The Resistance Mechanism Of Tall Fescue To The Stress Of Lead

At present,the situation of soil environmental problems in China is serious,especially the soil pollution caused by the excessive development and extensive use of lead-containing（Pb）resources.Phytoremediation is considered as an effective technical method for treating heavy metal soil pollution due to its environmental friendliness and no secondary pollution.Tall Fescue（Festuca arundinacea Schreb）has strong environmental adaptability,outstanding stress resistance and castration,and has great potential for repairing heavy metal contaminated soil.In this study,tall fescue was selected as the experimental material.Four concentration gradients of lead-containing solutions were used for hydroponic experiments to study the physiological response characteristics of tall fescue under different concentrations of Pb stress,the damage mechanism of photosynthetic systems and changes in root systems,and The effects of hydrogen peroxide on the root structure of tall fescue under Pb stress are as follows:（1）After 2 weeks of treatment in different concentrations of Pb²⁺solution,the fresh weight of tall fescue significantly decreased,the chlorophyll content also decreased significantly,and showed higher tolerance.The Pb²⁺content in its roots and leaves varied with the stress concentration.As the increase increases,the transfer coefficient gradually decreases.In addition,the physiological characteristics of the plant were also tested.It was found that when the concentration of Pb²⁺in the solution increased,the levels of hydrogen peroxide and superoxide anion in the roots and leaves of the plant increased,and the enzyme activities of SOD,POD and APX decreased.The enzyme activity of CAT and MDA content increased.After staining,it was found that the cell activity decreased and Pb²⁺was mainly distributed in the epidermis,middle pillars and vascular bundles and phloem of roots and leaves.（2）After 2 weeks of treatment in different concentrations of Pb²⁺,φ_P0 and ETR were lower than those in the control group,and obvious K-band and L-band appeared,indicating that the plant was exposed to root and Excessive active oxygen produced in the leaves caused oxidative stress,causing damage to the oxygen release complex（OEC）,damage to the hydrolysis system on the donor side of the electron chain in PS II,and hindering the original electron transfer of the quinone receptor（Q_A）,As a result,a large number of electrons are accumulated at Q_A,and it is also possible that Pb²⁺has replaced Ca²⁺in the oxygen releasing complex composition.It can be seen from the change of the slow fluorescence kinetics index that after being stressed by Pb²⁺,Y（II）and qP decrease,and NPQ and qN increase,that is,the photochemical efficiency gradually decreases,and the non-photochemical efficiency increases.This shows that the opening degree of PS II reaction center decreases,and the efficiency of photochemical activities of leaves becomes lower.This is because the plant has taken self-protection measures,and excreted the extra excitation energy in the form of heat and fluorescence to alleviate photoinhibition or light damage of the plant under Pb stress.（3）After 2 weeks of treatment in different concentrations of Pb²⁺solution,the root length,root surface area,root projection area,root volume,and root tip number of tall fescue decreased with the increase of Pb²⁺stress concentration,and the average diameter decreased with Pb²⁺stress concentration.It may be that Pb²⁺inhibits the division of root cells,resulting in root growth obstruction and destruction of the cell membrane so that the internal solution flows out,so the root system becomes thicker.It can be seen from the changes in the content of root cell wall components that after Pb²⁺stress,cell wall polysaccharides（Pectin,HC1,HC2,and Cellulose）all increase,consistent with the change in the content of Pb²⁺in the root,which is the adsorption of Pb²⁺Providing more binding sites is one of the reasons why tall fescue is resistant to Pb²⁺.After treatment in different concentrations of H₂O₂ solution for 2 weeks,the cell wall components of tall fescue roots showed the same changes as Pb²⁺stress,indicating that the active oxygen concentration also had a recombination effect on tall fescue root components.In addition,the content of carboxyl groups in root cell wall components after H₂O₂ stress was also analyzed,and Pb²⁺solution adsorption experiments were performed.It was found that the amount of Pb²⁺adsorbed by cell wall component was positively correlated with the carboxyl content.In pectin and hemicellulose 2,the H₂O₂ treatment group had higher carboxyl content than the control group,and the amount of Pb²⁺adsorbed was also greater.It indicated that H₂O₂ increased the carboxyl group content in the cell wall of tall fescue root,and also provided more binding sites for Pb²⁺adsorption.