ACTION-POTENTIALS AND VARIATION POTENTIALS IN SUNFLOWER - AN ANALYSISOF THEIR RELATIONSHIPS AND DISTINGUISHING CHARACTERISTICS

Sunflower plants (Helianthus annuus L.) were given an electrical stimu lus to the stem or a heat (flame)-wound to a single leaf or a cotyledo n. The resulting electrical activity was monitored with extracellular electrodes. An electrical stimulus applied to the stem frequently evok ed an action potential (AP), but never a variation potential (VP). In contrast, a hear-wound applied to a leaf virtually always elicited a V P, which was often accompanied by one or more superimposed spikes (put ative APs). The kinetic parameters of the AP and the VP were investiga ted. The AP appears to propagate without decrement in velocity or magn itude, whereas the VP parameters decrease significantly with distance. The heat stimulus triggered rapid alterations in stern elongation/con traction, which preceded changes in electrical potential, indicating t he transmission of a hydraulic signal. Light-off and light-on stimuli evoked negative- and positive-going changes in extracellular electrica l potential, respectively, corresponding to de- and hyper-polarization of the plasma membrane. Membrane depolarization (extracellularly mani fested 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 su ggest that APs are ''genuine'' electrical signals involving voltage-ga ted ion channels or pumps, which can be evoked directly by electrical stimulation or indirectly by changes in membrane potential occurring d uring the VP or after the light-off stimulus. In contrast, VPs appear to be a local (non-transmissible) electrical consequence of the passag e of a rapidly transmitted hydraulic signal in the xylem, presumably a cting on mechanosensitive ion channels or pumps in adjacent living cel ls.