Plants are subject to stimuli from the environment on which they strongly depend and in contrast to animals, they are unable to escape harmful influences. Therefore, being able to receive stimuli they have developed adequate responses to them. Such a reaction can occur in the area of a stimulus action or cover the whole plant or its parts. In the latter case, it is a systemic reaction. The plant reaction is expressed by various intensity, rate and kind of response. It is interesting to know the character of the signal informing about a stimulus, the routes of its propagation and the transmission mechanism. Three conceptions of excitation are distinguished: 1) propagation of chemical agents formed at the site of a stimulus action with the flow of the phloem sap or through the atmosphere (in the case of volatile substances) to other plant parts, 2) a very fast transmission by the xylem in the wave of hydraulic pressure formed after a plant damage. From combining the "hydraulic" and "chemical" hypothesis a conception of hydraulic dispersion has been formulated which assumes that chemical substances synthetized after an injury can be transferred very fast with the wave of hydraulic pressure changes in the whole plant, 3) a stimulus evokes the action potential (AP), and its transmission along the whole plant, plant organ or specialized tissue, by local circuits from cell to cell. Strong, damaging stimuli can evoke variation potentials (VPs), the character of which differs from APs. It is postulated that transmission of VP occurs by a hydraulic dispersion and electrical changes seem to be secondary phenomena.
Dziubińska, H. (2003). Ways of signal transmission and physiological role of electrical potentials in plants. Acta Societatis Botanicorum Poloniae, 72(4), 309–318.