Analytical Approach to the Study of Circadian Leaf Oscillations in Clover. II. Responses to Continuous Light and to Sinusoidal Light Oscillations

Abstract

The spectral characteristics of the circadian leaf oscillation of Trifolium repens L. were investigated in constant illumination from 1-10 000 lux. The natural frequency was found to fall from 0.92 cycles/day at 1 lux to 0.84 cycles/day at 10 000 lux, with a corresponding fall in damping coefficient from 0.29 day^-1 to 0.10 day^-1. Spectral characteristics of the leaf responses to rectangular wave light variations, and to sinusoidal variations in the logarithm of light intensity were also compared. The response to sinusoidal light variations was measured over the frequency range 0.67-20 cycles/day and Bode amplitude and phase plots indicated resonances at 0.85 cycles/day (the circadian frequency) and also at 9.5 cycles/day. A model consisting of two feedback loops and two phase advance elements was developed and its frequency characteristics approximated to those of the leaf. The physiological implications of the proposed model are considered.