Arduino project with open-source ideas for environmental monitoring Based on a project by Shahariar @ https://create.arduino.cc/projecthub/PSoC_Rocks/arduino-negative-voltmeter-993902
Goal is to borrow electrons from spruce trees, a widespread organism on this planet:
<<<<<<< HEAD Needs:
- Integration of SD card function to save datapoint
- sample rate for voltmeter
- some fine ASCII art
- aref_ voltage measure on board w/ battery power =======
We have fashioned this measurement device to monitor the environment: specifically the electrical environment within the context of the tree's physiology - which is, in turn, responding to the wider ecosystem the tree is rooted within. We anticipate a drifting, steady-state "resting" potential during most times with occasional nervous-system responses: action potentials, slow-wave potential, and others. The following design notes help describe both the steady-state and responsive components of the tree's physiology.
Living tree as viable system.
What is the "resting" electrophysical potential we can expect in a given tree, grounded in it's environment? Helpful insight is provided by Love et al. 2008:
Here we postulate a simpler hypothesis: the sustained voltage difference routinely observed between parts of trees and soil is mainly due to a difference in pH between the two. Specifically, the tree-root-soil system acts as a concentration pH cell, sometimes actively maintained by the tree’s homeostasis mechanisms. The potential from such a concentration cell is the Nernst potential, which only depends on a concentration gradient. At equilibrium (no net ionic flux across the interface), the Nernst potential is equal to the diffusion potential that results from charge separation across a permeable interface by diffusion down a concentration gradient [8].
From the same, evidence that "resting" potential is common to the entire body of the organism:
We observed no significant change in the electrical potential difference between the xylem of the trunk region of the tree and soil with changing height or cardinal orientation of electrode
Fromm and Lautner 2007 suggest the interior of plant cells have resting potential between -80 and -200 mV.
First of all, we quote Fromm and Lautner 2007 to clarify the extracellular measurements we are attempting:
In general, two different methods are being used to measure electric potentials in plants, viz. extracellular and intracellular recording. Extracellular potential measurements on the surface of higher plants have been widely performed in the past, and offer the advantage of being able to detect electrical potential differences over long periods of time (several days). By contrast, intracellular measurements with penetrating glass microelectrodes are only effective for short periods of time such as 1-2h...
The same authors provide a helpful summary of electrical signals in plants:
Action Potentials [sic] are rapidly propagated electrical messages that are well known in animals. They speed along the axons of the nervous system and over the surface of some muscle and glandular cells. In axons they are brief (in [millisecond] range), travel at a constant velocity and maintain a constant amplitude (Hille 1992). They usually have an all-or-nothing character, that is, after a stimulus reaches a certain threshold (which leads to membrane depolarization), further increases in stimulus strength do not change its amplitude and shape. The response is an all-or-none depolaraization that spreads passively from the excited region of membrane to the neighbouring non-excited region. Thus, [Action Potentials] propagate electrically, with depolarization being the stimulus for passive propagation.
Variation Potentials or Slow Wave Potentials are propagating electrical signals which also consist of a transient change in membrane potential (depolarization and subsequent repolarization). The main difference to Action Potentials lies in longer, delayed repolarizations and a large range of variation. The signal varies with the intensity of the stimulus, is non-self perpetuating and appears to be a local change to either a hydraulic pressure wave or chemical transmitted in the dead xylem. It can be generated by wounding, organ excision or flaming, and was studied in numerous plant species...
For these latter types of signals, the following mechanism is provided by the same authors:
The large propagating depolarizations of Variation Potentials are generated by a rapid loss of tension in the xylem vessels after wounding. This hydraulic wave is transduced into local changes in ion flux through mechanosensory channels in the adjacent living cells (Stankovic et al. 1998; Davies & Stankovic 2006). After being generated in these cells, the Variation Potential can move laterally, via plasmodesmata, into the sieve elements from where it can be transmitted over long distances. Alternatively, some wounding substance can also be transported in the xylem via the hydraulic shift and can evoke a Variation Potential via ligand-activated channels.
To summarize, the transmission of electrical signals within the plant depends on:
- the electrical conductance of plasmodesmata in lateral direction and
- on the high degree of electrical coupling via the sieve pores in longitudinal direction.