Effects Of Different Concentration Of Lead On Photosynthesis Of Ligustrum Lucidum

Soil heavy metal pollution has become one of the global environmental problems.Lead(Pb)pollution not only limits the growth of plants,but also poses a threat to human health through the food chain.Phytoremediation is one of the most effective and important methods to control Pb pollution.Woody plants have attracted great attention in the field of phytoremediation due to their large biomass and long growth cycle.Woody phytoremediation of heavy metal pollution is widely used in practice,but there are few reports on the potential of woody phytoremediation from the perspective of photosynthesis.Ligustrum lucidum(L.lucidum)is an evergreen broad-leaved tree species widely planted in southern China.It has been reported that L.lucidum has the potential of Pb contaminated soil remediation due to its large biomass and strong adaptability to adversity.In this study,2-year-old L.lucidum was selected as the research object and set four Pb pollution concentrations(CK:0;L1:10 mmol/L;L2:50 mmol/L;L3:100 mmol/L).The effects of different concentrations of Pb on photosynthesis of L.lucidum were studied by pot experiment.The changes of structure and function of leaf photosystems(PSⅡ and PSⅠ)were analyzed,and the reasons for the decrease of carbon assimilation rate and the ability of accumulator and translocation of heavy metal Pb in L.lucidum were discussed.In order to clarify the adaptation potential of L.lucidum under excess Pb pollution and provide scientific basis for the screening of tolerant woody plants in Pb polluted areas.The main conclusions are as follows:(1)The distribution of lead in organs of L.lucidum showed that root was the main part of lead accumulation,followed by stem and leaf.The enrichment factor of Pb in the underground part of L.lucidum was less than 0.1,while the enrichment factor of Pb in the underground part of L.lucidum was greater than 1.The enrichment factors of underground part of L.lucidum were 0.618,1.383,1.361 and 1.736 in CK,L1,L2 and L3 treatment groups,and the maximum value appeared in L3 treatment group.It means that with the increase of heavy metal content,it increases,which makes more heavy metals remain in the soil,and the content of heavy metals in leaves increases less,so as to effectively protect the leaves of L.lucidum from Pb stress.(2)The rapid chlorophyll fluorescence kinetic curve of L.lucidum under lead stress showed the characteristics of O-L-K-J-I-P.JIP test parameters showed that there was no significant difference in parameters(VJ、N、M0、K-band、ΨE0 and ΦE0)between L1 and L2 treatment groups on the 15th day compared with CK;only in L3 treatment group,JIP test parameters had significant difference with CK.There was no significant difference in energy allocation parameters of PSI between Pb treatment groups and CK.Energy allocation parameters of PSⅡ:Y(Ⅱ)decreased,Y(NPQ)increased and Y(NO)increased,which indicated that the electron transport chain of L.lucidum under Pb stress was inhibited and the efficiency decreased under severe pollution on 15th day,but the efficiency of electron transport chain was protected by adjusting the energy allocation under mild and moderate pollution.On the 30th day,K-band significantly increased and Φ(E0)、PIABs significantly decreased,indicating that the oxygen evolution complex of L.lucidum leaves was damaged in the electron transport chain;the electron transport was inhibited in the downstream process,and the function of PSⅡ active reaction center was damaged.Pb significantly inhibited the photochemical efficiency of PSI and PSⅡ,significantly changed the energy allocation of PSI and PSⅡ in the photosynthesis of L.lucidum,resulting in a serious imbalance of light energy allocation of PSI and PSⅡ,and more excitation energy was allocated to PSⅡ,which increased the degree of PSⅡ excess energy and aggravated PSⅡphotoinhibition.However,by significantly increasing Y(NPQ)and Y(NO),plants can protect the plants from further damage caused by excessive light energy to a certain extent;(3)Compared with CK,at L1,L2 and L3 treatment groups,Pn decreased by 31.3%,47.7%and 60.6%respectively,while CE decreased by 42.6%,66.8%and 80.4%respectively.CE、Pn、Vcmax、Jmax、Tp、Rp、Cisat and gm of L.lucidum leaves decreased with the increase of lead pollution.Under low and medium concentration of lead pollution,Pn and CE of L.lucidum had no significant decline,which indicated that L.lucidum could protect the process of carbon assimilation and net photosynthetic rate to a certain extent through its own coordination mechanism.Under high concentration of lead pollution,Pn and CE decreased significantly,while Vcmax decreased significantly,which indicated that Rubisco activity decreased,resulting in the decrease of carbon assimilation rate of L lucidum.At the same time,the decrease of gm,Cisat and Rp also indicated that the blockage of CO2 conduction process was one of the reasons for the decrease of carbon assimilation process;(4)The correlation analysis of Pb content with chlorophyll fluorescence parameters and carbon assimilation process parameters showed that Pb significantly affected the photosynthesis of L.lucidum leaves.At the same time,by analyzing the correlation between chlorophyll fluorescence parameters and carbon assimilation process parameters,it is concluded that Pm has a very significant correlation with PIABs and β/α-1,which means that under lead stress,the decrease of PSI activity is significantly related to the damage of PSⅡ and the imbalance of energy distribution between them.Pn and CE were significantly positive correlated with Y(Ⅱ),Vcmax,gm(P<0.05),and significantly negative correlated with β/α-1(P<0.05).In conclusion,the treatment of heavy metal lead inhibited the photosynthesis and CO2 response of L.lucidum leaves,and the inhibition was stronger with the increase of heavy metal lead concentration.However,under mild to moderate pollution conditions,L.lucidum still has high net photosynthetic capacity and carbon assimilation capacity,which can release excess energy through heat dissipation,reduce transport coefficient,avoid heavy metal accumulation in leaves,and change the way of energy distribution between photosystems to protect plant photosynthesis.Therefore,although under Pb polluted areas,L.lucidum grows slowly,it can still be used as a potential species for Pb pollution remediation.