CHANGES IN THE HEAT‐RESISTANCE OF ASCOSPORES OF NEUROSPORA UPON GERMINATION

Abstract

L ingappa , Y amuna , and A. S. S ussman . (U. Michigan, Ann Arbor.) Changes in the heat‐resistance of ascospores of Neurospora upon germination. Amer. Jour. Bot. 46 (9): 671–678. Illus. 1959.—A rapid loss in heat‐resistance accompanies activation of ascospores of Neurospora tetrasperma after incubation at 27°C. When activated spores are given a 5‐min. “heat‐flash” at 65°C. after only 5 min. at 27°C., fully % fail to germinate. Such treatment, if administered 25 min. after activation, results in the complete destruction of the spores. By contrast, when incubation at 27°C. is not interposed, more than ½ of the spores will germinate, even when they have been exposed to 65°C. for 30 min. Similar results were obtained with “heat‐flashes” at 50 and 60°C., although exposures of longer duration were required to affect the spores. Conidia respond very differently to “heat‐flashes” in that germination is stimulated if they are provided after an incubation period at 27°C. On the other hand, conidia are killed by short exposures to 60°C., so that they are far more susceptible to such treatment than are ascospores. A study of the cardinal temperatures of germination revealed that the maximum is about 44°C. for both conidia and ascospores. The maximum for the growth of two strains of N. tetrasperma and for one of N. crassa is between 40–45°C.; however, another strain of the latter species grows at 45°C. Dry heat was shown to be less effective than wet in activating ascospores. Removal of the exospore of ascospores results in the loss of considerable heat‐resistance. In addition, the requirement for heat‐activation is considerably mitigated in such spores, suggesting that the exospore, or an associated layer is the locus of the ascospore's heat‐resistance.