Studies on the Basic Physiological Conditions Affecting the Spore Germination of an Obligatorily Tonophilic Fungus
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Some basic physiological conditions affecting the ascospore germination of the obligatorily tonophilic fungus, Eurotium halophilicum forma A have been studied. This was facilitated by availability of a fungal strain which forms only ascospores. The results obtained are summarized as follows. 1. The process of ascospore germination in the obligatorily tonophilic fungus can be divided into two phases, swelling and sprouting. The swelling phase is divided into early and later stages. Spores in each stage can be distinguished morphologically. The early stage corresponds to the stage of exogenous swelling and the later to endogenous swelling of the conidiospores of Aspergillus niger. 2. The effect of environmental factors on the ascospore germination of the fungus have been studied in detail. The optimal conditions were 28°, 90% RH and pH 5.6-6.0. Dried malt-extract agar smeared on a slide-glass are found to be most favorable for aerial germination of ascospores. Spore germination occurs within the ranges of 15゜-37゜, 95-60% RH, and pH 1.8-10.6 on this culture medium. Heating of the spores at 50゜-100゜ before inoculation shows a suppressive and some times lethal effect. 3. Germination tests on dried simple agar medium have revealed that addition of various amino acids, vitamins (10-400μg/ml) and sugars (20mg/ml) to the basal suspension medium (Czapek's salt solution without C and N sources) are almost indifferent for germination. However, spore germination was promoted when a substance was added to the basal suspension medium together with sucrose (30mg per ml). And, pre-incubation of spores in the suspension medium at 28゜ for three weeks was most favorable for germination. It seems probable that external substances diffuse into the spore cells in an earlier stage, and certain metabolic change may take place at a later stage within the cell, whereupon the spores may be driven into the sprouting phase. 4. The effects of fifteen kinds of salts on germination have also been examined. On the whole, the salts, added to the suspension medium at higher concentrations showed suppressive effect on germination. The effect varies with the kind of salts and depends on a preincubation period. Among the salts tested, LiCl, LiNO_3 and NH_4NO_3 suppressed germination almost completely. However, the spores were able to germinate better, when salts were removed by washing. 5. Spores, whose germination was suppressed by highly concentrated salt solutions, recovered their capacity of germination after dilution of the salt solutions. In the case of NaCl, an optimal concentration (1.71M) lies between 5.13M and 0.005M salt solutions. As regards. LiCl and LiNO, suppressive effect decreased with an increase of dilution rate. On the other hand, decreases in spore number and a corresponding decreases in transmittance have been observed in the solutions of NaCl, MgCl_2 LiCl and NH_4NO_3 after pre-incubation at 28゜ for 7-9 hours in the concentrated solutions. From all these results, certain parallelism exist in the effects of salts during pre-incubation.
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