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.
A physiological study on the toxicity of culture filtrate of helminthosporium sativum P.K. & P.
When symptoms of the leaf-spot disease of barley caused byHelminthosporium sativum could be reproduced by dipping host plants in fungal filtrate, it was thought to trace any correlation that may exist between nitrogenous constituents of the filtrate and its wilt-inducing capacity to test plants (lupin seedlings). The fungus was grown on a modified Fries' No 3 medium with sodium nitrate or ammonium chloride or an equivalent mixture of both as nitrogen source. The nitrogen content of the culture medium and its wilt-inducing capacity were traced over 34 days. The nitrogen content, after reaching a minimum, was found to rise again; the rise being more rapid and bigger in nitrate and ammonium-nitrate media than in ammonium media. Only a partial apparent correlation could be noted between the trend in the wilt-inducing capacity of filtrates obtained at different periods of incubation, and the trend in the change in their nitrogen content. It is suggested that the toxicity of the culture filtrate may be attributed to more than one substance, one of them being a nitrogenous product (possibly of peptide nature). The observed degree of toxicity would therefore depend on the relative amounts of the nitrogenous product and other substances.
Investigations on the rôle of organic matter in plant nutrition
1. As the result of decomposition of green manure in a swamp soil, the coarse fractions (including coarse and fine sand) of the surface layer (first 3 inches) decreased by 11·2 per cent. while the fine ones (silt+clay) increased by 9·8 per cent. in the course of 43 days. The effect was less pronounced in the manured soil maintained at 60 per cent. saturation, while no alteration was to be noticed in the case of the unmanured specimens. 2. Treatment with dilute hydrochloric acid prior to dispersion had no appreciable effect on the estimates of mechanical composition obtained in the different cases. Loss in solution was also comparatively small. 3. The silica-sesquioxide ratio increased slightly as the result of decomposition of green manure in swamp soil. The composition of the fine fractions was also altered, the total percentages of the oxides of iron, aluminium and silicon increasing from 90·5 to 99·3 in the course of 71 days. 4. The practical significance of the above and related observations has been discussed.