Relative resistance of and delayed toxicity in undifferentiated plant cells to methyl mercury

Meristematic cells of carrot (Daucus carota L., Ca68-10) and lettuce (Lactuca sativa L., LE-67) cultured in 71-V medium (with 0.15 microgram/ml 2, 4-D) were given one initial dose (0.0001, 0.001, or 0.01 microgram/ml) of methyl mercury (MeHg) at 0 hr, cultured for 288 hr. and then subcultured for 4 weeks. In another experiment, MeHg was added in four daily doses (0.05, 0.10, 0.50, 1.0, 2.5, or 5.0 micrograms/ml) starting at 72 hr for 120 hr and then subcultured for 168 hr. The resulting 50% growth reduction (Gr50) levels were high--1570 and 540 micrograms Hg/g dry wt for carrot and lettuce, respectively. Methyl mercury was taken up completely by cells but retention decreased at higher MeHg concentrations in the medium. The resistance of undifferentiated cells of both species to MeHg was markedly greater than that observed in multicellular plants. Cells derived from Hg-treated cultures did not recover their growth potential when subcultured in Hg-free medium. The Gr50 levels were lowered during successive subculturing in both carrot (50 micrograms Hg/g) and lettuce (10 micrograms Hg/g), indicating increased sensitivity to residual Hg in cells. This effect depended on the initial Hg concentration and on the number of cell divisions. At low MeHg concentrations, it was observed in cells 12 generations after one initial dose of 0.01 microgram/ml MeHg.