Mechanisms of Plant Electrophysiology - Electro-Horticulture - page 79

slightlynegative.Nowthatthefloodgateshavebeenopened,theinteriorofthecellstartsto
experienceahugeincreaseinpositivecharge(avoltagespike).Iftheamountofthevoltage
is greater than a certain threshold (the depolarization limit), then the spike will hit a peak
valuecalledthe“actionpotential”.Notethatoncethelimithasbeenreached,anyincreases
in the voltage level will have no additional effect on the response. It simply causes the
voltage spike to fire.
This all-or-none type of response is indicative of typical AP behavior. It's like pressing
a needle against a piece of rubber. Until you penetrate the rubber, you can keep pushing
it but once the rubber is penetrated, that is it. You might push the needle further but
nothing dramatic will happen 24 . To assign a number to these voltages, in some plants, the
membrane depolarization level can reach upwards of +100 mV 25 .
Propagation
Now that we covered how APs are generated, let's go into how they spread. After the
membrane depolarization event occurs which causes a voltage spike, a massive exodus of
calcium ions (positive charges) flows back into the surrounding environment triggering
more VGCs to open up on the surrounding cells. It is in this way that the AP and the flood
of calcium ions cause a chain-reaction that spreads these signals to surrounding cells, that
if are also excitable, will spread far and wide, possibly throughout the entire plant.
It is for this reason that action potentials have been suggested as being information
carriers 26 capableofrelayingelectricalsignalsthroughoutmostoftheplant.Byfacilitating
high-speed message or information transfer, it helps the plant react in a timely manner in
response to most forms of internally or externally-triggered stimuli 27 .
Electrochemical Signaling
In addition to AP-based signaling, there are also chemical information carriers that can
relay information from one portion of the plant to another. In the next section we're going
to focus on the role of calcium as a regulator of plant physiological processes.
The Role of Calcium in Plant Biology
To start, as you may recall from earlier, the massive release of calcium after each AP
'pulse' causes waves of calcium ions to spread throughout the plant via each cell cluster
that the calcium ions come in contact with. What is the effect of this?