Action potentials and variation potentials in sunflower: An analysis of their relationships and distinguishing characteristics

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 hear-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 stern 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.