Copper Accumulation in Leucaena leucocephala by Mycorrhizae Glomus Sp. Zac–19 in Symbiosis with Rhizobium
Abstract
The Jose Antonio Alzate dam is the main man -made reservoir along the Lerma River in central Mexico. The water in this dam is heavily contaminated with organic and inorganic residues including copper and other heavy metals. For many years, people living in the vicinity of the dam use sediments as fertilizers, resulting in heavy soil contamination. This study focuses o n the use of native Leucadena leucocephala, a small, fast -growing mimosid tree cultivated under greenhouse conditions in an attempt to reduce the levels of copper. The plant was inoculated with a fungus of the Glomus sp. Zac -19 species that in a symbiotic manner increases the decontaminating properties of L. leucocephala. The study was carried out in three stages, starting with sequential extractions to assess the geochemical distribution of copper. Next, L. leucocephala wa s grown under controlled condition s using a factorial statistical model adding two known doses of Cu2+ and a third plantlet grown under normal conditions to be used as control. The plant growth was followed at random during 30, 60, 90 and 180 days to assess the level of copper bioaccumulat ion in leaves, stem and in the whole plant. The bioaccumulation index was evaluated using the initial six fractions, with fraction five revealing a direct relationship between the levels of copper in plant and sediment. Due to the relation of fraction V wi th humic material, it was finally considered for the calculation of the bioaccumulation index. Copper accumulation in stems was greater than in leaves and the metal concentration decreased with time. The percentage of arbuscules, vesicles and complete colo nization was affected by high doses of Cu2+,inhibiting the growth of stem and leaves of L. leucocephala. These results suggest that the plant can be useful for the biological removal o f copper in contaminated sediments or soils.
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Introduction
Anthropogenic activities such as agriculture, industry, urbanization and logging have alarmingly deteriorated extensive ecological areas throughout Mexico (López-Galván et al, 2011) Figure 1. The use of urban wastewater and sediments as fertilizers is a common practice in arid and semi-arid regions of developing countries and in areas where the human population is constantly increasing, as in the areas along the Lerma River in central Mexico (Avila et al, 2007). The Upper Lerma Basin is in the State of Mexico, with the Tejalpa and Temoaya rivers as its tributaries. These rivers are a depository of urban and industrial wastewater that are heavily contaminated with fertilizer and pesticide residues and large quantities of sedimentable materials that eventually end at the José Antonio Alzate dam (from here on referred to as the dam), with the inevitable deterioration due to accumulation of pollutants (Barceló-Quintal et al, 2013, Avila-Perez et al, 2011, Barceló et al, 2005). The dam sediments have a high content of organic matter and are used as fertilizers (Barceló, et al, 2012; PedrozaBenitez et al., 2010; Avila et al., 1999; Smith, 1991). The soil in the region is mainly vertisol of the phaeozem type. The aquifers in this basin sit on basaltic rocks and the sediments are mainly made up of and esite, basalt, pyroclastic, lahar and alluvial materials. Corn is the main crop grown in the surrounding fields and copper sulfate is used to avoid the growth of pathogenic fungi adding Cu2+ ions to those from industrial effluents. Phytoremediation is a possible via to directly curb the excess of Cu in these soils through absorption, sequestration or storage of the pollutant or by indirect methods using microorganisms such as bacteria or fungi that act in symbiosis with the plant for removal of pollutants (Gonzalez-Chavez et al., 2002).
Immobilization of metals by plants can be done with plants such as L. leucocephala, which is also an excellent host for arbuscular endomycorrhizal fungi (AMF; Gardezi et al, 2010, Gardezi, et al, 2011, George et al., 1994, Smith and Read, 1997). The symbiotic association between AMF and the roots of plants is prevalent in nature. The symbiosis occurs in approximately 80% of vascular plant species in all terrestrial biomes (Smith et al., 2010).The soil availability of phosphorus and AMF interact influencing the productivity of a plant community by mediating compensatory effects among plant species and functional groups, (Yang, et al, 2014).
Leep (1981) reported that plants have limited ability to minimize cooper absorption depending on accumulation of the element in soil. The Rhizobium strain plays an important role in the adsorption of nutrients (Gardezi et al, 1990). The importance of arbuscular symbiosis in crop production and natural ecosystems has been related to their ability to process mineral nutrition in soils, especially in soils lacking nutriments with low mobility, such as phosphorus, copper and chromium (Gardezi et al 2005, Gardezi et al, 2003). Rhizobium bacteria show resistance to metals, but they reduce its biological capacity to fix nitrogen (Gardezi et al, 2005).
Conclusion
Soil components such as feldspar, halloys ite and goethite have surfaces with greater potential of adsorption of humic materials. This allows greater Cu assimilation. Also, the soil is slightly acidic, with average pH = 5.7, which can promote mobility of Cu2+.Copperappears to be toxic to Leucaena leucocephala , Rhizobium and mycorrizal fungi at concentrations above 200 mg/kg. In aerial parts, the leaves accumulate less Cu than the stem. This is important for the use of Leucaena leucocephala as a food source for ruminants. The mycorrhizal fungus Glom us Sp. Zac -19 in symbiosis with Rhizobium can enrich the soil with nitrogen and phosphorus and reduce soil erosion and other soils of the region. The geochemical distribution or speciation of Cu in the soil allows determination of the fixed fraction of Cu and its bioavailability with greater precision.The results of this study support the idea of using Leucaena leucocephala doubly inoculated with Rhizobium and Glomus sp.Zac-19 for bioremediation of agricultural soils with levels of copper that do not surpas s the threshold for copper toxicity in such symbiotic system.
