Effects of cadmium and lead stress on somatic embryogenesis of coniferous species. Part II: Changes of thiol substances

Conifers are often used as biomarkers of industrial pollution; however, little is known about the effects of heavy metals on them because only a few species have been tested. The aim of this work was to investigate the effects of cadmium (Cd2+) and lead (Pb2+) at three different concentrations (50, 250, and 500 mu M) on the detoxification potential of Abies alba and Picea abies embryogenic cell masses throughout the 21-day proliferation period. Embryogenic cell masses of A. alba and P. abies responded to treatment with cadmium and lead by inducing phytochelatins and their biosynthetic intermediates. With increasing heavy metal concentrations, glutathione was used for the synthesis of phytochelatins enabling the tissues to bind to heavy metal ions and thereby avoiding the production of reactive oxygen species. Lead in A. alba and cadmium in both species caused similar increases of all antioxidative thiol compounds; thus, similar mechanisms involving a heavy metal-induced stress response can be assumed. In P. abies, the lowest lead concentration tested provoked the highest antioxidative response. Since a very low uptake of lead into the tissue was observed, the higher resistance of P. abies can be attributed to its ability to reduce lead uptake after longer exposure times. The results of cadmium treatment of both species and lead treatment of A. alba indicated the possibility of testing these coniferous species as potential phytoremediators. This is the first study to analyze the effects of heavy metals on the low-molecular-weight plant thiol content in A. alba embryogenic cell masses.