Mechanisms Of Arbuscular Mycorrhizal Fungi(AMF) Enhancing Salt Tolerance Of Lycium Barbarum L.

The arbuscular mycorrhizal fungi(AMF) communities in Ningxia were surveyed in this study. The edaphic factors driving the AMF community changes were revealed. The relationship among AMF community, edaphic factors and host plant identities were clarified. Furthermore, the effect of AMF on the hormone regulation, photosynthesis, antioxidant system, osmotic regulation, ultrastructure of mesophyll cells and nutraceutical quality of Lycium barbarum L. were assessed to reveal the physiological and biochemical mechanisms of AMF enhancing salt tolerance of L. barbarum L. The utilization of AMF as bio-fertilizer in order to promote the growth and quality of L. barbarum L. in salt-stressed field was appraised. The main results and conclusions are as follows:1. Investigation of AMF communities in Ningxia saline soilsL. barbarum L. and Elaeagnus angustifolia L. were chosen to characterize the AMF community in Ningxia saline soils. High AMF diversity was revealed, including a total of 33 AMF species from 10 genera and 7 families. Glomeraceae with 13 species was the most abundant family. The genera of Acaulospora(10 species) and Glomus(9 species) accounted for the largest proportion at genus level. Septoglomus constrictum was the most abundant species. The AMF community of L. barbarum L.was different from that of E. angustifolia L.(R=0.26, p<0.05). Twenty-seven AMF species were isolated from the rhizosphere soil of L. barbarum L., while 12 species were found from the rhizosphere soil of E. angustifolia L. The Shannon, Simpson and Evenness of L. barbarum L. were higher than that of E. angustifolia L, suggesting the effect of host plant on AMF community.2. Effect of edaphic factors on AMF community of Ningxia saline soilsSalinity negatively correlated with AMF diversity index, illustrating that salinity would decrease AMF diversity. However, available K/P/N and total P which represent soil fertility positively correlated with AMF diversity. This demonstrated that AMF diversity increased with elevated soil fertility. The alkaline soils facilitated the sporulation of AMF as soil pH positively correlated with spore richness. Available P positively correlated with the Evenness of AMF community, indicating that soil P could maintain the balance of AMF community. Taken together, edaphic factors had significant impact on AMF community in Ningxia saline soils.3. Effect of AMF inoculation on hormonal regulation and growth of L. barbarum L. under salt stressIAA in the leaves and roots of L. barbarum L. inoculated with Glomus versiforme under salt stress was increased by 59.0% and 39.1%, respectively. ABA in the leaves of mycorrhizal L. barbarum L. was increased by 13.8% compared with uninoculated control. The result of ANCOVA showed that G. versiforme had direct promotion effect on the levels of IAA and ABA of host plant, through which plant biomass was increased 1.75 fold, and the total and absorptive root areas were elevated by 2.17 and 2.66 fold, respectively. Inoculation with G. versiforme increased plant height, stem base diameter and leaf number per plant by 93.6%, 57.8% and 86.6%, suggesting better growth of mycorrhizal L. barbarum L. In summary, inoculation with G. versiforme alleviated the deleterious effect of salinity on L. barbarum L. through hormonal regulation with emphasis on root growth to maintain the water and nutrients uptake.4. Effect of AMF inoculation on the photosynthesis system of L. barbarum L. under salt stressThe impact of G. versiforme on chlorophyll, photosynthesis and chlorophyll fluorescence were estimated under salt stress. G. versiforme notably elevated the chlorophyll content of L. barbarum L. by 25.7% under no salt stress. The chlorophyll content was not affected by salinity, which demonstrated that L. barbarum L. had strong salt tolerance. The photosynthesis and chlorophyll fluorescence of L. barbarum L. were inhibited under salt stress. However, G. versiforme enhanced photosynthesis of L. barbarum L. at 100 mM salt level: the photosynthesis rate and transpiration of AM L. barbarum L. were increased by 56.0% and 28.7%, while the intercellular CO2 was decreased by 9.0%. At 200 mM salt level, L. barbarum L. inoculated with G. versiforme had higher ?PS?, qP and qN than control, suggesting G. versiforme augmented the energy capture and regulated the energy bifurcation between photochemical and non-photochemical events to efficiently prevent the photosynthesis apparatus from photo-oxidation.5. Effect of AMF inoculation on the antioxidant system and osmotic adjustment of L. barbarum L. under salt stressThe effects of G. versiforme on the antioxidant enzymes, osmotic regulation and water potential of L. barbarum L. under salt stress were appraised. Salinity caused membrane injury of L. barbarum L, which increased the electrolyte leakage of the leaves of L. barbarum L. at 100 mM and 200 mM salt levels by 1.29 and 1.13 folds, respectively. Meanwhile, salt stress increased the activities of SOD and POD by 11% and 7.6%. The activity of CAT was increased by 2.85 folds under 100 mM salt stress conditions. Hence, G. versiforme could improve the activities of antioxidant enzymes to avoid membrane injuries. G. versiforme increased soluble sugar and reducing sugar by 32.3% and 33.6%, but decreased starch by 23.8% in the leaves of L. barbarum L. This indicated that G. versiforme promoted the hydrolysis of starch into sugars to enhance the osmotic regulation of L. barbarum L, with leaf water potential increased by 32.6%. In this regard, the membrane of AM L. barbarum L. was protected from oxidation injury.6. Effect of AMF inoculation on the ultrastructure of L. barbarum L. leaves under salt stressThe effect of G. versiforme on the ultrastructure of L. barbarum L. mesophyll cells under salt stress was observed. Salt stress seriously disrupted the ultrastructure of membrane of mesophyll cells of control L. barbarum L, resulting in plasmolysis. Moreover, the thylakoids and stroma lamella of chloroplast of control L. barbarum L. were damaged by salt stress. However, the membrane of mesophyll cell of L. barbarum L. inoculated with G. versiforme was less damaged with weaker plasmolysis compared with non-inoculated control. The chloroplast of L. barbarum L. inoculated with G. versiforme was integrated relative to the non-inoculation plants under salt stress. These results demonstrated that G. versiforme protected the structures of mesophyll cells of L. barbarum L. to maintain the photosynthesis and other metabolic processes.7. Effect of AMF inoculation on the nutraceutical quality of L. barbarum L. leaves under salt stressThe effect of Rhizophagus irregularis on the nutraceutical quality of two successively harvested L. barbarum L. leaves under salt stress was assessed. R. irregularis promoted the regenerated bud number by 79% and 11.3 folds without and with salt stress, which facilitate the sustainable production of L. barbarum L. leaves. Under salt stress, R. irregularis elevated the rutin of the two harvests by 96.1% and 77.5%, improved the acidic polysaccharide of L. barbarum L. leaves by 66.7% and 103.1%. Moreover, the polysaccharide of mycorrhizal L. barbarum L. was increased by 9.9% and 32.5%. These results suggested that R. irregularis enhanced the nutraceutical quality of L. barbarum L. leaves.