Differential effects of elevated ozone on two hybrid aspen genotypes predisposed to chronic ozone fumigation. Role of ethylene and salicylic acid.

The role of ethylene (ET) signaling in the responses of two hybrid aspen (Populus tremula L. x P. tremuloides Michx.) clones to chronic ozone (O(3); 75 nL L(-1)) was investigated. The hormonal responses differed between the clones; the O(3)-sensitive clone 51 had higher ET evolution than the tolerant clone 200 during the exposure, whereas the free salicylic acid concentration in clone 200 was higher than in clone 51. The cellular redox status, measured as glutathione redox balance, did not differ between the clones suggesting that the O(3) lesions were not a result of deficient antioxidative capacity. The buildup of salicylic acid during chronic O(3) exposure might have prevented the up-regulation of ET biosynthesis in clone 200. Blocking of ET perception with 1-methylcyclopropene protected both clones from the decrease in net photosynthesis during chronic exposure to O(3). After a pretreatment with low O(3) for 9 d, an acute 1.5-fold O(3) elevation caused necrosis in the O(3)-sensitive clone 51, which increased substantially when ET perception was blocked. The results suggest that in hybrid aspen, ET signaling had a dual role depending on the severity of the stress. ET accelerated leaf senescence under low O(3), but under acute O(3) elevation, ET signaling seemed to be required for protection from necrotic cell death.

Characterisation of antioxidative systems in the ectomycorrhiza-building basidiomycete Paxillus involutus (Bartsch) Fr. and its reaction to cadmium.

The effect of different cadmium (Cd) concentrations (5, 50 and 500 microM) on growth, Cd accumulation and antioxidative systems was studied in Paxillus involutus, grown in liquid medium. Cd was rapidly accumulated by P. involutus and resulted in growth inhibition within 24 h. Antioxidative enzymes (superoxide dismutase (SOD), EC 1.15.1.1; catalase (CAT), EC 1.11.1.6; monodehydroascorbate radical reductase (MDAR), EC 1.6.5.4; dehydroascorbate reductase (DAR) glutathione reductase (GR), EC 1.8.1.7 and glutathione-dependent peroxidase (GPx), EC 1.11.1.9) were active in the investigated fungus. Furthermore, high concentrations of glutathione but no ascorbate were detected. Cd exposure resulted in a significant induction of SOD activity. However, activities of enzymes responsible for the detoxification of H2O2 showed no Cd-dependent increase or were only transiently induced (CAT, GPx) and no accumulation of H2O2 was detected. Exposure to low Cd concentrations (5 and 50 microM) caused an increase in GR, while 500 microM Cd led to an inhibition of GR and CAT. Increased glutathione concentrations were observed as a consequence of all Cd treatments. These results suggest that the antioxidative protection of the investigated strain of P. involutus was sufficient to avoid Cd-mediated oxidative stress. It is likely that this strain was able to detoxify high concentrations of Cd by transport of Cd into the vacuole because a high correlation between Cd and sulphur in the vacuole was detected by EDX.