Monitoring the electrical potential difference of pine tree

. The subject of the study is the time series of the difference in electrical potentials along the trunk of a pine tree and the time series of meteorological and geomagnetic parameters consistent with them, measured by hardware and software complexes of the geophysical observatory "Petrozavodsk" and the Botanical Garden of Petrozavodsk State University in 2017-2022, as well as the time series of the K-index database global geomagnetic activity. The existence of characteristic regimes in the difference in electrical potentials of pine trees is shown, depending on the season, temperature, precipitation, humidity, determined by physical environmental factors and physiological processes in the tree. The reaction of the electrical potential difference of pine trees to meteorological parameters, thunderstorms, and space weather phenomena: magnetic storms and solar eclipses is analyzed. The results of these observations are qualitatively interpreted using known features of electric field of the Earth.


Introduction
Unlike animals, whose behavior demonstrates sensitivity to changes in the environment, most plants do not outwardly respond to stimuli.Their reactions are manifested in electrical signals.The shape of the signals is determined by the processes of depolarization and repolarization of cell membranes [1].A single electrical impulse, by analogy with the excitation of a nerve fiber, is called an action potential, a signal that responds to strong stimuli: mechanical damage, a burn -a variable potential.This signal has an extended repolarization phase and contains repeated impulses of small amplitudes.The electrical activity of plants can manifest itself in the form of micro rhythms.
At the cellular level, the propagation of electrical signals has been studied in detail [2].In a tree, electrical signals propagate through small cells of the phloem, protoxylem, and vascular bundles that transport water and nutrients.The integrative bioelectrical indicators of plants in the natural environment have been less studied [3].They are trying to use the electrical response of plants to changes in the environment to determine optimal growing conditions, increase plant productivity, and study reactions to external stimuli.Compared to labor-intensive methods for assessing the condition of plants based on morphological characteristics, which give a 50% error, monitoring the electrical activity of plants seems to be more accurate, efficient and effective [4].Implemented in automated systems, such monitoring simplifies the solution of problems of operational diagnostics, warning and forecasting [5].In the context of global climate change and increasing anthropogenic influence on forest ecosystems, such monitoring is especially important.
The goal of the article is to describe the experience of recording the difference in electrical potentials along the trunk of a pine tree, studying the influence of meteorological and geophysical environmental factors on them.The effect of meteorological parameters, thunderstorms, magnetic storms, and solar eclipses on the difference in electrical potentials of pine tree was analyzed.
The study was carried out at the Geophysical Observatory "Petrozavodsk" on the basis of consistent time series of data obtained in 2017-2022 by special hardware and software systems [6].
The choice of Scots pine (Pinus sylvestris L.) as indicator in environmental monitoring systems is determined by its wide distribution area, nonspecific response to the level of pollution of atmospheric air and soil, and the preservation of photosynthetic activity at low temperatures.The branched, plastic, root system of a tree covers a significant part of the soil thickness and deep layers of soil.Pine energetically absorbs nutrients, uses them sparingly, maintaining the ability to grow with a limited supply of nitrogen from the soil.Its wood is durable and resistant to rot.The height of the tree, reaching several tens of meters, makes it possible to record the difference in electrical potentials at a large distance between the electrodes.

Materials and methods
The Geophysical Observatory "Petrozavodsk" is located in the Botanical Garden of Petrozavodsk State University (PetrSU) in a forest on the outskirts of the city.Located on the opposite shore of the Petrozavodsk Bay of Lake Onega, the observatory territory experiences weak man-made influence.The object of the study was a pine tree growing on rocky soil, about 25 m high.Brass electrodes were driven into the pine trunk on the southern side to a depth of 2 cm; the lower one was 10 cm from the surface of the Earth, the upper one was 10 m from the lower electrode.
The system for recording the difference in electrical potentials of pine tree and geomagnetic parameters similar to the system developed at the "Voekovo" geophysical observatory [7].To measure the difference in electrical potential between the electrodes, one of the channels of the GI-MTS1 geophysical complex [8], located in close proximity to the research object, was used.Other channels of the complex synchronously recorded the variations of H, D, Z components of the geomagnetic field and electrical components of magneto telluric currents.The sampling step of the recorded data was 1 s.Data of the Kp index of geomagnetic activity with a three-hour interval of discretization was taken from the site [9].
The Davis Vantage Pro2 Plus weather station was located at a distance of about 100 m from the pine tree under study [10].With a resolution of 30 minutes, the weather station automatically measured temperature, humidity, atmospheric pressure, precipitation, direction, wind strength, and intensity of solar radiation in the visible and UV range.
The measurement results in real time were received on the computer of the Botanical Garden and transmitted via mobile Internet to the servers of PetrSU and the Institute of Geology of the Karelian Research Center of the Russian Academy of Sciences.The high sensitivity of the GI-MTS1 complex led to impulse noise in its measurement data.During processing, noise was filtered out and replaced with interpolated values determined from the remaining data.The sampling step of the processed and averaged geophysical data was 10 minutes.

Results and discussion
In the seasonal records of variations in the electrical potential difference of pine tree in Figure 1, there is a daily peak (b, c), expressing the connection between electrophysiological processes and the transport of the substances in the tree.In late autumn and winter, this peak is absent in variations in the potential difference.It appears in early spring, increasing in amplitude, reaches a maximum in May-June, then decreases and disappears in the fall.Sometimes the peak has a double-humped shape.Its appearance and disappearance are associated with the times of awakening and completion of the active annual period in the life of the tree.With increasing air temperature, the amplitude of the peak increases, and with increasing relative humidity it decreases.In the absence of a daily peak (a, d), the influence of precipitation on the course of the measured potential difference is clearly manifested.Precipitation in the form of rain lowers the trend of records, while precipitation in the form of snow increases it.Similar to snow, fog affects variations in the potential difference of pine tree.Figure 2 shows the course of the pine potential difference (a) and meteorological parameters (b-d) during the week 09.17-09.23,2017.The thunderstorm at night on 09.19 was accompanied by heavy precipitation.In figure (a) impulse noise is preserved, some of which is associated with lightning discharges.In the recordings of the potential differences in the pine tree, one can notice an oscillation that arose 6-7 hours before the thunderstorm, an oscillation increasing in amplitude with a period of about 1.8 hours, and a daily peak that    Indications of the Kp-index and meteorological parameters in this time interval are given in Figure 5.
Increase in temperature, atmospheric pressure, precipitation, and relative humidity are associated with some magnetic storms.Significant decreases in the trend of the potential difference graph Figure 4    Registration of electrical activity provides an opportunity to study the plant as an integral organism with an operating regulatory system, assess its functional state and the intensity of metabolic processes.Both spontaneous (background) activity and signals responding to irritation are considered informative.
To monitor the condition of plant tissues, methods of electrical impedance and biopotentials are used.The frequency dependence of the electrical impedance of tissues is used to judge the values of intercellular (symplastic) and intracellular (apoplastic) resistance and membrane capacitance.At frequencies of 1 kHz-10 MHz, the current flows mainly through the cell, at low frequencies of 10 MHz-100 Hz -through the intercellular space.In the first case, the cell is considered a capacitor, in the second -an active resistance.At frequencies above 100 Hz, the electrode material does not affect the impedance values.The high-frequency component of the impedance characterizes the dielectric constant of the membranes and the polarization of the liquid dielectric of the cell.A decrease in the low-frequency component of the impedance is associated with membrane damage, for example, when exposed to low temperatures.Low-frequency impedance measurements, in particular, were used to study the seasonal dynamics of the precambial complex of tissues in pine trunks [11].
Compared to impedancemetry, the biopotential method is technically simpler.The method records the difference in electrical potential between tissue sections of higher plants that occurs during the transfer of potassium and chlorine ions through cell membranes.The values of biopotentials are used to characterize the physiological state of plants.With their help, planting material is diagnosed and dangerous trees are identified.The response of biopotentials is used in studying plant reactions to thermal stimuli, rhythmic, pulsed sound, light stimulation, and contact touch [12].
The potential difference along the pine trunk is formed by physical and biological processes.The physiological process that determines the daily peaks (Figure 1(b, c)) is transpiration -the movement of liquid from the roots up the plant and its evaporation through the stomata into the environment [13].By pumping and evaporating liquid, the plant regulates the temperature and flow of products into the photosynthetic zone.Solar radiation opens the stomata.Temperature, atmospheric pressure, wind speed, and humidity regulate the rate of diffusion of water molecules.The optimal combination of external and internal factors for transpiration in this area occurs at the end of May-June.
The potential difference depends on the geoelectric field and the electrical conductivity of the air gap between the electrodes.The geoelectric field is directly involved in the formation of the potential difference along the pine trunk.The electrical resistance of the interelectrode air gap shunts the resistance of plant tissues.The daily variation of the geoelectric field under "good weather" conditions is influenced by the morning convective generator, the distribution of aerosols over the height of the atmosphere, and the unitary variation, reflecting the change in the power of the generators of the global atmospheric electrical circuit [14].
Precipitation has a noticeable effect on the geoelectric field and electrical conductivity of air.During and after rain, sedentary heavy negative ions accumulating near the Earth's surface increase the negative charge of the surface, reduce the conductivity of the surface layer of the atmosphere, and ultimately increase the strength of the geoelectric field (Figure 1a).Under normal the strength of the Earth's electric field at the surface is about 130 V/m; during precipitation and thunderstorms it reaches values of the order of 16,000 V/m.During snow and fog, the presence of light mobile positive ions in the atmosphere increases its electrical conductivity, and their recombination with negative ions reduces the geoelectric field strength.In some cases, the polarity of the electrical potential difference changes (Figure 1d).
Radioactive emanations from rocks often precede earthquakes.Their effect on the Earth's electric field is similar to rain.They ionize the air, cause condensation of water vapor on the ions, and the precipitation of heavy negative ions to the earth's surface [15].
Despite long-term study, the study of plant responses to thunderstorms remains relevant [16].In the response of the electrical potential difference in Figure 2a to the thunderstorm of September 19, 2017, in addition to off-scale electrical impulses, there was an oscillation preceding the thunderstorm and the subsequent disappearance of the daily transpiration peak.The oscillation can be explained by the whirling of the thunderstorm center, the disappearance of transpiration -by intense precipitation (b), close to 100% relative humidity (c).A decrease in temperature (d), atmospheric pressure (e) during a thunderstorm is caused by shielding of solar radiation and rainfall.The decrease in geomagnetic activity Figure 3 (a-c) in the post-thunderstorm period can be considered both the result of the relaxing effect of lightning sprites on the ionosphere and magnetosphere of the Earth [17], and the general weakly perturbed picture of changes in the global Kp -index (g).
The tall pine tree with big inter electrodes distance is a convenient object for studying the effect of space weather phenomena on it.The correspondence of negative bays to the markers of magnetic storms in the time course of the pine electrical potential difference (Figure 4) is similar to the existence of the bays of magnetic storm in the time course of the geoelectric field [14].An increase in geomagnetic activity during a magnetic storm occur as a result of a chain of processes caused by an increase in the magnetic field of the Sun, flares, and eruptive emissions of solar cosmic rays.Having reached the Earth, these streams of charged particles interact with the ionosphere, disturb the geomagnetic field, and do not affect the electrical conductivity of the lower layers of the atmosphere.In contrast, energetic galactic cosmic rays penetrate the lower layers of the atmosphere and create aerosols throughout its thickness, increasing its electrical conductivity.During magnetic storms, galactic cosmic rays are more strongly deflected by the solar magnetic field (Forbush reduction [14]).The ratio of the contribution of these two components in the Earth's atmosphere changes in favor of solar cosmic rays, which enhance the Earth's electric field, which is confirmed by the negative bays of the difference in electrical potentials of pine on Figure 4.
The proximity of temporary disturbances in the regularity of atmospheric meteorological parameters and large values of the Kp index (Figure 5) is interpreted as follows.Intense solar flares during magnetic storms, accompanied by increased radiation in the near ultraviolet, visible, and infrared parts of the spectrum, are considered sources of additional heat into the lower atmosphere.This heat causes an abnormal increase in temperature (especially at night), humidity, and contributes to the formation of clouds, precipitation and thunderstorms.The graphs in Figure 5 generally confirm observations of meteorological parameters during magnetic storms made at the Paratunka geophysical observatory, with the exception of the negative daily atmospheric pressure drop on the second day after the geomagnetic storm [14].
A short-term space weather anomaly is created by a solar eclipse.A partial solar eclipse on October 25, 2022 was observed over a large area of the Earth, including in northwestern Russia.In the electrical potential difference of the pine tree, the eclipse effect is marked by a signal consisting of two pulses of positive and negative polarity, respectively (Figure 6a).By blocking part of the solar disk, the Moon screens the flow of solar cosmic rays without affecting galactic cosmic rays.The associated weakening of the Earth's electric field in the electrical potential difference of the pine tree in Figure 6a is represented by a positive impulse.With the exit of the Earth from the lunar shadow, the ratio of global ionizers is restored; however, the geoelectric field strengthens at new values of meteorological parameters, since the eclipse was accompanied by a lack of heat in the optical range, causing a drop in temperature by about 6°C (b), atmospheric pressure (c), an increase humidity (in).In the electrical potential difference of the pine tree, this phase of the process is expressed as a negative impulse.
Let us briefly look at the known mechanisms of the influence of electric fields on plants [16].According to one of them, the electric field facilitates the penetration of calcium ions through membranes into cells, thereby increasing the rate of metabolism, fermentation, and cell growth.Plants apparently use strong electrostatic fields from rain and thunderstorms as a signal to quickly utilize rain.Within a few hours after a thunderstorm, plant growth accelerates.According to A.L. Chizhevsky's electric field promotes the absorption of positive air ions by the tree, for example, the positive CO2 ion necessary for photosynthesis.Currents flowing through the soil ionize it, causing the activation of microorganisms and the decomposition of substances.As a result, chemical and biochemical reactions, moisture transfer, and the transformation of substances into forms easily digestible by plants are accelerated.

Conclusion
Based on the databases of the global Kp -index of geomagnetic activity and hardware and software complexes of the geophysical observatory "Petrozavodsk" and the Botanical Garden of PetrSU, time series of the difference in electrical potentials along the trunk of a pine tree for the period 01.01 2017 -12.31 2022 were built and, consistent with them, time series of meteorological and geomagnetic parameters were constructed and analyzed.The interpretation of the results is carried out under the assumption of the determining role of the geoelectric field in the formation of the electric potential difference along the pine trunk.
The time course of the potential difference depends on the season and type of precipitation.The spring-autumn regime, which has a daily cycle, is associated with transpiration -the movement of water and juices from the roots to the upper parts of the tree, followed by evaporation.Rainfall lowers the trend of the time course of the potential difference, precipitation in the form of snow and fog increases it.The influence of a thunderstorm on the potential difference of pine was manifested by previous oscillation, suppression of the daily peak of transpiration, changes in meteorological parameters, and a post-thunderstorm decrease in geomagnetic activity.The space weather phenomena: magnetic storms and solar eclipses act on the time course of the difference in electrical potentials of pine trees, similar to their influence on the Earth's electric field.
The incompleteness of the approach in biological terms is associated with the identification of one physical quantity of the difference in electrical potentials along the pine trunk and the analysis of its connections.A direction for its improvement, taking into account the diversity of physiological process in trees, could be the use of IoT technologies based on the Internet of Things [18] and wireless Tree Talker devices [19], which allow scanning of the processes of sap flow, photosynthesis, and trunk diameter growth, and provide prompt processing and transmission of data.Currently, these technologies are being introduced into scientific and practical research [20].

Fig. 2 .
Fig. 2. Variations in the potential difference of pine (a) and meteorological parameters (b-d) in interval 09.17-09.23,2017 Synchronous records of geomagnetic field components (Figure 3 a-c) and the time course of the global Kp index of geomagnetic activity (Figure 3 d) indicate calm dynamics of the geomagnetic field and a decrease in noise level in the post-storm period.

Figure 4
Figure 4 shows the time dependences of the pine potential difference (a) and the Kp index of global geomagnetic activity (b) in the two-month interval 21.03 -22.05 2021.This period of time is characterized by an increase in the value of the daily peak of the recorded potential difference and high geomagnetic activity.Magnetic storms (Kp index 5-7) are marked with arrows, which correspond to bay-shaped decreases in the trend of the time course of the recorded potential difference.Indications of the Kp-index and meteorological parameters in this time interval are given in Figure5.Increase in temperature, atmospheric pressure, precipitation, and relative humidity are associated with some magnetic storms.Significant decreases in the trend of the potential difference graph Figure4are observed with large values of the Kp index and intense rainfall

Fig. 4 Fig. 5
Figure 4 shows the time dependences of the pine potential difference (a) and the Kp index of global geomagnetic activity (b) in the two-month interval 21.03 -22.05 2021.This period of time is characterized by an increase in the value of the daily peak of the recorded potential difference and high geomagnetic activity.Magnetic storms (Kp index 5-7) are marked with arrows, which correspond to bay-shaped decreases in the trend of the time course of the recorded potential difference.Indications of the Kp-index and meteorological parameters in this time interval are given in Figure5.Increase in temperature, atmospheric pressure, precipitation, and relative humidity are associated with some magnetic storms.Significant decreases in the trend of the potential difference graph Figure4are observed with large values of the Kp index and intense rainfall

Figure 6
Figure 6 shows the course of the pine potential difference (a) and meteorological parameters (b-d) during 16.10-29.10,2022.The partial solar eclipse on October 25, 2022 caused a sharp rise and fall in the electrical potential difference (a), a decrease in temperature (b), pressure (c), and an increase in humidity (d).