Study On The Effect And Mechanism Of Arbuscular Mycorrhizal Fungi In Dought Resistance Of Citrus

Arbuscular mycorrhizas are mutualistic symbioses that form between arbuscular mycorrhizal (AM) fungi and the roots of host plants. The symbioses are characterized by bi-movement of nutrients where carbon flows to the symbiosis and inorganic nutrients or water move to the plants, thereby becoming a transported carrier between the roots of host plant and soil. Thus, AM symbioses are important for growth and development of host plants. Citrus is one of important fruit species in southern China. Most of citrus trees are cultivated in highland, and water is a restricted factor to acquire high yield and good quality of citrus. Citrus has rare and short root hairs in field and is fairly dependent on AM fungi that are most Glomus species. This study is to evaluate the effect and mechanism of AM fungi on drought resistance of citrus, and to select some effective fungi using the citrus. The main contents and results in the present study are as follows:(1) Selection of AM fungi for drought resistance of citrusExperiment was carried out in potted culture and effects of five Glomus species (Glomus mosseae, G. versiforme, G. diaphanum, G. geosporum and G. etunicatum) on mycorrhizal development, plant growth, carbohydrate and antioxidant enzymes in two citrus rootstocks species—trifoliate orange (Poncirus trifoliata (L.) Raf.) and tangerine (Citrus tangerine Hort. ex Tanaka)—under well-watered and water stress conditions were studied. Water stress significantly decreased the mycorrhizal infection of two citrus roots, whereas all structures of arbuscular mycorrhizae were more closely joined under water stress conditions than under well-watered conditions. Different AM fungi showed the difference of inter-species in drought resistance of citrus seedlings. Inoculation with G. versiforme showed the greatest effects in trifoliate orange seedlings under well-watered conditions and inoculated with G. mosseae the greatest effects under water stress conditions. G. diaphanum was a rather weak fungus for the drought resistance of trifoliate orange seedlings under well-watered and water stress conditions. Inoculation with G. mosseae or G. geosporum represented greater effects in tangerine seedlings whether water stressed or not, and inoculation with G. etunicatum did not represent effects in tangerine seedlings.(2) Effects of AM fungi on water and osmotic adjustment in citrusPotted experiments were conducted to research the effects of AM fungi on water status and osmotic adjustment of citrus seedlings under different water treatments conditions. G. mosseae-inoculation increased the total absorption and active absorption areas of roots in trifoliate orange seedlings under 20%, 16% and 12% water content of soil conditions. Trifoliate orange seedling colonized by G. mosseae had higher 20.0%～39.9% water use efficiency than non-mycorrhizal trifoliate orange seedlings, when the water content of soil was 20%, 16% and 12% respectively. The results also showed that G. versiforme- inoculation increased the soluble sugar contents, the soluble starch contents and the non-structural carbohydrate contents in leaves and roots of either trifoliate orange or tangerine seedlings, but G. versiforme-inoculation decreased the proline contents of two citrus rootstocks under well-watered and water stress conditions, respectively. There were greater the glucose, fructose and sucrose contents of roots of trifoliate orange seedlings, as well as greater the K⁺ and Ca²⁺ levels in leaves and roots of trifoliate orange or tangerine seedlings grown under well-watered and water stress conditions. However, the responses to inoculation with G. versiforme on Mg²⁺ levels of two rootstocks and the glucose, fructose and sucrose contents of leaves of trifoliate orange seedlings were varied. AM tangerine seedlings had higher leaf water potential, transpiration rates, photosynthetic rates, stomatal conductance, and lower leaf temperature (0.80℃～1.82℃) than corresponding non-AM control. AM colonization improved the osmotic adjustment originating not from proline but from non-structural carbohydrate, K⁺, Ca²⁺, Mg²⁺, glucose, fructose and sucrose, resulting in the enhancement of drought resistance.(3) Effect of G. versiforme inoculation on reactive oxygen metabolism in trifoliate orange and tangerine seedlings in potted culture under well-watered and water stress conditionsWhether water stressed or not, AM symbiosis decreased the concentrations of malondialdehyde, superoxide anion radical and hydrogen peroxide in leaves or roots of two citrus rootstocks. AM infection also increased the activities of antioxidant enzymes including superoxide dismustase, guaiacol peroxidase, catalase, glutathione reductase and ascorbate peroxide and enhanced the contents of antioxidants including reduced ascorbate, reduced glutathione, total ascorbate and total glutathione regardless of water status. The more antioxidant enzymes and antioxidants in AM seedlings would result in faster removal of hydrogen peroxide through ascorbate-glutathione cycle, helping to alleviate oxidative damage and enhancing drought resistance of host plants.(4) Effects of phosphorus and glomalin on drought resistance of trifoliate orange seedlings colonized by G. versiforme, G. mosseae and G. diaphanum in potted culture and relationship between phosphorus and anti-oxidation under well-watered and water stress conditionsWater stress enhanced fungal density and glomalin in rhizosphere of AM citrus seedlings and decreased alkaline phosphatase and succinate dehydrogenase activies of citrus roots. In AM roots, the ranking of AM colonization, alkaline phosphatase and succinate dehydrogenase was as follows: AM colonization＞succinate dehydrogenase＞alkaline phosphatase. The activities of acid phosphorylase and total phosphorylase were increased by AM colonization, resulting in more organic phosphorus translated into inorganic phosphorus for providing host plants. As a result, the contents of phosphorus in leaves and roots of AM citrus seedlings were increased by AM colonization, and the content of available phosphorus in rhizosphere was decreased. When analysed relativity, the results showed that mycorrhizal structures, soil phosphorylase and succinate dehydrogenase together affected the phosphorus absorption of plant. There was indirect correlation between contribute of AM to phosphorus and contribute of AM to antioxidant enzymes and antioxidants of roots, as well as no correlation between contribute of AM to plant growth or nutrient element (excluding Ca and Mg) and contribute of AM to both antixodant enzymes and antioxidants of roots.Whether water stressed or not, glomalin was firstly observed in rhizosphere of AM seedlings, and its levels were from 1.63 mg/g to 1.93 mg/g. As a result of "super glue" of glomalin, small aggregations were agglutinated big aggregation, resulting in the increments of 2 cm, 1 cm and＞0.25 cm water-stable aggregations and the decreases of 0.25 cm water-stable aggregation. Thus, mycorrhizal soils presented considerable characterists of interspace and holding water. Water would be delayed to flow to soil capillary, and drought resistance of AM host plants was enhanced.(5) Study on AM characterists of citrus in fieldModel AM structures including vesicles, arbuscles, entry points, internal hyphae and external hyphae were observed in roots of citrus in filed. This showed that Citrus belonged to AM plants. Both AM colonization and spore density showed "A" shape on Guoqing No.1 (Citrus unshiu Marc cv Guoqing No.1)/trifoliate orange (Poncirus trifoliata (L.) Raf.) and Guoqing No. 4 (C. unshiu Marc Guoqing No.4)/trifoliate orange, reaching lower status in Feb. and Dec., middle status in Apr. and Oct., and higher status in Jun. and Aug. "V" sharp was found in soil avail phosphorus or litmusless phosphatase. The highest AM colonization and spore density were within the soil depth of 10 cm～20 cm in Guoqing No.1 and Guoqing No.4 in field. Covered ruderals significantly enhanced AM colonization and spore density of two citrus species in field. However, AM colonization and spore density of two citrus in field were decreased by application of N fertilization and tillage. AM development of tangerine+trifoliate orange was highest in five citrus rootstocks (tangerine+trifoliate orange, trifoliate orange, troyer citrange, swingle citrumelo, tangerine+rough lemon) and was significantly higher than that of other rootstocks.The study indicated that AM colonization was significantly positive correlation with spore density (p＜0.01) and notably negative correlation with soil avail phosphorus (p＜0.01) on Guoqing No.1 and Guoqing No. 4. Thus, the higher spore density and the lower soil avail phosphorus could accelerate mycorrhizai infection to roots. Spore density was significantly negative correlation with soil avail phosphorus (p＜0.01) and markedly positive correlation with litmusless phosphatase (p＜0.01) or total phosphatase (p＜0.01) on Guoqing No.1 and Guoqing No. 4. Therefore, we deduced that higher soil avail phosphorus had a function of restraint, and both higher litmusless phosphatase and total phosphatase had function of stimulation to spore density. The mineralization of organic phosphorus gave priority to litmusless phosphatase in citrus trees in field.Finally, model figure of mechanisms about drought resistance of host plants by AM fungi was primarily established according to both this study and former results. The study also discussed the possibility of cross-tolerance in mycorrhizal plants.