Effects Of Arbuscular Mycorrhizal Fungus On The Growth And Nitrogen Uptake And Utilization Of Catalpa Bungei And Related Mechanisms

Catalpa bungei is a unique precious timber tree with excellent material and wide application.In recent years,the construction area of C.bungei nurseries and plantations has increased greatly,and the growth of C.bungei plantations is promoted mainly by increasing nitrogen（N）fertilizer.However,this not only increases the economic costs but also causes serious environmental pollution due to low N use efficiency.Therefore,it is particularly necessary to find new ways to enhance the N use efficiency of C.bungei.N efficiency,including N absorption efficiency and N use efficiency,is an important index to evaluate the absorption and utilization capacity of plants,and is also the key to efficient N management.Under natural conditions,the roots of C.bungei can form a good symbiotic relationship with arbuscular mycorrhizal fungi（AMF）,but the effect and mechanism of AMF on N absorption and use efficiency of C.bungei are still unclear.Understanding the effect of mycorrhizal symbiosis on N absorption and utilization in forest trees,as well as the regulatory pathways,is crucial for reducing fertilizer application,improving N fertilizer use efficiency,and promoting tree growth.This paper analyzed the effects of inoculation with Rhizophagus intraradices on the growth,N absorption and use efficiency of C.bungei seedlings under different N application levels（low,medium,and high N）.The changes of photosynthetic and nutritional physiology,anatomical characteristics,antioxidant system,root architecture,hormone levels,NH₄⁺and NO₃^-uptake kinetics,N metabolism-related enzyme activities and expression of key genes of N metabolism in C.bungei seedlings were studied.The related mechanisms of AMF affecting the growth,N absorption and use efficiency of C.bungei were elucidated.Finally,by constructing a structural equation modeling,the key driving factors and main pathways of AMF inoculation affecting N absorption and use efficiency of C.bungei under different N levels were revealed.The main findings were listed as follows:（1）AMF can form a good symbiosis with C.bungei and promote the growth of C.bungei seedlings.The AMF colonization rate in the roots of C.bungei seedlings significantly increased at medium N levels（P<0.05）,but significantly decreased at high N levels.Under low N levels,inoculation with AMF significantly increased the plant height,leaf area of seedlings.Under medium N levels,inoculation with AMF significantly increased plant height,basal diameter,leaf area and total biomass of seedlings.Under high N levels,AMF inoculation significantly increased seedling leaf area and total biomass.Inoculation with AMF significantly increased the N use efficiency of C.bungei seedlings at low N levels,notably increased the N absorption efficiency at medium N levels,but had no significant effect on N absorption and use efficiency at high N levels.（2）AMF inoculation increased photosynthetic gas exchange parameters,photosynthetic pigment content,specific leaf area and chlorophyll fluorescence parameters,promoted N and phosphorus absorption and enhanced photosynthetic and nutritional physiology of C.bungei seedlings at low and medium N levels.（3）Under low and medium N levels,inoculation with AMF could improve the water and gas exchange performance of C.bungei seedlings by changing the characteristics of stomata and lenticels,as well as the anatomical characteristics of stems and leaves.This includes increasing the length,width,area,and density of stomata and lenticels,as well as increasing the thickness of the epidermis and cortex of stems,pith diameter,the density and diameter of vessels,reducing the thickness of the upper and lower epidermis and sponge tissue of leaves.（4）At the same N level,inoculation with AMF reduced the reactive oxygen species content and increased the peroxidase activity and root soluble protein content in C.bungei seedlings,thus enhancing the antioxidant system and osmoregulation ability of C.bungei seedlings and alleviating the oxidative stress in seedlings.（5）Inoculation with AMF significantly improved the root architecture of C.bungei seedlings by increasing the content of root auxin and gibberellin at medium N levels.（6）AMF inoculation improved N absorption and assimilation by seedlings at low and medium N levels by regulating the activity of key enzymes of N metabolism and the expression levels of key enzyme genes of N metabolism and nitrate transporter protein genes（NRT）in seedling leaves and roots.（7）Structural equation models were constructed from the perspective of physiology and biochemistry,morphological anatomy,and gene expression levels with N use efficiency as the target variable at low N levels and N absorption efficiency at medium N levels.By constructing a structural equation model,the key driving factors and pathways of AMF affecting N absorption and use efficiency in C.bungei seedlings were clarified.Under low N levels,there was a pathway in the model that significantly affected the N use efficiency of C.bungei.That is,AMF inoculation significantly increased the N use efficiency of C.bungei by down-regulating the expression levels of N metabolism enzyme genes NADH-GOGAT1 and Fd-GOGAT in roots,and up-regulating the expression levels of NRT2.5 in leaves.Under medium N levels,inoculation with AMF significantly increased the N absorption efficiency of C.bungei,but the pathway by which AMF affected the N absorption efficiency of C.bungei was not significant and the pathway of action was unclear.This study can lay the foundation for the application of AMF to promote the growth of C.bungei,provide a reference for revealing the effect of mycorrhizal symbiosis on N absorption and utilization characteristics of forest trees,and also provide a theoretical basis for using mycorrhizal technology to improve N efficient utilization and reduce N fertilizer application in C.bungei plantation forests.