Abstract
Short-term changes in the cytoplasmic pH in the plasmodium of Physarum polycephalum were measured with special reference to its con-traction-relaxation cycle. The measurement was done photometrically in a plasmodial strand segment loaded with fluorescein diacetate. To eliminate possible errors caused by volume changes of the plasmodial strand in the course of its contraction-relaxation cycle, fluorescence intensities at two excitation wavelengths, 493 nm (pH sensitive) and 448 nm (pH insensitive), were recorded alternately. Simultaneously, tension production of the same strand segment was recorded under isometric conditions. The results were : 1) The cytoplasmic pH averaged 6.6 ± 0.5, for 34 specimens. 2) The cytoplasmic pH changed cyclically with the same period as the tensile force. 3) The average peak-to-peak amplitude of pH change was 0.13 ± 0.06 pH unit. 4) The phase of the pH oscillation was just opposite to that of the force production cycle; i.e., the cytoplasmic pH decreased during the contraction phase and increased during the relaxation phase.
