Study On Physiological Response Of Hibiscus Syriacus To Pb Stress

Soil heavy metal Pb pollution affects the ecological environment,and phytoremediation is simple and easy to operate,low cost,and is beneficial to maintaining the ecological environment of the soil.Hibiscus(Hibiscus syriacus Linn.)is a deciduous shrub or small tree of the Malvaceae(Malvaceae)Hibiscus.It grows fast,has a well-developed root system,has good adaptability and stress resistance,and has a certain degree of phytoremediation potential.However,the physiological and biochemical properties of different species of Hibiscus vary greatly,and their Pb tolerance needs further study.In this study,three Hibiscus species were used as experimental materials to study the growth and physiological changes of Hibiscus species under different Pb stress conditions,and their growth and physiological change indexes were measured to evaluate the strength of their Pb tolerance ability through principal component analysis and affiliation function analysis,so as to screen out the species with strong Pb tolerance.It can not only provide a theoretical basis for the screening of woody plants,but also further provide more options and theoretical references for the promotion of Hibiscus species and their restoration in lead-polluted areas.The main findings results are as follows.(1)The effect of Pb stress on the growth and morphology of Hibiscus syriacus:Pb stress inhibited the plant height increment as well as rootstock and leaf dry weight of the three Hibiscus species.The root stem and leaf dry weight decreased with the increase of Pb concentration,in which the root dry weight content of ’Elegantissixnus’ decreased most significantly in the concentration range of 1500 mg·kg-1,with a decrease of 75.63%compared to CK,followed by the original species with a decrease of 68.11%and’Elegantissixnus’ ’Elegantissixnus’ was the lowest with a decrease of 61.23%.Its relative water content showed a decreasing trend.(2)The effect of Pb stress on photosynthetic characteristics of Hibiscus syriacus:The Photochemical quenching factor(qP)and electron transfer rate(ETR)of the three Hibiscus species decreased with increasing concentration,the minimum initial fluorescence(Fo)continued to increase,the net photosynthetic rate(Pn)of Hibiscus syriacus showed a trend of increasing and then decreasing with increasing light intensity,the maximum net photosynthetic rate(Pnmax),light saturation point(LSP)and dark respiration rate(Rd)of Hibiscus syriacus leaves showed a trend of increasing and then decreasing,and the light compensation point(LCP)showed a trend of increasing and then decreasing.Among them,‘Elegantissixnus’was the most affected,followed by‘Paeoniflorus’,with the smallest original species.(3)The effect of Pb stress on the physiology of Hibiscus syriacus:The physiological indexes such as soluble protein(SP),MDA content(MDA)and SOD activity(SOD)of the leaves of the three Hibiscus species under Pb stress showed the same trend,with the increase of Pb stress concentration,they all showed the trend of "rising then falling",but the change of proline content was not consistent in response to Pb stress.However,the proline content of the original species and ’Paeoniflorus’ showed a trend of increasing and then decreasing,while the proline content of ’Elegantissixnu’ showed an increasing trend.(4)Comprehensive evaluation of Pb stress tolerance in Hibiscus species:Principal component analysis divided 13 physiological indicators into three independent composite indicators(cumulative contribution of 83.899%),among which total chlorophyll content,Chl a,Chl b,Fv/Fm,Fo,qP,ETR,Fv’/Fm’,MDA,Pro and SP were the most reflective of the physiological response of Hibiscus to Pb stress.The identification indexes of Hibiscus.Combining the affiliation function values and the composite scores of the principal components,we can conclude that in the low Pb concentration range(250～500 mg·kg-1),the Pb tolerance is in the order of ’Elegantissixnus’>’Paeoniflorus’>original species,and in the moderate Pb concentration range(750～1000 mg·kg-1),the Pb concentration tolerance is in the order of ’Elegantissixnus’>’Paeoniflorus’>original species,’Paeoniflorus’>’Elegantissixnus’>original species in order of Pb tolerance,high Pb range(≥1500 mg·kg-1),original species>’Paeoniflorus’ in order of Pb tolerance>’Elegantissixnu’,the strongest Pb tolerance in the late stage of stress was the original species,followed by ’Paeoniflorus’,and the weakest Pb tolerance was found in ’Elegantissixnus’.