Effects Of Mycorrhizal Fungi On Growth And Physiological Indexes Of Pinus Thunbergii Seedlings Under NaCl Stress

The high NaCl content of soil in coastal saline-alkali land of Shandong Province resulted in fewer species of trees,low forest coverage,simple structure of forest ecosystem and fragile ecosystem.Pinus thunbergii is a salt-alkali-tolerant and stress-resistant tree species.It is an evergreen coniferous tree species commonly used in the construction of warm temperate coastal shelterbelt,and it grows slowly at seedling stage under NaCl stress.But artificial inoculation of mycorrhizal fungi can effectively improve the seedling growth,survival rate and seedling quality of plants and improve NaCl tolerance of plants.In this study,we use Pinus thunbergii seedlings and endophytic fungi Piriformospora indica?PI?and exogenous fungi Lacaria bicolor?LB?as experimental materials,using potted seedlings,inoculation treatment methods,Set inoculation?inoculation with Piriformospora indica,inoculation with Lacaria bicolor,and use non-inoculated Pinus thunbergii seedlings as control group?and NaCl stress?NaCl concentration gradient of 0 mM,25 mM,50 mM,75 mM,100 mM?two factors.The seedling growth index?plant height and biomass?,and root parameters?total root length,root surface area,root volume,etc.?were determined after NaCl stress for 15 d,30 d,60 d.At the end of the NaCl stress treatment,the physiological indexes such as osmotic adjustment substance content,antioxidant enzyme activity,MDA content and photosynthetic index of Pinus thunbergii seedlings were determined.The differences of growth and physiological indexes between two strains of Pinus thunbergii seedlings under different NaCl stress were investigated.The effects of two strains on NaCl tolerance of Pinus thunbergii seedlings under different NaCl stresses were evaluated by membership function method.Use grey correlation analysis method to select the primary index for measuring NaCl tolerance of Pinus thunbergii seedlings.The results showed that:?1?Inoculation with Piriformospora indica and Lacaria bicolor could all alleviate the growth inhibition of Pinus thunbergii seedlings under NaCl stress,and increase the plant height and biomass of Pinus thunbergii seedlings under NaCl stress,showing obvious promoting effect under NaCl stress.Symptoms of NaCl damage in Pinus thunbergii seedlings are alleviated,such as yellowing and dryness.The symptoms of NaCl damage in the uninoculated control group were earlier and more obvious than those in the inoculated seedlings.The highest MGR?mycorrhizal growth response?of Piriformospora indica on plant height,total dry weight,above-ground dry weight and root dry weight of Pinus thunbergii seedlings can reach 42%,37%,41%,and 18%,respectively;The highest MGR of Lacaria bicolor were 41%,56%,60%,and 41%,respectively.The order of the membership function values of each processing growth indicator is:LB?0.5690?>PI?0.4879?>CK?0.3348?.Among them,the inoculation of Lacaria bicolor had the best growth effect on the growth of Pinus thunbergii seedlings under NaCl stress,and the uninoculated control group had the worst growth condition.?2?Inoculation with Piriformospora indica and Lacaria bicolor promoted the growth of roots of Pinus thunbergii seedlings,and the total root length,root surface area,volume,forks,number of root tips,and root fractal dimension were all bigger in both the salt and salt-free conditions than the uninoculated control group,and the lateral roots were increased,and the average root diameter was decreased.The inoculation treatment made the root system more developed,and had a larger absorption area,which promoted the growth of Pinus thunbergii seedlings.The order of membership function values of root parameters was:LB?0.6109?>PI?0.5021?>CK?0.3534?,indicating that the promotion effect of Lacaria bicolor on the root growth of Pinus thunbergii seedlings was better than other treatments.?3?Under NaCl stress,the Pinus thunbergii seedlings inoculated with Piriformospora indica and Lacaria bicolor had more osmotic adjustment substance and stronger antioxidant enzyme activity than the control group,which enhanced the osmotic adjustment and tolerance of Pinus thunbergii seedlings to NaCl stress.The order of MDA content was:CK>PI>LB,indicating that the inoculation treatment significantly reduced the content of MDA in Pinus thunbergii seedlings and enhanced the tolerance of Pinus thunbergii seedlings to NaCl.In general,the Pinus thunbergii seedlings inoculated with Lacaria bicolor had the least damage.?4?The inoculation treatment increased the total chlorophyll content,chlorophyll a content and chlorophyll b content of Pinus thunbergii seedlings under NaCl stress,alleviate stomatal limitation and non-stomatal limitation caused by NaCl stress,enhanced transpiration rate and water use efficiency,and the net photosynthetic rate of Pinus thunbergii seedlings was increased.When the NaCl concentration reached 100 mM,the net photosynthetic rate of the three treatment groups was expressed as follows:PI(4.80?molCO₂·m^-2·s^-1)>LB(4.07?molCO₂·m^-2·s^-1)>CK(3.57?molCO₂·m^-2·s^-1).It seems that the inoculation of Piriformospora indica was superior to other treatments in promoting photosynthesis of Pinus thunbergii seedlings.?5?Correlation analysis of the infection rate of different inoculated Pinus thunbergii seedlings and other 23 indicators showed that there was a significant or extremely significant correlation between the mycorrhizal infection rate and most NaCl tolerance indicators.Based on all the NaCl treatment,the calculation results of the membership function value of 23indicators for each treatment were:LB?0.5424?>PI?0.5255?>CK?0.3279?,indicating that the NaCl tolerance of the inoculated Pinus thunbergii seedlings was lower than that of the uninoculated control group,and inoculation of the Lacaria bicolor on the NaCl tolerance of Pinus thunbergii seedlings was better than inoculation of Piriformospora indica.?6?The calculation of the grey correlation degree of 23 indicators in each treatment showed that the soluble sugar content,proline content,chlorophyll a content,plant height and total chlorophyll content were highly correlated with the NaCl tolerance of the Pinus thunbergii seedlings under each treatment.It is the primary indicator for measuring the NaCl tolerance of Pinus thunbergii seedlings.In general,the effect of inoculation of Lacaria bicolor on the salt tolerance of Pinus thunbergii seedlings was better than that of Piriformospora indica.The Lacaria bicolor had a better effect on the growth of Pinus thunbergii when the NaCl concentration was 25 mM,50 mM,75 mM.Piriformospora indica is more conducive to the growth of Pinus thunbergii seedlings at a NaCl concentration of 100mM.Therefore,when planting Pinus thunbergii seedlings and inoculate mycorrhizal fungi in coastal saline-alkali soil,suitable strains should be selected in combination with the concentration of NaCl in the soil.