Weed
may 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 crop
production (Qasem and Foy 2001). According to the International Society
of Allelopathy defined the term allelopathy as any process involving secondary
metabolites produced by plants, microorganisms, viruses and fungi that
influence the growth and the development of agricultural and biological system
(Allelopathy Journal 2009). It is studied that not only biological and chemical
complex 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 inhibitory
or stimulatory effect  of one plant to
another plant called allelochemicals (Torres et. Al. 1996) which are present in different plant
organs i.e. plant tissues, leaves, flowers, fruits, stems, roots, rhizomes and
seeds (Putnum, 1987).

The
allelochemicals that are released into the soil environment (Rovira, 1969,
Ghafoor and Sadiq, 1991) through roots and its system are those organic
constituents, most of which are mainly low molecular weight carbon , composed
primarily of carboxylic acids, amino acids, sugars, phenols, flavonoids, as
well as an array of secondary metabolites account for much of the diversity of
root exudates (Cesco et. al. 2010,
2012, Phillips et. al. 2012). On an
average, between 30-60% of the net assimilated carbon is allocated to the roots
in 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 the
rhizosphere environment (Badri 2009, Jones
et. al. 2011, Badri et at. 2013a Badri et at. 2013b). It was
demonstrated that root apex is the predominant site of exudation and secretion
in healthy young plants which is clearly separated from older tissues in terms
of metabolic fingerprinting (Bowen 1979). The root exudates has implications to
the 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 negative
interactions on the root-root, root-insect, and root-microbe interactions in
the immediate proximity of the roots (Eilers
et al., 2010; Shi et. al. 2011b). It was found that phytochemicals
such as phenolics that released from the weed roots or residues in the soil
facilitate on the activity of microbial population and seed germination and
seedling growth, thus these chemicals have a profound effects on the plant health
and its ecosystem (Batish et al., 2005). During physiological activity in case
of growth and development of plants, some known allelopathic compounds
synthesized where cinnamic and benzoic acid showed most inhibitory effect on seed
germination (Xia et.al. 2014). Moreover, some exudate compounds can inhibit
weed species which act as natural harbicides. For example, root exudate of host
plant may stimulate to germinate Orobanche ramosa L. seeds under its favourable
condition (Hameed et. al. 2006).Furthermore, Brown et al. (1951a) reported that non-host plants may
stimulate Orobanche seed germination
without being parasitized due to presence of an acidic grouping components,
lactone.

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Most of the allelopathy studies on crop plants were mainly focused
with different plant organs. There is limited information about the
interactions of root exudates on crop plants. The allelophathic effects of root
exudate from weed species depend on target host plant. Thus, it is crucial to
evaluate the interaction of root exudates of weed species with various crop
plants to understand more deeply about the allelopathic interaction between
weed and crop plants.

The purpose of this study was to collect root exudates
from five weed species grown in natural condition available around the crop
plants, namely a)
Ageratum conyzoides b)Leucas aspera
c)Scoparia dulcis d)Spilanthes acmella e)Vernonia patula on seed
germination of Radish (Raphanus sativus)
and Cucumber (Cucumis sativus) under
laboratory conditions.