Study On Physiological Changes And Response Mechanisms Of Tea Plants Under Heavy Metal Cadmium Stres

Cadmium has features of difficult degradation,long half-life period,easy migration,etc and shows greater toxicity than other heavy metals within a certain concentration range.Now,excessive cadmium of tea garden is showing a trend of aggravation.Cadmium stress not only has a negative influence on growth of tea trees,but also affects food safety of tea leaves,which is not helpful for healthy development of tea industry.There is few research on physiological change and response mechanism of tea trees under cadmium stress.Therefore,this research took tea seedlings as materials to study the physiological change and molecular regulation mechanism of tea trees under high concentration cadmium stress.The main experimental results are described as follows:1.Tea seedlings showed high resistance for cadmium.When cadmium stress concentration was 160mg/kg and 320mg/kg,tea seedlings h no significant phenotypic change compared with control materials at day 50 of treatment.When cadmium stress concentration was 640mg/kg,tea leaves had obvious necrotic lesions at day 50 of treatment,the growth of lateral root was restrained,and obvious browning happened to both the main root and the lateral root,which indicated that high concentration cadmium would cause serious damage to the growth of tea seedlings.2.The contents of cadmium in root,stem,leaves and other tissues of unprocessed tea seedlings and tea seedlings under 160mg/kg,320mg/kg and640mg/kg cadmium stress treatment for 50 days were measured.The results showed that under different cadmium stress concentrations,the cadmium accumulation degree sequence of the parts of tea seedlings was root > stem > leaf.Compared with control materials,cadmium contents in every part of tea seedlings after cadmium stress treatment significantly increased.Cadmium contents in tea seedlings after 160mg/kg,320mg/kg and 640mg/kg cadmium stress treatment were compared,and the results showed the sequence was 320mg/kg>160mg/kg>640mg/kg.When cadmium stress concentration didn’t exceed the defense threshold of tea seedlings,cadmium content in tea seedlings increased as cadmium content in soil increased.When cadmium stress concentration caused obvious damage to the root of tea seedlings,the absorption and transportation ability of tea seedlings for cadmium reduced,and cadmium content in tea seedlings also reduced accordingly.As the main organ of accumulating cadmium,the root of tea seedlings shielded and buffered the transfer of cadmium from soil to above-ground part.3.When cadmium stress concentration was 160mg/kg and 320mg/kg,SOD,POD,CAT activity and MDA content in seedlings showed an overall trend of increase first and then decrease,which indicated that the antioxidant system in tea seedlings would actively remove the active oxygen in the face of cadmium stress and protect the tea seedlings.When cadmium stress concentration was 640mg/kg,SOD,POD and CAT activity in tea seedlings showed a linear decrease and MDA showed a linear increase.At day 50 of treatment,SOD activity decreased by 26.23% compared with control materials,POD activity decreased by 41.93%,CAT activity decreased by30.95%,and MDA content increased by 107.98% significantly.It indicated that if cadmium stress concentration was too high,the antioxidant system in tea seedlings would break down,SOD,POD and CAT activity showed a linear decrease,and MDA content showed a linear increase.4.Transcriptome sequencing was done for leaves and root of control materials and tea seedlings at day 30,day 40 and day 50 of 640mg/kg cadmium stress treatment.The results showed that the differential genes in leaf comparison were significantly more than the differential genes in root comparison.Compared with control leaves,there were 5,207,4,398 and 5,655 up-regulated differential genes in leaves of tea seedlings at day 30,day 40 and day 50 of 640mg/kg cadmium stress treatment respectively,and there were 4,742,3,275 and 4,635 down-regulated differential genes.Compared with control root,there were 1,430,1,239 and 2,676 up-regulated differential genes in root of tea seedlings at day 30,day 40 and day 50 of 640mg/kg cadmium stress treatment respectively,and there were 1,640,856 and 2,708down-regulated differential genes.5.Differential gene GO functional annotation results showed that photosynthetic system and oxidation-reduction system in tea seedlings would actively participated in psysiological and biochemical reaction of tea seedlings in the face of cadmium stress.According to differential gene KEGG enrichment analysis results,biosynthetic pathway of phenylpropane,biosynthetic pathway of flavonoid,glycometabolism and other approaches could improve the antioxidant ability and chemical defense ability of tea seedlings under cadmium stress and played a positive role for cell structure and function stabilization.Analysis on transcription factor families with differential gene expression under cadmium stress showed that there were many differential genes specifically expressed by four transcription factor families of AP2/ERF,bHLH,MYB and WRKY in both leaves and root.It was speculated that four transcription factor families of AP2/ERF,bHLH,MYB and WRKY played a positive regulation role when tea seedlings faced cadmium stress.6.The candidate genes CSS0044673 and CSS0027444 could significantly improve the tolerance of cadmium-sensitive strain Δycf1 for cadmium,so CSS0044673 and CSS0027444 were expected to improve the tolerance of tea seedlings for cadmium in the face of cadmium stress.