Tadeusz Zawadzki

Transmission route for action potentials and variation potentials in Helianthus annuus L.

Summary Helianthus annuus generates two types of electrical responses to different kinds of stimuli. Non-damaging electrical stimulation evokes action potentials. Damaging, thermal stimuli trigger complex responses: long lasting variation potentials with superimposed action potentials. Electrically evoked action potentials are transmitted along the stem both acro- and basipetally, and on its way, excitation covers nearly all living tissues. Action potentials usually do not enter leaves and cease in petioles. Variation potentials appearing in response to thermal stimulation of one leaf spread preferentially down the leaf trace, probably along conducting bundles. Electrodes inserted into the vicinity of conducting bundles coming from the stimulated leaf register variation potentials, while those inserted on the opposite side of the stem register no change or responses of strongly reduced amplitudes. We point out that electrode arrangement during the monitoring of the transmission of electrical signals in plants is of great importance when physiological responses of excited cells are simultaneously investigated.

Complex relationship between growth and circumnutations in Helianthus annuus stem

The growth and circumnutation of the stem of three-week old Helianthus annuus in the 16:8 h light:dark photoperiod were monitored using an angular position-sensing transducer and a time lapse photography system. It was found that the rate of growth and circumnutation reached a high level in the dark stage; in the light stage, however, only the growth rate reached the same high level, whereas the circumnutations were weak. These results showed that in the light stage the stem circumnutation was down-regulated more strongly than the growth. Short-term stem responses to darkening and illumination were a further display of the relation between growth and circumnutations. Switching off the light caused an increase in the growth and circumnutation rate. In some cases it was accompanied by changes in the rotation direction. On the other hand, switching the light on caused an immediate transient (several-minute long) decrease in the growth rate resulting in stem contraction, and this was accompanied by an almost complete pause of circumnutation. Additionally, under light, there occurred a subsequent decrease in the magnitude, disturbance of circumnutation trajectory and, in some cases, changes in the direction of rotation. The observed stem contraction and disturbance of circumnutation imply the occurrence of turgor changes in sunflower stem, which may be caused by a non-wounding, darkening or illumination stimulus. Our experiments indicate that the disturbances of the growth rate are accompanied by changes in circumnutation parameters but we have also seen that there is no simple quantitative relation between growth rate and circumnutation rate.

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.

Does circumnutation follow tidal plumb line variations

The Morlet wavelet coherences and spectro-temporal coherences were applied to detect common oscillations in sunflower autonomic movements (circumnutations) and variations of the direction of the gravity vector. The mean coherence between time series of sunflower coordinates and the theoretical plumb line model was of the order of 0.6–0.8 for oscillations with periods greater than two days. We also detected weak common oscillations with periods of 0.5 and 1.0 days, which may correspond to semidiurnal and diurnal tides, respectively. Computing the common signal in the time series of sunflowers used in the experiment revealed a low, but statistically significant coherence for semidiurnal and diurnal oscillations. Although we cannot determine if plumb line variations indeed influence circumnutation in sunflower, we prove that there is a significant correlation between these two phenomena.