Physiological Response Of Robinia Pseudoacacia Seedlings To NaCl Stress

Soil salinization is becoming more and more serious,which has seriously affected agricultural security,ecological environment,resource utilization and so on.It is the most effective and ecological method to use salt tolerant plants to improve and utilize saline alkali land.Robinia pseudoacacia is resistant to salt and alkali,drought and smoke,and has high medical value.It is a pioneer tree species to prevent wind and sand,improve hydrology and environmental greening.Because R.pseudoacacia has a certain salt tolerance,it is used as a greening tree species in saline soil.However,the mechanism of salt tolerance,especially the effect of salt on photosynthetic structure,is rarely reported.In order to explore the physiological response of R.pseudoacacia to salt stress and discover its salt tolerance mechanism,the effects of salt stress on its growth,antioxidant system and photosynthetic structure were studied.The main findings are as follows:1.Effects of salt stress on physiological characteristics of R.pseudoacacia seedlings:Under NaCl stress,the plant growth of R.pseudoacacia seedlings were significantly inhibited;the contents of Na⁺,Cl^-in plants significantly increased,while the contents of K⁺significantly decreased.The content of soluble sugar,soluble protein,proline and organic acid increased significantly.Compared with the control,the activities of SOD,POD,CAT and APX antioxidant enzymes significantly increased,while the contents of H₂O₂ and O₂^- didn’t significantly changed under low concentration of NaCl（50 mM）,indicating that the increased antioxidant enzyme activities cleared reactive oxygen species in time.With the increase of NaCl concentration,the activities of SOD,POD and and so on antioxidant enzymes decreased significantly,while the contents of H₂O₂ and O₂^- increased significantly,indicating that plants were under oxidative stress.2.The effect of salt stress on chlorophyll content and photosynthetic parameters of R.pseudoacacia seedlings:Under NaCl stress,the contents of chlorophyll a,chlorophyll b and total chlorophyll in leaves of R.pseudoacacia seedlings decreased significantly.Pn,Tr and Gs decreased with the increase of NaCl concentration,while Ci increased with the increase of NaCl concentration.Fv/Fm decreased with the increasing of salt concentration,which indicated that PSII primary light energy conversion efficiency decreased with the increasing of NaCl stress.Both Fo and NPQ increased with the increasing of NaCl concentration.3.Effects of salt stress on chloroplast structure of R.pseudoacacia seedlings:Under NaCl stress,the ultrastructure of chloroplast of R.pseudoacacia leaves was damaged obviously.Under control,the chloroplast structure was intact,the chloroplast shape was full,and the thylakoids and grana lamellae were arranged orderly;When treated with 100 mM NaCl,the thylakoid began to swell and deform,the starch grains swelled and irregular,and lipid globules appeared in the thylakoid;However,the thylakoid was further distorted and deformed under 200 mM NaCl treatment,and the filamentous lamellae appeared,the starch grains were expanded and irregular,and the volume of lipid spheres increased.Further study showed that the expression of key genes psaA、psaB、psbA、psbD、ndhE、ndhH、psaC、psbC、rps7、ropA,were significantly upregulated under 50 mM NaCl treatment,which showed the species has a certain salt tolerance.However,the expression of these genes was significantly down regulated under 100,150 and 200 mM NaCl treatment.The results showed that high concentration of NaCl treatment affected the development of chloroplast,destroyed the structure of chloroplast,and then affected its function,which was consistent with the physiological index.This laid a foundation for the molecular mechanism of salt tolerance of R.pseudoacacia seedlings.