HP0-773 training material

Plants rely on three known types of electrical signals - Action Potentials (AP), Variation Potentials (VP), and System Potentials (SP) to carry out critical life functions (e.g. respiration, photosynthesis, water uptake, etc.) and to respond to environmental stimuli and stresses. All three types of electrical signals travel intracellularly (within a cell) and extra/inter-cellularly or apoplastically (outside or between cells) and per Matthias R. Zimmermann et al System Potentials, a Novel Electrical Long-Distance Apoplastic Signal in Plants, Induced by Wounding, may act as forerunners of slower traveling chemical signals while serving as a back up to each other and overlapping in some instances.

APs are rapid all or nothing self-propagating electrical signals that occur in both plants and animals characterized (based on Eric Davies, Electrical Signals in Plants: Facts and Hypotheses (Plant Electrophysiology - Theory & Methods, Springer-Verlag Berlin Heidelberg, 2006)) by a sharp rise, brief peak, and sharp return to near baseline. They travel at a constant speed and amplitude regardless of distance when stimuli reach a certain threshold (consistent with AP activity in cnidaria) that triggers membrane depolarization. Based on Electrical signals and their physiological significance in plants (Plant, Cell and Environment, Blackwell Publishing Ltd. 2007) by Jörg Fromm and Silke Lautner, an AP can propagate over short distances through plasmodesmata, and after it has reached the sieve element/companion cell complex, it can travel over long distances along the sieve element plasma membrane (in the phloem, which extends throughout a vascular plant) in both directions. APs travel at a speed of between 20-400 cm per minute. The duration of a typical AP is less than 20 seconds. Once an AP passes, a period of delay called a refractory period follows in which an additional AP cannot be generated.

VPs are slower non-self-propagating electrical signals that are elicited by stimuli that trigger a change in potential (depolarization and subsequent repolarization) at the plasma membrane of parenchyma cells that reside adjacent to xylem vessels due to a rapid loss of turgor. They are characterized (based on Eric Davies, Electrical Signals in Plants: Facts and Hypotheses (Plant Electrophysiology) by a sharp rise followed by a lingering decline, often with spikes. Unlike APs, membrane repolarization is delayed and the signals are graded and travel at varying amplitudes (which are reduced as the distance increases) based on intensity of the stimulus. VPs also utilize the sieve element plasma membrane and plasmodesmata. VPs travel at a speed of between Electrical Signals in Plants: Facts and Hypotheses APs are generally caused by non-damaging stimuli such as electrical stimulation, light/dark transitions, brief cooling, pollination [and sometimes] excision and VPs are caused by damaging stimuli such as severe wounding, [organ excision, and flaming]. SPs are generally caused by cutting and cations.