Study On The Tolerance Mechanism Of Castor Cultivars To Cd And Accumulation To Cd By The Addition Of Organic Acids

With the development of industrial and agricultural production, cadmium (Cd) pollution of soil is becoming increasingly serious. Castor (Ricinus communis L.) is a renewable energy crop with high tolerance and accumulation to Cd, so the use of castor for phytoremediation of Cd contaminated soil is one kind of feasible way. This paper studied that the tolerance and enrichment characteristics of castor cultivars under different Cd treatment stress to screen the Cd tolerance and accumulation castor cultivars, the differences in tolerance and detoxification mechanism of castor cultivars for Cd from oxidation stress, subcellular compartmentalization and subcellular chemical form of Cd and the differences of the organic acids in regulation castor cultivars to accumulate Cd were studied. The results were as follows:1.12 castor cultivars showed that different degree of Cd tolerance, as indicated by the different growth index for response to stress with the treatments of 0,10,20,30,40 and 50 mg/kg Cd. By comprehensive evaluating of relative plant height, root length, chlorophyll content, aboveground biomass and underground biomass, with principal component analysis, subordinative function and cluster analysis,12 castor cultivars were divided into high tolerance and low tolerance of Cd.2. The order of the Cd concentrations in the aboveground parts of castor was stem>leaf. According to the castor cultivars aboveground extraction efficiencies of Cd, and combining with castor cultivars tolerance,12 cultivars can be divided into low tolerance and high accumulation, low tolerance and low accumulation, high tolerance and high accumulation, high tolerance and low accumulation.3. The difference in antioxidant enzyme activity, subcellular distribution and chemical form of Cd for "JinBi-2" (low tolerance and high accumulation), "ZheBi-2" (low tolerance and low accumulation), "TongBi-5" (high tolerance and low accumulation) and "TongBi-6" (high tolerance and high accumulation) led to the distinction in tolerance and accumulation mechanism.The antioxidant enzyme activity of "TongBi-5" and "TongBi-6" (high tolerance) was higher than "JinBi-2" and "ZheBi-2" (low tolerance). With the increase of Cd concentration, SOD, POD and CAT of the high tolerance castor hold the line or elevated, so active oxygen can be cleared timely, while SOD, POD and CAT of the low tolerance castor were inhibited under high Cd concentration stress, so active oxygen could not be cleared timely. With the increase of Cd concentration, the change of SOD, POD and CAT in leaves of high accumulation castor varieties was higher than that in low accumulation castor varieties, while the change of SOD and POD activities in roots was lower than that in low accumulation varieties.The antioxidant enzyme activity in leaf of castor was relatively higher than antioxidant enzymes activity in root.Catalase (CAT) played a leading role in root of castor cultivars to antioxidate stress, superoxide dismutase (SOD) and peroxidase (POD) played a leading role in leaf of castor cultivars to antioxidate defense.4. Subcellular distribution and chemical form of Cd was also the main tolerance and detoxification mechanism of castor. Subcellular content of castor cultivar was listed in descending order,as followsrcell soluble fraction>cell small fraction>cell organelle fraction.The low tolerance of "JinBi-2" and "ZheBi-2" had the higher percentage of Cd in the cell organelle fraction than high tolerance of "TongBi-5" and "TongBi-6", while the high accumulation of "JinBi-2" and "TongBi-6" had the higher percentage of Cd in the cell soluble fraction than the low accumulation of "ZheBi-2" and "TongBi-5". Subcellular chemical forms of Cd in castor are also considered as important factors influencing the characteristics of Cd accumulation and phytotoxicity in different cultivar.The percentage of 80% ethanol-extractable and d-H2O-extractable Cd in the leaves of low tolerance of "JinBi-2" and "ZheBi-2" were predominant, while 2% HAC-extractable Cd were responsible for the adaptation of high tolerance of "TongBi-5" and "TongBi-6" to Cd stress.The percentage of residual Cd in the root of low accumulation of "ZheBi-2" and "TongBi-5"were predominant,while high accumulation of "JinBi-2" and "TongBi-6" had higher percentage of d-H2O-extractable Cd in the root than low accumulation of "ZheBi-2" and "TongBi-5".5. The pot experiment was conducted to study the difference in accumulation and transfer ability of "JinBi-2", "ZheBi-2", "TongBi-5" and "TongBi-6" of Cd through the addition of organic acids. The study showed that low concentration organic acids could improve the ability of castor in accumulation and transfer Cd, the effect of the order was EDTA> CA> TA, but high concentration of organic acids decreased castor’s ability to transport and accumulate Cd.The effect of improving the concentration of Cd in castor leaves and stems were listed in descending order, as follows:EDTA> CA> TA, The effect of improving the concentration of Cd in castor root were listed, as follows:EDTA≈TA> CA.The influence of organic acids on Cd transport and enrichment ability of different castor cultivars were listed in descending order,as follows: "TongBi-6">"JinBi-2">"TongBi-5>"ZheBi-2".6. The differences in biomass and total Cd extraction of "JinBi-2", "ZheBi-2", "TongBi-6" and "TongBi-5" with the addition of organic acid were studied by pot experiment."TongBi-5" and "TongBi-6" showed better EDTA tolerance than "JinBi-2", "ZheBi-2".Low concentration of CA and TA relieved the toxicity of Cd for castor, and enhanced the biomass of castor, especialaly for "TongBi-5" and "TongBi-6". The organic acid could effectively improved the total Cd extraction of different castor cultivars as follows:lmmol/kg EDTA could increased total Cd extraction of "JinBi-2" to 70.82μg/plan,1mmol/kg CA could enhance total Cd extraction of "TongBi-5" to 49.77μg/plan,5mmol/kg EDTA cand improve total Cd extraction of "TongBi-6" to 102.88μg/plan, while the organic acids had little impact on Cd extraction of "ZheBi-2".