PRELIMINARY STUDIES OF PHYTOREMEDIATION BY LEUCAENA LEUCOCEPHALA IN ASSOCIATION WITH ARBUSCULAR ENDOMYCORRHIZA AND RHIZOBIUM IN SOIL POLLUTED BY CU AND CR

摘要

Heavy metals are discharged to water and soils, creating a severe contamination which endangers the biota and man. Actions to preserve natural resources and rehabilitate soils are necessary. 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 metals are complex. The bacterial reduction of sulfate to sulfur tends to immobilize metals like copper, cadmium, lead, chromium and mercury by changing them to sulphur compounds. There is a leguminous plant, L. leucocephala, from the south of Mexico and Guatemala that grows in a wide diversity of soils. This plant is capable of surviving frosts. The effects of double innoculation with Rhizobium and endomycorhiza in a Leucaena leucocephala host were studied. In this study the results obtained showed that the initial amount of Cr and Cu added to the soil in the pot decreased sharply. This tendency was similar at both concentrations evaluated (20 and 200 ppm). This means that the plant absorption for both metals was similar either at 20 or 200 ppm. The pots were 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 metals in the soil could be attributed mainly to the plant uptake. In this case mycorrhizal fungi Glomus sp. Zac 19 and Glomus intraradix helped to partially alleviate Cr and Cu toxicity as indicated by greater plant growth. A considerable Cr and Cu accumulation was shown in non-colonized plants with 200 ppm. Mycorrhizal colonization, arbuscular, vesicles, and total colonization percentage were affected more by high doses of Cr and Cu; 200 ppm reduced the vegetative growth and negatively affected esseential: elements (N, P, K) in the aerial part. The will help to design strategies for Leucaena leucocephala associated arbuscular endomycorrhiza and Rhizobu bioremediation of Cr and Cu accumulation.