Subcellular Localization and Responses of Antioxidant Systems of Three Rhododendron Cultivars under Salt Stress

Effects of NaCl stress on reactive oxygen species (ROS) and the antioxidant systems in the mitochondria, cytosol and chloroplasts of the three Rhododendron cultivars were studied for elucidating the response mechanism of the antioxidant systems in Rhododendron to salinity stress at the subcellular level. According to the morphological damage index, the order of the resistance to salt stress was as follows: 'Yanzhi Mi' > 'Hong Shan Hu' > 'Hong Yue'. ROS including hydrogen peroxide (H_2O_2) and superoxide radicals (O_2 ^<・–>) was mainly located in the cytosol, and the level in the mitochondria and chloroplasts was lower, and the differences of the two subcellular compartments were not significantly. Salinity stress induced the increase of production rate of O_2 ^ <・–> and H2O2 level in three subcellular compartments and resulted in the accumulation of malondialdehyde (MDA). MDA in the three subcellular compartments of 'Yanzhi Mi' leaves did not change significantly under salt stress, which further indicated that 'Yanzhi Mi' had better salt tolerance. The activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD) and glutathione reductase (GR), and the level of non–enzymatic antioxidants including ascorbate (AsA) and glutathione (GSH) were higher in the cytosol of the three cultivars than the other two subcellular compartments. Ascorbate peroxidase (APX) in the chloroplasts and catalase (CAT) in the mitochondria were highest. With the exception of CAT, other antioxidants were enhanced to varying degrees under salinity stress. The results indicated that the salt–sensitive cultivars were more severely damaged by peroxidation under salt stress, which caused the stress response of ROS–scavenging system, especially the significant increase of SOD and POD in salt–sensitive rhododendrons. But this did not effectively alleviate the peroxidation damage under salt stress, which may be related to the decrease of CAT activity, and also indicated the importance of CAT activity in salt tolerance of rhododendron.