Physiological Response Of Winter Oilseed Rape Seedling (Brassica Napus L.) To Waterlogging Stress

Waterlogging stress is one of the major factors limiting crop growth and foodstuff production all over the world. Oilseed rape is the second biggest crop in Yangtze River. Since the frequency and intensity of waterlogging, especially in spring and autumn, is significantly increased in the recent years, which is severely influenced the oilseed growth and rapeseed production and introduced disease. Many researches showed that exogenous application of abscisic acid (ABA) and brassionsteroid (BR) could regulate the responses and increase the resistance of plants to various abiotic stresses, such as cold, drought and waterlogging. However, the influences and basic mechanisms of exogenous application of ABA and BR on winter oilseed rape growth and physiological responses to waterlogging are still unclear. Therefore, a series of pot experiments were conducted to investigate the influences of waterlogging on the growth and physiological properties of winter oilseed rape (Brassica napus L.), and the effects and basic mechanisms of the exogenous application of ABA and BR on the responses of winter oilseed rape to waterlogging stress. The objects of the present study were: (1) the influences of different waterlogging time on winter oilseed rape seedlings growth and physiological properties; (2) the influences of exogenous application of ABA and BR on winter oilseed rape seedlings growth and physiological properties under waterlogging stress; (3) the effects of exogenous application of ABA on the active oxygen metabolism of winter oilseed rape under waterlogging stress. The main results were shown as following:1. The influences of waterlogging on the growth and physiological properties of winter oilseed rape (Brassica napus L. cv. Zhongshuang 9 and GHO1) were studied through a pot experiment. Results showed that the growth and physiological properties, such as relative growth rate, root activity (RA), soluble sugar content, malondialdehyde (MDA) content, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity of winter oilseed rape seedlings were significantly increased until the 12^th day of waterlogging time, nitrate reductase activity reached the maximum at the 6^th day, and then significantly decreased with experiment time. While, the chlorophyll contents were significantly decreased with experiment time. These results indicated that waterlogging stress significantly influenced the growth of both shoots and roots of winter oilseed rape seedlings. Plants growth was maintained under short-term waterlogging stress through regulated their metabolisms by themselves, but significantly suppressed by long-term waterlogging stress.2. Exogenous application of ABA (75.67μmol/L) and BR (0.21μmol/L) significantly alleviated the injures of waterlogging stress and significantly increased the dry matter accumulation of winter oilseed rape (Brassica napus L. cv. Zhongshuang 9 and GHO1) seedlings, which might be due to exogenous application of ABA and BR alleviated chlorophyll degradation and increased the photosynthetic rate, increased the antioxidant enzymes (SOD, POD and CAT) activities, decreased the MDA content and the peroxidation of membrane lipids, and increased the soluble sugar contents. These results indicated that exogenous application of ABA and BR could alleviate the negative influence of waterlogging stress on winter oilseed rape seedlings growth, especially75.67μmol/LABA treatment at which the plant growth was increased by 23.58% compared with CK.3. Results showed that ABA alleviated waterlogging damage in winter oilseed rape via participatiing in the process of reactive oxygen species metabolism. The synthetic rate of O₂^- and MDA content were significantly increased in winter oilseed rape (Brassica napus L. cv. Zhongshuang 9 and Zhongyouza 12) seedlings roots and leaves compared with control under waterlogging stress, which indicated that waterlogging stress caused lipid peroxidation damage in plant cell membrane. While exogenous application of 50 mmol/LABA biosynthesis inhibitor sodium tungstate significantly decreased the synthetic rate of O₂^- and increased the MDA content of winter oilseed rape seedlings roots and leaves. That may be because ABA synthesis inhibitor sodium tungstate can decrease the activities of antioxidant enzymes (SOD, POD and CAT), so that too much ROS participate in the membrane lipid peroxidation. On the contrary, exogenous application of 113 mmol/LABA significantly decreased MDA content of winter oilseed rape seedlings roots and leaves, and increased synthetic rate of O₂^-, which was correlated with increasing activities of antioxidant enzymes (SOD, POD and CAT), decreaseing membrane lipid peroxidation and inducing ROS generation by exogenous ABA. These results indicated that, ABA enhanced the resistance of waterlogging tolerance by participating in the process of ROS metabolism in winter oilseed rape seedlings under waterlogging stress, while the mechanisms were unclear.