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Contraction rhythm in the plasmodium of Physarum polycephalum: dependence of the period on the amplitude, temperature and chemical environment.

The plasmodium of Physarum polycephalum exhibits a characteristic protoplasmic shuttle streaming and generates periodical tension. We determined the factors which govern the period of the contraction rhythm by measuring isometric tension of the plasmodial strand and the motive force of the protoplasmic streaming under a variety of conditions by changing the size of plasmodia, chemical composition of environment and surrounding temperatures. The results are: (1) The period of contraction rhythm, tau, increased linearly with the amplitude of oscillating tension, F, and was expressed by the following empirical equation when F was lower than a certain critical value, Fc, i.e., tau = aF+tau O. Above Fc, tau stayed at a constant level of tau S. There, a tau O and tau S are numerical constants which are independent of F. A similar relationship is valid for the amplitude of the motive force of the protoplasmic streaming, delta P. (2) Values of tau O were 1.0 min in air and 1.6 min in an aqueous medium and they were independent of temperature. (3) The Arrhenius plots of the parameter "a" exhibited different straight lines in air and in aqueous medium, from which the values of Q10 were determined to be 4.0 and 10, respectively. (4) Lowering of temperature decreased Fc, and eventually diminished it at Tc (= 15 degrees C). (5) The presence of glucose, CaCl2, MgCl2 and NaCl gave the identical tau-F relationship. Applications od D2O and ethanol slowed down the contraction rhythm, while that of KCl accelerated the rhythm. However, all these chemicals did not affect the tau O value. All results described above indicate that the contraction rhythm in the plasmodium is influenced by three components: a limit cycle which is independent of tension generation, a component which is strongly linked to the amplitude of the tension, F, and the part which is independent of the value of F. External factors appear to influence separately these three components of the contraction rhythm.