Physarum polycephalum, a ''slime mold'' widely used in laboratory experimental studies, becomes less resistant to heat stress during transformation from sclerotium to plasmodium in an interval between 60 to 120 min after hydration as evidenced by decreased respiratory recovery and delayed excystment. The mean respiratory rate of non heat-treated preparations during transformation from sclerotium to plasmodium has an initial short lag period, followed by a rapid rise in respiration reaching a peak at approximately 60 min post-hydration. This is followed by a decline to a lower steady state level at about 90 to 120 min. Fine structural data show that the period of rapid increase in respiration precedes dissolution of the walls surrounding the spherical cysts and subsequent formation of cytoplasmic cross linkages. The post-peak plateau in respiratory activity occurs when the cysts progressively become merged into a motile plasmodial network. The mean respiratory rate of the plasmodial stage is 0.51 (S.E. = +/-0.1) micromoles O-2 per mg dry weight per hour. Heat treatment at 60 and 120 min post-hydration reveals increasing susceptibility of respiration to heat shock. Heat treatment at 60 min after hydration resulted in a 48% reduction in respiration rate while heat treatment at 120 min after hydration resulted in 81% reduction in respiration. The difference was significant (p = 0.01). Sclerotia exposed to heat shock at 120 min are delayed in excystment compared to those exposed at 60 min post-hydration, further indicating a decreased resistance to heat shock during the excystment process.
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