| Chromium pollution is an important problem to be solved in agricultural production.Chromium can enter the food chain through plants and do all kinds of harm to the human body.Chromium in the environment mainly exists in two oxidation forms:Cr3+and Cr6+.Cr3+ is an essential nutrient element for human beings and animals,but the biological toxicity of Cr6+ is very high.In the natural environment,Cr3+ and Cr6+ are easy to transform each other through redox reaction and then be absorbed by plants,changing the conformation of biological macromolecules in plants,resulting in loss of enzyme activity and membrane lipid peroxidation.At present,we have a certain understanding of the toxic effects of Cr6+ stress on plant growth and development and physiological metabolism,but there is still a lack of in-depth research on the mechanism of Cr6+ tolerance in wheat.Melatonin exists widely in plants and plays an important role in promoting plant growth and development and enhancing plant stress resistance.therefore,it is of great significance to ensure the food security of our country that melatonin alleviates the toxic effect of chromium on plants.In this study,using Yumai51(sensitive type)and Yangmai16(Cr6+ tolerant type)as materials,from seedling growth to wheat grain maturity,the response mechanis1 of wheat to Cr6+ stress was studied systematically from seedling growth and photosynthesis,Cr6+absorption and transport,seedling antioxidant system and yield formation,and the mitigation effect of exogenous melatonin on Cr6+ stress was analyzed.The main research results are as follows:1.The effects of Cr6+on the growth and photosynthetic characteristics of wheat and the mitigation effect of exogenous melatonin were clarified.Compared with the control(CK),Cr6+ stress inhibited the growth of wheat seedlings.which mainly showed that the dry matter accumulation of wheat seedlings decreased,the plants dwarfed,the roots were damaged.The decrease of Yangmail6 was significantly lower than that of Yumai51.and the Cr6+ tolerance of Yangmai16 was stronger than that of Yumai51.The addition of exogenous melatonin significantly increased plant dry weight.plant height and total root length of two wheat varieties under Cr6+ stress.Cr6+ stress significantly decreased the content and ratio of chlorophyll an and chlorophyll b in leaves of two wheat varieties.At the same time,the leaf chlorophyll content of the two wheat varieties increased significantly after the addition of exogenous melatonin.With the extension of Cr6+ treatment time,the net photosynthetic rate(Pn)and stomatal conductance(Gs)of wheat leaves decreased and significantly lower than those of the control,and the intercellular carbon dioxide concentration(Ci)decreased at first and then increased,which indicated that the decrease of net photosynthetic rate of wheat leaves was affected by both stomatal and non-stomatal factors.Actual photochemical efficiency(PSII),maximum photochemical efficiency(Fv/Fm),PSII potential activity(Fv/Fo),photochemical quenching coefficient(qP),electron transfer rate(ETR)and non-photochemical quenching coefficient(NPQ)decreased,indicating that photoinhibition occurred,and the inhibition degree of Yumai51 was higher than that of Yangmai 16.At the same time,the net photosynthetic rate and the above chlorophyll fluorescence parameters increased significantly after the addition of exogenous melatonin.These changes indicate that exogenous melatonin can effectively dissipate non-photosynthetic electron transport energy and reduce photo inhibition in wheat under Cr6+ stress,thus maintaining the operation of photosynthetic system.2.The effects of Cr6+ stress and exogenous melatonin on the absorption and transport of Cr6+in wheat seedlings were investigated.The characteristics of absorption and distribution of Cr in wheat seedlings and the effects of exogenous melatonin were studied by water culture experiment.The results showed that the distribution of Cr.Cr in all organs of wheat plants under Cr6+ stress was root>stem>leaf,indicating that wheat roots had the strongest ability to accumulate Cr.Compared with Cr6+stress,after exogenous melatonin was added at the same time,the Cr content in all organs of wheat seedlings decreased,and the enrichment coefficient of roots,stems and leaves decreased.Compared with the control,under Cr6+ stress alone,the subcellular distribution of Cr in wheat seedlings was F1(cell wall and residue)>F2(organelle and membrane part)>F3(soluble part),indicating that the cell wall had a strong retention effect on Cr.Under Cr6+ stress,the ratio of Cr content to total Cr content in cell wall was increased by adding melatonin at the same time.In addition.Cr6+ stress induced a large amount of total sulfhydryl.non-protein sulfhydryl and chelating peptides in the leaves and roots of the two wheat varieties,and the ability of free movement of Cr in the cytoplasm was limited,thus reducing Cr6+ toxicity and enhancing wheat tolerance to Cr6+.3.The effects of Cr6+ stress on antioxidant system of wheat and the mitigating effect of exogenous melatonin were clarified.Cr6+ stress induced apoptosis in root tip cells of two wheat,and the contents of superoxide anion(O2-),hydrogen peroxide(H2O2)and malondialdehyde(MDA)in wheat leaves and roots increased significantly.The production rate of reactive oxygen species in Yumai51 was significantly higher than that in Yangmai16.At the same time,the addition of exogenous melatonin significantly reduced the production of reactive oxygen species in wheat plants under Cr6+ stress,decreased the level of membrane lipid peroxidation,and enhanced the stability of membrane structure and function.Under Cr6+ stress,the activities of peroxidase(POD),superoxide dismutase(SOD),ascorbate peroxidase(APX),glutathione reductase(GR)and root catalase(CAT)in leaves and roots of the two varieties increased significantly,while the addition of exogenous melatonin significantly decreased the activities of these enzymes and significantly increased the activity of CAT in leaves.In addition,exogenous melatonin significantly decreased the contents of reduced glutathione(GSSH)and dehydroascorbate(DHA),and increased the contents of oxidized glutathione(GSH),ascorbic acid(AsA)and the ratio of GSH/GSSH to AsA/DHA in two wheat varieties under Cr6+ stress.The results showed that exogenous melatonin enhanced the antioxidant capacity of wheat,inhibited the accumulation of reactive ROS under Cr6+ stress,maintained the redox homeostasis and alleviated the oxidative damage of wheat plants under Cr6+ stress by regulating the activities of antioxidant enzymes and increasing the contents of GSH,AsA and other non-enzymatic antioxidants in wheat plants under Cr6+stress.4.To analyze the effect of Cr6+ stress on wheat yield and the mitigation effect of exogenous melatonin.Cr6+ stress decreased the number of grains per spike and 100-grain weight of wheat,resulting in a decrease in wheat yield,which increased with the increase of soil Cr6+concentration.Cr6+ stress inhibited dry matter accumlation after anthesis and significantly decreased the Pn of wheat flag leaves and SPAD.In addition,the actual photochemical efficiency(photochemical PSII)and maximum photochemical efficiency(Fv/Fm)also decreased with the increase of treatment days and Cr6+ concentration.At the same time,the addition of exogenous melatonin alleviated the decrease of photosynthesis,ensured the production of photosvnthates under Cr6+ stress.and increased dry matter accumulation after anthesis.In terms of carbon and nitrogen metabolism,exogenous melatonin increased the contents of sugar and nitrogen and the activities of related metabolic enzymes in flag leaves under Cr6+ stress,indicating that exogenous melatonin was beneficial to maintaining the ability of carbon and nitrogen metabolism in wheat.In addition,Cr is mainly distributed in wheat roots,followed by stems,leaves,spikes,and finally grains.Root enrichment of large amounts of Cr,becomes an important barrier to reduce Cr transport,which is also one of the main reasons why exogenous melatonin alleviates the inhibition of Cr6+ stress on wheat yield.In summary,the growth and photosynthesis of the two wheat varieties were inhibited under Cr6+ stress,and Yumai51 was more inhibited.The addition of exogenous melatonin can reduce the transfer of Cr6+ to the shoot by reducing the absorption of Cr6+and increasing the fixation of Cr6+ in roots,and improve the ability of scavenging reactive oxygen species,thus alleviating the inhibition of photosynthesis and growth of wheat leaves under Cr6+ stress.In addition,exogenous melatonin improved the photosynthetic fluorescence characteristics and carbon and nitrogen metabolism of wheat flag leaves after anthesis under Cr6+stress,reduced the transport of Cr to grains,and finally reduced the inhibition on wheat yield and improved the safety of wheat production. |
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