Weedmay define as the plants which are grown in undesirable place (Jethro 1731),compete with crop plants for nutrients,moisture, light and space. Thus weeds affect the seedling growth and cropproduction (Qasem and Foy 2001). According to the International Societyof Allelopathy defined the term allelopathy as any process involving secondarymetabolites produced by plants, microorganisms, viruses and fungi thatinfluence the growth and the development of agricultural and biological system(Allelopathy Journal 2009).
It is studied that not only biological and chemicalcomplex systems of the individuals or in the community but also edaphic,climatic factors influence to regulate the allelopathic interactions Inderjit and Weiner (2001). Chemicals having inhibitoryor stimulatory effect of one plant toanother plant called allelochemicals (Torres et. Al. 1996) which are present in different plantorgans i.e.
plant tissues, leaves, flowers, fruits, stems, roots, rhizomes andseeds (Putnum, 1987). Theallelochemicals that are released into the soil environment (Rovira, 1969,Ghafoor and Sadiq, 1991) through roots and its system are those organicconstituents, most of which are mainly low molecular weight carbon , composedprimarily of carboxylic acids, amino acids, sugars, phenols, flavonoids, aswell as an array of secondary metabolites account for much of the diversity ofroot exudates (Cesco et. al. 2010,2012, Phillips et.
al. 2012). On anaverage, between 30-60% of the net assimilated carbon is allocated to the rootsin annual species (Lynch and Whipps 1990, Marschner 1995). Of this carbon,about 5%-21% of the total portion can be released as root exudates into therhizosphere environment (Badri 2009, Joneset. al. 2011, Badri et at.
2013a Badri et at. 2013b). It wasdemonstrated that root apex is the predominant site of exudation and secretionin healthy young plants which is clearly separated from older tissues in termsof metabolic fingerprinting (Bowen 1979). The root exudates has implications tothe ecological relevance in the rhizospheres environment, particularly,soil-root contact, affecting the physical and chemical properties of the soil,mediating chemical signalling, and establish both positive and negativeinteractions on the root-root, root-insect, and root-microbe interactions inthe immediate proximity of the roots (Eilerset al., 2010; Shi et. al.
2011b). It was found that phytochemicalssuch as phenolics that released from the weed roots or residues in the soilfacilitate on the activity of microbial population and seed germination andseedling growth, thus these chemicals have a profound effects on the plant healthand its ecosystem (Batish et al., 2005). During physiological activity in caseof growth and development of plants, some known allelopathic compoundssynthesized where cinnamic and benzoic acid showed most inhibitory effect on seedgermination (Xia et.al. 2014). Moreover, some exudate compounds can inhibitweed species which act as natural harbicides. For example, root exudate of hostplant may stimulate to germinate Orobanche ramosa L.
seeds under its favourablecondition (Hameed et. al. 2006).Furthermore, Brown et al. (1951a) reported that non-host plants maystimulate Orobanche seed germinationwithout being parasitized due to presence of an acidic grouping components,lactone.Most of the allelopathy studies on crop plants were mainly focusedwith different plant organs. There is limited information about theinteractions of root exudates on crop plants.
The allelophathic effects of rootexudate from weed species depend on target host plant. Thus, it is crucial toevaluate the interaction of root exudates of weed species with various cropplants to understand more deeply about the allelopathic interaction betweenweed and crop plants.The purpose of this study was to collect root exudatesfrom five weed species grown in natural condition available around the cropplants, namely a)Ageratum conyzoides b)Leucas asperac)Scoparia dulcis d)Spilanthes acmella e)Vernonia patula on seedgermination of Radish (Raphanus sativus)and Cucumber (Cucumis sativus) underlaboratory conditions.