Summary
The step-down photophobic response ofEuglena induced by lowering the light intensity was studied quantitatively with individual cells. One cell was isolated in a small agar chamber and its response to a rectangular pulse of the stimulating light was observed with red light under a low magnification (objective × 4). 1. The mode of the phobic response changed according to the strength of the stimulus or the degree of attenuation of the light intensity, indicating that at least the first and the last steps of signal processing are of a graded and not an all-or-none nature. 2. The threshold stimulus to induce the phobic response was minimum at midday in the culture light regime, supporting the idea that the circadian clock is involved in the photophobic response. 3. The relationship between the illuminating time and the threshold light intensity was a rectangular hyperbola, indicating that absorption of a certain amount of light energy is necessary before induction of the phobic response. 4. Weber's law was valid at higher light intensities. The value of Δ I/I0 was 0.5–0.65. 5. Re-illumination of the cell stopped the phobic response at an intensity of only 10–20% of the light pulse whose switching off had induced the response.
Based on these results, a mechanism of accumulation of cells in the light area is discussed.
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Shimmen, T. Quantitative studies on step-down photophobic response ofEuglena in an individual cell. Protoplasma 106, 37–48 (1981). https://doi.org/10.1007/BF02115960
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DOI: https://doi.org/10.1007/BF02115960