Effects Of Arbuscular Mycorrhizal Fungi Inoculation On Ponkan Growth Under High Temperature And Drought Condition

Proper temperature and sufficient water are the basic conditions necessary for the healthy growth of Ponkan.However,in natural environment,Ponkan often has to face the adversity of high temperature and water shortage,and the two kinds of adversity often occur simultaneously,which seriously affects the growth of Ponkan.Arbuscular mycorrhizal fungi（AMF）formed symbionts with the roots of rhizal fungi to enhance the stress resistance of Ponkan trees and promote the growth of Ponkan under stress.The effect of Funneliformis mosseae（Funneliformis mosseae）inoculation on Ponkan（Citrus reticulata Blanco cv.Ponkan）on growth and physiological indexes of above ground and root systems,physicochemical properties of rhizosphere soil,root transcriptome,etc.The main research results are as follows:1.Inoculation with AMF could improve the resistance ability of Ponkan overground to the combined stress of high temperature and drought,and promote its growth.After 15 days of high temperature and drought treatment,the growth indexes such as growth phenotype,biomass and other growth indexes,as well as the relative water content,water use efficiency and some photosynthesis related indicators of Ponkan were reduced,and the plants could not be recovered by rehydration.The results showed that high temperature and drought inhibited the growth of Ponkan by reducing water content and inhibiting photosynthesis.AMF inoculation under high temperature and drought combined with stress increased biomass,relative leaf water content,water use efficiency of Ponkan and promoted plant photosynthesis.The results showed that inoculation of AMF could promote the growth of Ponkan overshoot,increase its water content,and finally enhance the resistance ability of Ponkan to combined stress of high temperature and drought,and promote its growth under combined stress.2.Inoculation of AMF could promote the root system to resist the combined stress of high temperature and drought,and promote its growth.Under the combined stress of high temperature and drought,the number of first-order new roots,root shoot ratio and root activity of Ponkan root significantly decreased,and the contents of available phosphorus,available potassium and activities of urease,acid phosphatase and catalase in rhizosphere soil were decreased.The results showed that the combined stress of high temperature and drought damaged plant roots and affected rhizosphere soil environment,which was not conducive to plant growth.Under combined stress,the root growth index,the content of available phosphorus and available potassium in rhizosphere soil and the soil enzyme activity of Ponkan after inoculation with AMF were higher than those without inoculation,but the urease reflecting soil nitrogen was lower than that without inoculation.The results showed that AMF inoculation improved the physical and chemical properties of the rhizosphere soil,increased the enzyme activity of the rhizosphere soil,promoted the absorption of water and nutrients in the soil,and finally promoted the growth and development of the root system,and improved the resistance of the Ponkan to the combined stress of high temperature and drought.3.Transcriptome sequencing analysis was conducted on root systems of three treated Ponkans 15 days after treatment.It was found that GO items such as Oxidoreductase enzyme activity,Redox process and MAPK signaling pathway--plant,phosphatidyl inositol signaling system played a role in the response of AMF inoculated Ponkan to high temperature and drought combined stress.The above pathways are all related to active oxygen metabolism.Differential gene heat map analysis of some antioxidant enzymes showed that the combined stress of high temperature and drought significantly down-regulated the expression levels of related genes,while AMF inoculation under the same conditions up-regulated the gene expressions of antioxidant enzymes in Ponkan.The results showed that the AMF-inoculated Ponkan was activated through MAPK signal transduction pathway and phosphatidyl inosiol signal system under the combined stress of high temperature and drought.The gene expressions of antioxidant enzymes such as SOD,POD,CAT and APX are upregulated.Finally,the change of active oxygen content in root system of Ponkan was regulated,so as to improve plant stress resistance and promote the growth of Ponkan under the combined stress of high temperature and drought.4.The contents of superoxide anion radicals（O₂·）,hydrogen peroxide（H₂O₂）,malondialdehyde（MDA）and the activities of superoxide dismutase（SOD）,catalase（CAT）,peroxidase（POD）and ascorbate peroxidase（APX）in Ponkan leaves and roots were determined.It was found that the combined stress of high temperature and drought increased the contents of O₂·,H₂O₂and MDA in the leaves and roots of Ponkan,and decreased the activity of antioxidant enzymes.The active oxygen content in the leaves and roots of Ponkan inoculated with AMF was lower than that without inoculation,and the activity of antioxidant enzymes was higher,and varied with the stress time.The antioxidant system analysis of leaves and roots showed that the combined stress of high temperature and drought increased the content of active oxygen and decreased the activity of antioxidant enzymes in Ponkan.Under combined stress,inoculation of arbuscular mycorrhizal fungi decreased the content of active oxygen and increased the activity of antioxidant enzymes in Ponkan leaves and roots,which was consistent with the transcriptome results.The results showed that AMF could improve the antioxidant enzyme activity of Ponkan,reduce the content of reactive oxygen species,reduce the oxidative damage caused by the combined stress of high temperature and drought,and finally promote the growth of Ponkan under the combined stress.