Physiology of thick-walled spores of soil-borne plant pathogenic fungi. I. Respiration and germination of Helminthosporium sativum and H. pedicellatum spores

Abstract

Spores of Helminthosporium sativum germinated well at temperatures 30–39 °C while spores of H. pedicellatum germinated best at 39 °C with a lower percentage germination at lower or higher temperatures. Although germination of spores of both species occurred in the absence of an exogenous energy source, it was enhanced by various mono- and di-saccharides as well as by various plant extracts such as orange juice. The viability of these spores remained high (above 80%) for over 2 years when stored with a moisture content of 11% at 4 °C. The constituents of spores of both fungi were quite similar with lipids composing about 10% of the dry weight and carbohydrates 5.5–8.3%. Trehalose and mannitol were the most abundant carbohydrates. Spores of H. sativum had an endogenous respiration rate that was about 2–3 times greater than that of H. pedicellatum spores. The respiration rate of spores of both fungi could be stimulated by various energy sources, including sugars and some polyols. The greatest stimulator of respiration was buffered orange juice. As spores aged up to 25 months the respiration rates varied but to no significant extent. Respiration rate of spores of both fungi was not greatly affected by temperature but spores of H. pedicellatum that had been incubated for 15 h at either 30° or 39 °C did exhibit a greater oxygen uptake at 39 °C than at 30 °C. Use of various metabolic inhibitors indicated the presence of a functioning oxidative phosphorylation system in spores of both fungi. Lack of response of H. pedicellatum spores to high concentrations of dinitrophenol and azide may indicate that these inhibitors may have difficulty in penetrating into the cytoplasm.