Germinação e crescimento de Brachiaria decumbens na revegetação de solo contaminado por mercúrio

Abstract

Mercury (Hg) is a metal that accumulates in the soil due to environmental pollution, causing a series of problems, such as soil erosion due to reduced vegetation cover. The introduction of plant species with high biomass production, such as Brachiaria decumbens, is a potential strategy to stabilize the metal and revegetate soils contaminated with Hg. However, studies documenting the complete development of this plant are still lacking. The revegetation of soils contaminated with Hg can promote soil stabilization and reduce the bioavailability of Hg. Therefore, this research aimed to evaluate the response of B. decumbens to Hg, considering (i) biometric, biochemical, and physiological parameters of seeds exposed to Hg; (ii) physiological and morphological parameters of plants exposed to Hg; and (iii) the efficiency of Hg phytostabilization by measuring the concentration of the metal in the soil, roots, and aerial parts of the plants. The germination experiment was conducted in a completely randomized design, with B. decumbens seeds exposed to six doses of Hg (0; 1,2; 7,2; 21,6; 43,2; and 86,4 mg L−1) for 21 days. In a second experiment, conducted in pots containing contaminated soil, B. decumbens plants were subjected to the same doses of Hg (in mg kg−1 of soil) for 150 days, in a randomized block design. The experiments included four replications. The germination of B. decumbens was reduced by exposure to Hg. The parameters of shoot and root length, germination speed index (GSI), mean germination time (MGT), and germination uncertainty (GU) showed no significant differences. The activity of the enzymes superoxide dismutase (SOD), peroxidase (PO), esterase (EST), and malate dehydrogenase (MDH) increased from the Hg dose of 21,6 mg L−1, while catalase (CAT) activity was lower at all doses compared to the control treatment. In the adult phase, the concentration of Hg in the aerial part was not significantly altered, unlike the linear increase observed in the roots of B. decumbens. The bioconcentration (BCF) and translocation (TF) factors indicated that, at the Hg dose of 1,2 mg kg−1, there was greater transport of Hg to the aerial part, while at higher doses, the plants reduced Hg transport to the aerial part. The number of tillers, leaves, fresh and dry shoot mass, and dry root mass were not significantly affected by Hg, except for fresh root mass, which decreased with increasing doses. Among the nutrients, only copper (Cu) in the roots showed a significant reduction due to increasing Hg doses. Chlorophyll content, photosynthetic flux, internal CO2 concentration, transpiration, and carboxylation efficiency decreased with increasing Hg doses. Hg reduces germination by causing oxidative stress and reduces the growth of B. decumbens by affecting the photosynthetic process. B. decumbens demonstrated the ability to develop in the presence of up to 43,2 mg kg−1 of Hg in the soil, showing promise for the revegetation of contaminated soils.