The role of root organic acids in the tolerance of Festuca rubra to zinc, lead and cadmium

Festuca rubra L. plants are pseudometallophytes colonizing abandoned Pb/Zn mine areas, successfully employed in phytostabilization. To study the contribution of low-molecular weight organic acids to metal tolerance, F. rubra plants were grown for three months in hydroponics with Cd (1.8, 18 and 36 mu mol L-1), Pb (50, 250 and 500 mu mol L-1) and Zn (0.3, 3 and 6 mmol L-1), separately, and in ternary combination (18 mu mol L-1 Cd + 250 mu mol L-1 Pb + 0.3 mmol L-1 Zn). The roots retained most of the metals but their distribution from shoot to root was altered when the plants were treated with the ternary combination. The main organic acids in roots were citrate and malate. At the lowest concentrations, the metals caused small reductions in biomass, had no effects on photosynthetic pigments nor on malondialdehyde, but led to increases in root organic acids. At higher concentrations, phytotoxic responses were observed, associated with a decline of citrate and malate in the roots.