Identification Of The Copper Resistant Genotypes And The Antioxidant Protection Mechanism In Cotton

At present,soil copper pollution has drawn more and more attentions.Cotton has the characteristics of large biomass production and strong resistance to abiotic stress,and its main economic product is fiber.Copper accumulating in cotton plants will not be enriched in human body through eating,so planting cotton in copper-contaminated land is of great significance for sustainable use of farmland.In this experiment,forty three cotton varieties（lines）were evaluated for copper tolerance and accumulation abilities at the seedling stage,and the indicators suitable for identification of copper tolerance genotype were determined,which provided basic materials for the study of copper tolerance mechanism in cotton.The growth parameters and antioxidant metabolism related indices in cotton varieties（lines1318,1413,1415 and 2304）treated with 0（CK）,50,100 and 200μM copper were measured and analyzed.The subcellular distribution and chemical forms of copper in cotton cultivars 1415 and 2304 treated with 100μM copper were determined and analyzed,and the copper tolerance mechanism of cotton seedlings under copper stress was preliminarily explored.the results of the study are as following:1、Four representative varieties（lines）were screened:1413 and 2216,2304,1415 and1318.Three copper tolerance indexes were identified:total root length,root branch number and root surface area.2、The toxic effects of copper stress on cotton seedlings were evidenced as copper stress significantly inhibited the growth and development of cotton seedlings,and the growth parameters were decreased significantly.Copper stress resulted in the accumulation of Hydrogen peroxide（H₂O₂）and Superoxide anion（O²·-）in roots and leaves of cotton seedlings,and the lipid peroxidation of cell membrane and the increase of Malondialdehyde（MDA）and Pro contents.With the increase of copper concentration,the toxic degree of cotton seedling also was deepened.Copper stress activated the antioxidant system in cotton seedlings,and the activities of Superoxide dismutase（SOD）and Peroxidase（POD）in cotton seedling leaves were increased significantly and consistently with the increase of copper concentration.The activities of Monodehydroascorbate Reductase（MDHAR）and Dehydroascorbate Reductase（DHAR）were increased at the lower Cu concentration,but decreased at the higher Cu concentration compared to the control.Copper stress inhibited Catalase（CAT）,Ascorbate Peroxidase（APX）and Glutathione Reductase（GR）activities.Ascorbate Acid（As A）content increased in a copper dose dependent way（significantly higher than that of control）.The change trend of Reduced Glutathione（GSH）content was similar to that of GR activity,both of which firstly decreased and then increased with the increase of copper ion concentration.The activities of SOD,POD and CAT in cotton seedling roots increased significantly under copper stress compared with the control.They were originally increased and then decreased with the increase of copper ion concentration.The activity of APX increased in parallel to the increase of copper ion concentration.Copper stress inhibited GR activity,while MDHAR and DHAR activities were enhanced at the lower Cu level,but declined at the higher Cu level.The contents of As A and GSH were first increased and then decreased.The leaves and roots of cotton seedlings consistently maintained a higher GSH/GSSSG ratio under different concentrations of copper.3、The tolerance advantage of copper tolerant cultivars over copper sensitive ones in response to copper stress is elucidated by the lower copper uptake amount,higher SOD,POD,CAT and APX activities in the former,which can contribute to increased elimination of reactive oxygen species.The higher GR activity promoted the regeneration of GSH,which was conducive to maintain a higher GSH/GSSG ratio and a higher As A content,and in turn led to the scavenging of reactive oxygen species and the improvement of the resistant ability of cells against oxidative stress.4、Most of the copper in the roots of cotton seedlings accumulate in the cell walls,which can effectively block the transport of copper ion into the aboveground parts.A small amount of copper ion move along the stem and enter the leaf.Most of the copper ion in the leaf are sequestered in the vacuole,and the others are deposited in the cell wall.Only a small amount of copper may be accumulated in the organelles.In cotton roots the most chemical forms of copper is Cu in inorganic forms followed by Cu in water soluble organic forms and copper oxalate.In the case of leaf blade copper in inorganic forms and residues were predominant.Independent of either roots or leaves,less phytotoxic Cu chemical forms other than inorganic and water soluble forms represented more than 50%indicating a potential tolerance mechanism underlying cotton seedlings in response to copper toxicity.