The Rhizosphere Zone of Soil

The rhizosphere is the zone of soil that surrounds the root. In comparison to the bulk soil, the rhizosphere is characterized by an intense microbial activity and growth that is stimulated by the released biologically active chemicals from the roots (Hiltner 1904, Lynch and Whipps, 1990,). 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 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 play an important role to improve 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). Moreover, exudation of these compounds increased nutrient uptake and improve the stabilization of soil aggregation (Nelson and Mele, 2007, Bais et. al. 2006, Repley et. al. 2008, Vogan et. al. 2011).

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). Concurrently, 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 found 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 by H+-ATPase activity as it is involved with different membrane bound proteins 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).

The main objective of this study was to determine the effect of water stress on the quality of root exudates in six species belonging to Solanaceae, Poaceae, Brassicaceae and Fabaceae by the use of different analytical techniques. Although several hydroponic culture systems were frequently used (Conn et. al. 2013, Oburger et. al. 2013). But using this techniques could produce unreal figures of root exudation since plants are devoid of natural conditions. Hence, it is important to grow the plants closely related to environmental conditions. For this, the soil grown plants under controlled environment was used which has the potential to give more insight feature of plant health and climate changes.