Cadmium-induced changes in antioxidative systems, hydrogen peroxide content, and differentiation in Scots pine roots

To investigate whether Cd induces common plant defense pathways or unspecif ic necrosis, the temporal sequence of physiological reactions, including hy drogen peroxide (H2O2) production, changes in ascorbate-glutathione-related antioxidant systems, secondary metabolism (peroxidases, phenolics, and lig nification), and developmental changes, was characterized in roots of hydro ponically grown Scots pine (Pinus sylvestris) seedlings. Cd (50 mum, 6 h) i nitially increased superoxide dismutase, inhibited the systems involved in H2O2 removal (glutathione/glutathione reductase, catalase [CAT], and ascorb ate peroxidase [APX]), and caused H2O2 accumulation. Elongation of the root s was completely inhibited within 12 h. After 24 h, glutathione reductase a ctivities recovered to control levels; APX and CAT were stimulated by facto rs of 5.5 and 1.5. Cell death was increased. After 48 h, nonspecific peroxi dases and lignification were increased, and APX and CAT activities were dec reased. Histochemical analysis showed that soluble phenolics accumulated in the cytosol of Cd-treated roots but lignification was confined to newly fo rmed protoxylem elements, which were found in the region of the root tip th at normally constitutes the elongation zone. Roots exposed to 5 mum Cd show ed less pronounced responses and only a small decrease in the elongation ra te. These results suggest that in cells challenged by Cd at concentrations exceeding the detoxification capacity, H2O2 accumulated because of an imbal ance of redox systems. This, in turn, may have triggered the developmental program leading to xylogenesis. In conclusion, Cd did not cause necrotic in jury in root tips but appeared to expedite differentiation, thus leading to accelerated aging.