Modification temporaire des relations fonctionelles entre pulvinus primaire et petiole chez des feuilles jeunes de Mimosa pudica L.

Abstract

Des expériences de longue durée pendant lesquelles des mouvements de sensibilité sont déclenchés par stimulation directe du pulvinus primaire à intervalles réguliers de 30 minutes montrent que la transmission de l’état d’excitation du pulvinus primaire vers le pétiole (propagation d’un potentiel d’action), lorsqu'elle existe, se situe à un moment déterminé de la journée («temps des relations modifiées»). Des changements dans les conditions de photopériode, (décalages des passages Lumière/ Obscurité ou des passages Obscurité/Lumière), montrent que l’apparition du temps des relations modifiées est déterminée par le dernier passage Obscurité/Lumière. Cependant la persistance du phénomène en conditions de libre cours, avec une période propre de 25 h 30, indique l’intervention d’un rythme interne circadien. In Mimosa, action potentials are normally propagated basipetally from the petiole to the primary pulvinus. However, under certain circumstances, the physiological relationship between pulvinus and petiole is modified in such a way that also acropetal propagation can be observed. The conditions of this «inverse propagation» of action potentials from the primary pulvinus to the petiole are investigated in this paper. In long-time experiments, leaf movement is induced every half hour by direct stimulation of the primary pulvinus. The «inverse propagation» occurs at a particular time of the day only («phase of modified relationship»). By changing the pattern of the daily light-dark cycle, it can be shown that the «phase of modified relationship» is determined by the preceding onset of light (fig. 5 to 7). Besides, the timing of this «phase of modified relationship» is also determined by a circadian rhythm. This can be demonstrated in continuous illumination, where a free running period of 25,5 hours appears (fig. 8), but resetting of the «clock» is obtained after one single dark period. The «phase of modified relationship» shows a biphasic pattern (fig. 3), the second maximum of which is rather sensitive to changes in the experimental conditions: it tends to disappear in lower temperature (25 instead of 30 °C; fig. 4), or sometimes during longlasting experiments. In contrast, the first part of the «phase of modified relationship» is quite insensitive to change in conditions. Finally, the daily pattern of the «phase of modified relationship» is compared with the nyctinastic leaf movement (fig. 9). Whenever the period of one phenomena is changed, the same holds true for the other. Since the nyctinastic movement is based on membrane properties in the pulvinus, it is concluded that also the «phase of modified relationship», i.e. the appearence of «inverse propagation» of action potentials is controlled by membrane effects which are located in the pulvinus.