Graduate Thesis Or Dissertation
 

Rhizosphere microfloras associated with mycorrhizae of Douglas-fir and red alder

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  • The mycorrhizal rootlets of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco.) and red alder (Alnus rubra Bong.) were extensively investigated. A jet-black mycorrhiza was found to be dominant on Douglas-fir rootlets. The fungal symbiont was identified as Cenococcum graniforme (Sow.) Ferd. and Winge. Two forms of mycorrhizae predominated on root systems of red alder. Detailed morphological studies, the first for this alder species, revealed distinct characteristic differences between the fungal symbionts. One common mycorrhizal form was clavate with a dark -brown roughened fungal mantle. The Hartig net was well developed. The other predominant form was pale brown and glabrous. The Hartig net was weakly developed and sporadic. Rhizosphere microflora of three morphologically different mycorrhizae of a Douglas-fir were examined and compared with microflora surrounding adjacent suberized roots and with that in nonrhizosphere soil. Populations of bacteria, molds, and Streptomyces were different for each microhabitat. Bacteria varied in total numbers and in distribution of morphological and physiological types, A low percentage of Streptomyces and fewer molds were found in mycorrhizal and adjacent suberized root rhizospheres than in the soil mass. Rhizosphere fungi were predominantly members of the genus Penicillium, which also were abundant, together with representatives of Aspergillus and Trichoderma, in nonrhizosphere soil. Rhizosphere differences between the morphologically distinct white, gray, and yellow mycorrhizae are attributed to influence of the different associated fungal symbionts. Rhizosphere microfloras of Cenococcum graniforme mycorrhizae of Douglas -fir, of one type of ectotrophic mycorrhiza of red alder, and of nonmycorrhizal suberized roots of both tree species were investigated. Microbial populations and the most probable numbers of ammonifying and nitrate reducing microbes differed qualitatively and quantitatively among rhizosphere microhabitats. Nutritional classification of isolates from each rhizosphere micro-habitat revealed that a large proportion was capable of synthesizing their growth requirements from glucose-salts media or from basal media supplemented with amino acids. In contrast, complex substances in yeast and soil extracts were required for growth by most of nonrhizosphere soil isolates. In manometric studies, homogenized Douglas-fir nonmycorrhizal suberized root and red alder mycorrhizal root suspensions highly stimulated respiration of nonrhizosphere microbes, especially in the presence of glucose. Glucose oxidation, however, was suppressed in the presence of Douglas-fir mycorrhizal root suspension, probably by the antibiotic which the fungal symbiont, Cenococcum graniforme, is reported to produce. Glucose oxidation by nonrhizosphere microbes was similarly repressed in the presence of red alder nonmycorrhizal root suspension. An antibacterial substance, found in red alder root and nodule suspensions, inhibited growth of Bacillus subtilis and B. cereus on glucose-salts agar. These experimental results are discussed with reference to the influence of mycorrhizal and adjacent nonmycorrhizal suberized roots upon rhizosphere microfloras.
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