SPORE GERMINATION AND CARBON METABOLISM IN FUSARIUM SOLANI. II. ENDOGENOUS RESPIRATION IN RELATION TO GERMINATION

COCHRANE, V. W., JEAN C. COCHRANE, C. B. COLLINS, and F. G. SERAFIN. (Wesleyan U., Middletown, Conn.) Spore germination and carbon rmetabolism in Fusarium solani. II. Endogenous respiration in relation to germination. Amer. Jour. Bot. 50(8): 806-814. Illus. 1963.Endogenous oxygen uptake by ungerminated macroconidia of Fusarium solani f. phaseoli is more than doubled by exogenous ammonium ion and is increased about 7-fold by germination. The respiratory quotient is halved by the provision of ammonia, which has essentially no effect on the endogenous formation of carbon dioxide. Stimulation by azide and 2,4-dinitrophenol suggests that the supply of phosphate acceptors regulates the rate of endogenous respiration. Mercurials poison the endogenous respiration, cyanide accelerates it, and iodoacetate, arsenite, fluoride, and chelating agents have little effect. Respiration is little affected by changes in pH, external phosphate, oxygen concentration, and spore density, within the limits tested. Spore lipid concentration is increased by cultivation in a high-glucose medium, but this variation in lipid content of spores does not affect the endogenous Qo2, nor does a high lipid content abolish the requirement for exogenous carbon for germination. Lipid utilization during endogenous respiration accounts for 37% of the loss in dry weight; lipid is also utilized during germination, but such utilization amounts to only about 5% of the carbon requirement. Neither mannitol nor nitrogenous compounds are significant substrates of endogenous respiration. The oxidation of the exogenous substrates tested does not measurably affect the concomitant rate of endogenous respiration. It is proposed that endogenous respiration can contribute to the synthetic processes of spore germination, although quantitatively insufficient to support germination without exogenous carbon. It is also questioned whether the respiratory quotient is an adequate index of the substrate of endogenous respiration. IN THE first paper of this series (Cochrane et al., 1963), it was shown that endogenous respiration by itself does not support germination in macroconidia of Fusarium solani f. phaseoli, i.e., that an exogenous carbon source is required. There was, however, a suggestion that endogenous reserves may contribute energy or carbon to the germination process, inasmuch as "high-reserve" spores (grown in a medium high in carbohydrate) have a slightly lower carbon requirement, on a dry weight basis, than normal spores, and inasmuch as prior assimilation of glucose appears also to lower the requirement for exogenous carbon. Species of Fusarium are known to accumulate large amounts of lipid in mycelial cells (Murray, Woodbine, and Walker, 1953). Accumulation in spores has not been studied quantitatively; Sisler and Cox (1954) found that macroconidia of F. roseumn after incubation in glucose contain fatstaining granules. Lipids accumulated during growth and spore formation have, therefore, been examined as possible sources of energy for germination, a role suggested in recent studies on the 1 Received for publication January 25, 1963. 2 This investigation was supported in part by the United States Atomic Energy Commission, under Contract AT (30-1)-1813, and in part by the National Science Foundation, under Research Grant G-9087. germination of rust fungus uredospores (Shu, Tanner, and Ledinghamn, 1954; Farkas and Ledingham, 1959). In one sense, the characteristic and necessary biochemical event in spore germination is protein synthesis, preceded or accompanied by synthesis of amino acids and amides (Yanagita, 1957). The synthesis of nitrogenous compounds, especially amino acids, occurs, of course, at the expense of respiratory intermediates and in turn may affect, qualitatively or quantitatively, the patterns of respiration. Now, if endogenous respiration contributes to amino acid synthesis, it should be affected by provision of exogenous nitrogen. This question has been considered along with the effect of germination itself on the rate of exogenous respiration and on the endogenous respiratory quotient. Endogenous respiration in fungi has 2 other facets of interest. First, it is commonly so active that there is a real question whether it does not compete with substrate metabolism in respiration; such competition would result either in suppression of endogenous respiration by substrate or in the less often considered but equally possible suppression of substrate respiration by endogenous metabolism (Dawes and Ribbons, 1962).