STUDIES OF PHYTOREMEDIATION BY LEUCAENA LEUCOCEPHALA IN ASSOCIATION WITH ARBUSCULAR ENDOMYCORRHIZA AND RHIZOBIUM IN SOIL POLLUTED BY Cu

摘要

Copper (Cu) is one of the most toxic heavy metals in polluted soils. It can be absorbed and translocated to feed and food crops. Copper is also strongly held on inorganic and organic sites and in complexes with organic matter. As such a large proportion of the total copper content of soil is not available for uptake by plants, deficiencies in crops may be due to an inherently low total copper content of the soils or to only a small amount of it being in an available form. Deficiencies may be aggravated by soil microorganisms. Although copper held in exchange sites is not readily available to plants, cation exchange from Cu~(2+) and CuOH~+ can take place and is best effected by H~+. The amount of copper in soil solution decreases with increasing pH due to stronger copper absorption. As pH increases with the application of lime to soil, availability of copper to plant usually, but not always, decreases. In organic soil, availability of copper depends not only on the concentration in soil solution, but also on the form in which the copper is present. Copper complexes of molecular weight < 1000 were found to be more available to plants than those with molecular weight exceeding 5000. Microorganisms may have a tolerance to different metal concentrations. This enhances selection of resistant species, such as those like mycorrhizic fungi, Rhizobium and actinomicetes. The mechanisms of microorganisms that cause the immobilization of metal are complex. The bacterial reduction of sulfate to sulfur tends to immobilize metals like copper, cadmium, lead, chromium and mercury by changing them to sulfur compounds. There is a leguminous plant, Leucaena leucocephala, from the south of Mexico and Guatemala that grows in a wide diversity of soils. This plant is capable of surviving frosts. The objective of this research was to determinate the effect of double inoculation with Rhizobium and endomycorrhiza in a Leucaena leucocephala host were studied. This study showed mat initial amount of Cu added to the soil in the pot decreased gradually. This tendency was similar at both concentrations evaluated (20 and 200 mg.kg~(-1)). The pot was watered with a little excess, and thus there was possibly a lixiviation of the metals. However this was rather small so the decrease of the metal in the soil could be attributed mainly to plant uptake. In this case mycorrhizal fungi Glomus sp. Zac-19 and Glomus intraradices helped to partially alleviate copper toxicity as indicated by greater plant growth. Arnconsiderable copper accumulation was shown in non-colonized plants with 200 mg.kg~(-1) mycorrhizal colonization. Total colonization percentage was affected more by high doses of copper; 200 mg.kg~(-1) affected negatively essential elements (N, P, K) in the aerial part and reduced the vegetative growth in different parameters, such as plant height, stem diameter, number of leaves, number of leaflets, root volume, dry weight of roots, leaf area, dry weight of aerial parts, number of nodules, dry weight of nodules and soil speciation. The results will help to design strategies for using Leucaena leucocephala associated with arbuscular endomycorrhiza and Rhizobium in bioremediation of Cu accumulation.