| In recent years,with the acceleration of urbanization and industrialization,heavy metal content in soil exceeds the standard,which seriously affects vegetable safety production and human health.The cadmium(Cd)contaminated area is up to 200,000 hectares,accounting for 7.0%of the contaminated soil in China.Chinese cabbage(Brassica pekinensis)originated in China and is the most widely distributed leaf vegetable with the largest planting area in China.Therefore,it is of practical significance to take Chinese cabbage as the breakthrough point to carry out research on the prevention and control of cadmium pollution in vegetables and the safe production of vegetables.Based on the screening results of our research group,this experiment studied the effects of cadmium stress on the physiological and biochemical characteristics of the plants of the high cadmium accumulation variety "Beichen" and the low cadmium accumulation variety "Jinfeng",and analyzed the relative expression of antioxidant enzyme genes and genes related to cadmium transport accumulation in the tissues of the two kinds of Chinese cabbage.In order to provide a theoretical basis for the safety of vegetable production,the reasons for the difference in response of high and low accumulation Chinese cabbage varieties to cadmium stress were explored at the molecular level.The main research results are as follows:(1)The effects of cadmium stress on the biomass and cadmium accumulation of high and low accumulation Chinese cabbage: although Cd stress can inhibit the growth of Chinese cabbage,the responses of high and low accumulation Chinese cabbage varieties are different under different Cd stress levels.when the cadmium stress level is 1.0 mg/kg,the growth rate and biomass accumulation of two Chinese cabbages are inhibited.Under three cadmium stress levels of 0.5.0mg/kg,1.0 mg/kg and 5.0 mg/kg,the Cd accumulation in the shoot of Beichen was 88.2%,81.9%and 58.9% higher than that of Jin Feng,and the Cd accumulation in the root was 80.8%,83.8% and60% higher respectively Compared with the low accumulation variety "Jin Feng",the high accumulation variety "Beichen" has stronger Cd enrichment ability.(2)Effects of Cd stress on chlorophyll content and photosynthetic respiratory system of high and low accumulation Chinese cabbage: Chlorophyll content in leaves of high accumulation Chinese cabbage "Beichen" and low accumulation Chinese cabbage "Jin Feng" decreased with the increase of Cd treatment concentration.Chlorophyll synthesis in leaf cells of the two varieties was seriously disturbed under high concentration of Cd stress,which affected plant photosynthesis.From the inhibition rate of chlorophyll content of the two varieties,it can be seen that photosynthetic respiratory system of leaf cells of high accumulation Chinese cabbage "Beichen".Under Cd s Tress,the net photosynthetic rate(Pn),stomatal conductance(Gs)and transpiration rate(tr)of Chinese cabbage varieties decreased with the increase of Cd treatment concentration,while the intercellular CO2 concentration(Ci)of the two varieties showed an upward trend,indicating that the limitation of Cd treatment on photosynthesis of Chinese cabbage was caused by non-stomatal limitation,which led to the destruction of photosynthetic apparatus,thus inhibiting the enzyme activity in the dark reaction process and reducing the photosynthesis intensity.Under certain cadmium stress,the photochemical dissipation(P)of low-accumulation varieties was significantly inhibited,and the antenna heat dissipation(D)of high-accumulation varieties had no significant difference compared with the control,but both varieties remained above 70%,indicating that the light energy absorbed by PSII antenna pigment in leaves of both varieties was mainly dissipated in the form of heat,thus weakening the excessive excitation energy of reaction centers.(3)Effects of Cadmium Stress on MDA Content and Antioxidant Enzyme System in Chinese Cabbage with High and Low Accumulation of Cd The MDA activity in Chinese Cabbage with high and low accumulation of Cd increased with the increase of Cd stress.The MDA content of Jinfeng is always significantly higher than that of Beichen,indicating that Jinfeng’s membrane damage is more serious.The levels of peroxidase(POD),catalase(CAT),ascorbate peroxidase(APX),dehydroascorbate reductase(DHAR),and glutathione reductase(GR)in the leaves of low-accumulation varieties The activity was significantly higher than that of the high accumulation variety "Beichen" under various stress levels.In addition,the activities of CAT,APX,GR and DHAR in low-accumulation cabbage "Jinfeng" only under 5.0 mg/kg cadmium stress are significantly different from those of the control,indicating that the low-accumulation variety "Jinfeng" has an active antioxidant system and ascorbic acid The glutathione system makes it less susceptible to cadmium poisoning.Beichen maintains certain activities of SOD,POD,CAT,APX,and DHAR under 5.0 mg/kg cadmium stress,indicating that Beichen also has certain ability to tolerate cadmium stress.(4)Comparison of relative expression of antioxidant enzyme genes between high and low accumulation Chinese cabbages under Cd stress: With the increase of Cd stress time,Chinese cabbages with different Cd accumulation ability have great differences in gene expression due to different mechanisms of resisting Cd stress.The high accumulation variety "Beichen" still had higher relative gene expression of SOD and DHAR after the seventh day of cadmium stress,indicating that the high expression of SOD and DHAR genes in Beichen is one of the reasons for the cadmium tolerance of Beichen cabbage.The low accumulation variety "Jinfeng" can still maintain a certain relative expression level of APX gene after the fifth and seventh days of cadmium stress.It shows that the strong resistance to cadmium stress of variety Jinfeng is due to the higher expression of APX gene in Jinfeng tissue.Cadmium stress inhibited the expression of antioxidant enzyme-related genes(SOD,APX,GR and DHAR)in the tissues of low-accumulation varieties,indicating that low-accumulation varieties were more susceptible to cadmium at the level of antioxidant enzyme gene expression.On the seventh day of cadmium stress The inhibition rate of SOD,GR,and DHAR can be seen that the expression of antioxidant enzyme-related genes in high accumulation varieties has little influence.(5)Comparison of relative expression of cadmium transport and accumulation related genes between high and low accumulation Chinese cabbage varieties under cadmium stress: The related genes of NRAMP gene family,HMA gene family and ZIP gene family(NRAMP2.1,NRAMP4.2,HMA2,HMA4 and ZIP)play an important role in regulating cadmium accumulation ability of high and low accumulation Chinese cabbage varieties.With the increase of Cd stress time,the relative expression of NRAMP2.1 and HMA4 genes in the tissues of low accumulation Chinese cabbage varieties were down-regulated to some extent.The relative expressions of NRAMP2.1 and HMA4 genes in the tissues of high accumulation Chinese cabbage varieties were up-regulated to a certain extent,and the relative expressions of NRAMP4.2 and ZIP genes in the tissues of the two Chinese cabbages were up-regulated to varying degrees,while the relative expressions of HMA2 genes remained basically stable.The relative expression of five genes in the tissues of high accumulation varieties was significantly higher than that of Jinfeng.From the perspective of gene expression level,this may be the fundamental reason that more Cd was absorbed by high accumulation varieties and transported to their aboveground accumulation. |
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