Physiological Response Characteristics Of Four Eucalyptus Seedings Under Aluminum Stress And Mitigation Mechanism Of Hydrogen Peroxide Pretreatment

Aluminum（Al）is a neurotoxic agent whose bioavailability and toxicity manifest under acidic conditions.30%of the world’s land area and 50%of its arable land have a p H below 5.5.Eucalyptus is a common fast-growing tree species in tropical and subtropical areas and an important strategic wood fiber raw material in China.At present,aluminum toxicity stress in acid soil is a serious threat to the normal growth of eucalyptus plantations in southern China.Meanwhile,the signal regulation mechanism of eucalyptus adaptation to aluminum toxicity stress is not completely clear.Among them,hydrogen peroxide（H₂O₂）is a key signal molecule that plays an important regulatory role in the response to stress.However,the response of H₂O₂ pretreatment to aluminum stress and its mitigating effect on aluminum tolerance of eucalyptus are not completely clear.Therefore,four kinds of eucalyptus clones commonly used in south China were selected as experimental materials in this study:Three purebred E.grandis,E.urophylla,E.tereticornis,and a hybrid clone E.urophylla×E.Grandis with E.urophylla as the female parent and E.grandis as the male parent.The effects of different concentrations of aluminum stress on the physiological indexes of four eucalyptus species under different time effects,the physiological indexes induced by different concentrations of hydrogen peroxide pretreatment on four eucalyptus species of aluminum toxicity stress,and the alleviating effects of H₂O₂ pretreatment on four eucalyptus species of aluminum toxicity stress and its mechanism were studied.The purpose of this study was to investigate the response of four eucalyptus cultivars to aluminum stress and their antioxidant regulation mechanism in aluminum tolerance,as well as the alleviating effect mechanism of H₂O₂ pretreatment on aluminum toxicity stress of four eucalyptus cultivars.The results provided scientific basis for the study of aluminum tolerance and productivity improvement of eucalyptus under acidic soil in south China.The main results obtained are as follows:（1）There were differences in the response mechanism of aluminum induced by different aluminum tolerant eucalyptus varieties,and various antioxidant indexes of eucalyptus showed repeated fluctuations with the extension of Al stress time.Cluster analysis showed that the four eucalyptus species could be divided into two groups with different regulation mechanisms of aluminum tolerance.The increase of POD,SOD and DPPH activities of E.urophylla and E.urophylla×E.Grandis was significantly higher than that of E.tereticornis and E.grandis under Al stress compared with CK,suggesting that Al stress could stimulate the stronger antioxidant capacity of E.urophylla and E.urophylla×E.Grandis.However,120mg/L aluminum stress is more sensitive to the activation of the antioxidant mechanism of eucalyptus.The reaction mechanism of the antioxidant system in eucalyptus is a cyclic process of first excitation,then equilibrium,and then reactivation.It can be seen that there exists a complex relationship of mutual restriction and balance among various antioxidant substances in the eucalyptus system.（2）H₂O₂ pretreatment alleviated aluminum induced oxidative stress by effectively activating antioxidant enzyme activity of eucalyptus under aluminum stress.However,under aluminum stress,different genotypes of eucalyptus showed different levels of activation of oxidase activity under H₂O₂ pretreatment.2.4mmol/L H₂O₂ pretreatment had a good ability to distinguish the mechanism of antioxidant regulation of four eucalyptus varieties.Among them,2.4mmol/L H₂O₂ pretreatment under aluminum stress could induce the early occurrence of SOD at 3 h in E.grandis and E.urophylla.At the same time,the early burst of As A and GSH was induced at 3 h in E.tereticornis and E.urophylla.In conclusion,H₂O₂ pretreatment under aluminum stress mediates a variety of signal transduction pathways of aluminum tolerance of eucalyptus,and synergies with the dynamic balance among antioxidant indexes to regulate aluminum tolerance of eucalyptus.（3）H₂O₂ pretreatment decreased MDA content and alleviated membrane lipid peroxidation by increasing the activities of ASA-GSH cycling-related antioxidant enzymes of E.grandis and E.tereticornis leaves.Further studies showed that the root tips of E.grandis and E.urophylla×E.Grandis were more sensitive to aluminum toxicity than leaves,and H₂O₂ pretreatment had a stronger alleviating effect on roots.Among them,the antioxidant activity and content of root tips of four eucalyptus species were significantly lower than that of leaves.Pretreatment with hydrogen peroxide could improve the GSH content and the activities of DPPH and FRAP in E.grandis leaves,the MDHAR activity in E.urophylla leaves,the FRAP activity in E.tereticornis leaves,the As A content and the activities of MDHAR and DPPH in E.urophylla×E.grandis leaves under aluminum stress.