The Effects Of Lead-Zinc Combined Stress On Photosystem Performance In Ligustrum Lucidum Leaves

Heavy metal pollution in soil is one of the most serious environmental problems in the world.At present,excessive exploitation of lead and zinc mineral resources in China has caused serious problem.Lead and zinc pollution poisons plant photosynthesis.Woody plants are excellent candidates for phytoremediation of heavy metal contaminated soils due to their well-developed roots,rapid growth and large above-ground biomass.Previous studies have found that Ligustrum lucidum(L.lucidum)has strong resistance to stress.However,little research has been done on the effects of heavy metals on its photosystems.L.lucidum,an evergreen broad-leaved garden tree species in southern China,was selected as the object in this study.Four concentration gradients(CK:0:100%,L1:10%:90%,L2:25%:75%,L3:50%:50%)were mixed with the non-polluted soil for indoor pot experiment according to the mass ratio.The response characteristics of photosystem ?(PS?)and photosystem I(PSI)of L.lucidum leaves to lead-zinc combined stress were studied under different concentration treatments,and the damage mechanism of photosystem was analyzed.The results provide theoretical support for the further study of the physiological adaptability of photosynthetic system of L.lucidum to heavy metal stress and for the screening of tolerant woody plants in lead and zinc polluted areas.The main conclusions as below:1.Compared with the control,Fo increased significantly and Fm decreased significantly.PS? operating efficiency(?PS?),PS? maximum efficiency(Fv'/Fm"),electron transport rate ETR was significantly lower than that of the control,and the light saturation point appeared in the light response curves of ETR at all treatments on the 75th day,indicating that photoinhibition and inactivation of PSII reaction center in leaves,and there was an imbalance of excitation energy between PSII reaction center and antenna chlorophyll.2.The K point and I-P phase ampiltude increased with the prolongation of stress time and the increase of concentration.The values of normalized total complementary area above the OJIP transient(Sm)and turnover number of QA reduction events(N)decreased significantly compared with the control,while the values of approximated initial slope(Mo)and time to reach Fm(tFm)increased significantly.This indicated that electron transfer in PSII donor side and receptor side of L.lucidum leaves has been significantly inhibited.The absorbed light energy per unit reaction center(ABS/RC),the dissipated heat energy per unit reaction center(DIO/RC)and the energy captured per unit reaction center(TRO/RC)increased significantly,but the electron transfer per unit reaction center(ETO/RC)decreased significantly compared with the control.The value of maximum quantum yield(?PO)and probability that a trapped exciton(?EO)decreased significantly,the value of quantum yield for energy dissipation(?DO)and efficiency with which an electron can move from the reduced intersystem electron(?RO)increased significantly.This means that the ability to primary reaction in PSII gradually decreased after trapping light energy,the heat dissipation in PSII increased,and electron transfer from QA-to downstream was much lower than that of PSII donor side.3.The absorption of light energy by PSI was L1>L2>L3.Maximum decrease in slope of MR/MRO(VPS?),maximum increase in slope of MR/MRO(VPS?+PS?),activity of PS?(VPS?)decreased significantly with the increase of treatment concentration,and all showed CK>L1>L2>L3.This indicated that the rate of redox of PSI reaction center(P700)was slowed down after lead-zinc combined stress,while the photosynthetic activity of PSI was gradually weakened,which reduced the redox rate of plastodquinol(PHQ2).The rate of redox of P700 and plastocyanin(PC)from PSII was slowed down.At the same time,it hindered the process of transferring electrons from the end of PSII receptor to PSI.As a result,the connectivity of electron transfer decreased between two photosystems.4.The contents of lead(Pb)and zinc(Zn)in the aboveground and underground parts were negatively correlated with the photosystem performance parameters.In summary,Pb and Zn increased the energy absorbed by the inactive reaction centers of PSII and decreased the energy absorbed by the active reaction centers,thus affecting the primary photochemical reaction and reducing the performance of PSII.At the same time,the weakening of redox capacity of PSI reaction center results in the decrease of PSI performance,which hindered the electron transfer process between them and ultimately destroyed the coordination between the two photosystems.