RESPIRATORY RATE AND EFFECTS OF HEAT-STRESS IN PHYSARUM-POLYCEPHALUM DURING TRANSFORMATION FROM SCLEROTIUM TO PLASMODIUM

Physarum polycephalum, a ''slime mold'' widely used in laboratory expe rimental studies, becomes less resistant to heat stress during transfo rmation from sclerotium to plasmodium in an interval between 60 to 120 min after hydration as evidenced by decreased respiratory recovery an d delayed excystment. The mean respiratory rate of non heat-treated pr eparations during transformation from sclerotium to plasmodium has an initial short lag period, followed by a rapid rise in respiration reac hing a peak at approximately 60 min post-hydration. This is followed b y 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 pla teau in respiratory activity occurs when the cysts progressively becom e merged into a motile plasmodial network. The mean respiratory rate o f the plasmodial stage is 0.51 (S.E. = +/-0.1) micromoles O-2 per mg d ry weight per hour. Heat treatment at 60 and 120 min post-hydration re veals increasing susceptibility of respiration to heat shock. Heat tre atment at 60 min after hydration resulted in a 48% reduction in respir ation rate while heat treatment at 120 min after hydration resulted in 81% reduction in respiration. The difference was significant (p = 0.0 1). Sclerotia exposed to heat shock at 120 min are delayed in excystme nt compared to those exposed at 60 min post-hydration, further indicat ing a decreased resistance to heat shock during the excystment process .