The effect of pH value on the toxicity of culture filtrates ofHelminthosporium Sativum and on its possible correlation to extracellular nitrogen accumulation
Helminthosporium sativum P.K. & B. was grown on modified Fries No. 3 medium buffered at pH 4.7, 5.5, 6.7, and 7.8. The nitrogen content of the culture filtrate, mycelial growth and the toxicity of the filtrate (as measured by its wilt inducing capacity to lupin seedlings) were traced over 24 days. The correlation between nitrogen accumulation and the toxicity of the filtrates was clear in case of media originally buffered at 4.7, 5.5 and 7.8. On the other hand, there was no correlation between growth of the mycelial mats and toxin production. The toxicity of the filtrate was attributed to two substances at least, one of unknown composition produced during the accelerated growth phase, while the other is nitrogenous and produced during the decline growth phase i.e. autolytic in nature. Acidity of the medium favoured toxin production during the first two weeks of incubation. The toxicity of the filtrate remained stable within a wide range of pH between pH 3–9.4.
THE pH FACTOR IN THE METABOLISM OF FUSARIUM MONILIFORME SHELDON ON SUCROSE
A study has been made of the effects of varying the pH value of the culture media on sucrose inversion and absorption, on respiration, and on growth of Fusarium moniliforme Sheldon. The study was carried out on 1-week-old fungal mats that were then subjected to experimentation for 48 hours. Sucrose inversion was almost unaffected by pH values higher than 5.4 but was inhibited at pH 4.0. Sugar uptake was influenced more by lowering than by raising pH beyond 5.4. Carbon dioxide output per unit dry weight of mycelial mat was lowest at the pH optimum for growth, as measured by mat dry weight. At the same optimum pH there was also the highest rate of sucrose inversion and uptake. It seemed, therefore, that the effect of pH on growth was the additive effects on the different enzymatic processes operating in the metabolism of the fungus. It is also suggested that the external pH of the medium might act by the invasion of hydrogen or hydroxyl ions into the mycelial cells, thus changing the internal pH.
The assimilation by plants of various forms of nitrogen
SOME YEARS AGO (1926) the author became interested in the possible effect of hydrogen-ion concentration on the assimilation by plants of nitrogen as ammonia or as nitrate. In an experiment 2 designed to attack this problem, mineral nutrient solutions containing dextrose were prepared in which the nitrogen was furnished as ammonium nitrate, ammonium sulfate, or sodium nitrate, and the hydrogen-ion concentration of the solutions was varied by intervals of 0.2 or 0.3 pH from pH 4.0 to pH 7.6. The organism used, Rhizopus nigricans, grew well at all reactions in the solutions which contained ammonium sulfate or ammonium nitrate (somewhat better in the more alkaline solutions) but not at all in those which contained sodium nitrate. This confirmed earlier reports (Bach, 1927; Ritter, 1909) of the inability of Rhizopus nigricans to assimilate nitrogen as nitrate (at least under ordinary laboratory conditions) and encouraged the writer to plan an investigation of the ability of organisms to use various forms of nitrogen. Circumstances made it impossible to carry out the investigation as planned. Nevertheless, the preliminary work may be of sufficient interest to encourage further investigations in this field. A survey of the literature (space precludes summarizing all the available literature) suggested to the author that organisms may be arranged in four groups 3 according to their ability to assimilate various forms of nitrogen. GROIJP I. NITROGEN-FIXING ORGANISMS. These organisms are capable of assimilating gaseous nitrogen and also nitrates, ammonium salts, and certain organic nitrogenous compounds. Attention should be called to the assumption that everv organism capable of fixing gaseous nitrogen is capable also of assimilating nitrogen in the form of nitrates, ammonium compounds, and 'suitable forms of organic nitrogen. Though the evidence available supports it, this assumption needs further confirmation since considerably more attention has been devoted to the relation to gaseous nitrogen of the organisms generally accepted as nitrogen fixers than to their ability to assimilate combined nitrogen. However, it is generally agreed (Waksman, 1927; Lohnis, 1926; Stephenson, 1930) that Azotobacter can assimilate nitrogen as nitrates, as ammonium salts. and as orgLanic nitrogen (for