The respiration and germination of activated ascospores of Neurospora tetrasperma have been shown to be almost completely inhibited by concentrations of ethylene diaminetetraacetic acid (EDTA) as low as 0.0035 M. In contrast, however, dormant ascospores are insensitive to this chelating agent. At any time up to about 150 minutes after activation Ca++ or Mg++ can completely reverse this toxicity but Cu++, Co++, and Mn++ only partially reverse it. After this time, the minerals of the Neurospora "minimal" medium taken singly, or in various combinations cannot reverse this effect. Adding EDTA at 120 minutes after activation eliminates the lag period associated with its effect upon respiration. Inhibition occurs even though the cells seem to be impermeable to EDTA. Cationic exchange resins, as another example of a non-penetrating metal-binding agent, gave effects similar to those noted with EDTA. Of the resins used the H+ form of IR-120 and the Na+ and K+ forms of amberlite IRC-50 were the most toxic to activated ascospores. On the other hand, dormant ascospores were entirely unaffected by the resins. The release of Ca++ from activated ascospores coincided with the period of maximum sensitivity to EDTA. More than 60 per cent of the cell's content of K+ is released by EDTA-inhibited ascospores. A low pH decreased the effectiveness of EDTA as a poison. The data are consistent with the possibility that non-penetrating metal-binding agents are toxic because of the irreversible removal of essential cations from the cell. The kinetic data for the inhibitory effects, and for the release of Ca++ establish that the permeability of germinating ascospores to minerals changes drastically as a result of activation.

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