Combined effects of hypoxia and excess Mn2+ on oxidative stress and antioxidant enzymes in tomato seedlings

Effects of high level of Mn²⁺ on the changes in ROS generation, root cell viability, antioxidant enzyme activities, and related gene expression in tomato (Solanum lycopersicum L., cv. Zhongza 9) seedlings were studied under normoxic and hypoxia conditions. Mn²⁺ concentrations, ranged between 10 and 200 ??M, led to significantly higher activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APOD), glutathione reductase (GR), and also ascorbic acid (AsA) content in leaves and roots, improved root cell viability, and decreased O ₂ ^·? accumulation compared with the higher Mn²⁺ level under hypoxia stress, which indicated that low Mn²⁺ could eliminate the active oxygen and protect the membrane lipid from the hypoxia hurt. When the concentration of Mn²⁺ reached 400?C600 ??M under hypoxia stress, the activities of SOD, POD, APOD, and GR and AsA content were decreased remarkably. In contrast, the MDA content was increased at the higher Mn²⁺ concentration. A number of antioxidant-related genes showed high expression at the lower level of Mn²⁺. The expression levels of SOD, POD, CAT, APOD, and GR genes were 7.95, 5.27, 3.18, 5.54, and 8.81 times compared to control, respectively. These results illustrated that the appropriate amount of Mn²⁺ could alleviate the detrimental effects of hypoxia stress, but reversely, the high level of Mn²⁺ just aggravated the existing damage to the tomato seedlings.