Induction of metal binding compounds and antioxidative defence in callus cultures of two black poplar clones (P.nigra)with different tolerance to cadmium

Callus cultures of two parental clones of Populus nigra L., Poli and 58-861, originating from contrasting environments, were exposed to different cadmium concentrations (0, 150 and 250 lM CdSO4). Clones showed different growth responses to cadmium, evaluated by the tolerance index (Ti), with Poli being more tolerant to the metal at both concentrations. The cadmium concentration at the end of the treatment was very similar between clones at 150 lM CdSO4, while a higher value in 58-861 compared to Poli was detected at 250 lM CdSO4. The bioconcentration factor evidenced the lowest value in Poli at 250 lM CdSO4. Unlike 58-861, cadmium provoked a strong induction of thiols and phytochelatins in clone Poli. In both clones, organic acid concentration differed notably in untreated calli and cadmium treatment induced a general lowering of these compounds. A notably higher antioxidant enzyme activity (ascorbate peroxidase, APX; catalase, CAT; guaiacol peroxidase, GPX) was measured in control calli of clone Poli compared to 58-861. Cadmium induced a remarkable enhancement of APX and CAT, but not GPX, activity at 150 lM CdSO4 in Poli. Conversely, in 58-861 at 150 lM CdSO4, and in both clones at 250 lM CdSO4, a decrease in the antioxidant activity occurred. This investigation provided evidence that these two contrasting genotypes of P. nigra are characterised by a different response to cadmium in callus cultures. In particular, in Poli, the higher tolerance to cadmium is associated with a higher activity of antioxidative enzymes and the ability to strongly increase thiol and PC concentration in response to metal exposure