Varietal Difference In Cadmium Tolerance And Its Mechanism In Cabbage

The contamination of soils with heavy metals is becoming a major environmental concern with the rapid development of agriculture and industries throughout the world. Cadmium (Cd) is considered to be one of the most toxic pollutants for humans due to its longevity and accumulation in their organs. Due to its high mobility in the soil-plant system, Cd is readily taken up by the roots and transported to other parts of plants. Leafy vegetables are an important component of human diet after cereals in China. They have an ability to take up heavy metals from contaminated soils and transport these metals to the shoots, which may lead to a risk for the health of the people. However, there is little information about varietal difference in Cd tolerance, uptake and accumulation, and which has in some extent limited the developments of its related breeding and cultivation programs to prevent Cd accumulation in crops. The investigation was carried out to study the physiological mechanism of varietal difference in Cd-uptake, accumulation and tolerance of cabbage and detrimental effects of Cd on growth, based on the research of varietal variation of Cd tolerance and Cd concentration, by using four levels of Cd hydroponic experiments. Meanwhile, the possibility of reducing Cd uptake and accumulation in cabbage by application of chemical regulators was also studied in the investigation. The major results were summarized as follows:1. Response of different varieties of cabbage to cadmium stressPot experiment was conduced to investigate the reaction of31cabbage varieties to Cd toxicity. The results showed that these varieties had significant difference in Cd tolerance, uptake and accumulation when plants grew in the soil contaminated100mg·kg-1Cd. Among the varieties, the inhibition percentage of shoots dry weight varied from-15.4%to91.3%. The detrimental effects of Cd on shoots growth exceeded on roots. The range of Cd concentration in roots was from617to1789mg·kg-1and in shoots from47to183mg·kg-1, which roots exceeded shoots. Based on pot experiment,10varieties screened preliminarily as Cd-tolerance and Cd-sensitivity respectively were planted in a solution with50μmo·L-1 Cd. According to plant tolerance to Cd and the capacity to transport Cd from the roots to the shoots, Chunfeng and lufeng were eventually chosen as the Cd resistant and low Cd transportion variety and the Cd sensitive and high Cd transportion variety, respectively, to investigate the Cd tolerance mechanisms in cabbage.2. Effects of Cd on growth and Cd uptake of cabbage seedlingsIn order to study possible Cd resistance mechanisms in cabbage, several parameters on metal uptake and distribution were compared between the variety Chunfeng (Cd-resistance) and Lufeng (Cd-sensitive).The results showed that low Cd (20μM) exposure for both varieties caused a sharp decline (P<0.05) in the root elongation and the dry weight of plant, with a much more severe responsein the Cd-sensitive varieties. Cd taken up by cabbage plant was mainly distributed in roots compared to shoots, and the variety Chunfeng contained significantly lower Cd concentrations in the leaves and higher Cd ones in roots than Lvfeng. About70-74%and66-68%of the Cd taken up by Lvfeng and Chunfeng, respectively, was transported to shoots. The majority of the Cd accumulated in the leaves, stems and roots of both varieties appeared to be bound on the cell walls. The higher Cd resistance in Chunfeng than in Lufeng may be attributed to higher capacity to limit Cd uptake into shoots and to complex Cd on cell walls.3. Effects of Cd on photosynthesis, chlorophyll fluorescence and nutrient uptake of cabbage seedlingsCadmium stress impacted photosynthesis and chlorophyll fluorescence of cabbage plants. Low Cd (20μM) exposure for the Cd-sensitive variety Lufeng caused a sharp decline (P<0.05) in the net photosynthetic rate (Pn), stomatal conductance (Gs), photochemical efficiency of PS Ⅱ (Fv/Fm) and quantum yield of electron flow through PS Ⅱ (OPS Ⅱ) compared to the control. However, these deteriorative effects for the Cd-tolerant variety Chunfeng could be found only at high Cd (50～100μM) levels. At same Cd level, the inhibitions of these parameters were more pronounced in Lufeng than in Chunfeng. Cd stress reduced the concentration of chlorophyll a and chlorophyll b, especially for chlorophyll b, which could be an important factor of photosynthesis inhibition. For these two varieties, Cd stress significantly inhibited the Mn uptake, promoted Zn and S uptake, restrained the transportion of Zn and S (for Lufeng) from roots to shoots, and the decreases of nutrient uptake in lufeng were more obvious than in Chunfeng. Moreover, Cd stress also significantly inhibited Liifeng on Fe and Mg uptake, reduced its Fe and Mg concentration in leaves, but not significant effect on Chunfeng. Cd stress promoted the P uptake by Chunfeng, but opposite results were observed in Lufeng. Therefore, physiologically, the decrease of Mn, Fe, Mg, Zn、Ca and P concentrations in leaves of Lufeng under Cd stress was the key reason for the restraint of leaf photosynthesis, and the decrease of plant growth.4. Effects of Cd on lipid peroxidation and activities of antioxidant capacity of cabbage seedlingsExcess Cd can cause an increased formation of reactive oxygen species (ROS). However, plants have evolved antioxidantive defence systems to keep ROS under control. The concentration of MDA in Lufeng and Chunfeng plants had not been significantly affected2d and4d, respectively, after Cd treatments, suggesting the antioxidantive defence systems played an importance role, mainly SOD and POD activities significantly increased. Apart from GR activity in leaves of Chunfneg increasing, the SOD, POD, CAT, APX and GR in leaves and roots of two varieties all increased firstly and then decreased with extended Cd exposure of time, but the concentration of MDA significantly increased, which indicated that the increased formation of ROS had exceeded the capacity of antioxidantive defence systems to scavenge ROS, resulting in membrane lipid peroxidation. The activities of antioxidant enzymes in Lufeng plants rised sharply within a short time and then declined significantly compared to that in Chunfeng, to the10th day exposed to Cd, the enzymes all were significantly lower than that in Chunfeng, but the concentration of MDA markedly higher than in Chunfeng, indicating that Lufeng was subjected to a greater degree of oxidative stress. Taken together, Cd induces the formation of ROS, resulting in a lipid peroxidation, which was an important mechanism of Cd toxicity to cabbage plants. The difference of antioxidant capacity might partly explain the Cd resistance of Chunfeng and Lufeng.5. Role of non-protein thiols and organic acid in Cd tolerance of cabbageA hydroponic experiment was carried out to examine the role of non-protein thiols and organic acid in Cd tolerance of cabbage. The results showed that compared with the control groups, Cd treatment significantly increased concentrations of non-protein thiol, PCs, citric acids and proline in the leaves and roots of two varieties. The increases were more pronounced in Chunfeng than in Lufeng. Taken together, it is suggested the higher Cd resistance in Chunfeng than in Lvfeng may be attributed to metal binding ligands such as PCs and citric acid, and osmotic adjustment ability. However, the contributions of SH-PCs and citric acid to Cd detoxification might be smaller than that of the cell walls.6. Role of root cell wall in Cd tolerance of cabbagePrevious research has showed that the cell wall of roots played a important role in cabbage resisting Cd toxicity. To further understand the Cd resistance mechanism in cabbage, changes in cell wall properties of roots under Cd treatments were examined. The results showed that Cd stress evidently increased the concentrations of pectin, hemicellulose I and hemicellulose II in the cell wall of Cd-sensitive variety Liifeng and pectin and hemicellulose II in that of Cd-resistant one Chunfeng. The increase was more pronounced in Lufeng than in Chunfeng. The concentration of the uronic acid in pectin, hemicellulose I and hemicellulose II in Chunfeng and in the pectin and hemicellulose II in Lufeng increased markedly under Cd treatments, but not vary much in hemicellulose I in Lufeng. Chunfeng contained significant higher the uronic acid content than Liifeng, which suggested an important role of the uronic acid in Cd tolerance in cabbage. In addition, among the cell wall polysaccharides in two varieties roots, the Cd content in pectin was the highest, suggesting the majority of the Cd accumulated in root cell walls was tightly bound to the pectin. The Cd concentration in pectin of the variety Chunfeng was significantly higher than that of the variety Lufeng. The results suggested the higher Cd resistance in Chunfeng than in Lvfeng may be attributed to Cd-bound on the uronic acid in pectin.7. Sodium nitroprusside (as nitric oxide donator) supplementation ameliorates cadmium-toxicity in cabbageA hydroponic experiment was performed using Cd-sensitive (var. Lufeng) and Cd-tolerant (var. Chunfeng) cabbage seedling to evaluate how different varieties responsed to Cd toxicity in the presence of sodium nitroprusside (SNP), a nitric oxide (NO) donor. Results showed that20-400μM Cd levels inhibited seed germination, and50μM Cd reduced the biomass and chlorophyll concent, with a much more severe response in the Cd-sensitive variety. Cd+SNP treatment restored the Cd-inhibited growth. Exogenous SNP dramatically alleviated Cd toxicity, mainly observed:(a) markedly reduced Cd uptake by plants, obviously diminished Cd accumulation in subcellular fraction of leaves and roots of two varieties,(b) restored the increase of Cd-induced polysaccharides and uronic acid in polysaccharides contents in cell walls of roots,(c) enhanced the activities of antioxidant enzymes and the concentration of antioxidant, whereas decreased the concentrations of malondialdehyde (MDA) and hydrogen peroxide (H2O2), and helped to maintain stability and integrity of membrane system.(d) elevated the increased NP-SH contents, whereas declined slightly in the PCs content, which suggested the GSH may play a important role in protecting cabbage plants from Cd toxicity.(e) promoted nutrients uptake and increased nutrient elements distribution in leavese, and upregulated Cd-induced increase in S content in both varieties, inferring S increment may be involved in NP-SH synthesis;(f) Exogenous SNP reduced the inhibition of Cd on net photosynthetic rate (P n), which might partly be attributed to the NO protection to photosynthetic apparatus such as increasing the chlorophyll content, Fv/Fm, OPS Ⅱ and PSI Ⅰ activity. Taken together, these data suggest that the protection of NO to cabbage seedling against Cd toxicity may be involved in complex signal transduction in plants and cross regulation of physiological and molecular mechanisms.