DIURNAL-VARIATIONS IN BIOELECTRICAL ACTIVITY OF PLANT-LEAVES

Variations of amplitude and repolarization rate in heat-pulse-induced bioelectrical response during an one day-light period were studied in cucumber (Cucumis sativus L.) leaves. Transfer from night (21-degrees-C) to daytime temperature (26-degrees-C) under continuous illumination caused a decrease in the response amplitude, followed by its gradual increase, and did not affect the repolarization rate. Switching on illumination at a constant temperature resulted in a gradual increase in the repolarization rate, with no effect on the bioelectrical response amplitude. Lowering temperature to 18-degrees-C led to an increase of the response amplitude, which was eliminated by blocking anion channels with ethacrynic acid. Under these conditions, substitution of Ca2+ in the nutrient solution with Mg2+ led to the same delay in repolarization as that observed at 26-degrees-C without light. It is proposed that this similarity arises from the absence of anionic current inactivation during excitation, and the increased response amplitude at lowered temperature results from stimulation of the anion channels activity. To avoid the undesirable effect of diurnal variations in the bioelectrical response parameters on the reproducibility of the results, we suggest 14- to 16-h precultivation of plants under constant temperature and illumination.