Physiological And Root Exudation Response Of Maize Seedling To Nanoparticles Exposure And The Regulation Of H₂S On Root Exudation

Large-scale production and release of nanoparticles(NPs)into environment can affect living and nonliving components of ecosystems.Some advantage and disadvantage function of NPs for plants were reported contradictorily.In the present study,the effects of ZnO,TiO2 and SiO2 NPs and series NPs with various size,shape and composition(including carbon nanotubes)on the growth and physiology of maize seedlings cultivated hydroponically for 6 days were investigated.Plant biomass,pigment contents,membrane lipid peroxidation in terms of malondialdehyde(MDA)content and reactive oxygen species(ROS)production were measured.While the contents of organic acids in root exudates under various kinds of NPs treatment were analyzed by high performance liquid chromatography(HPLC).Besides,due to the physiological function of hydrogen sulfide(H2S)was widely reported,we further investigated the possible regulation role of H2S in organic acids changes in maize seedling root exudates caused by NPs.Growth and physiological results showed that ZnO,SiO2 NPs treatment significantly reduced seedling length and biomass of maize seedling,while TiO2 NPs have no significant toxic effect.In terms of photosynthetic pigments,there were no significant differences between the CK and ZnO NPs treatment for chlorophyll a,b and total chlorophyll content,however the content of carotenoid was significantly decreased and chlorophyll/carotenoid ratio was profoundly increased under ZnO NPs treatment compared to the CK.There were significant differences in pigments for the seedlings treated by TiO2 NPs compare to the CK,such as chlorophyll a,b,total chlorophyll content,chlorophyll/carotenoid ratio decreased respectively,while the content of carotenoid was increased.In case of SiO2 NPs treatment,there was no significant increase in chlorophyll a but chlorophyll b increased,while chlorophyll a/b ratio and the content of carotenoid decreased respectively.Membrane lipid peroxidation in terms of MDA content in roots and leaves were no significant differences between CK,ZnO and TiO2 NPs treatments,while it significantly diminished in roots and leaves respectively under SiO2 NPs.Content of H2O2 significantly diminished in roots and no significant change in leaves under ZnO NPs,while TiO2 and SiO2 NPs treatment showed no significant increase in roots or leaves.Besides,O2·-production increased in roots and leaves under ZnO NPs treatment,in case of TiO2 and SiO2 NPs treatment increased respectively in roots while no significant change in leaves compare to the CK.The pH of root exudates was declined under ZnO,TiO2 and SiO2NPs treatment compare to the CK,which means the rhizosphere was acidified under NPs treatment.The contents of organic acids in root exudates of maize seedlings treated by different NPs were measured by HPLC.Oxalic acid was found to be significantly decreased under ZnCl2,ZnO nano,ZnO bulk,SiO2-15 nm-1(Phyllic),SiO2-15 nm-2(Phobic),TiO2-15 nm,TiO2-30 nm,MWCNT treatments,and significantly increased at SiO2-30 nm(Phobic),a non-significant change under CB nano treatment,compare to the CK.Formic acids content was found significantly reduced at ZnO nano,ZnO bulk,SiO215 nm-1(Phyllic),SiO2-15 nm-2(Phobic),TiO2-15 nm,TiO2-30 nm,MWCNT treatments respectively,while non-significant change under ZnCl2,SiO2-30 nm(Phobic),CB nano treatment respectively compare to the CK.Malic acid were found significantly diminished under ZnO nano,ZnO bulk,SiO2-15 nm-1(Phyllic),SiO2-15 nm-2(Phobic),SiO2-30 nm(Phobic),TiO2-15 nm,MWCNT respectively,while heightened at CB nano and non-significant by ZnCl2,TiO2-30 nm treatment respectively.Lactic acid contents showed non-significant change at under ZnO nano,ZnO bulk,SiO2-15 nm-1(Phyllic),SiO2-30 nm(Phobic),TiO2-15 nm,CB nano,while it significantly enhanced under TiO230 nm and significantly lessened under ZnCl2,SiO2-15 nm-2(Phobic),MWCNT respectively.Citric acid content significantly minified under ZnCl2,SiO2-15 nm-1(Phyllic),SiO2-15 nm-2(Phobic),SiO2-30 nm(Phobic),MWCNT,while raised with TiO2-30 nm,and no significant change under ZnO nano,ZnO bulk,TiO2-15 nm,CB nano respectively compare tothe CK.Fumaric acid contents showed significant augmented under ZnCl2,ZnO nano,ZnO bulk,while exposure under SiO2-15 nm-1(Phyllic),SiO2-15 nm-2(Phobic),SiO2-30 nm(Phobic),TiO2-15 nm,TiO2-30 nm,CB nano and MWCNT showed no significant changes.Finally,tartaric acid content significantly diminished under ZnO nano and ZnO bulk treatment by of 64.8%,74.4%,respectively compare to the CK.H2S treatment alone showed no significant differences for all pigments in maize seedlings compare to the CK.Moreover,MDA content in roots significantly decreased under 500 or 1000 μM NaHS respectively compare to CK.H2O2 content in roots significantly decreased under 500 or 1000 μM NaHS respectively,while no significant change in leaves under H2S treatment.O2·-production decreased in roots at NaHS treatment,whileno significant change in leaves under H2S treatment.The pH of root exudates was declined under H2S in dose-dependent manner,which means the rhizosphere was acidified under H2S treatment.The contents of organic acids in root exudates under H2S treatment were analyzed.Oxalic acid contents significantly diminished at all NaHS concentrations compare to the CK.In addition,formic,malic acids increased respectively at 100 μM NaHS.Lactic acids increased at 100,200 μM NaHS and decreased at 50,1000 μM NaHS respectively.At 200,500,and 1000 μM NaHS treatment the content of acetic acid decreased respectively comparison to the CK.Citric acid content increased at 100 μM NaHS,while decreased at 1000μM NaHS respectively.Lastly fumaric acid was increased at 100 μM NaHS compare to CK.In the NPs and H2S combined treatment,we further investigated the possible role of H2S in regulation of some organic acids content changes caused by NPs in the roots exudates from maize seedlings.The contents of tartaric and fumaric acids increased with the addition of H2S under the selected NPs treatments,while formic and oxalic acids contents showed decreased,suggesting H2S may relief the phytotoxicity of NPs by organic acids secretion.Besides this,in lactic,malic and citric acids contents showed variations to their selected treatments.Generally,all different NPs brings variation in root exudation,related to oxalic,formic,malic,lactic,citric,fumaric and tartaric acids compare to CK.Additional research should be conducted to deepen cognition around the root exudation and their pathway in response to NPs and H2S.We conclude that ZnO NPs,TiO2 and SiO2 NPs has different positive and negative growth and physiological effects on maize including biomass,pigments,MDA,ROS,while H2S under 100,200 μM NaHS has positive effects implying the detoxify response of H2S.In case of root exudates it showed variation to their NPs and H2S exposure.These answers furnish the first manifest of root exudates of maize seedling to their mentioned concentration of NPs and H2S.