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The Rhizobium sp. strain PR389 was isolated from the root nodules of (Willd.) Ohwi, which grows in acidic (pH 4.6) yellow soil of the Jinyun Mountains of Beibei, Chongqing, China. While rhizobia generally have a pH range of 6.5-7.5 for optimum growth, strain PR389 grew in a liquid yeast extract - mannitol agar medium at pH 4.6, as well as in a pH 4.1 soil suspension, suggesting acid tolerance in this specific strain of rhizobium . However, at pH 4.6, the lag phase before vigorous growth was 40 h compared with 4 h under neutral conditions (pH 7.0). For PR389, the generation time after the lag phase remained the same at different pH levels despite the different durations of the lag phase. Except in the pH 4.4 treatment, the pH of the culturing media increased from 4.6, 4.8, 5.0, and 5.5 to neutral and slightly alkaline after 70 h of culture. Chloramphenicol was added to determine if protein production was involved in the increasing pH process. Chloramphenicol significantly inhibited PR389 growth under acid stress but had little effect under neutral conditions. Proton flux measured during a short acid shock (pH 3.8) revealed that this strain has an intrinsic ability to prevent H+ from entering cells when compared with acid-sensitive rhizobia. We propose that the mechanism for acid tolerance in PR389 involves both intracellular and extracellular processes. When the extracellular pH is lower than pH 4.4, the cell membrane blocks hydrogen from entering the cell. When the pH exceeds 4.4, the rhizobium strain has the ability to raise the extracellular pH, thereby, potentially decreasing the toxicity of aluminum in acid soil. [ABSTRACT FROM AUTHOR]

The extent to which pretransplant colonization of seedlings with the arbuscular mycorrhizal fungus (AMF) Schenck and Smith emend. Koske could enhance the post-transplant growth of two cultivars of (Lam.) de Wit (cv. K-8 and cv. K-636) in Al- and Mn-rich acid soils was evaluated in a greenhouse. Arbuscular mycorrhizal colonization measured at the end of the experiment was significantly stimulated by inoculation in both cultivars at all pH levels tested, although colonization was most stimulated if cv. K-8 was grown in the Al-rich soil at the lowest pH. Symbiotic effectiveness measured as P content of pinnules was significantly suppressed in both cultivars if they were grown at the lowest pH. Symbiotic effectiveness measured as pinnule P content and shoot biomass yield was enhanced in both cultivars by liming. The trends in effectiveness were similar in both cultivars, but cultivar effect was significant in the Mn-rich Oxisol (Wahaiawa soil) but not in the Al-rich Ultisol (Leilehua soil). The tolerance of the cultivars to acid soil toxicity in the Wahiawa soil varied with the pretransplant mycorrhizal status of their seedlings. The effect of pretransplant colonization of seedlings was to eliminate the differences in the tolerance of the cultivars to acid soil toxicity. Our data suggest that AMF could offset some of the growth reduction associated with soil acidity and that host genotype could play a role in this regard. [ABSTRACT FROM AUTHOR]

Recently, sugar maple (Acer saccharum Marsh.) decline in northeastern North America has been regarded as a major factor structuring hardwood forests by favouring American beech (Fagus grandifolia Ehrh.) in the understory of maple-dominated stands. To determine whether soil fertility differences associated with sugar maple decline may have promoted the expansion of American beech, we explored the relationships between the soil base status and the sapling and tree strata density and composition, using data from 426 permanent sample plots distributed throughout Quebec. Our results indicate that American beech is currently expanding in the sugar maple range of Quebec. The abundance and proportion of American beech in the sapling stratum are mainly associated with the proportion of American beech in the tree stratum, the relative basal area of dead sugar maple trees, and the base status of soils. In accordance with the many studies reporting on the high sensitivity of sugar maple to the acid–base status of soils and the decline of the sugar maple population, this study supports the hypothesis that soil base cation depletion, caused in part by atmospheric acid deposition, is among the main factors involved in the present-day expansion of American beech over a large area in Quebec. [ABSTRACT FROM AUTHOR]