Effects of Salicylic Acid on Oxidative Stress and Thermotolerance in Tobacco

Tobacco (Nicotiana tabacum L.) plants grown in vitro for 4 weeks on medium containing 10 μmol/L salicylic acid (SA) displayed enhanced tolerance of a 4.5 h heat-shock at 49°C. However, 100 μmol/L SA did not enhance thermotolerance, and caused reduced shoot growth and leaf epidermal cell size. Endogenous glucosylated SA content was enhanced in shoots of plants grown on 10 or 100 μmol/L SA, while free SA was enhanced in those on 100 μmol/L SA. Shoot H2O2 content increased, while catalase activity declined, as the SA concentration in the medium increased. Both SA concentrations increased shoot glutathione reductase and dehydroascorbate reductase activity, although there was no significant effect on ascorbate peroxidase. SA at 100 μmol/L also increased monodehydroascorbate reductase activity. The ascorbate redox-ratio AA/(DHA+AA) was maintained in shoots of plants on 10 μmol/L SA, despite increases in ascorbate and dehydroascorbate levels. However, on 100 μmol/L SA, the ascorbate redox-ratio decreased by over 40 % due to a 300 % increase in dehydroascorbate. The glutathione redox-ratio GSH/(GSH+GSSG) was maintained in shoots of plants on either SA concentration, despite increases in GSH and GSSG levels.