Sensors and Materials, Vol. 26, No. 7 (2014) 495–504
MYU Tokyo
S & M 1016

Purification Capability of White Radishfor Gaseous Ethyl Alcoholand Bioelectric Potential under LED Light
Takashi Oyabu*, Masanori Kawauchi1, Hiromi Ban2 and Hidehito Nanto3
Kokusai Business Gakuin College, Minamicyo 6-12, Kanazawa, 920-0919 Japan
1Kanazawa Kiko, Muryoji, Ha38-2, Kanazawa, 920-0332 Japan
2Nagaoka University of Technology, Nagaoka, 940-2188 Japan
3Kanazawa Institute of Technology, Yatsukaho, Hakusan, 924-0838 Japan(Received January 20, 2014; accepted April 7, 2014)
Key words:

bioelectric potential, LED, white radish, SPA, absorption capability

Bioelectric potential is caused by ion stream in vegetation tissue. This potential
could be applied as a speaking plant approach (SPA) technique. In this study, thepotential of white radish is measured in an ethyl alcohol atmosphere to establish the SPA
technique. The absolute summation (vm1) of the potential for 1 min is adopted to analyze
the potential characteristic, and the frequency of an LED panel and the ethyl alcohol
concentration are adopted as parameters. Moreover, the absorption capability (Pa) ofwhite radish for the alcohol is derived and its relationship with vm1 is examined. As aresult, it is clarified that there is a correlation between vm1 and Pa, namely, Pa increases
with vm1. The distribution of Pa in relation to the light frequency and concentration
indicates a normal distribution, and it is considered that the potential is related to thegrowth rate of vegetation. Other summations vm30 and vh1 for 30 min and 1 h are also
adopted to derive their relationship with the absorption capability (Pa) for the substance.

1.

Introduction

Plants have various functions. A plant can sense the surrounding luminosity, odor,
and even the touch of a person. Recently, it has been found that a plant has both sensory
and memory functions.(1) Plant capabilities can be recognized by studying various plant
functions. A plant has the capability to purify soil, water, and airborne chemicals.(2–4) It
can grow by absorbing air-polluting substances, such as carbon dioxide, nitrogen oxide,and hydrocarbon, which are exhausted by cars. Vegetation also has the informationtransmission capability, for instance, bioelectric potential.(5–7) There is a technology forgathering a lot of information from plants, which is called the speaking plant approach (SPA)
in Japan, and the technique has been adopted in plant factories.
Corresponding author: e-mail: oyabu24@gmail.com

*

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Plants have been produced as food since ancient times. Food self-sufficiency has
become unbalanced owing to the rapid population increase on Earth. Therefore, plantfactories, which can produce and provide vegetation as food, have received a lot of
attention. Many vegetations produced in these factories are leafy vegetables such as
lettuce. They have a high growth rate and market competitiveness because of their low
unit price. One of the challenges in the plant factories is how to reduce the production
cost. The production of a value-added vegetable in the factories contributes to theincrease in food production. They have considered that the plant growth rate andfertilization period could be determined through the signal emitted by the plant, and amore efficient production in a vegetable factory will be achieved. Labor saving and cost
reduction are accomplished in this factory. These are great advantages.(8)In this study, the bioelectric potential of white radish is examined under eightirradiation light frequencies including darkness, and its absorption capability in an
atmosphere of gaseous ethyl alcohol is also investigated. The potential is one of theimportant signals emitted by the plant. It is considered that plant growth and absorption
capability for carbon dioxide differ depending on the irradiation light frequency.(9–11)
White radish is adopted as the subject plant. There was a record showing that it was animportant food for laborers working in pre-Hispanic pyramids. It is known to nearly
everyone and is also used as a medicinal plant. Its big root is eaten and it is a type oftypical root crop. Ethyl alcohol in the atmosphere is used as a nutrient component.
This component is safe and cheap to use in an experiment. The bioelectric potential
varies hourly, so the analysis of the potential is carried out using a summation value
for unit intervals, for example, a minute and half an hour. As a result, it is obvious thatthe absorption capability for the atmospheric component (ethyl alcohol) increases as
the potential summation value increases. Major differences in absorption capability
could not be recognized between the summation values for homogeneous and complex
lights. However, the capability differs depending on the ethyl alcohol concentration. It
is considered that the results of this research can be applied to a vegetation factory in the
future, and that the production efficiency can be improved.

2.

Experimental Procedure

Plants have many sensory functions; thus, it necessary that a steady condition is
maintained during an experiment on plant bioelectric potential. Such an experiment iscarried out in a 300 l acrylic chamber. The subject plant and various sensors are installedin the chamber. The bioelectric potential varies hourly rapidly. It is difficult to derivesome features from this change. The potential is inputted every 0.1 s and its absolutevalue is used in the analysis. The summation values of the inputted absolute potentials
per unit of time [for example, a minute (vm1), half an hour (vm30), and an hour (vh1)] areadopted in this study. Two electrodes are connected to two neighboring leaves of thesubject plant and the ground is the potted soil. The electrode material is aluminum (0.3
cm2) and the difference in voltage between the electrodes is amplified 100 times. The
value is inputted into a low-pass filter that cuts off the frequency component over 10 Hz.
Then, it is inputted into an AD converter and data logger. Finally, it is inputted into a PC

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and analyzed. The following sensors are installed in the experimental chamber: tin oxide
gas sensor (Figaro TGS#800), temperature and humidity sensor with an atmosphericpressure sensor (T&D, TR-73U), and illuminance sensor (T&D, PHR-51). The gas
sensor can be used to measure the concentration of ethyl alcohol. The following eighttypes of irradiation light are adopted in the experiment: darkness, red (660 nm), green(525 nm), and blue (475 nm), which are monochromatic lights, and white light, red andgreen, red and blue, and green and blue, which are mixed lights. LED panels (CCS
Inc., ISL-150×150 series) are used in the experiment as the irradiation light source.
The experimental system used is shown in Fig. 1. In this figure, “Action” is performed
depending on the “Growth model”. That is, the environmental factors for the subject
plant are monitored using a PC and the vegetation condition is identified. For example,
fertilization as an action is carried out through an interface if necessary. At that time, thecondition is coordinated with the optimum growth model and the best action is carriedout. The action is derived by cross-checking with the vital phenomenon of the plant,
which is a type of feedback. In the future, it is necessary to adopt “system biology”,
which can analyze plant vital phenomena as systems engineering.
White radish (scientific name: Raphanus sativus var. longipinnatus) is adopted asa subject plant. A photograph is shown in Fig. 2. It has many special characteristics(e.g., medicinal effect) and is consumed widely since early times. In particular, it is
known to the Japanese and is served frequently. The moisture content of radish isabout 95%: therefore, adequate moisture in the potted soil should be considered in theexperiment. Plants can absorb exhaust fumes in the atmosphere as a growth substance.
In this investigation, the plant absorption capability (Pa) for ethyl alcohol is adopted as aparameter. The capability is defined as follows. First, ethyl alcohol is injected to reacha specified concentration using a microsyringe. The output of the gas sensor increases atthe same time as the injection and reaches a peak level. After that, the output decreases

Fig. 1. (Color online) Experimental system.

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Fig. 2. (Color online) Photograph of subject plant (Japanese radish).

gradually owing to the absorption capability of the radish for the alcohol. The peak value (h)from the offset level of the output is derived and the time from the injection to the halfvalue of h is measured, which is called the half width (tw). The capability (Pa) is derived
using h and tw, and the equation can be represented as eq. (1). The relationship between
Pa and vm1 is introduced. The frequency of irradiation light is adopted as a parameter.
Pa = h/tw × 100

3.

Experimental Results

3.1

Output characteristic of gas sensor(1)

A tin oxide gas sensor is used to detect the concentration of ethyl alcohol in theexperimental chamber. The sensor is sensitive to various types of reducing gas, that is, itdoes not have selectivity to a specific gas. The subject plant is installed in the chamber
at 15:00 and ethyl alcohol is injected at 17:00 every experiment. The experiment isfinished the next morning at 7:00. The data for 16:00 to 6:00 are used to analyze theplant characteristics. A typical gas sensor output characteristic is shown in Fig. 3. The
concentration of ethyl alcohol is 15 ppm and the irradiation light is red (intensity 73 lx).
The output increases rapidly until 0.313 (V). The offset level, which means the levelright before the injection, is 0.0174 V and h is 0.2956 V. The absorption capability (Pa) is
about 8.3 (V/h) because tw is 3.556 h. The concentrations of 0, 8, 12, and 15 ppm are set
in the experiments. The relationship between the sensor output h and each concentrationis shown in Fig. 4. It increases linearly as the concentration increases. The characteristic
saturates over 25 ppm. The value of Pa can be derived until 20 ppm, which is the limit,
and the plant is susceptible partially to high concentration. Thus, the experiment iscarried out until 15 ppm.

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Fig. 3 (left). Typical gas sensor output characteristic.
Fig. 4 (right). Relationship between gas sensor output h and ethyl alcohol concentration.

3.2

Characteristic of bioelectric potential

3.3

Absorption characteristic of ethyl alcohol

Two electrodes are connected to two neighboring leaves of the subject plant andthe ground is the potted soil. The bioelectric potential, which is a differential voltage
between the two electrodes, is inputted into a PC every 0.1 s. This characteristic is
indicated in Fig. 5. It is a unique characteristic of the subject plant illuminated withwhite light for 1 min. It is difficult to derive some features in the figure because thecharacteristic oscillates sharply. Therefore, the summation value of the potential for 1
min (vm1) is adopted for analyzing this plant characteristic and is shown in Fig. 6. The
figure shows the vm1 characteristic between 15:00 and 6:00 the next morning. In fact,
the average value (vm1-ave) of vm1, which is located near the half width in the gas sensor
characteristic, is adopted and analyzed. In addition, vm30 and vh1 are also adopted in theanalysis. They indicate the summation values for 30 min and 1 h. The vm30 characteristic
is shown in Fig. 7, and the vh1 characteristic is also indicated in Fig. 8. These two
characteristics correspond to that in Fig. 6, which is smoothed by expanding thesummation time. In the figures, the characteristics are higher at the start and end of theexperiment to some degree.
Plants grow by absorbing various gaseous substances in the atmosphere, thatis, plants have a purifying capability. Moreover, they can take in nutrients throughthe roots. It is obvious that formaldehyde is taken in as a growth substance.(12–14)
Formaldehyde is a substance responsible for the sick house syndrome. It is thought that
alcohol is also a nutrient. Pa is used as an absorbing capability index in this study. This
index increases larger when the half width (tw) of the gas sensor output decreases for the
peak level, namely, the plant can absorb the chemical rapidly. The relationship between
the bioelectric potential and the plant growth rate can be revealed by examining therelationship between vml and Pa. The growth rate can be controlled using the relationship500

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Fig. 5 (left). Original bioelectric potential of a Japanese radish for 1 min.
Fig. 6 (right). vm1 characteristic of Japanese radish from 15:00 to 6:00 the next morning.

Fig. 7 (left). vm30 characteristic of Japanese radish.
Fig. 8 (right). vh1 characteristic of Japanese radish.

between vm1 and Pa in a vegetable factory. The summed bioelectric potential near thehalf width (tw) of the gas sensor characteristic is adopted to introduce some features.
Every average bioelectric potential (vm1, vm30, and vh1) from the injection to the end of
the experiment was compared with that near the half width under various experimental
conditions. It became clear that the values are nearly equal with respect to eachsummation time; thus, the average values of vm1, vm30, and vh1 near each half width canbe used for analyzing the plant characteristics. There is a possibility that the taste andtexture of vegetation change according to the gaseous species injected as nutrients. The
relationship between vm1-ave and Pa in 12 ppm ethyl alcohol is revealed in Fig. 9. The
correlation coefficient is γ ≈ 0.55, and there is some degree of correlation. Similarly, therelationship between vm30-ave and Pa is shown in Fig. 10 and that between vh1-ave and Pa isindicated in Fig. 11. Each horizontal axis means the average bioelectric potential near
the half width of the gas sensor output, which represents the chemical concentration. Itshows the relationship under eight types of irradiation light. The correlation coefficientSensors and Materials, Vol. 26, No. 7 (2014)

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Fig. 9 (left). Relationship between Pa and vm1-ave at 12 ppm concentration.
Fig. 10 (right). Relationship between Pa and vm30-ave at 12 ppm concentration.

Fig. 11. Relationship between Pa and vh1-ave at 12 ppm concentration.

is γ ≈ 0.55 in each case. The scattering plots in Figs. 10 and 11 closely resemble eachother. Therefore, the potential and Pa could be indicated using the vh1-ave characteristic.
The average bioelectric potential near the half width could be used in analysis. The half
width could also be estimated. This is an important advantage.

3.4

Relationship between irradiation light and Pa
The vegetable physiology varies depending on the irradiation light. It is reported
that the growth rate and taste of plants such as vegetables change owing to the quantityand frequency of the irradiation light. The bioelectric potential can be regarded as aneffective sensor to detect the ripeness and freshness of fruits and vegetables. It became
clear that the relationships between the potentials (vm1-ave, vm30-ave, and vh1-ave) and theirradiation light frequency have positive correlations. In this section, Pa is examined
under eight types of irradiation light. It is thought that Pa is related to the plant growthrate. It is clarified that the growth rate can be controlled by varying Pa. However, anotable difference in potential is not desired. It is effective when a difference of several
percent (5 to 6%) in Pa could be derived by the irradiation light frequency.

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The change in Pa is derived by varying the alcohol concentration in darkness andunder three homogeneous lights. The result is shown in Fig. 12. In this figure, “R”
indicates red light, “G” indicates green light, and “B” indicates blue light. “Black” is“in darkness”. The experiment is carried out at three concentrations, namely, 8, 12, and15 ppm. Pa tends to be minimum at 12 ppm concentration under homogenous light.
Its value in darkness is nearly the same as that under light. It could not be determinedthat Pa in ethyl alcohol atmosphere increases owing to the photosynthesis of whiteradish, which is a root crop. It is considered that plants have the capability to purify theatmosphere, and Pa increases with the concentration. Therefore, Pa becomes higher at ahigher concentration. A similar experiment is carried out under mixed light. The resultis shown in Fig. 13. It is maximum at 12 ppm concentration. This result is different
from that in Fig. 12. The variation is less under mixed light than under homogenous
light. The coefficient of variation (cv) of all the plots in Fig. 12 is 0.36, and that in Fig.
13 is 0.16. There is little change in Pa under white light, and cv is small under mixed
light. It is considered that plants can fulfill the absorption function for ethyl alcohol
stably under mixed light. The function becomes stable by receiving mixed light, which
includes various frequencies.
The histogram of Pa shown in Figs. 12 and 13 is introduced, and the result is shownin Fig. 14. The vertical axis indicates the statistical frequency. The average Pa is 9.33
and the standard deviation σ is 2.87. cv is about 0.31. It is relatively small and the plantfulfills the function stably. It is important to establish the light environment in which theplant can fulfill stable functions.

Fig. 12 (left). Absorption capacity Pa of Japanese radish for ethyl alcohol under monochromatic
light.
Fig. 13 (right). Absorption capacity Pa of Japanese radish for ethyl alcohol under compound light.

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Fig. 14. (Color online) Histogram of Pa under all types of irradiation light.

4.

Conclusions

White radish has been eaten from time immemorial and has a medicinal effect. It is
a common vegetable in the world. In Japan, it is one of the seven herbs of spring andis widely consumed. It is also used as seasoning. It is categorized as a root crop andthe white radish’s fleshy root contains about 95% moisture. It is always necessary toprovide fresh radish in the market. In this study, the relationship between the bioelectric
potential and the absorption capability for ethyl alcohol is examined by producing radish
in a plant factory. Suitable production management and cost consciousness in the factory
are desired. Therefore, this study is conducted as a preliminary experiment to determine
the plant characteristics that can be applied to the factory.
Our results show that the plant bioelectric potential varies hourly, and it was clarified
that vh1-ave can effectively indicate the characteristics, which means the summation value
for one hour. vm1-ave and vm30-ave, which mean the summation values for 1 min and half
an hour, are also effective in deriving the bioelectric features. These values show aclose correlation with absorption capability (Pa). The coefficient is about 0.55, which is
not very large. The absorption function is stable under mixed light compared with that
under homogeneous light, and the value is about 10. It somewhat varies depending on
the light frequency. There is a difference in potential because the atmospheric pressure
and the vegetable physiology are different in each experiment. In each experiment, theconditions are fixed wherever possible. The temperature is about 26 °C and the relative
humidity (RH) is over 95% RH in the experimental chamber.
It is necessary to collect and accumulate bioelectric potential data from the standpointof vegetable physiology. Many institutions have studied the microphysiology of plants.
It is also necessary to investigate the macrophysiology when the production efficiency
is increased. The production efficiency of vegetables will be increased by synergistic
effects on plant physiology in the vegetable factory.

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