| Zinc(Zn)is an essential micronutrient element for plant growth.It has important functions such as regulating photosynthesis and participating in chlorophyll synthesis.Photosynthesis is the process by which green plants use sunlight to synthesize foods from carbon dioxide and water,and is sensitive to heavy metal stress.Excessive Zn not only causes toxicity to plants,but also inhibits photosynthesis of plants,destroys physiological processes,and hinders plant growth and development.In recent years,woody plants have attracted great attention and been widely used in phytoremediation because of their large biomass and long growth cyclic,which are the leading functions of vegetation restoration.How the photosynthesis process of woody plants grown in soil contaminated by heavy metal Zn responds to Zn stress,and the mechanism of adaptation to Zn stress remains unclear.For this purpose,we selected two-year-old seedlings of Melia azedarach,common in the subtropical and native tree species in Hunan province with strong ecological adaptability,potted under different concentrations of ZnCl2 solution(L1<L2<L3)in the greenhouse.The response mechanism of the photosynthetic process of M.azedarach under Zn stress were studied,including photosynthetic electron flow transmission pathway,photosystem activity and the expression patterns of related genes.Furthermore,we elucidated the adaptive mechanism of photosynthesis to Zn stress from the aspects of photoprotective such as heat dissipation,cyclic electron flow and antioxidant level.This study aims to reveal the adaptability of tree growth under heavy metal stress from the perspective of photosynthesis,and provide theoretical basis for the application of heavy metal pollution remediation woody plants.The main results are as follows:(1)After 30 days of Zn treatments,the growth and chlorophyll content of M.azedarach seedlings were not significantly different from the control(CK);with prolonged stress,the growth and chlorophyll content of M.azedarach seedlings were significantly affected after the 60 days of Zn treatments,and the decreasing trend was significant.The growth and chlorophyll content were negatively correlated with Zn uptake and accumulation in M.azedarach seedlings,respectively.The Zn subcellular distribution in leaf showed that after 30 days of Zn treatments,Zn was localized in cell wall component(F1)and soluble component(F4);after the 60 days of Zn treatments,a large amount of Zn was migrated in chloroplast component(F2),which had a significant effect on chlorophyll a,chlorophyll b and carotenoid content of M.azedarach seedlings.(2)The gas exchange parameters of M.azedarach seedlings were not significantly affected when compared CK after 30 days of Zn treatments,but changed significantly after long-term Zn stress treatment.After 60 days of Zn treatments,the net photosynthetic rate(Pn),stomatal conductance(Gs),stomatal restriction(Ls)and carboxylation efficiency(CE)of the three different Zn treatment levels decreased by decreased by 16.62-57.78%,41.67-67.21%,14.46-36.71%and 22.78-59.52%,respectively compared with the CK,but intercellular CO2 concentration(Ci)increased by 15.92-26.83%,these results indicating that the stomatal and non-stomata factors were the main reason for the decrease of photosynthetic rate of caused M.azedarach seedlings under Zn stress.Stomatal conductance limitation(SL),mesophyll conductance limitation(MCL)and biochemical limitation(BL)are three factors that limit the net photosynthetic rate.The three factors changed with the concentration and time of Zn treatment.After 30 days of Zn treatments,the photosynthesis was less restricted by SL,MCL and.BL,but after 60 days of Zn stress,SL is the main factor limiting the net photosynthetic rate at the concentration of L1 and L2,while at the concentration of L3,BL becomes the main factor limiting the photosynthetic rate.(3)M.azedarach seedlings was suffered photodamage by Zn stress.Although after 30 days of Zn treatments,the electron transport rates(ETR Ⅰ and Ⅱ),photochemical efficiency(Fv/Fm)and photochemical quenching(qP)of M.azedarach seedlings leaves decreased significantly in L3 treatment compared with the CK,but PS Ⅰ and PSⅡ were not significantly change when compared to the CK,indicating that the photosynthetic apparatus was not significantly affected;After 60 days of Zn treatments,the photosynthetic system performance,electron transport rates,light distribution,energy balance and allocation were significantly inhibited.The results of related functional proteins and genes expression on the thylakoid membrane showed that after 30 days of Zn treatments,membrane proteins D1,D2,PsbO,Lhcb1,PsaA and ATP-βand related coding genes PsbA,PsbD,PsbO,Lhcb-1,PsaA,ATP-β was not significantly affected by Zn stress when compared with CK.However,the expression level of proteins and genes were decreased significantly after 60 days of Zn treatments.The above results indicate that Zn has significantly impacted on the PSⅡ and PSⅠ reaction centers,and PS Ⅰ was more affected by Zn.Zn impedes the transport of photosynthetic electrons,the balance of light energy distribution is broken,eventually photosynthesis is inhibited.(4)After 30 days of Zn treatments,only the content of H2O2 in the ROS increased significantly compared with the CK,but the content of malondialdehyde(MDA)had no significant difference compared with the CK,indicating that H2O2 might be act as a signal molecule to adaptively regulate the physiological function of M.azedarach seedlings;however,with stress prolong,the "ROS burst" appeared in leaves of M.azedarach seedlings,and the content of MDA also increased significantly,at this time ROS act as the main toxic molecule to cause oxidative damage to the photosystem.The results showed that the activities of superoxide dismutase(SOD),peroxidase(POD),catalase(CAT),ascorbate peroxidase(APX)and glutathione reductase(GR)under Zn stress were significantly higher than those in the CK.The results indicated that M.azedarach seedlings could tolerate the oxidative stress induced by Zn by upregulation of the antioxidant system.However,the activities of SOD and GR decreased,but the activities of POD,CAT and GR continued to increase under high concentration of Zn stress,indicating that the antioxidant system can still play the ROS scavenging function under high concentration of Zn stress.(5)In order to prevent excessive reduction of electron transport chain,the heat dissipation(NPQ)and cyclic electron transfer(CEF)were increased.Although the NPQ and CEF were decreased unnder high concentration of Zn stress after 60 days of Zn treatments,resulting in photodamage,higher NPQ and CEF were beneficial to maintain photosynthesis under Zn stress,which is an important defense mechanism of photosynthetic apparatus in M.azedarach to adapt to Zn stress.(6)The NPQ,CEF and antioxidant systems were activated to protect the photosynthesis process of M.azedarach under Zn stress.These protective mechanisms can reduce the degree of damage and improve the adaptability to Zn stress.The structural equation model(SEM)fitting shows that NPQ and CEF have a higher protective effect on the photochemical activity and function of PS Ⅱ were higher than those of antioxidant system;CEF is most contribute to protect the photochemical activity and function of PSI;the protective effect of antioxidant system on PS Ⅱphotochemical activity,photosynthetic rate and carbon assimilation was higher than that on PS Ⅰ photochemical activity.In addition,the above results also showed that although the photosynthesis of M.azedarach was affected under Zn stress,it could still be used as an ideal tree species for Zn contaminated soil remediation. |
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