Physiology of the causal fungi of stem rot of rice plant

Abstract

In the present paper, the results of a study on the nitrogen metabolism of the causal fungi of stem rot of rice plant, Leptosphaeria salvinii Catt. and Helminthosporium sigmoideum Cav. irregulare Cralley et Tullis. Both fungi were cultured in liquid media at 25°C for 3 weeks. The nutrient solutions contained one of the following substances respectively, as a nitrogen source: KNO3, (NH4)2SO4, (NH4)2HPO3, NH4NO3, NaNO2, urea and peptone. Decoction of straws and leaves of rice plant was added to the nutrient solution at a rate of 10%, as growth promoting substance.Both fungi could not utilize NO2-N, but absorb NH3-N, NO3-N, urea and peptone. Growth of the fungi was checked by NO2-N. Difference in growth between the two fungi·was observed according to the kinds of nitrogen source. Leptosphaeria-fungus could grow better than Helminthosporium-fungus in the presence of (NH4)2SO4, NH4NO3 and peptone, while Helminthosporium preferred (NH4)2HPO4. The two fungi grew vigorously in the presence of nitrogenous substances which have not decreased the pH of the nutrient solution. Above all, peptone gave the best growth of them. When NH3-N and NO3-N co-exit, both fungi absorb NH3-N selectively and quickly, and absorb NO3-N later. Helminthosporium-fungus begins to absorb and utilize NO3-N earlier than Leptosphaeria-fungus.