| With the rapid development of industry and agriculture in China,soil contamination with cadmium?Cd?has become an increasing serious environmental issue.The emission of waste gas,water and residues produced by mining,smelting and electroplating process and application of chemical fertilizers,pesticides and sewage sludge have led to the excessive accumulation of Cd in soils.Since it is a non-essential element,excessive amounts of Cd absorbed by plants will induce toxic effects on the plant and also constitute a serious threat to human health through food chain.Therefore,the safe utilization of Cd contaminated soil in line with the food safety and hygiene standards has important practical and theoretical significance.Currently,previous studies have mainly focused on food crop and leaf vegetables,little information is available on root vegetables.Radish?Raphanus sativus L.?,an important annual or biennial root vegetable crop of the Brassicaceae family,has a long plantation history and abundant germplasm resources and its planting area ranks second in all vegetable crops.So in this study,20 radish cultivars were used to evaluate and screen for high-Cd and low-Cd accumulator cultivar under germination experiment,hydroponic experiment and field experiment.Then the two cultivars were used to systematic compare the responses of physiological and biochemical mechanism and the differences of rhizosphere effect.The main results are as follows:?1?Screening for high or low Cd accumulation cultivars20 radish cultivars were used to investigate the responses of seed germination,seedling and growth periods to Cd exposure and screen for high o r low accumulation cultivars using germination experiment,hydroponic experiment and filed experiment.The results of germination experiment indicated seed germination rate was less affected under 0.5 mg/L Cd,while 1.0 mg/L Cd inhibited significantly the germination of most cultivars.Cultivar Pin-4 and 6 were sensitive to Cd and other varieties are relatively stronger to Cd.The results of hydroponic experiment showed that Cd had larger inhibitory effects on shoot growth than root growth.Cd concentrations and accumulation in the root increased obviously with increasing Cd levels and there were significant differences in Cd concentration and accumulation among different radish cultivars.The results of field experiment also showed that Cd concentrations in the root obviously increased with increasing Cd levels and there were significant differences in Cd concentration among different radish cultivars.Considering the results of germination experiment,hydroponic experiment and filed experiment,cultivar 4?H4?,a high-Cd concentration cultivar,and cultivar 19?L19?,a low-Cd concentration cultivar,were chosen to carry out follow-up studies.?2?Differences in photosynthetic and antioxidant system between the high and low Cd cultivars Soil culture experiment of 30 days indicated that:the biomass of L19 was initially increased and then decreased,while that of H4 decreased significantly.Cd concentrations and accumulations in shoot and root of H4 were both higher than for L19 under 1.0 and5.0 mg/kg Cd stress,showing that H4 exhibits a better ability to absorb and accumulate Cd and it is more sensitive to Cd pollution than L19.With increasing Cd level,photosynthetic rate?Pn?,quantum yield of PSII photochemistry(?PSII),the electron transport rate through PSII?ETR?and photochemical quenching?qP;estimating a fraction of“open”PSII centers?were all decreased significantly and non-photochemical quenching?qN;the non-radiative energy dissipation to heat?was increased obviously.The declines in?PSII,ETR and qP were 35%,35%and 18%in H4 and 11%,11%and 13%in L19 and the increase in qN was 36%in H4 and 2%in L19 by the 5.0 mg/kg Cd treatment,respectively,in comparison to the control.These suggest that less light energy utilized by photochemical reactions and the block of electron transport result in the reduction of Pn from Cd exposure in H4.With increasing Cd levels,a significant increase in malonaldehyde?MDA?concentrations,a trend of first increase and then decrease in superoxide dismutase?SOD?and peroxidase?POD?activities and an obvious decrease in ascorbic acid?As A?concentration were observed in the leaf of H4,an obvious decrease in SOD activity and a significant increase in MDA concentration,POD and catalase?CAT?activities were observed in the root of H4.A trend of first increase and then decrease in SOD activity and reduced glutathione?GSH?concentrations,a significant decreased in AsA concentration and an obvious increase in POD activity and MDA concentration were observed in the leaf of L19.A significant decrease in SOD activity and an obvious increase in MDA and POD activities were observed in the root of L19.The increase of MDA concentration was sharper in the leaf and root of H4,exhibiting that H4 experiences a greater oxidative damage than L19.In leaf,POD and CAT activities of L19 were all remarkably higher than for H4 by all the Cd treatments and As A concentration decreased shaper in L19 than H4 with increasing Cd levels.In root,the reduction of SOD was shaper in H4 and H4 exhibited higher GSH and As A concentrations compared with L19.These results suggest that antioxidant enzyme plays a leading role in L19 and GSH-AsA cycle plays a major role in H4.?3?Differences in Cd absorption,transportation and detoxification mechanism between the high and low Cd cultivars Kinetic assays showed that the different Cd accumulation between the two cultivars might be ascribed to root absorption and translocation from root to shoot.The investigations of subcellular distribution verified that most of the Cd was associated with cell walls in the root of H4,representing 46%-49%of the total amount,but the Cd in the root of L19 and leaf of the two cultivars was mainly stored in soluble fraction,comprised44%?51%and 44%?50%,respectively of the total amount,which could be one possible mechanism of tolerance to Cd toxicity.Meanwhile,Cd concentrations of all subcellular fractions in H4 were all higher than in L19.The investigations of chemical forms suggested that Cd fractions extracted by NaCl and HAC were predominant in root and leaf of both cultivars,representing 64.1%and 27.6%of the total amount and the concentrations and proportions extracted by water and 80%ethanol in root and NaCl in leaf were all higher in H4 than in L19.These results indicated that the Cd in H4 was more active than L19,which could be responsible for the sensibility of H4 to Cd damage.?4?Differences in Cd fractionation and enzyme activities in the rhizosphere soil between the high and low Cd cultivars The shoot and root biomasses of H4 were significantly reduced with increasing Cd levels,whereas no obvious changes were observed for L19 compared to their controls respectively.In addition,the root biomass of H4 was significantly higher than for L19.With increasing Cd levels,the pH of the soil from H4 was decreased significantly,while the soil p H from L19 remained similar to the control.The predominant fractions,carbonated Cd?CA-Cd?and exchangeable Cd?EXC-Cd?,comprised 32%?43%and15%?21%,respectively of the total soil Cd at the two treatment levels.The EXC-Cd concentration of soil from H4 was significantly higher than for soil from L19.Enzyme activities were more susceptible to Cd in soil from H4.These results indicated that acidification of the rhizosphere prompted a transformation from a less bioavailable form to more bioavailable forms,together with its high root biomass,potentially affecting the sensitivity of soil enzymes. |
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