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Introduction to Electrotropism Experiment in Pollen Tubes
Electrical fields have been shown to influence a gamut of cellular processes and responses. Animals, plants, and bacteria have a range of responses to electrical structures. The electrophysiology in humans consists of the nervous system regulating our actions and behaviors through controlled responses. Action potentials in our nerves and our heart are regulated based on our sodium and potassium levels. Pressure applied to our skin opens up mechanosensitive sodium channels. With the right amount of stimulus it can cause the action potential to reach threshold and cause an influx of sodium during the depolarization phase. After a couple of seconds, the membrane potential becomes positive and causes potassium ions to exit the cell during the repolarization phase and go below the threshold level into the hyperpolarization phase. The leaky sodium and potassium channels bring back the membrane potential to resting. The electrical signaling in humans allows us to perform rapid movements during periods of stress and anxiety. Similarly in plants, electrotropism is used in plant defense signaling and growth.

Plant growth in response to electric signals and fields has been studied by some researchers; however, it has not been as widely tested on pollen tubes. Specifically, pollen tubes are plants that are able to grow quickly in response to mechanical, electrical and chemical cues. This behavioral response allows pollen tubes to attack flower pistils and drop off sperm cells to ovules for fertilization. Carlos Agudelo and colleagues investigated the relationship between electrical signaling and pollen tube growth. The model organism used by the researcher was Camellia japonica pollen, because it displayed a differential sensitivity to the electrical fields when different parts of the tube were exposed. This flower is found in the wild areas of mainland China and Taiwan at elevations of 300-1100 meters. The plant grows at temperatures of 45-61 Fahrenheit and forms buds in the autumn and winter time.

Experimental Conditions
Analyzing the plant’s homeostatic conditions and implementing it in the experiment, the researchers exposed parts of the pollen tube that were either the whole cell or the growing tip to see how growth occurs in response to an external field. The pollen tube serves as a useful model because it is similar to a nerve ending which conducts electrical signaling in humans and animals. Using the tip as a place for growth allows the cell to invade a substrate and for tropism. The experiment that the researchers conducted to support their hypothesis was that they suspended Camellia japonica pollen into an electrical field. Camellia japonica pollen was collected, dehydrated, and stored on silica gel at −20 °C until use. Pollen was thawed and rehydrated in a humid atmosphere for one hour before submersion in liquid growth medium and injection into the chip. By doing this setup Once the pollen is positioned in the ELoC, the growth medium flow is stopped and the electric field is turned on. The ELoc system is used to mimic the conditions surrounding a pollen tube when its grown in a plant. Then the pollen tubes are left to germinate and grow for 2 hours undisturbed unless otherwise stated. After the pollen tubes germinated, they were placed in DC and AC electric fields to see how an external field affected the growth of pollen tubes and the grains inside of them. The researchers applied varying voltages and frequencies to the pollen tubes and the grains to see how this affected their growth rate.

To ensure reproducibility of test conditions, no dyes were implemented, no extreme voltages were applied, and pollen from the same plant and flowering season was used as not to be confounders in the experiment. The researchers applied an increasing voltage to not wear out the microelectrodes and not cause the pollen tubes and grains to burst. It was difficult to remove the air bubbles involved in the process, but they tried to reduce the amount of water that was present in the microchamber.

Results and Discussion
The authors had some compelling results based on their experimental procedure. In the zero-voltage test all zones within the electric chamber showed a similar average tube length indicating that the simple vicinity to the aluminium electrode did not affect pollen tube growth. As the electric fields increased the average pollen tube length decreased. Notably, the percentage of pollen germination decreased when the applied electric field increased. Germination was not as affected by small electric fields but was decreased when the electric field was raised above the threshold of 8 V/cm. The authors of this paper concluded that the presence of external electric fields on the behavior of Camellia japonica pollen tubes interfered with pollen germination and growth in a dose dependent manner. AC fields restored pollen tube growth for frequencies greater than 100 mHz. Importantly, this recovery of growth was achieved under the same strong field strengths (up to 10.71 V/cm) that caused complete growth inhibition at lower frequencies and with DC fields. This indicates that pollen cells can tolerate strong electric fields and perform normal growth—as long as these are applied in the form of high frequency AC fields. The critical field strength that inhibited pollen performance when the entire cell (including grain) was exposed was approximately 10 V/cm. By contrast under a DC field, a much stronger field of 30 V/cm was necessary to impede pollen tube growth when only the growing tip of the cell was exposed. This suggests that pollen tubes can endure stronger fields than grains. This finding may be explained by differences in ion transport behaviour in these two cellular regions, and is consistent with the extremely polar organization of the cell. Ions are being transported when an electric field is applied to cells that are producing the necessary nutrients for growth.

Proposed Physiology
Although the authors did not delve deep in the physiology of how electric fields affect plants, they did propose that ions are being regulated during this experiment. The researchers stated that an electric field’s signal is the stimulus that binds to a receptor on the pollen tube. The electrical signal causes a signal cascade that leads to the increased production of sodium and potassium ions in the cell. These ions accumulate in the cell wall of the pollen tube which causes the expansion of the cell wall due to the buildup of the ions. With a strong electric field, it allows the plant to grow in the direction of the electric field.

Conclusions
This experiment performed by the researchers shows that electrical fields and forces that exist in plants can shape their external and internal structures. Plants have the ability to detect small electrical fields resulting from wounds or structures within their organelles. The magnetic field on Earth and in the electrical signals in plants can affect plant growth and crop yield. Photosynthesis may be affected by the electrical field as conducted by Hebda and colleagues. It is important to take into consideration the plant’s electrical signaling system when assessing its growth and behavior.