant reduced the P, K, Mg, Mn, contents in shoots and roots. Patterson et al. [50] reported that caffeic, coumaric, ferulic, cinnamic, and vanillic acid significantly inhibit the growth of soybean G. max by reducing the photosynthetic products and chlorophyll content. In another study Baziramakenga et al. [51] studied the allelochemical effect on the G. max he found that soybean plant growth is affected by the interference of allelochemicals with nucleic acid and protein metabolism. He also found that phosphorus uptake was reduced in G. max by benzoic, cinnamic, vanillic and ferulic acids. Mersie and Singh [52] also mentioned that allelochemicals affect the photosynthesis and protein synthesis in M. pruriens.

Soil sickness and replanting injuries are very common in successive cultures [7] [53] that affect the growth and yield of crops. Replanting problems in continuous cropping is occurred when the chemical interference from previous crop or residue to the same crop or other crop of the next culture. Allelopathic effects from crop residues and root exudates have extensively studied in legume such as alfalfa [43] [54] - [56] , bean [25] . Generally, leguminous crop such as broad bean, garden pea and snap bean are intensively cultivated in the same farmland continuously without maintaining any crop rotation which hamper the production of bean due to in replanting conditions owing to autotoxicity [11] - [13] . In such a condition suitable succeeding crop with a crop rotation has good control over it. It can limit the soil autotoxicity due to all allelopathy to a greater extend [57] . Assaduzzaman et al. [58] investigated sixty seven cultivars of 42 vegetable crop species from 14 families through seedling growth bioassay using replanting soil for succeeding crops of V. faba L., P. sativum L. and P. vulgaris L. replanting soil to suggest their possible succeeding crops. He suggested that most of the cultivars tested can be planted after three beans with fewer to be affected. Among the methods of bioassay he used nutrient solution, direct seed sowing and seedling transplanting in the replanting soil; nutrient solution bioassay has more sensitivity than replanting soil bioassay. However, in the field condition the results of nutrient solution bioassay may not be reproducible. Therefore, he suggested seedling transplanting method as an easy and practical bioassay method for suggesting succeeding crops for a field with replanting problems.

In the field of the legume allelopathy extensive research has been done and a great numbers of allelochemicals responsible for allelopathy have identified. For sustainable crop production in agriculture, allelopathy research is very important. As allelopathy cause a great loss in legume crops in monoculture system in such a condition, allelopathy research is inevitable. For a successful allelopathy research knowledge on plant physiology and ecology is necessary. This will help to know the synthesis and release of allelochemicals of plants in its surrounding environment. Lack of information on this above two aspects of plants, chemical analysis is worthless. Nowa- days by the application of genetic approaches in allelopathy has started which open a new era in allelopathy research. To understand the allelopathic mechanism in legume further studies on the production, role, and fate of allelochemicals in many legume species with the surrounding ecosystem are necessary by using all the mentioned knowledge and techniques. Progress is this research will help in making a crop rotation schedule to improve the crop growth and yield in a sustainable manner.