Control theory approach to the study of circadian leaf movement in trifolium repens
Creators: name: Robinson, GR
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis describes a study of natural and forced leaf oscillations in white clover (Trifolium repens L.). Apparatus was constructed which permitted the accurate control and programmed variation of environmental conditions surrounding clover plants and also the automatic recording of changes in clover leaf angle. Records of leaf movement were examined and interpreted by applying the techniques of Fourier analysis and of control theory. Problems associated with the analysis of short oscillatory data records are discussed and the advantages and limitations of Fourier analysis are described. Particular attention is given to possible errors in spectral estimates. Experiments are described in which clover leaves were treated with constant intensity light, single light pulses, rectangular wave light changes and sinusoidal log-light variations; i.e. light oscillations forwhich the logarithm of light intensity varied sinusoidally with time. For continuous light treatment the characteristics of the natural (circadian) leaf oscillation (e.g. frequency, damping rate, harmonic content) are considered for different light intensities. Experiments with sinusoidal log-light oscillations of various amplitude indicate that entrainment is not an all-or-none response but that frequency components due to both the natural and forcing oscillation may contribute to the leaf movement. As part of a control theory approach to analysing the clover leaf oscillator plants were treated with sinusoidal log-light oscillations ranging in frequency from 2/3 c/d (cycle per day) to 20 c/d. The amplitude and phase of the component in the leaf movement at the frequency of the forcing oscillation were determined for each case. These frequency response data are graphed as separate amplitude and phase plots (Bode plots). A feedback model having a transfer function which is consistent with these data is developed. Some nonlinear features of this system relating light changes to leaf movement are also described. A characteristic of a circadian system considered to be important by many investigators is its light-pulse phase response. This is shown graphically by plotting the change in phase of a circadian oscillator caused by a light pulse applied at different times during the natural cycle. Pulse phase response studies are considered and the relationship between these and a frequency response analysis is examined. Results for 2 hour light pulses are given and are shown to be consistent with the mathematical model developed from the frequency response data. A phase response curve for-2.5 mM 2 hour azide pulses is also presented and is shown to have a form similar to. the light phase response curve. Repeated KCl and NaCl pulses do not entrain the cloverleaf oscillatorbut do have some effect on leaf movement. Frequency changes caused by the presence of ethanol are reported. Finally circadian-rhythm for excised leaves in continuous light is shown to be markedly less heavily damped if the leaves are supplied with sucrose. Attempts were made to continuously monitor pulvinule transmembrane potential. Although such monitoring proved unsuccessful it is shown that the potential for both adaxial and abaxial pulvinule cortical cells varies in a circadian fashion. These results and in particular the mathematical model developed are discussed in terms of possible physiological models for the circadian oscillator.