Can Artificial Soil be Enhanced by Electric Components?

The use of electric fields to increase plant growth in a variety of configurations has been proposed several times in the past decades. This broad family of methods, which often goes under the generic name of “electro-culture,” has produced a mixed bag of results dispersed across a long list of plant species and experimental conditions. Although a significant body of work indicates a broad range of quantifiable physiological effects of external electric fields on plants, a consistent and reproducible increase in growth and yield remains largely unclear. A comprehensive list of publications on the topic is beyond the scope of this brief commentary, but the interested reader can refer to recent reviews on the topic.^1,2

Methods developed to test historical claims of electro-culture often involve setting up a high-voltage electric field between the atmosphere and soil where the plants are grown, perhaps inspired by the voltage difference measured between these two environments in a variety of natural conditions.^3,4

In other commonly described configurations, at least one electrode is directly in contact with the plant tissue, generating ionic currents on its surface or within its cellular network. From a purely experimental point of view, it is generally more difficult to attach an electrode to a root rather than to a leaf or a stem, so the published data about electrostimulation specifically aimed at the root system is sparse.

Recently, Oikonomou et al., from Linköping University in Sweden, published a clever twist on the topic.⁵ Instead of inserting a solid electrode (usually a rod, a plate, or a foil) in the soil or the liquid medium of hydroponic cultivations, they describe a diffused conducting material integrated within an artificial substrate. Crucially, this geometric configuration increases the chances that the plant roots will grow in contact with the electrode instead of simply next to it. The substrate, called eSoil by the authors, is porous and immersed in a liquid medium (hydroponic cultivation), which effectively acts as an electrolyte.

This is a truly original approach since, to our knowledge, only a very limited number of precedents exist in diffusing an electrode in soil. One notable example would be to realise the electrode as a periodic grid,^6,7 although this solution still maintains a limited 2D configuration, which is surpassed by the 3D version described by Oikonomou and colleagues.

This close integration of electric stimulation with mechanical support results in physical contact between the roots and the charged elements of the substrate, creating a difference in electric potential between the roots and the surrounding liquid medium.

This is relevant from a bioelectricity perspective because it contributes to the open discussion about the dominant nature of the perturbations induced by electrodes inserted in growth substrates: without physical contact, the complexity of the electrode-medium and medium-root interfaces points to electrochemical reactions in the medium as the prevalent cause of physiological responses; instead, a persistent physical connection between roots and electrode opens up the arguably more interesting scenario where voltage potentials and ionic currents within the living tissue are perturbed.

With the integrated electrode in contact with the roots and a grounded electrode in the liquid medium, the authors report that +0.5 V between the roots and the medium was sufficient to increase the biomass in barley plantlets grown after 5 days of continuous electrostimulation in eSoil, when compared with plants grown in the same system but without electrostimulation. Interestingly, the effect was detected in both shoot and root tissues, with no measurable difference between them. Since the stimulation was applied specifically to the root system, a systemic effect can be deduced from this result. Whether the propagation of the stimulation happens through molecular, mechanical, or bioelectric mechanisms, or a combination of these, remains to be discovered.

Overall, this is an original method that could find applications in laboratories focused on fundamental topics in plant electrophysiology and bioelectric phenomena in general, but also in some potential translational applications. For example, one popular aspect of root biology that could be studied with eSoil is the effect of induced bioelectric perturbations on the geometrical (e.g., volume, maximum extension, length distribution, symmetry) and topological (e.g., branching distribution, number of root tips) traits of the entire root system architecture.

Moreover, the integrated conducting material in contact with the growing roots could be used to measure variations in electric potentials between the root’s outermost layer (epidermis) and soil, or between the surfaces of root and shoot tissues in the same plant. These measurements have a long history in plant electrophysiology,⁸ but a diffused electrode potentially in contact with many parts of the same root system at once could offer new opportunities for research and agritech applications, for example as biosensors integrated into the growth substrate.

A different set of tools could also emerge by combining the configuration described by Oikonomou and colleagues, with a supporting material whose mechanical properties vary when the conducting element is charged, indirectly controlling root growth in the eSoil.

Finally, this setup could open up new approaches to explore how electric fields could affect soil microbiota and root-microbe interactions.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.