Ionizing irradiation sensitizes spores to subsequent heating, though the converse is much less effective. The magnitude of the effect varies with irradiation dose rate and is increased if the spores are additionally preincubated at low pH values, as described above. It has been suggested that the effect derives from radiation-induced decarboxylation of peptidoglycan in the spore cortex, which is known to play a role in maintainance of the low water content in the enclosed spore protoplast, but this is not proven.