The Resistance Mechanism Of Tall Fescue To Heavy Metal Cd²⁺ Stress

The plant tolerance to Cadmium?Cd?is one of the important issue in phytoremediation.Some turfgrass species showed high tolerance to soil Cd contamination and great potential in phytoremediation.In our study,the accumulation characteristics of Cadmium?Cd?and toxic effects of Cd²⁺on the growth and physiological characteristics of tall fescue?Festuca arundinacea Schreb?were studied.Moreover,the role of NO in Cd²⁺detoxification of tall fescue was proved,and the resistance of tall fescue to Cd²⁺stress increased with exogenous NO application.The results were summarized as follows.?1?After 1,5,50,150 mg/L Cd²⁺treatment for 14 days under hydroponic conditions,the cadmium contents in leaves were 0.026,0.08,0.18,0.73 mg/g DW,respectively.And the cadmium contents in roots were 0.54,1.57,5.78,25.73 mg/g DW,respectively.The cadmium transport coefficients were 0.048,0.051,0.031,and 0.028,respectively.Tall fescue showed a good cadmium tolerance,and the contents of Cd in roots and leaves of tall fescue increased significantly with the increase of Cd treatment.In addition,the toxic effects of Cd²⁺on the physiological characteristics of tall fescue were also studied.The biomass and chlorophyll contents of tall fescue decreased,and the production of reactive oxygen species resulted in oxidative stress,MDA contents of tall fescue increased lead to lipid peroxidation,under high concentration of Cd treatment.At 5 mg/L Cd²⁺treatment,a large number of cell nuclei were stained in the roots of tall fescue by PI,but we did not find this phenomenon in the control group.It indicated that the integrity of the root cells membranes of tall fescue were destroyed,root cells appeared apoptosis.?2?After different concentrations of Cd²⁺14 day's treatment,the?_P0 of tall fescue decreased with the increase of Cd²⁺treatment concentration,which was consistent with the trend of?P0 of tall fescue under heat stress.F0 and?RC decreased with the increase of Cd²⁺treatment,indicating that PS II reaction center of tall fescue were inactivated after being affected by Cd²⁺stress.The K-band and L-band amplitudes at5,50,and 150 mg/L Cd²⁺treatment were significantly higher than those in the control group,indicated that at 5,50,150 mg/L Cd²⁺treatment,electron transfers from the oxygen release complex?OEC?to tyrosine?Yz,D1-Tyr161?were inhibited,and OEC was damaged due to Cd stress.These were due to the increase of ROS content in tall fescue leaves and the replacement of Ca²⁺by Cd²⁺in the oxygen releasing complex after Cd stress.PI_ABS,PI_total and ETR decreased with the increase cadmium treatment because of the inhibition of PS II electron donor side activity after Cd²⁺stress.The change of slow fluorescence kinetics of tall fescue showed that the photochemical quantum efficiency Y?II?,the photochemical quenching qP and qL of PS II decreased with the increase Cd treatment.It indicated that the proportion of photon used in photochemical reactions and the photochemical reactivity of photosynthesis system decreased with the increased Cd treatment.In addition,non-photochemical quenching NPQ and qN increased under the stress of cadmium,indicating that the self-protection mechanism of tall fescue.In JIP-test,we also found that the transfer efficiency from the secondary ruthenium electron acceptor?Q_B?to the PS I electron acceptor increased with the increase of cadmium treatment concentration(?_R0),it also indicated that the self-protection mechanism of tall fescue.However,electrons transferred to the secondary ruthenium electron acceptor?Q_B?were still only a small fraction of the electrons absorbed by the PS II antenna,so it cannot alleviate the toxicity of cadmium.?3?Effects of the Cd²⁺and NO treatments on the tall fescue growth and physiological characteristics were studied under the condition of soil culture.Tall Fescue showed high cadmium tolerance under Cd stress,and the addition of exogenous NO could improve the Cd tolerance of tall fescue.Tall fescue has similar properties to Indian mustard with super accumulation of cadmium,the biomass reduction was similar under Cd stress,and the transfer coefficient increased with the increase of Cd treatment,indicated that tall fescue has ideal plant characteristics for phytoremediation of heavy metal pollution,and the addition of exogenous NO could increase the dry weight and fresh weight of tall fescue.The addition of exogenous NO could protect plants from the toxicity of heavy metals.The addition of exogenous NO could also increase the contents of chlorophyll in tall fescue and the germination rates of seeds.But the addition of L-NAME will decrease the chlorophyll contents of tall fescue.These reasons were exogenous NO can alleviate the toxicity of Cd²⁺by scavenging reactive oxygen species?ROS?,increasing mineral element absorption,regulating cell membrane proton pumps,and maintaining hormone balance in plants.In addition,the addition of SNP reduced the content of cadmium in the shoots of tall fescue under the treatment of 50 and 300 mg/L Cd²⁺.In summary,it was sure that NO plays an important role in regulating the absorption and transport of Cd²⁺in tall fescue.The toxicity of Cd to the tall fescue photosynthetic system was mainly on the PS II electron donor side.The electron acceptor side of PS II has more resistant to heavy metal toxicity,and exogenous NO can improve the tolerance of PS II electron donor side to heavy metal toxicity.After adding exogenous NO,the amplitude of K-band and L-band were lower than that of Control and the addition of L-NAME under 300mg/L Cd treatment,indicated that NO has a protective effect on photosynthesis reaction center.The addition of exogenous nitric oxide increased the value of the maximum quantum efficiency?_P0 of PS II and reduced the light energy absorbed by the unit reaction center ASB/RC,indicated NO has a protective effect on the PSII electron donor side of tall fescue.