The which represent a significant energy loss for

The rhizosphere is
the zone of soil that surrounds the root constantly released a variety of organic compounds as root
exudates (Badri and Vivanco, 2009, Dixon and Strack
2003, Baetz and Martinoia 2014,). The process of releasing these compounds
have been termed rhizodeposition, which represent a significant energy loss for
the plants (Marschner 1995). 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 into the rhizosphere environment (Badri 2009,
Jones et. al. 2011, Badri et at. 2013a Badri et at. 2013b). Such
compounds are accumulated and persist for some period of time in the soil
environment, have a profound effects on the seed and plant health and its
ecosystem (Chaparro et. al 2013a, Doornbo et. al. 2012).
Therefore, the nature and relative abundance of these compounds, its
performance and regulation are a major field of interest in root biology
research. However, a more comprehensive view on metabolic diversity composition
are tremendously being elucidated through the recent advancement of metabolomics
and the development of non-destructive sampling techniques (Bakker et. al. 2012, Chaparro 2013a, Nicole and Harro 2016).
Alongside, genes and biosynthetic pathways are also contribute to enrich
the knowledge of root exudation process which could open prospective for
practical application in agriculture and plant protection (Liu et. al. 2009, Ishimaru et. al. 2011).

 

An
inclusive list of root exudate components found in the literature, most of
which are mainly low molecular weight carbon (LMW) compounds, composed
primarily of carboxylic acids, amino acids, amides, sugars, phenolic, phytosiderophores,
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), Few reports have
shown the qualitative
and quantitative proportional contribution of the
different compound classes to total root exudation
(Azaizeh et. al. 1995). Typically, the concentration of organic acids in
roots is generally about 10–20 mM which is account 1%–4% of total dry matter,
and the concentration of sugars is ca. 90 mM (Jones, 1998, Farrar et. al. 2003). Schneckenberger
(2008) showed that the
concentration of organic compounds ranging from 0.1 to 10 mM to reflect carbon
concentrations in the naturally occurring soil solutions (Owen and Jones, 2001). However, concentrations of LMW
carbon compounds can be increased in a rooted soil (van
Hees et. al. 2005). It has been shown that the release of organic comprises
two mechanisms: passive efflux of organic anions or simply diffused by
apoplastic pathways via specific carrier proteins that depends on membrane
permeability, the polarity of the exuded compounds (Weston
et. al. 2012, Badri and Vivanco 2009). On
the other hand, high molecular weight (HMW) compounds consisted of mucilage
(polysaccharides) and proteins (Bais et. al. 2006,
Badri and Vivanco 2009) are less diverse but often compose a larger proportion
of the exudates by mass. Studies showed that the
involvement of Golgi apparatus in the hyper secretory
root cells resulted in the excretion of mucilage by degeneration with
subsequent sloughed off root cells (Hirsch and Kapulnik, 1998). There are
reports demonstrating that many secretory enzymes such as acid phosphatase,
phytase, peroxidase, phenoloxidase are transported through the vacuolar
compartment and consequently
secreted by the vesicle from the root cells (Oldroyd et. al. 2005). Overall,  the process is energy-dependent or active
proton pumping as it is involved with different membrane bound proteins

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for the secretion of HMW (Zhu et. al. 2005, Weston et. al.
2012, Reddy et. al. 2012).

 

Various factors such as plant species, age, environmental and cultural
conditions, level of chemical, physical and biological conditions, soil
structure, pH, O2 concentration, temperature and organisms present
in the rhizosphere and possibly the mycorrhizal symbiont affecting on
root exudation composition and process (Badri and Vivanco 2009; Mimmo et al., 2011,
Nihorimbere et. al. 2011). Additionally, cultural
manipulations to induce stress may change the quantity of root exudates and
thus increase the effectiveness of phytoremediation (Henry
et al. 2007). These biotic and abiotic factors generate a certain degree of
specificity for each plant species and even in cultivars (Uren, 2007; Neumann
and Römheld 2007, Uren 2000).

 

Among the plants, genus Amaranthus having 60
species distributed in
tropical, subtropical and temperate regions (Uphof 1968, Willis et al.1973, Sigh et
al. 1983, Wiersema and Leon 1999), of which  Amaranthus
gangeticus, a red amaranth is  a very popular leafy
vegetable in Bangladesh. Several compounds such as
carotenoids, ascorbic acid, steroids, alkaloids, aldehydes, xyloids,
chlorogenic acid and saponins flavonoids and phenolic acids etc have been
reported  responsible on the growth
retardation on various crops (Shahrokhi et al.,
2011; Shahrokhi et al., 2012).

 

The present study was conducted to evaluate
the effect of different concentration of aquous extract of root exudate from Amaranthus gangeticus on seed
germination of randomly selected of two species in fabaceae family i. e. in order
to evaluate the sensitivity of root exudate components.