Tolerance Mechanism Of Two Oilseed Rape Species Under Cadmium Stress

With the development of industry and mining, cadmium (Cd) pollution has become an environmental problem that threatens human health. Oilseed rape is a major oil crop widely grown in the world. In recent years, more and more studies showed that oilseed rape can be used as a potential plant resource for phytoremediation of Cd contaminated soil. Thus, further study on the physiological and biochemical mechanism of Cd tolerance in different oilseed rape varieties has important realistic guiding significance for phytoremediation of Cd contaminated soil. In this paper, Brassica napus L., with a widely planting area and high output, and Brassica juncea L., with the strong Cd tolerance, were served as the experimental materials. To investigate the effects of Cd stress on plant growth, characteristics of Cd uptake and accumulation, antioxidant systems and subcellular distribution of Cd, hydroponic experiments were conducted. The mechanisms of Cd tolerance between two oilseed rape species were discussed. The main results were as follows：(1)The effects of Cd stress on the growth and Cd content of oilseed rapeDifferences in Cd content and growth were showed between Brassica napus L.and Brassica juncea L. Under high Cd concentrations (above 50 μM) stress, shoots and roots biomass of Brassica juncea L. were significantly higher than that of Brassica napus L. (P<0.05). The dose-dependent response was found in the root of two species under Cd stress. The exact result showed that the growth of roots was slightly stimulated under low Cd concentration (10 μM Cd), while were severely inhibited under high Cd concentrations (above 25 μM Cd). Compared with the Brassica napus L., the total length and volume of root, number of root hairs were higher in Brassica juncea L. under 200 μM Cd treatment. The Cd content in shoots and roots of Brassica juncea L. were significantly higher than Brassica napus L. Brassica juncea L. exhibited higher Cd transfer coefficient and relative coefficient compared with Brassica napus L. We also found that Cd accumulation in shoot was higher than root of Brassica juncea L. These results showed that the capability of Cd uptake and translocation in Brassica juncea L.was stronger than Brassica napus L. The content of photosynthetic pigments and mineral elements declined in all the two species under high Cd concentration treatment, especially in Brassica napus L. Taken as a whole, Brassica juncea L. showed strong Cd resistance.(2) Oxidative damage and antioxidant systems induced by Cd stressUnder Cd stress, contents of hydrogen peroxide (H2O2), superoxide anion (O2·-) and malonaldehyde (MDA) increased, which indicated oxidative damage in oilseed rapes.Under 50 μM Cd treatment, rapid accumulation of H2O2 and O2·- in Brassica napus L. showed antioxidant systems were insufficient to alleviate the oxidative damage. Compared with Brassica juncea L., Brassica napus L. produced more reactive oxygen species (ROS), showed stronger plasma membrane peroxidation, contained less glutathione(GSH) and ascorbic acid (ASA), lower the AsA/DHA ratios and lower activity of antioxidant under Cd stress. These results indicated that Brassica napus L. was sensitive to Cd, and showed lower antioxidant capacity and Cd tolerance than Brassica juncea L. Furthermore, under Cd stress, contents of non-protein thiols (NPTs) and phytochelatins (PCs) increased in all the two species, especially in Brassica juncea L. These results showed that compared with Brassica napus L., Brassica juncea L. had a higher Cd2+ chelating capability.(3)Subcellular distribution of Cd in two oilseed rape species under Cd stressCadmium absorbed by Brassica napus L. accumulated about 46%-77% in cell walls and about 16%-40% in cytoplasm, and the lowest in cell organ, while more than 50% of Cd in Brassica juncea L. was localized in cytoplasm. Under Cd stress, the subcellular distribution ratio (SDR) in cell walls of shoot tissues declined in all species, especially in Brassica napus L. The SDR of Cd in cytoplasm of Brassica juncea L. was significantly higher than Brassica napus L., while lower in cell wall. Under 200 μM Cd stress, the fluorescent staining for leaves and roots tips showed large amount of Cd in Brassica napus L. distributed in cell wall tissues and intercellular spaces, but in Brassica juncea L. the Cd distributed in every site of intracellular. In conclusion, Brassica napus L. limited Cd accumulation in shoots with the higher SDR in cell wall of roots tissues, while Brassica juncea L. showed a high absorptive capacity and accumulation ability to Cd because of the higher SDR in cytoplasm of roots tissues. Thus, Brassica juncea L. can be used as potential restoration plant for Cd-contaminated farmland.