| Arbuscular mycorrhizal(AM) fungi and dark septate endophyte(DSE) are diverse and widely distributed in natural ecosystems. They are functionally important for hosts and may aid plants surviving in highly competitive and endangered environments, which may be used as sensitive indicators of soil ecological quality. Impoundment of the Three Gorges Reservoir(TGR) has dramatically influenced riparian environments and shaped a special riparian vegetation zone, which has been subjected to long-term winter flooding and short periods of summer flooding. Due to the artificial control of the Dam, the riparian environments of the Jialing River and the Yangtze River are completely different, which may influence the AM and DSE colonized in riparian plants. Furthermore, as the prevalence in extreme habitats and their alleged roles in protecting plants under waterlogged conditions make studies on AM and DSE even more compelling. Thus, the objectives of this paper are to(1) investigate the AM and DSE fungal status and the morphology of the former in common riparian plants collected from the water-level-fluctuate-zone of TGR,(2) quantify levels of AM and DSE colonization,(3) assess the relationship between AM and DSE colonization in plants occurring in TGR,(4) isolate indigenous dark septate endophyte from native C. dactylon, as well as(5) assess the ability of DSE to alleviate waterlogging stress in C. dactylon through effects on root traits, root activity, anti-oxidant enzymes. The main results are as follows:1. In this study, 66 plant species of 27 families were examined for the presence of AM and DSE associations. For 30 plant species surveyed, this is the first report on their mycorrhizal status. Overall, 52 species had AM association in at least one individual at one or more sites. Significant variations existed among species for arbuscules, vesicles, hyphae and total colonization. Mean total colonization levels ranged from a low of 4.1 ± 2.3 in Nepeta fordii(Labiatae) at site Caijia to a high of 76.6 ± 3.5 in Selaginella nipponica(Selaginellaceae) at site Fengdu. In plants colonized by AM, hyphae colonization was the highest component of AM total root colonization compared with arbuscular or vesicular colonization, and internal and external hyphae accounted for most of the structures detected.2. In contrast, the DSE structures were observed in 42 species. Like AM, significant variations also existed among species for dark septate hyphae, microsclerotia and total colonization. Average percentage of total colonization ranged from 0.4 ± 0.2 in Rumex acetosa(Polygonaceae) at site Jiangjin to 58.0 ± 2.1 in Kalimeris indica(Compositae) at site Caijia. Inspite of the presence of a wide range of colonization levels, most species colonized by DSE had mean total colonization levels less than 10%. For most of them, the main component of total colonization rate was still hyphae colonization. However, it was different for Salix variegata, Veronica undulata, Myricaria laxiflora and K. indica, in which total root colonization mainly consisted of microsclerotia colonization.3. It was noted that AM and DSE colonization co-occurred in 33 plant species surveyed. For them, typically one type of colonization was obviously superior to the other. However, there was no correlation between AM and DSE fungal colonization in species which were colonized by both AM and DSE.4. Of the 52 plant species colonized by AM, 24 were of the Arum-type, 22 were of the Paris-type and 6 taxa revealed Intermediate morphology. It was worth noting that the occurrence of resting spores of Olpidium spp. were observed in Polygonum nepalense and Galium aparine, and spore-like structure in cortical cells were also observed in plant Ficus tikoua.5. AM and DSE colonization levels tended to have largely contrasting responses to the Yangtze and Jialing River for those plant species collected repeatedly from different sites. AM had higher average colonization levels in plants of Yangtze River. There was significant variations in the percent root length with hyphae and total colonization between two rivers of TGR. In contrast, mean DSE colonization level was higher in Jialing River plants than in plant species from Yangtze River. Unlike the AM, significant variations were detected between two rivers of TGR in root length with microsclerotia and total colonization.6. In our present work, among 18 endophytic strains isolated from surface-sterilized roots of C. dactylon, only one was found to be DSE fungi. The fungal endophyte CD-31 grew rapidly on PDA. After 1 week, colonies were 5 – 5.5 cm in diameter, dark green to black, usually with loose shallow margins, flattened with low, slightly lanose, or radiating aerial mycelium. Our molecular analysis of the ITS nuclear r DNA region showed that it closely resembles Pleosporales sp.(99% similarity to the Gen Bank sequence from Pleosporales sp. HQ914851) and sequence alignment showed that CD-31 and Pleosporales sp. formed a cluster of 100%.7. The root growth traits of Cynodon dactylon such as fresh weight, average length, volume and stolon number had a tendency to reduce with increasing flooding stress, but these reductions were not significant. Under the three waterlogging conditions, colonization by Pleosporales sp. CD-31 did not affect the root average length, volume and stolon number in C. dactylon seedings, but significantly increased root fresh weight, relative to the dead fungi-inoculation control.8. The waterlogged stress significantly decreased the root activity of C. dactylon seedlings regardless of DSE colonization. Under 30% and 50% waterlogging treatment, a significant increase of root activity was observed in Pleosporales sp. CD-31-colonized seedlings, relative to the dead fungi-inoculation control. With regard to 80% waterlogging treatment, the root activities were not significantly different between DSE and non-DSE seedlings.9. The soluble carbohydrate concentrations of C. dactylon roots were significantly decreased with increasing flooding stress, whether the Pleosporales sp. CD-31 colonized or not. Compared with the dead fungi-inoculation control, colonization with CD-31 did not significantly affect soluble carbohydrate concentration of C. dactylon roots under the 30% and 50% of waterlogging conditions. But for the 80% waterlogging treatment, C. dactylon seedlings inoculated by Pleosporales sp. CD-31 significantly decreased soluble carbohydrate concentration by 42.2%.10. The SOD, CAT and POD activity of C. dactylon seedlings were positively correlated with the intensity of waterlogging stress. Compared with the non-DSE seedlings, the DSE seedlings displayed significantly higher SOD activity. Corresponding to 50% and 80% waterlogging treatment, the root SOD activity in C. dactylon seedlings inoculated by Pleosporales sp. CD-31 was 21.0% and 22.0% higher than that inoculated by dead Pleosporales sp. CD-31. For the CAT activity, C. dactylon seedlings which inoculated by CD-31 showed higher activity on the CAT activity by 36.0%, 36.3% and 17.0% under 30%, 50% and 80% waterlogged condition, respectively, relative to the dead fungi-inoculation control. Compared with the non-DSE seedlings, the increase in POD activity was greater in DSE seedlings under 50% and 80% waterlogging stress: 30.9% in root under the 50% waterlogging, and 46.6% in root under the 80% waterlogging conditions. |
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