Cadmium Tolerance Evaluation And Physiological Mechanisms Of Chrysanthemum In Responce To Cadmium Stress

Chrysanthemum（Chrysanthemum morifolium）,as one of the ten most famous in China and one of the four most popular cut flowers around the world,finds a wide use in potted plants,cut flowers and gardening.Potted mum has the traits of high ornamental value,stress tolerance,vigorous growth,high branching ability,rich flowers,long flowering period,which is commonly used in landscaping.The study used a series of ’Zhong shan’chrysanthemum varieties as materials,established a synthetic evaluation system for assessing the cadmium resistance of 17 chrysanthemum varieties.The evaluation system was established under comprehensive evaluation with the application of Principal Component Analysis and Subordinate Function Value Method.The tolerance to cadmium was graded using Hierarchical Clustering Analysis.Selected Cd-tolerant ’Zhongshanjinkui’and Cd-sensitive ’Zhongshandangui’ were used to investigate the effect of Cd stress on the growth,metabolism,membrane lipid peroxidation,antioxidant system,photosynthesis,accumulation,transportation and distribution of Cd.We also studied the effects of exogenous β-Aminobutyric Acid（BABA）on alleviating cadmium toxicity in chrysanthemum.The main results are as follows:1.Cadmium tolerance evaluation of 17 varieties of chrysanthemumSeventeen ’Zhong shan’ series varieties of chrysanthemum was subjected to hydroponic culture in 1/2 Hogland nutrient solution supplemented with 200μmol·L^<sup>-1 CdCl2.Eight indexes,i.e.,ratio of injured leaf area,plant height,root length,overground fresh weight,underground fresh weight,overground dry weight,underground dry weight,and relative electrolyte conductivity were measured.The evaluation system was established under comprehensive evaluation with the application of Principal Component Analysis and Subordinate Function Value Method.The tolerance to cadmium was graded using Hierarchical Clustering Analysis.Eight evaluating factors closely related to cadmium resistance were selected and the weight of each factor was calculated.According to the rating criteria,17 varieties were endowed with different scores and then multiplied the weight of each factor to obtain the comprehensive score of each variety.The higher the score is,the more is resistant to cadmium stress.Finally,by Hierarchical Clustering Analysis,using the clustering center value of synthetical scores,17 varieties were clustered into 4 grades:high tolerant,tolerant,low tolerant and sensitive.The synthetic evaluation system can be used to evaluate and classify cadmium tolerance of ’Zhong shan’ varieties of chrysanthemum.Cadmium tolerance was different among varieties.In present study,one high cadmium tolerant,3 cadmium tolerant,8 low cadmium tolerant varieties were selected,while the remaining 5 varieties were cadmium sensitive.2.Effects of different concentrations of Cd and BABA treatment on physiological characteristics of chrysanthemumTo illustrate whether the toxicity of cadmium to chrysanthemum on the growth and physiological metabolism could be reduced or not by exogenous β-aminobutyric acid（BABA）,chrysanthemum ’Zhongshanj inkui’（Cd-tolerant）and ’Zhongshandangui’（Cd-sensitive）were subjected to hydroponic culture in 1/2 Hogland nutrient solution supplemented with low concentration Cd(50μmol·L^<sup>-1),high concentration Cd(200μmol·L^<sup>-1)and 200μmol·L^<sup>-1 Cd with 200μmol·L^<sup>-1 BABA.The apparent morphology and biomass,Cd accumulation,transportation and distribution,and leaf physiological indexes such as relative electrolyte conductivity,antioxidase activities,contents of MDA,chlorophyll,GSH,proline and soluble protein contents were measured.The results showed that Cd stress inhibited growth,biomass and chlorophyll synthesis,destroyed the integrity of the cell membrane,and induced membrane lipid peroxidation damage in both varieties.The toxicity of Cd increased with higher Cd concentration and longer duration.’Zhongshanjinkui’ suffered less toxicity than ’Zhongshandangui’.Leaf SOD activities of both varieties first rose followed by a thereafter decrease under 50μmol·L^<sup>-1 Cd stress,while declined with time period under 200μmol·L^<sup>-1 Cd Stress.POD activities first rose then fell under both concentrations of Cd,’Zhongshanjinkui’ has a higher POD activity than that of ’Zhongshandangui’.The contents of GSH,proline and soluble protein of’Zhongshanjinkui’ increased more than ’Zhongshandangui’ under Cd stress.Cd accumulated mainly in the underground part and the ratio of Cd in aboveground and underground part increased at higher Cd treatment(200μmol·L^<sup>-1)in both varieties,but’Zhongshanjinkui’ accumulated more Cd and the ability of transfering Cd to aboveground is higher than ’Zhongshandangui’.Accumulation assay of Cd in different leaf positions showed that Cd in ’Zhongshanjinkui’ mainly distributed in base mature leaves,while distributed evenly in ’Zhongshandangui’.BABA improved the growth,biomass and photosystheses of chrysanthemum under Cd stress.Meanwhile,BABA reduced the increase of MDA content and relative conductivity in leaves,but no increase in proline and soluble proteins were observed with the presence of BABA.It is concluded that Cd-tolerant variety’Zhongshanjinkui’ could quickly synthesis a large number of GSH and osmotic adjustment substances including proline and soluble protein,and can alleviate the toxicity of Cd stress by improving effective osmotic regulation,antioxidant system and Cd distribution.BABA accelerated the absorption of Cd in both varieties and promoted transportation of Cd from root to aboveground,but has no effect on distribution of Cd in different leaf positions,inferring application of BABA could contribute to the phytoremediating of cadmium contaminated soil by chrysanthemum.