Circadian Modulation of Circumnutation Length, Period, and Shape in Helianthus annuus

Modulation of a turgor-growth movement called circumnutation in sunflower (Helianthus annuus L.) was investigated using a picture analysis system. Two photoperiod conditions were applied: light–darkness conditions (LD) 8:8 and LD 20:10. After about 3 weeks of these regimes, the plants were placed under constant light to determine whether circadian regulation of circumnutation existed or not. The rhythms of movement activity with regard to the trajectory length, period, and shape of individual circumnutations were examined. Data were processed by Fourier spectral analysis. All the parameters, trajectory length, period, and shape, revealed the ability to entrain to the administered daily cycles (16 h or 30 h). We observed diurnal fluctuations of the circumnutation parameters with the phase of the highest trajectory length, the shortest period, and the highest shape coefficient (the most circular form) during the dark period. After the LD–LL transition, the parameters revealed periodicity, which was close to 24 h. After several days of a clear circadian free running rhythm, a gradual decrease of the amplitude of the rhythm was observed. However, the rhythm did not disappear completely. The trajectory length manifested the strongest entrainment; the circumnutation period and the circumnutation shape were less modulated by photoperiod. These findings indicate for the first time that different parameters of circumnutation in sunflower are circadian-regulated rhythms, not solely ultradian as had been thought previously.