<i>Brachiaria decumbens</i> for phytostabilization of mercury-polluted soils: from seed germination to plant growth

As soil pollution by mercury (Hg) is a serious socio-environmental problem, we assessed the ability of <i>Brachiaria decumbens</i> Stapf. (Syn. <i>Urochloa decumbens</i>) cv. Basilisk to germinate and grow under high Hg exposure. Germination tests were conducted in germination boxes for 21 days, while the long-term effects of Hg on the growth of <i>B. decumbens</i> were evaluated after cultivating plants for 150 days in pots containing Hg-polluted soil. Seed germination decreased with increasing Hg exposure. However, more than 50% of the seeds still germinated under very high Hg exposure (86.4 mg L⁻¹). We postulate that malate dehydrogenase and esterase (important in generating energy and mobilizing seed reserves), as well as superoxide dismutase and peroxidase (mitigating cellular Hg-induced oxidative stress) considerably contribute to this. Plant growth was severely restricted at a Hg concentration of 86.4 mg kg⁻¹, ultimately leading to plant death. At lower Hg concentrations, <i>B. decumbens</i> stored most of the Hg in its roots, with limited translocation to the shoots. Nevertheless, photosynthesis was impaired, although nutrient content was generally not significantly affected by Hg exposure. Overall, our findings suggest that <i>B. decumbens</i> can be used for the revegetation of Hg-polluted soils with Hg concentrations of up to 43.2 mg kg⁻¹. Our findings indicate that the activities of malate dehydrogenase (MDH) and esterase (EST) enzymes were reduced only under conditions of very high Hg exposure (86.4 mg L⁻¹), suggesting that the Hg-induced impairment on seed germination probably is linked to other processes than energy generation and reserve mobilization. Moreover, our results show that seed germination is not a limiting factor for using <i>Brachiaria decumbens</i> to vegetate Hg-polluted soils, and this species can be used for the revegetation of Hg-polluted soils with Hg concentrations of up to 43.2 mg kg⁻¹. These findings are important to better understand the mechanisms related to the impairment Hg-induced on seed germination and growth of species suitable for Hg phytostabilization.