Action Potentials and Slow Wave Potentials Exhibit Distinct Phloem Propagation Velocities in Vicia faba

Plant vascular bundles are the main pathways for the propagation of electrical signals such as action potentials (APs) and slow-wave potentials (SWPs), also known as variation potentials (VPs). Despite substantial descriptive evidence on the propagation of electrical signals, their underlying mechanisms remain elusive. Using aphids as bioelectrodes and the herbaceous Vicia faba as a biological model, we studied how physical stimuli such as cutting and burning induce propagating signals within the plants in the form of APs or VPs. Cut-induced APs were characterized by fast propagation (≈2.6 cm s^-1), short duration (≈3 s), and an amplitude of about 12 mV, frequently followed by a hyperpolarization phase. In contrast, flame-triggered VPs were significantly slower (≈0.38 cm s^-1), longer (≈45 s), with amplitudes comparable to APs. Notably, APs propagated at similar velocities in both acropetal and basipetal directions, whereas VPs propagated faster acropetally, suggesting different propagation mechanisms of APs and VPs. Interestingly, flame-induced VPs altered aphid feeding behavior: in 20% of cases, aphids switched from phloem ingestion (E2) to salivation (E1) within 30-320 s of signal onset, while APs did not induce any behavioral changes. This study provides the first evidence that both APs and VPs propagate through the phloem at different velocities in acropetal and basipetal directions following leaf damage. These results highlight the different functional roles that APs and VPs may play in systemic communication of external physical stimuli.