# Plant Electrical Signal Classification Based on Waveform Similarity
## 1 Introduction  
The research of plant electrical signals has lasted for more than 100 years since [PERSON] discovered the existence of plant action potential (AP) in the Venus flytrap in 1873. In higher plants, there are three main types of electrical signals, i.e., action potential (AP), variation potential (VP), and system potential (SP). APs are evoked by non-invasive stimuli and follow all-or-nothing kinetic characteristics, which depend on the movement of ions (Cl\({}^{-}\), Ca\({}^{2+}\), K\({}^{+}\), H\({}^{+}\)) through a cell membrane by voltage-dependent ion channels and the activity of electrogenic pump within the plasma membrane, and the ionic mechanism makes an AP an identifiable waveform within the various plant species. In theory, once a membrane potential threshold is exceeded, APs propagate with defined amplitudes, propagation speed, and a refractory period [1; 2; 3; 4; 5]. Unlike APs, VPs are induced by damaging stimulation--e.g., cutting, burning--and they are slower propagating than APs, having a speed of 0.5-5 mm/s in general. They are varied in diverse distance-dependent waveforms, whichshow longer, delayed repolarizations and a large range of variation with attenuation of amplitude and velocity. In 2009, [PERSON] and his coworkers defined a novel electrical long-distance apoplastic signal in plants--i.e., SP. They suggested that hyperpolarization events of a plasma membrane are systemically transmitted via propagating SPs (2009) [7]. For higher plants, [PERSON] and [PERSON]'s research group established mathematic models to describe action potential and variation potential based on ions kinetics [8,9]. Plant electrical signals are an initial physiological response to the stimuli of the environment, and have an important effect on plant physiological activities [10, 11, 12, 13]. Both AP and VP can influence photosynthesis [14,15] and their influence is possible to be connected with Ca\({}^{2+}\)[16] and (or) H\({}^{+}\) influxes [17,18]. In addition, electrical signals can affect physiological processes [11], e.g., respiration [18,19], changes of ATP content [20], transpiration [21], plant resistance to heating [22]. Changing of environmental conditions or stress will effect on the plasma membrane potential of plant cells. As is well known, plant action potential (AP) is a rapid response to abiotic and biotic stresses [23]. It is generated by the movement of ions (Cl\({}^{-}\), Ca\({}^{2+}\), K\({}^{+}\), H\({}^{+}\)) through a cell membrane. Ions move via different ion channels and pumps in the plant, under the control of concentration and electric gradients [8,24]. As a result, we think that AP can be selected as a kind of plant physiological phenotype. It is probable that genotyping and external phenotyping should be linked to physiology at the cellular and tissue level to reduce the genotype-to-phenotype knowledge gap in stress biology [25, 26, 27]. In fact, plant electrical signals are typical time series data. Different from the animal electrical signal, plant electrical signal is induced by a change of environment factors (such as temperature, light, water, etc.) [27, 28, 29, 30, 31, 32, 33, 34]. In general, when the environment is in steady state, the electrical signal response will not occur except for imposing external stimuli. Also, plant electrical signals induced by stimulation or environmental changes are variable and will not respond to stimuli in the refractory period. Even the signals acquired from one plant by the same stimulus will not be completely similar. Therefore, how to recognize the APs quickly and accurately from plant electrical signals with noise and artifacts is a challenge for researchers. In previous studies, some electrophysiological recording technologies have been used to collect amount of plant electrical signal data [1, 2, 35, 36, 37, 38, 39, 40, 41], such as fluorescence images of bioelectrical activity, multi-electrode arrays (MEA) data, extracellular recording data, etc.