Phasic response of the photoperiodic clock to wavelength and intensity of light in the redheaded bunting, Emberiza bruniceps.

We have investigated phasic response of the photoperiodic clock to wavelength (color) and intensity of light in the male redheaded bunting (Emberiza bruniceps). Two experiments were performed. Experiment I examined whether varying the wavelength and intensity of first (entraining, E) and/or second (inducing, I) light pulse will alter the effects of a skeleton photoperiod. Birds were subjected for a period of 6 weeks to skeleton photoschedules (6L:5D:1L:12D; 6 h E-light pulse, 1 h I-light pulse) containing either an E-pulse at 5 or 20 lx coupled with I-pulse at 100 lx, or an E-pulse at 100 lx, coupled with I-pulse at 5- or 20-lx intensity. Additional two groups that received both E- and I-pulses at 100 lx served as controls. All photoschedules were employed in two colors-white and red (654 nm). There was the wavelength- and intensity-dependent stimulation of the testis growth and development. Long wavelengths of light (red light) induced faster and greater gonadal response, but the effects were also intensity dependent. Experiment II tested whether in the photostimulated birds held on long photoperiods the change in wavelength and intensity of light hours in the morning (entraining period) or the evening (inducing period) will influence the maintenance of the photoperiodic sensitivity. Birds were subjected initially to a long photoperiod of 14L:10D (L = approximately 500 lx; D = 0 lx) and then after 3 weeks, a 4-h light period in the morning (zeitgeber time, zt, 0-4) or in the evening (zt 10-14) of 14 L was substituted with white, green (528 nm), or red (654 nm) light at approximately 20-lx intensity. One group maintained on 14L:10D and other exposed to 10L:14D served as controls. After another 7 weeks, all birds were subjected to 16L:8D for a further 4 weeks to test for their responsivity to long-day photostimulation as a consequence of exposure to different experimental photoperiods. Testes regrew under 16L:8D only in birds that were exposed to 10L:14D or to 14L:10D with a green light pulse. However, there was no effect of the timing (morning, evening) of the light pulse. Taken together, the results from both the experiments indicate that in the redheaded bunting (1) the photoperiodic clock responds differentially to different wavelengths (colors, spectra) and intensities of light, and (2) the effects of wavelength and intensity of light on the clock are phase dependent, and such phasic effects can be seen in skeleton photoperiods in which light is applied discretely at different circadian phases.