Effects Of Zinc Or Iron Treatment On Chloroplast Physiological Characteristics Of Wheat Seedlings

Zinc?Zn?and iron?Fe?are indispensable trace elements for plant growth and development,which they play an extremely important role in regulating the life activities of plants.In recent years,due to the unreasonable use of chemical fertilizers and pesticides,sewage irrigation as well as industrial and mining wastewater discharge,large amount of Zn and Fe has accumulated in the soil,which seriously affects crop yields.As a place for plant photosynthesis and a“nutrient manufacturing space”,chloroplasts provide material basis and energy for plant growth and development.Therefore,the physiological metabolism of chloroplasts exerts a direct impact on crop yields and qualities.In this study,“Xihan 3”wheat?Triticum aestivum L.?seedlings were used as material to analyze and to compare growth and chloroplast physiological characteristics in response to different Zn concentrations(50?mol·L^-1,250?mol·L^-1)or 300?mol·L^-1 Fe alone or Zn-Fe composite(300?mol·L^-1 Fe+50?mol·L^-1 Zn,300?mol·L^-1 Fe+250?mol·L^-1 Zn)treatments,and to explore the physiological responses of plants to Zn and Fe stress as well as alleviative effect of Zn on Fe toxicity,thus providing scientific basis for biological evaluation and treatment of heavy metal pollution.The main findings are as follows:1.50?mol·L^-1 Zn promoted root growth and increased the dry weight?DW?of leaves,250?mol·L^-1 Zn or Fe treatment alone inhibited the root and shoot growth and reduced DW and fresh weight?FW?of roots and leaves.The addition of 50?mol·L^-1 Zn significantly alleviated the inhibition of Fe on root and shoot growth and the reduction of DW and FW,while the addition of 250?mol·L^-1 Zn only relieved the Fe-induced decrease in root DW.2.Different Zn concentrations promoted the accumulation of Zn in wheat seedlings.Fe treatment alone promoted the accumulation of Fe element and reduced Zn content.Compared with Fe treatment alone,the addition of 50?mol·L^-1 or 250?mol·L^-1 Zn significantly reduced Fe level in wheat seedlings.,and increased the amount of Zn.3.50?mol·L^-1 Zn increased the levels of chlorophyll b and lutein,Electron transport rate?ETR?,Net photosynthesis rate?Pn?and Transpiration rate?Tr?,while decreased chlorophyll a/b value;250?mol·L^-1 Zn or Fe treatment alone reduced photosynthetic pigment content,Effective photochemical quantum yield?Y?II??,ETR,Coefficient of photochemical quenching?qP?,Intercellular carbon dioxide concentration?Ci?,but increased Non-photochemical quenching?NPQ?and Quantum yield of regulatory energy dissipation?Y?NPQ??;Moreover,Fe treatment alone also decreased Maximal photochemical efficiency?Fv/Fm?,Pn,Tr,and Stomatal conductance?Gs?.The addition of 50?mol·L^-11 Zn increased the content of carotenoids and lutein,Y?II?,ETR,Tr and Gs in the leaves of seedlings under Fe treatment,but decreased the value of chlorophyll a/b.Besides,the addition of 250?mol·L^-1 Zn increased the xanthophyll content and Tr of seedling leaves.4.The 50?mol·L^-1 Zn reduced the content of chloroplast hydrogen peroxide?H₂O₂?and the activity of chloroplast peroxidase?POD?and chloroplast ascorbate peroxidate?APX?.The 250?mol·L^-1 Zn and Fe treatment alone increased the content of chloroplast MDA?Molondialdehyde,MDA?,the generation rate of superoxideanion(O₂^·?)and the activity of chloroplast superoxide dismutase?SOD?and APX;In addition,Fe also induced an increase in chloroplast H₂O₂ content and POD activity.Compared with Fe treatment alone,the addition of low concentration of Zn significantly inhibited the chloroplast H₂O₂ content and generation rate of chloroplast O₂^·?as well as reduced the amount of MDA and the activities of SOD and APX,while the addition of high-concentration of Zn only relieved the increase of MDA content caused by Fe.5.Compared with the control group,50?mol·L^-1 Zn increased the activity of chloroplast Ca²⁺-ATPase,and 250?mol·L^-1 Zn and Fe treatment alone reduced chloroplast Hill reaction activity,Mg²⁺-ATPase and Ca²⁺-ATPase activities.Besides,the addition of 50?mol·L^-11 Zn significantly alleviated the reaction activity of Fe-induced Hill and the decrease of Ca²⁺-ATPase activity,while the mitigation effect of250?mol·L^-1 Zn was not outstanding.The above research results show that low concentration of Zn could effectively increase the openness of the photosystem II?Photosystem,PSII?reaction center by increasing the synthesis of photosynthetic pigments and chloroplast ATPase activities,improved the electron transfer rate and light energy conversion efficiency to promoted wheat seedling growth.High concentrations of Zn and Fe could induce the accumulation of ROS and MDA content in the chloroplasts of wheat seedlings,which caused damage to the normal structure and function of the chloroplast,and reduced the photosynthetic pigment content,Hill reaction activity,chloroplast ATPase activities and photosynthetic electron transfer efficiency.Besides,most of the light energy absorbed by plants is dissipated in the form of heat energy but not used for photosynthesis,which resulted in the inhibition of photosynthesis.Therefore,the growth of wheat seedlings was inhibited.However,the increase of the chloroplast antioxidant enzyme SOD,POD and APX activities alleviated the accumulation of ROS in chloroplast to a certain extent.Low concentrations of Zn alleviated the decreasing of Fe-induced photosynthetic pigment content and ATPase activities,improved electron transfer rate and light energy conversion efficiency,and reduced the ratio of energy and heat dissipation.photosynthesis was enhanced accordingly,which ultimately relieved the Fe poisoning inhibition of the growth of wheat seedlings;However,the high concentration of Zn had a weakening effect on iron toxicity.In addition,the addition of low concentrations of Zn alleviated the accumulation of chloroplast ROS and MDA under Fe stress,affected the changes of chloroplast antioxidant enzyme activities,and decreased the activities of SOD and APX.