Screening of In Vitro Heavy Metal Tolerance in Tocoyena brasiliensis Mart. (Rubiaceae)

Zinc (Zn: 0-400 mg L^-1 zinc acetate), lead (Pb: 0-400 mg L^-1 lead acetate), and cadmium (Cd: 0-8 mg L^-1 cadmium chloride) tolerance in stem explants of Tocoyena brasiliensis Mart. from seeds collected in the Brazilian Cerrado were studied under controlled conditions. The explants showed a regular growth activity in a metal-free medium. All metals did not affect explant survival, except for 400 mg L^-1 Zn, which resulted in lethality. Shoot number was not affected by metal treatment, while shoot length and leaf number varied depending on the metal. Cd induced a gradual reduction in leaf number without affecting shoot length. Pb gradually reduced the shoot length at concentrations beyond 200 mg L^-1, while no effects were found in Zn concentrations from 0 to 200 mg L^-1. Pb and Zn induced leaf production at 50 mg L^-1, while a gradual reduction was observed with increasing concentration. Callus formation was not affected by Cd, while increasing Zn and Pb concentrations reduced this cell division and organization with Zn, showing drastic effects. Altogether, T. brasiliensis explants demonstrated high tolerance to Cd and Pb. However, further studies are needed to explore the phytoextraction capacity of this species at in vitro and planta levels.