The physiological characters of Chinese cabbage (Brassica rapa L. var. pekinensis Rupr.) seedlings under heat and/or waterlogging stresses

Hot and wet summer in Taiwan limited the production of Chinese cabbage (Brassica rapa L. var. pekinensis Rupr.). Selecting heat- and waterlogging-tolerant Chinese cabbage were needed for current industry. Proper physiological indexes of seedlings could be useful tools to screen heat and/or waterlogging tolerant cultivars. This research used 17 cultivars/lines of Chinese cabbage as materials, conducted the field yield trail in summer and autumn and the physiological index experiments on 21 days seedlings treated with 25°C/20°C, waterlogging at 25°C/20°C, 35°C/30°C, and waterlogging at 35°C/30°C, respectively. Comparing with the head fresh weight in 2014 autumn and 2015 summer trails, ʻChing-Long 200ʼ and ʻYu-Fong ʼcould be regarded as the heat-tolerant cultivars, due to high head fresh weight in summer. Seedlings of these two cultivars also showed significantly lower contents of malondialdehyde (MDA) and hydrogen peroxide (H2O2) than the heat-sensitive cultivars ʻRuey-Hua No. 12ʼ and ʻRuey-Long 720ʼ after 35°C/30°C and waterlogging at 35°C/30°C treatments in seedling stage. The relative ascorbate peroxidase (APX) activity in ʻChing-Long 200ʼ seedlings were higher than the heat sensitive cultivar, in the waterlogging at 35°C/30°C treatment. The relative catalase activity in ʻChing-Long 200ʼ also raised faster than ʻRuey-Long 720ʼ. Inbred lines cc-st 03, cc-st 21, cc-st 23, cc-st 24, cc-st 25, cc-st 26 and cc-st 29 could be regarded as the potential heat-tolerant lines, since they formed head in the summer. Inbred lines cc-st 09 and cc-st 22, which had relatively low head fresh weight in autumn waterlogging trail, chlorophyll b content declined but chlorophyll a/b raised after waterlogging at 25°C/20°C treatment followed by recovery 48 h in the seedling stage. However, the seedling indexes in heat and/or waterlogging treatments, such as the relative dry weight of shoot and root, SPAD, chlorophyll fluorescence Fv/Fm and cell membrane relative injury, did not show significant correlations with the head fresh weight in summer and autumn. In conclusion, the physiological index, relative activity of leaf APX and CAT, chlorophyll a/b, MDA and H2O2 contents, in 21 days Chinese cabbage seedlings were consistent with the head fresh weight in autumn and summer. These physiological indexes might be used to screen heat- and/or waterlogging-tolerant Chinese cabbage lines and shorten the breeding time.