| Water is one of the necessary environmental conditions for plant growth,development and reproduction,and it is also a major factor restricting plant yield and quality.China is one of the countries with the highest frequency and most serious impact of drought disasters,and the direct economic losses caused by droughts every year are as high as 44 billion yuan,which seriously restricts China’s agricultural development and ecological security.As the largest fruit in China,citrus occupies a pivotal position in the revitalization of China’s national economy and rural industry.However,although the annual rainfall in the main citrus producing areas in China,represented by Chongqing,is above 1000 mm,due to the serious uneven distribution of rainfall time and space,the seasonal drought represented by the high temperature and drought in summer has become the main problem in the production of citrus producing areas in China.In addition,citrus root hairs are small and short,sensitive to soil moisture,and excessive drying and excessive humidity will cause serious damage to the root system,resulting in a decline in citrus yield and quality.Seasonal drought has become a prominent problem restricting the high-quality development of China’s citrus industry.Arbuscular mycorrhizal fungi(AMF)can form a good symbiotic relationship with citrus,effectively alleviating the damage caused by drought stress to plants and improving the drought resistance of citrus.However,so far,the research reports on the effects of AMF on the drought resistance of citrus have mainly focused on a few citrus varieties such as trifoliate orange,tangerine and Ponkan,and there is a lack of research on Ziyang Xiangcheng.At the same time,the research mainly focuses on the apparent physiological mechanism,and the research on the relevant molecular regulation mechanism,especially the expression of the aquaporin genes,is relatively lacking.As the most important citrus rootstock variety that is being vigorously promoted,it is of great significance to study the impact of inoculation AMF on the drought resistance of Ziyang Xiangcheng.This experiment selected Funneliformis mosseae(recorded as: Fm)and Claroideoglomus etunicatum(recorded as: Ce)as test bacteria.One-year-old nontoxic Ziyang Xiangcheng(Citrus junos Sieb.ex Tanaka)plant seedlings were used as test materials,different water treatments of citrus seedlings by artificial drought simulation.By studying the effects of two AMF on the growth and development,photosynthetic ability,antioxidant and osmotic regulation ability,endogenous hormone metabolism,and the aquaporin gene expression of citrus seedlings under different water conditions,the apparent physiological mechanism and molecular mechanism of AMF in improving the drought resistance of citrus were further explored.The specific findings of this study are as follows:1.Whether under normal water supply or drought stress,the infestation rate of AMF has always been maintained at 51.38%-63.82%,which shows that Fm and Ce can effectively infect the root system of citrus seedlings to form a symbiote.At the same time,with the increase of drought stress,the mycorrhizal dependence of citrus seedlings showed an upward trend,reaching the maximum under severe drought.In the drought stress duration test,the drought symptoms of the seedlings in the Fm group and the Ce group appeared later,the relative moisture content of the soil and the relative water content of the leaves were higher,the survival time was 8 days longer than that of the NAM control,and the mycorrhizal effect of Fm was better than that of Ce.2.Both AMF promote seedling growth and root development in citrus.Under drought stress,both Fm and Ce can increase the plant height,stem thickness,leaf area,dry weight and root-to-crown ratio of seedlings to varying degrees,and promote the growth of citrus seedlings.At the same time,the number of new roots,the root length,root volume and root viability of the seedlings in the AMF group were significantly higher than those in the NAM control group.AMF improves the root morphology of seedlings,enhances root vitality,and promotes root life activities to improve drought resistance.3.Both AMF species can improve the photosynthetic capacity of citrus seedlings and improve water and nutrient conditions.Under drought stress,Fm and Ce can increase the chlorophyll(a,b,a+b),relative leaf moisture content,water potential,net photosynthetic rate(Pn),stomatal conductivity(Gs),transpiration rate(Tr),and reduce the intercellular CO2 concentration(Ci)in different degrees.AMF regulates stomatal opening and closing,accelerates leaf gas exchange rate,optimizes moisture conditions,enhances photosynthetic efficiency,and mitigates plant damage caused by drought.At the same time,the total N,P,K,Mg,Ca,and ammonium nitrogen contents in the leaves and roots of the AMF-inoculated seedlings were significantly higher than those in the NAM control.AMF promotes the absorption of nutrients by seedlings,accumulating more nutrients to enhance drought resistance.4.Under different experimental periods and different degrees of drought,both AMF species significantly improved the antioxidant and osmotic regulation ability of citrus seedlings.Under drought conditions,Fm and Ce significantly reduced the malondialdehyde(MDA)content and hydrogen peroxide(H2O2)content in seedling leaves and roots,increased the activity of superoxide dismutase(SOD),peroxidase(POD),catalase(CAT),ascorbic acid peroxidase(APX)and other antioxidant enzymes,and promoted the accumulation of soluble sugars,free proline,soluble polyamines,soluble proteins and other osmotic regulatory substances.At the same time,the content of substances related to antioxidant and osmotic regulation in leaves was basically higher than that in roots,but the mycorrhizal benefits of Fm and Ce in different organs at different degrees of drought and at different times were different,and the dominant strains were different.5.Under different experimental periods and different degrees of drought,both AMF can effectively regulate endogenous hormone metabolism.Under drought stress,inoculation of Fe and Ce significantly promoted the accumulation of abscisic acid(ABA),indoleacetic acid(IAA),gibberellin(GA),jascular acid(JA),and strigolactones(SLs)in citrus seedlings in roots and leaves,and coordinated interaction between various hormones to jointly promote drought resistance enhancement.At the same time,in addition to GA,ABA,IAA,JA,and SLs were higher in leaves than in roots,and the mycorrhizal benefits of Fm were higher than those of Ce;overall,the mycorrhizal effect in roots was greater and the benefits were higher in drought.6.Drought stress induces upregulated expression of three hydroforin genes(Ci PIP1;1、Ci PIP2;1 and Ci NIP4;2)in citrus seedling leaves,but inhibits the expression of Ci NIP4;2 in roots.Under drought stress,inoculation with AMF can effectively promote the upregulated expression of Ci PIP1;1、Ci PIP2;1 and Ci NIP4;2 in the leaves and roots of citrus seedlings.Among them,the upregulation of Ci PIP2;1 is the most significant,indicating that inoculation with AMF can improve drought resistance by promoting aqua protein gene expression.Overall,the mycorrhizal benefit roots were higher than the leaves,and the mycorrhizal benefits were higher under moderate drought stress,and the mycorrhizal benefits of Fm and Ce were different in different genes. |
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