Abstract: In order to explore the alkali-resistant mechanism of Rhododendron，the seedlings of the three Rhododendron cultivars，‘Yanzhimi’，‘Hongshanhu’and‘Hongyue’were treated with mixed NaHCO3 and Na2CO3 for 2 months. The morphological changes of the seedlings under alkali stress were observed，and ROS and antioxidant systems in the chloroplasts，mitochondria and cytosol of the leaves were determined. The results showed that the alkali tolerance of the three cultivars from high to low was ‘Yanzhimi’>‘Hongshanhu’>‘Hongyue’according to the alkali damage index. Under alkali stress，the H2O2 and superoxide anion levels in the three subcellular fractions of the three cultivars were significantly increased，which resulted in the increase in MDA content. There were differences in the activity of antioxidant enzymes among the three cultivars under alkali stress. SOD，POD，CAT and GR activities in the three subcellular fractions of‘Yanzhimi’were markedly increased after alkali stress，and the significant increase of APX activity in the chloroplasts and cytosol was observed in the chloroplasts and cytosol，but not in the mitochondria. Under alkali stress，the activities of CAT and APX in the chloroplasts，SOD，POD and CAT in the mitochondria and SOD，POD，and CAT in the cytosol of‘Hongshanhu’were significantly increased. SOD activity of‘Hongyue’increased significantly in the mitochondria and cytosol，CAT activity increased significantly in chloroplasts and mitochondria，POD and APX activities decreased significantly，GR activity did not change significantly. The contents of non-enzymatic antioxidants，AsA and GSH in the chloroplasts of the three cultivars decreased，and increased in the mitochondria and cytosols under alkali stress. Under alkali stress，the contents of non-enzymatic antioxidants AsA and GSH in the three Rhododendron cultivars decreased in chloroplasts and increased in mitochondria and cytosol.‘Yanzhimi’with the stronger tolerance maintained relatively more stable ROS level and lower MDA content and higher SOD，POD，APX and GR activities than the other two cultivars under alkali stress. ROS and antioxidant systems in the three subcellular fractions，chloroplasts，mitochondria and cytosol were determined and found that the levels of H2O2 and superoxide anion in the cytosol were significantly higher than that in the mitochondria and chloroplasts，corresponding antioxidant enzymes SOD，POD and GR and antioxidants AsA and GSH were mainly distributed in the cytosol，subcellular distribution of ROS and antioxidant systems had higher consistency. APX was mainly distributed in the chloroplasts. CAT was the most active in the mitochondria. The results indicated that ROS accumulation in the different subcellular fractions and their peroxidative damage degree varied under alkali stress，and the responses of the antioxidant defense systems in each subcellular fraction to alkali stress were also different. Cytosol was the main subcellular fraction where ROS were generated and removed.

Key words: Rhododendron, alkali stress, subcellular distribution, antioxidant systems