Influence of NaCl and mannitol on peroxidase activity and lipid peroxidation in Centaurea ragusina L. roots and shoots

Centaurea ragusina L. is a Croatian endemic plant species growing on cliffs above the Adriatic Sea, but there is no information about its physiological behavior or stress tolerance. To investigate the response of C. ragusina plants to salinity and drought, we have analysed soluble peroxidase (POD; EC 1.11.1.7) activity, anionic isoperoxidase pattern, levels of malondialdehyde (MDA) and hydrogen peroxide in C. ragusina plants exposed to these stresses. Rooted plantlets grown on MS 1/2 nutrient medium supplemented with mannitol (300 mM) and different concentrations of NaCl (150, 300, 450 or 600 mM) were harvested after 5, 10 and 15 days. Both osmotic treatments significantly increased MDA and hydrogen peroxide contents in C. ragusina shoots after 10 days of stress, while in roots these parameters showed no significant difference compared to control in overall. POD activity of salt-stressed plants changed with respect to different saline treatments and plant organs - in shoots enzymatic activity markedly increased in response to lower saline treatments, especially 300 mM NaCl; otherwise it was similar as in control plants while in roots of plants grown under 450 and 600 mM NaCl it significantly decreased. Drought increased POD activity of both shoots and roots especially after 10 days of experiment. Generally, change in the POD isoenzyme pattern of treated plants was in accordance with the activity change in time. Several POD isoforms (P3, P4 and P9) were specifically induced by salinity and drought.     

Hydrogen peroxide- and nitric oxide-induced systemic antioxidant prime-like activity under NaCl-stress and stress-free conditions in citrus plants

We tested whether pre-treatments of roots with H₂O₂ (10 mM for 8 h) or sodium nitroprusside (SNP; 100 μM for 48 h), a donor of NO, could induce prime antioxidant defense responses in the leaves of citrus plants grown in the absence or presence of 150 mM NaCl for 16 d. Both root pre-treatments increased leaf superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) activities, and induced related-isoform(s) expression under non-NaCl-stress conditions. When followed by salinity, certain enzymatic activities also exhibited an up-regulation in response to H₂O₂ or SNP pre-exposure. An NaCl-stress-provoked decrease in the ascorbate redox state was partially prevented by both pre-treatments, whereas the glutathione redox state under normal and NaCl-stress conditions was increased by SNP. Real-time imaging of NO production was found in vascular tissues and epidermal cells. Furthermore, NaCl-induced inhibition in OH scavenging activity and promotion of OH-mediated DNA strand cleavage was partially prevented by SNP. Moreover, NaCl-dependent protein oxidation (carbonylation) was totally reversed by both pre-treatments as revealed by quantitative assay and protein blotting analysis. These results provide strong evidence that H₂O₂ and NO elicit long-lasting systemic primer-like antioxidant activity in citrus plants under physiological and NaCl-stress conditions.     

Hydrolyzable tannins as “quantitative defenses”: Limited impact against Lymantria dispar caterpillars on hybrid poplar

The high levels of tannins in many tree leaves are believed to cause decreased insect performance, but few controlled studies have been done. This study tested the hypothesis that higher foliar tannin levels produce higher concentrations of semiquinone radicals (from tannin oxidation) in caterpillar midguts, and that elevated levels of radicals are associated with increased oxidative stress in midgut tissues and decreased larval performance. The tannin-free leaves of hybrid poplar (Populus tremula × P. alba) were treated with hydrolyzable tannins, producing concentrations of 0%, 7.5% or 15% dry weight, and fed to Lymantria dispar caterpillars. As expected, larvae that ingested control leaves contained no measurable semiquinone radicals in the midgut, those that ingested 7.5% hydrolyzable tannin contained low levels of semiquinone radicals, and those that ingested 15% tannin contained greatly increased levels of semiquinone radicals. Ingested hydrolyzable tannins were also partially hydrolyzed in the midgut. However, increased levels of semiquinone radicals in the midgut were not associated with oxidative stress in midgut tissues. Instead, it appears that tannin consumption was associated with increased metabolic costs, as measured by the decreased efficiency of conversion of digested matter to body mass (ECD). Decreased ECD, in turn, decreased the overall efficiency of conversion of ingested matter to body mass (ECI). Contrary to our hypothesis, L. dispar larvae were able to maintain similar growth rates across all tannin treatment levels, in part, because of compensatory feeding. We conclude that hydrolyzable tannins act as “quantitative defenses” in the sense that high levels appear to be necessary to increase levels of semiquinone radicals in the midguts of caterpillars. However, these putative resistance factors are not sufficient to decrease the performance of tannin-tolerant caterpillars such as L. dispar.