| The function of mycorrhizal fungi(MF) in environmental bioremediation has become a focus for ecology research in recent years. MF have gained increasing importance due to its significant role in promoting plant growth, improving plant resistance to disease, restoring degraded and contaminated soil, maintaining ecological balance, and stabilizing ecological cycles. Karst rocky desertification in Southwest China has resulted in an ecologically fragile zone in this region. Water shortage is the key factor preventing restoration of vegetation, in addition to high levels of calcium, lack of adequate nitrogen and phosphorus content in the surface soil, and soil erosion, which has resulted in decreased microbial diversity in surface soil. These factors are unfavorable for restoration of vegetation. The physiological and ecological characteristics of MF can overcome the factors that cause rocky desertification. In this study, we inoculated Cyclobalanopsis glauca, a common afforestation species in the karst area, with MF to determine the effect and the mechanism of drought tolerance of the afforestation species as well as to explore the ecological impact of MF in a degraded ecosystem. This would provide a foundation for widespread application of mycorrhizal technology in restoration of vegetation in a region with rocky desertification.Three MF, Glomus mosseae, Glomus intraradices, and Pisolithus tinctorius were used to inoculate C. glauca seedlings in a greenhouse and pot soil moisture was used to simulate normal water(NW), moderate drought(MD), and severe drought(SD) conditions. Effects of MF on growth, chlorophyll content, photosynthesis, nutrient uptake, osmotic regulators content, and antioxidant enzyme activity were studied under these 3 soil-water conditions. The total root length, surface area, volume, and other root morphology indices were analyzed using a root scanner Epson Perfection V700 and Win Rhizo Pro software. To investigate the growth-promoting effects of external hyphae on the p H value, nutrient content, soil enzyme activity, microbial biomass carbon/nitrogen, and soil-water stable aggregates of rhizosphere and non-rhizosphere soil, 2-compartment pots were used. Promoting effects of common mycorrhizal networks(CMNs) on hydraulic lift(HL) and water redistribution of C. glauca were studied using deuterium isotope tracer to trace water transport in CMNs. The contribution of CMNs to water absorption by C. glauca was also estimated under drought conditions. The results and conclusions were as follows: 1) All 3 mycorrhizal fungi, namely G. mosseae(Gm), G. intraradices(Gi), and P. tinctorius(Pt), as well as mixed fungi(Mi), infected C. glauca roots effectively and formed mycorrhizal symbionts. The biomass, height, base diameter, total chlorophyll content, and relative water content of leaves in the inoculated C. glauca seedlings significantly improved as compared to control, under moderate drought stress. However, growth index and relative water content of C. glauca seedling leaves inoculated with P. tinctorius showed no significant change under severe drought stress conditions as compared to control. Under moderate drought conditions, the order of mycorrhizal dependency in the four inoculation treatments was as follows: Pt(47.68%) > Gm(46.27%) > Mi(45.47%) > Gi(40.87%). Under severe drought conditions, mycorrhizal dependency of Gm and Gi treatment increased, however that of Pt treatment decreased to about 24.03%. The water utilization efficiency of C. glauca seedlings inoculated with Gm, Gi, Pt, and Mi increased by 23.6%, 15.1%, 21.7%, and 18.8% compared to the uninoculated treatment under soil drought conditions, while it increased by 41.1%, 39.6%, 9.5%, and 32.6% compared to the control treatment under severe drought conditions. 2) Total nitrogen content of Pt treatment significantly increased compared to uninoculated treatment under moderate drought conditions, while Gi treatment significantly increased the seedling nitrogen content under severe drought conditions. However, total phosphorus and total potassium content of all inoculated treatments were significantly increased, regardless of whether the soil experienced drought or otherwise. The soluble sugar content of inoculated treatments increased while proline content decreased as compared to uninoculated treatment under drought stress, indicating that mycorrhizal symbionts promote moisture retention by the plant to avoid dehydration stress. Antioxidant enzyme activities of superoxide dismutase(SOD), peroxidase(POD), and catalase(CAT) under inoculated treatment were enhanced as compared to activities with uninoculated treatment. The activity of POD enzyme with inoculated treatment was significantly higher than that with uninoculated treatment condition, especially under severe drought stress. SOD enzyme activity in Gm, Gi, and Mi treatments was significantly higher than that with uninoculated treatment, but no difference was observed between Pt and uninoculated treatment. 3) Compared with the uninoculated treatment, the root biomass, length, volume, and surface area increased significantly, while the average root diameter decreased in the C. glauca seedlings with inoculated treatment. Under moderate drought stress, the root biomass of seedlings with Gm, Pt, Gi, and Mi treatments increased by 86.6%, 66.1%, 88.4%, and 84.7%, respectively, as compared to uninoculated treatment. Under severe drought, the uninoculated treatment and Pt-treatment showed significant decrease in root biomass, while the remaining treatments showed no significant change. Under moderate drought conditions, seedlings showed maximum total root length with Gm, Gi, Pt, and Mi treatment, namely an increase of 128.3%, 120.9%, 139.7%, and 132.5%, respectively, as compared to the uninoculated treatment. However, under severe drought conditions, all seedlings with inoculated treatment showed a significant decrease in total root length. Under moderate drought conditions, the average root diameter with uninoculated treatment was significantly higher than that with inoculated treatment, where Gm, Gi, Pt, and Mi treatments showed a decrease in root diameter by 22.4%, 31.1%, 45.3%, and 46%, respectively, as compared to uninoculated treatment. Under severe drought conditions, only Gm and Gi treatment showed a reduction in root average diameter when compared with the uninoculated treatment. The results indicate that MF inoculation caused the C. glauca roots to become finer, so that the absorption of water and nutrients becomes easier. 4) Compared with uninoculated treatment, the p H value of rhizosphere soil reduced significantly with inoculated treatment, while, alkali hydrolysable nitrogen, available phosphorus, and organic content of rhizosphere and non-rhizosphere soil increased significantly with inoculated treatment. The hyphal density of rhizosphere soil with inoculated treatment was obtained in the order of Pt > Gm > Gi, and for non-rhizosphere soil the order was Gm > Gi > Pt. With MF treatment, urease and acid phosphatase activity was significantly higher in rhizosphere soil than in non-rhizosphere soil. Activity of urease and acid phosphatase in rhizosphere soil with Pt and uninoculated treatment was significantly lower than that with Gm and Gi treatment. MF inoculation had no significant effect on the activity of catalase and sucrase. Microbial biomass carbon content of rhizosphere soil was significantly increased by inoculated treatment, but the microbial biomass nitrogen was not affected. For non-rhizosphere soil, microbial biomass carbon/nitrogen content with Pt and uninoculated treatment were significantly lower than that with Gm and Gi treatment. Meanwhile, inoculated treatment also significantly increased the amount of >0.25-mm, soil-water stable aggregates in the rhizosphere and non-rhizosphere soil. 5) The results from isotope tracing suggest that C. glauca roots have hydraulic lifting and water redistribution functions, where water is transported from donor seedlings to receptor seedlings though the common mycorrhizal networks during drought. According to the mixed linear model, the capacity of CMN to transmit moisture was estimated and about 1.66% of water in receptor seedlings was found to be absorbed by CMN from the donor chamber. However, the actual water content from CMN transmission may have been higher than that observed, since the barrier effect of the nylon net was not considered, where a portion of the soil water from the receptor chamber could have seeped in from the donor chamber, which was not accounted for. |
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