To investigate whether Cd induces common plant defense pathways or unspecific necrosis, the temporal sequence of physiological reactions, including hydrogen peroxide (H 2 O 2 ) production, changes in ascorbate-glutathione-related antioxidant systems, secondary metabolism (peroxidases, phenolics, and lignification), and developmental changes, was characterized in roots of hydroponically grown Scots pine (Pinus sylvestris) seedlings. Cd (50 m, 6 h) initially increased superoxide dismutase, inhibited the systems involved in H 2 O 2 removal (glutathione/glutathione reductase, catalase [CAT], and ascorbate peroxidase [APX]), and caused H 2 O 2 accumulation. Elongation of the roots was completely inhibited within 12 h. After 24 h, glutathione reductase activities recovered to control levels; APX and CAT were stimulated by factors of 5.5 and 1.5. Cell death was increased. After 48 h, nonspecific peroxidases and lignification were increased, and APX and CAT activities were decreased. Histochemical analysis showed that soluble phenolics accumulated in the cytosol of Cd-treated roots but lignification was confined to newly formed protoxylem elements, which were found in the region of the root tip that normally constitutes the elongation zone. Roots exposed to 5 m Cd showed less pronounced responses and only a small decrease in the elongation rate. These results suggest that in cells challenged by Cd at concentrations exceeding the detoxification capacity, H 2 O 2 accumulated because of an imbalance of redox systems. This, in turn, may have triggered the developmental program leading to xylogenesis. In conclusion, Cd did not cause necrotic injury in root tips but appeared to expedite differentiation, thus leading to accelerated aging.

The aim of the present study was t o investigate the effects of an enhanced CO, concentration alone or in combination with drought stress on antioxidative systems of a deciduous (oak; Quercus robur) and an evergreen (pine; Pinos pinaster) tree species. The seedlings were grown for one season in a greenhouse in tunnels supplied with 350 or 700 p L L-' CO,. The experiment was repeated i n a second year. Antioxidants, protective enzymes, soluble protein, and pigments showed considerable fluctuations in different years. Elevated CO, caused significant reductions in the activities of superoxide dismutases i n both oak and pine. The activities of ascorbate peroxidase and catalase were also reduced in most cases. The activities of dehydroascorbate reductase, monodehydroascorbate radical reductase, glutathione reductase, and guaiacol peroxidase were affected little or not at all by elevated CO,. When the trees were subjected to drought stress by withholding water, the activities of antioxidative enzymes decreased in leaves of pine and oak grown at ambient CO, and increased in plants grown at elevated CO, concentrations. The present results suggest that growth i n elevated CO, might reduce oxidative stress to which leaf tissues are normally exposed and enhance metabolic flexibility t o encounter increased stress by increases in antioxidative capacity.Most predictions suggest that the current mean ambient CO, concentration of 355 pL L-' will approximately double by the end of the next century (Roeckner, 1992). Increasing concentrations of CO, and of other greenhouse gases will result in an increase in mean temperature and cause changes in precipitation patterns (Roeckner, 1992). Climate models predict large-scale drought periods during summer for northern mid-latitudes (Roeckner, 1992). The availability of water is one of the most important factors determining vegetation diversity and plant productivity (Rochefort and Woodward, 1992). The effects of water deficits on plant performance and growth are mediated through decreases in stomatal conductance and photosynthesis and depend on the severity and duration of the drought period, the '

Maritime pine (Pinus pinaster), a drought-avoiding species, contained 2--4-fold lower activities of superoxide dismutase, ascorbate peroxidase, catalase, dehydroascorbate reductase, and glutathione reductase than pendunculate oak (Quercus robur), a drought-tolerant species. The levels of ascorbate, monodehydroascorbate radical reductase activity, and glutathione in pine needles were similar to those in oak leaves. In both species the development of drought stress, characterized by decreasing predawn water potentials, caused gradual reductions in antioxidant protection, increased lipid peroxidation, increased oxidation of ascorbate and glutathione and in pine also significant loss in soluble proteins and carotenoids. These results support the idea that increased drought-tolerance in oak as compared with pine is related to increased biochemical protection at the tissue level. To test the hypothesis that elevated CO(2) ameliorated drought-induced injury, young oak and pine trees acclimated to high CO(2) were subjected to drought stress. Analysis of plots of enzymatic activities and metabolites against predawn water potentials revealed that the drought stress-induced decreases in antioxidant protection and increases in lipid peroxidation were dampened at high CO(2). In pine, protein and pigment degradation were also slowed down. At high CO(2), superoxide dismutase activities increased transiently in drought-stressed trees, but collapsed in pine faster than in oak. These observations suggest that the alleviation of drought-induced injury under elevated CO(2) is related to a higher stability of antioxidative enzymes and an increased responsiveness of SOD to stressful conditions. This ameliorating mechanism existed independently from the effects of elevated CO(2) on plant water relations and is limited within a species-specific metabolic window.

Maritime pine (Pinus pinaster), a drought-avoiding species, contained 2--4-fold lower activities of superoxide dismutase, ascorbate peroxidase, catalase, dehydroascorbate reductase, and glutathione reductase than pendunculate oak (Quercus robur), a drought-tolerant species. The levels of ascorbate, monodehydroascorbate radical reductase activity, and glutathione in pine needles were similar to those in oak leaves. In both species the development of drought stress, characterized by decreasing predawn water potentials, caused gradual reductions in antioxidant protection, increased lipid peroxidation, increased oxidation of ascorbate and glutathione and in pine also significant loss in soluble proteins and carotenoids. These results support the idea that increased drought-tolerance in oak as compared with pine is related to increased biochemical protection at the tissue level. To test the hypothesis that elevated CO(2) ameliorated drought-induced injury, young oak and pine trees acclimated to high CO(2) were subjected to drought stress. Analysis of plots of enzymatic activities and metabolites against predawn water potentials revealed that the drought stress-induced decreases in antioxidant protection and increases in lipid peroxidation were dampened at high CO(2). In pine, protein and pigment degradation were also slowed down. At high CO(2), superoxide dismutase activities increased transiently in drought-stressed trees, but collapsed in pine faster than in oak. These observations suggest that the alleviation of drought-induced injury under elevated CO(2) is related to a higher stability of antioxidative enzymes and an increased responsiveness of SOD to stressful conditions. This ameliorating mechanism existed independently from the effects of elevated CO(2) on plant water relations and is limited within a species-specific metabolic window.

The development of beech leaves ( Fagus sylvatica L.) was characterized by determination of the pigment and electrolyte concentrations as well as the accumulation of dry mass and specific leaf mass from bud break to senescence. To test the hypothesis that stress tolerance and responsiveness of defences show developmental and/or seasonal changes, leaf discs were either incubated in the absence (control) or presence of paraquat to induce oxidative stress. Controls displayed developmental changes in stress susceptibility ranging from less than 15% of maximum electrolyte leakage in mature leaves to more than20% leakage in senescent and 36-46% in immature leaves. Paraquat concentrations were chosen to result in about 95% of maximum electrolyte conductivity within 24 h in all developmental stages. Paraquat accumulation was about two-fold lower in senescent as compared with immature leaves, whereas stress susceptibility, as characterized by the kinetics of the increase in relative leakage, was similar in these developmental stages with 50% of maximum electrolyte conductivity (EC 50 ) = 6·5 h in immature and 7·5 h in senescent leaves. In mature leaves with intermediate paraquat accumulation rates, two classes of stress-sensitivity were distinguished, namely stress-resistant and stress-susceptible leaves with EC 50 = 9·5 and 5·2 h, respectively. Stress-resistance of mature leaves was accompanied by a rapid, approximately two-fold induction of superoxide dismutase activity. Stresssensitive mature leaves initially contained high superoxide dismutase activities but showed a rapid, more than sixfold loss in activity in 24 h. Correlation of meteorological data with leakage rates suggested that high air temperatures and low precipitation might have been predisposing for loss of resistance against oxidative stress in beech leaves.