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

Abstract

As soil pollution by mercury (Hg) is a serious socio-environmental problem, we assessed the ability of Brachiaria decumbens Stapf. (Syn. Urochloa decumbens) 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 B. decumbens 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, B. decumbens 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 B. decumbens can be used for the revegetation of Hg-polluted soils with Hg concentrations of up to 43.2 mg kg-1.