This paper analyzes published and unpublished data on phase resetting of the circadian oscillator in the fungus Neurospora crassa and demonstrates a correlation between period and resetting behavior in several mutants with altered periods: As the period increases, the apparent sensitivity to resetting by light and by cycloheximide decreases. Sensitivity to resetting by temperature pulses may also decrease. We suggest that these mutations affect the amplitude of the oscillator and that a change in amplitude is responsible for the observed changes in both period and resetting by several stimuli. As a secondary hypothesis, we propose that temperature compensation of period in Neurospora can be explained by changes in amplitude: As temperature increases, the compensation mechanism may increase the amplitude of the oscillator to maintain a constant period. A number of testable predictions arising from these two hypotheses are discussed. To demonstrate these hypotheses, a mathematical model of a time-delay oscillator is presented in which both period and amplitude can be increased by a change in a single parameter. The model exhibits the predicted resetting behavior: With a standard perturbation, a smaller amplitude produces type 0 resetting and a larger amplitude produces type 1 resetting. Correlations between period, amplitude, and resetting can also be demonstrated in other types of oscillators. Examples of correlated changes in period and resetting behavior in Drosophila and hamsters raise the possibility that amplitude changes are a general phenomenon in circadian oscillators.

Leaflet movement in legumes depends on rhythmic, light-regulated ion fluxes in opposing regions of the leaf-moving organ. In flexor and extensor protoplasts from Samanea saman Merrill, opening and closing of K(+) channels were rhythmic in constant darkness. When channels were open in flexor protoplasts they were closed in extensor protoplasts, and vice versa. The rhythms were shifted by a delay in the onset of constant darkness, a response typical of endogenous circadian rhythms. During the light period, the channels in flexor protoplasts were sensitive to red light that was followed by premature darkness; phytochrome was implicated as the photoreceptor.

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Leaflet movements in the legume Samanea saman are under joint control by light and a circadian oscillator. The movements are driven by massive fluxes of K+, Cl−, and H+ through pulvinar motor cell membranes. Light and the oscillator affect leaflet movements by altering the activity of ion transport systems. Some effects of light on ion transport may be mediated by the phosphatidylinositol (PI) cycle, since brief irradiation of the pulvinus with white light accelerates PI turnover.