Sunflower plants (Helianthus annuus L.) were given an electrical stimulus to the stem or a heat (flame)‐wound to a single leaf or a cotyledon. The resulting electrical activity was monitored with extracellular electrodes. An electrical stimulus applied to the stem frequently evoked an action potential (AP), but never a variation potential (VP). In contrast, a heat‐wound applied to a leaf virtually always elicited a VP, which was often accompanied by one or more superimposed spikes (putative APs). The kinetic parameters of the AP and the VP were investigated. The AP appears to propagate without decrement in velocity or magnitude, whereas the VP parameters decrease significantly with distance. The heat stimulus triggered rapid alterations in stem elongation/contraction, which preceded changes in electrical potential, indicating the transmission of a hydraulic signal. Light‐off and light‐on stimuli evoked negative‐ and positive‐going changes in extracellular electrical potential, respectively, corresponding to de‐ and hyper‐polarization of the plasma membrane. Membrane depolarization (extracellularly manifested as a VP) evoked by both the light‐off and heat‐wounding stimuli was able to trigger one or more APs. We interpret these results to suggest that APs are “genuine” electrical signals involving voltage‐gated ion channels or pumps, which can be evoked directly by electrical stimulation or indirectly by changes in membrane potential occurring during the VP or after the light‐off stimulus. In contrast, VPs appear to be a local (non‐transmissible) electrical consequence of the passage of a rapidly transmitted hydraulic signal in the xylem, presumably acting on mechanosensitive ion channels or pumps in adjacent living cells.

First, using tomato, Fisahn and co-workers [9] showed that electrical stimulation did, indeed, evoke an AP followed by the accumulation of pin2 mRNA some 5-6 h later. Unfortunately, this time period is sufficiently long to allow transport of a wound hormone from the wounded site to the region analyzed; thus the crucial role of the of the AP could only be inferred. Second, recent work has confirmed that a heatinduced VP can evoke gene expression (calmodulin in Bidens) as can another electrical-like signal, which was tentatively described as a 'non-propagated AP' [10]. However, a 'nonpropagated AP' is almost a contradiction in terms and the electrical response seen [10] was most likely a local hydraulically induced change in membrane potential.Here, by flame wounding or electrically stimulating individual plants and monitoring for the passage of an electrical signal, either a VP or an AP, through the petiole or the blade of the leaf analyzed or into the lamina proper, we directly tested whether passage of either of these electrical signals is followed shortly thereafter by transcript accumulation. The results show unequivocally that both the systemic electrical signal (AP) and the systemic hydraulic signal and its local electrical aftermath (VP) precede and may thus evoke the expression of pin2.

Abstract. Localized burning of a leaf causes a rapid change in apoplastic electrical potential throughout the shoot of wheat seedlings (‘variation potential’). It also causes marked increases in turgor pressure in epidermal cells of adjoining leaves. These turgor increases indicate rapid propagation throughout the seedling, of a hydraulic pressure wave from the site of wounding. Evidence is presented that this pressure wave is caused by relief of xylem tension, by water released from damaged cells in the wounded region. It is demonstrated that, in the absence of wounding, pressure waves imposed at the tip of one leaf can travel to neighbouring leaves, and can there induce change in apoplastic electrical potential similar to a ‘variation potential’. This indicates that the hydraulic event produced by wounding is the signal responsible for systemic induction of the ‘variation potential’. This signal has been termed ‘Ricca's factor’. It is suggested that arrival of the hydraulic wave alters leaf water potential and thereby induces stomatal activity. Leaf surface potential may be dominated by electrogenic ion pumping or flux at stomatal cells, and the ‘variation potential’ may therefore be a reflection of stomatal activity induced by the hydraulic signal.

Tomato (Lycopersicon esculentum L.) plants accumulate proteinase inhibitor ( pin) mRNA in response to various stimuli in leaves distant from those treated. Most earlier work suggests that the intercellular wound signals are chemical; we have tried to determine whether electrical or hydraulic signals can also evoke systemic pin expression. We used a mild¯ame to evoke a hydraulic signal and its local electrical aftermath, the variation potential (VP), and an electric stimulus to trigger an action potential. Under medium light, wounding evoked a 3-to 5-fold systemic increase in pin mRNA within 15 min, suggesting involvement of a rapidly transmitted signal. Wounding also triggered a transient systemic increase in calmodulin (cal ) mRNA, under medium light conditions. Wounded plants exhibited electrical responses (VP) and yielded 5-to 15-fold increases in pin mRNA within 1 h. Electrically stimulated plants that transmitted an action potential to the analyzed leaf exhibited similarly large, rapid increases in pin transcript. Plants which generated no signal had unchanged levels of pin mRNA. Thus, in addition to the previously shown chemical signals, both hydraulically induced VPs and electrically induced action potentials can elicit systemic pin expression.

A major candidate for intercellular signaling in higher plants is the stimulus-induced systemic change in membrane potential known as variation potential (VP). We investigated the mechanism of occurrence and long-distance propagation of VP in sunflower (Helianthus annuus 1.) plants. Here we present evidence of the relationship among injury-induced changes in xylem tension, turgor pressure, and electrical potential. Although locally applied wounding did trigger a change in membrane potential, it evoked even faster changes in tissue deformation, apparently resulting from pressure surges rapidly transmitted through the xylem and experienced throughout the plant. Externally applied pressure mimicked flame wounding by triggering an electrical response resembling VP. Our findings suggest that VP in sunflower is not a propagating change in electrical potential and not the consequence of chemicals transmitted via the xylem, affecting ligand-modulated ion channels. Instead, VP appears to result from the surge in pressure in the xylem causing a change in activity of mechanosensitive, stretch-responsive ion channels or pumps in adjacent, living cells. The ensuing ion flux evokes local plasma membrane depolarization, which is monitored extracellularly as VP.