Characterization of bacterial communities associated with Brassica napus L. growing on a Zn-contaminated soil and their effects on root growth.

The interaction between plant growth-promoting bacteria (PGPB) and plants can enhance biomass production and metal tolerance of the host plants. This work aimed at isolating and characterizing the cultivable bacterial community associated with Brassica napus growing on a Zn-contaminated site, for selecting cultivable PGPB that might enhance biomass production and metal tolerance of energy crops. The effects of some of these bacterial strains on root growth of B. napus exposed to increasing Zn and Cd concentrations were assessed. A total of 426 morphologically different bacterial strains were isolated from the soil, the rhizosphere, and the roots and stems of B. napus. The diversity of the isolated bacterial populations was similar in rhizosphere and roots, but lower in soil and stem compartments. Burkoholderia, Alcaligenes, Agrococcus, Polaromonas, Stenotrophomonas, Serratia, Microbacterium, and Caulobacter were found as root endophytes exclusively. The inoculation of seeds with Pseudomonas sp. strains 228 and 256, and Serratia sp. strain 246 facilitated the root development of B. napus at 1,000 µM Zn. Arthrobacter sp. strain 222, Serratia sp. strain 246, and Pseudomonas sp. 228 and 262 increased the root length at 300 µM Cd.

Mercury uptake by Silene vulgaris grown on contaminated spiked soils.

Mercury is a highly toxic pollutant with expensive clean up, because of its accumulative and persistent character in the biota. The objective of this work was to evaluate the effectiveness of Silene vulgaris, facultative metallophyte which have populations on both non-contaminated and metalliferous soils, to uptake Hg from artificially polluted soils. A pot experiment was carried out in a rain shelter for a full growth period. Two soils (C pH = 8.55 O.M. 0.63% and A pH = 7.07 O.M. 0.16%) were used, previously contaminated with Hg as HgCl(2) (0.6 and 5.5 mg Hg kg(-1) soil). Plants grew healthy and showed good appearance throughout the study without significantly decreasing biomass production. Mercury uptake by plants increased with the mercury concentration found in both soils. Differences were statistically significant between high dosage and untreated soil. The fact that S. vulgaris retains more mercury in root than in shoot and also, the well known effectiveness of these plants in the recovering of contaminated soils makes S. vulgaris a good candidate to phytostabilization technologies.

Role of the polycarboxylic compounds in the response of Silene vulgaris to chromium.

This work aims to investigate the nature and the specific mechanisms by which polycarboxylic compounds participate in the tolerance of Silene vulgaris to Cr with special attention given to the rhizosphere system. This knowledge is important to use this species in the implementation of phytoremediation technologies in Cr-polluted soils. According to the results, chromium is chelated and mobilized by the citric and malic acids in plant tissues, while oxalic acid might participate in the reduction and chelation of Cr in the rhizosphere. At the applied doses, the response of both exudation rate and root exudate composition (total polyphenols and quercitin) seems to involve a rearrangement in the lignification of the plant cell wall to immobilize Cr. Quercetin-3-dirhamnosyl-galactoside and apiin (apigenin-7-O-apiosyl-glucoside) have been identified as the major polyphenols in the root exudates of S. vulgaris. The increments found in the apiin concentration in root exudates seem to be related to the protection against Cr toxicity by chelation of Cr or by free radical scavenging. Though earlier response is detected in plant tissues, results from this work together with previous studies in S. vulgaris indicate that exudation might be a regulated mechanism of protection under Cr exposition in S. vulgaris that may involve mainly Cr reduction and chelation.