Sensors and Materials, Vol. 26, No. 7 (2014) 461–470
MYU Tokyo
S & M 1013

Ambient Temperature Effects
on Evaluation of Plant Physiological Activity
Using Plant Bioelectric Potential
Yuki Hasegawa*, Genta Yamanaka, Ki Ando1 and Hidekazu Uchida
Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, JapanTokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan

1(Received March 3, 2014; accepted May 1, 2014)
Key words: plant bioelectric potential, plant factory, photosynthesis, ambient temperature, plant
physiological activity

The plant factory has been in practical use for ensuring a stable food supply andcultivating high-quality food. However, it has problems with capital and operating
expenditures, because it is necessary to prepare a big room of the same level ofcleanliness as that of a semiconductor factory and to use a full environment control
system that includes an air conditioner, a feed nutrient solution line, and sensing devices
in continuous operation. Therefore, we focused on the bioelectric potential response as apromising and low-cost approach of evaluating plant physiological activities. Bioelectric
potential is generated by ions in a plant cell and the potential is related to plant
physiological activities. Although we already reported that the potential response differs
depending on the on-off cycle of illumination and is related to photosynthetic activity,
there is no report on the effects of ambient temperature and light interruption time on
the measurement of bioelectric potential. In this study, we investigated the effects ofambient temperature and light interruption time on the evaluation of plant physiologicalactivities. In the experiments, we measured bioelectric potential responses and CO2
concentration when light irradiation was started or stopped at several temperatures. In
addition, we investigated the effect of the light interruption time on the evaluation ofplant physiological activities using plant bioelectric potential. These results contributeto the improvement of the precision of the evaluation system for plant physiologicalactivities using bioelectric potential responses.

Corresponding author: e-mail: yuki@ees.saitama-u.ac.jp

*

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Sensors and Materials, Vol. 26, No. 7 (2014)

Introduction

Recently, it has been a global issue of securing safe and abundant food. The plantfactory has been in practical use for ensuring a stable food supply and cultivating highquality food. In the plant factory, a system controls artificial light, temperature, humidity,
water including nutrition, and carbon dioxide concentration to create an optimal
cultivation environment in an indoor space, and does not use pesticides.(1–3) Thus, theessential characteristic of the plant factory is not only productivity but also the ability to
produce a stable amount of clean and safe food all year round.
However, the plant factory has problems with capital and operating expenditures,
because it needs to maintain a stable and full environment control system that includes
an air conditioner, a feed nutrient solution line, and sensing devices. Therefore, theimportant issues are cost reduction and productivity of the improvement.(4,5) Various
studies have been carried out to investigate how to reduce these costs and improve theproductivity of cultivation.(6–8)
In our previous work, we focused on the measurement of bioelectric potential as a
low-cost technique of evaluating plant physiological activities, such as photosynthesis,
respiration, and transpiration, and supporting effective cultivation. The bioelectric
potential of a plant is generated by ions inside the plant cells.(9) Measurement of the plantbioelectric potential is a promising method of real-time evaluation and monitoring of theplant’s state of activity as a physical quantity. We reported that the bioelectric potential
varies when the illumination is started or stopped, that the amplitude of this variationis correlated with the photosynthetic rate, and that the characteristics of the bioelectric
potential depend on the illumination period, wavelength, and intensity.(10–12) Although
we also showed that the potential response differs depending on the illumination
conditions, there is little discussion on the effect of ambient temperature on the behaviorof the potential response. To improve the photosynthetic rate and productivity in a plantfactory, it is desirable that the light interruption time is as short as possible. However, in
our present evaluation method, light has to be interrupted for at least 30 min to observe asignificant potential response when the illumination is started. Thus, the photosynthetic
reaction is reasonably disturbed during light interruption time. Therefore, it is necessary
to study the shortest optimal light interruption time for the evaluation of using thebioelectric potential response.
In this study, we aim to clarify the effect of ambient temperature on the evaluationof plant activity using plant bioelectric potential. The optimal temperature differs
depending on the plant species, and the cultivation temperature affects the photosynthetic
rate. Several studies have been carried out on the temperature dependence of thephotosynthetic rate and other physiological activities of plants.(13–15) In our experiments,
the bioelectric potential response under light irradiation and interruption was observed
at several cultivation temperatures. We also observed the bioelectric potential responsewhen the light interruption time was gradually shortened. These results contribute to theimprovement of the precision of the evaluation system for plant physiological activities
using bioelectric potential responses.

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2.

Materials and Methods

2.1

Measurements of plant bioelectric potential and photosynthetic rate

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Figure 1 shows the measurement system of the plant bioelectric potential. The plant
was placed in a 22.4 L closed vessel, and the change in CO2 concentration caused byplant metabolic processes, such as photosynthesis and respiration, was measured using a
nondispersive infrared (NDIR)-type CO2 analyzer (LI-840, Meiwafosis Co., Ltd.). The
ambient temperature and humidity were controlled by the growth chamber (BAC-130H,
Espec Corp.) enclosing the vessel. The measurement temperature was controlled atevery 5 °C from 15 to 35 °C if necessary, and the humidity was controlled at 60 to 70%
RH. The light source consisted of blue (470 nm), green (525 nm), red (660 nm), andinfrared (735 nm) LEDs, and light irradiation patterns were controlled by a control unit.
The photosynthetically active photon flux density (PPFD) on the leaf surface was 150
μmol∙m2∙s−1.
The sample plants were some golden pothos plants (Epipremunum aureum) grownin hydroponics in 0.5 L plastic pots and have two to three leaflets. The bioelectric
potential was measured using electroencephalographic (EEG) disk-type electrodes of
9 mm diameter (NE-155A, Nihon Kohden Corp.) attached to the plant surface with aconductive paste for EEG (Z-401CE, Nihon Kohden Corp.). To detect the potential
response induced by photosynthetic reactions, an electrode was attached to the leafstalk
as a reference electrode and another electrode was attached to the leaf surface, where
photosynthesis takes place. The potential difference between the two electrodes wasmeasured with a high-input-impedance (> 1 GΩ) digital multimeter (DMM in Fig. 1,
R6552A, ADC Corp.) and recorded by a computer at a sampling interval of 1 s. The
measurement was started 60 min after the electrode attachment because of the exclusionof a noise signal generated immediately after electrode attachment.
To determine the photosynthetic rate, an NDIR-type CO 2 analyzer (LI-840,
Meiwafosis Co., Ltd.) was used. First, we measured the increase in CO2 concentration
caused by the respiration of the plant in the dark, and defined it as the respiration rate (ppm/h).
Then, we determined the sum of the observed decrease rate in CO2 concentration owing

Fig. 1. (Color online) Measurement system of bioelectric potential.

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to photosynthetic activity under illumination and the respiration rate, and defined it as thephotosynthetic rate (ppm/h).

2.2

Evaluation parameters and measurement conditions

We measured the bioelectric potential response of the sample plant to a stimulusof light irradiation or interruption. Figure 2 shows the typical bioelectric potential
responses when illumination was started and stopped. The potential rose immediately
after light irradiation and passed a peak, and slowly returned to approximately the initialvalue. On the other hand, when the illumination was stopped, the potential dropped
sharply, and then rose toward the steady state. We defined the difference Von [Fig. 2(a)]as that between the largest potential increase and the value before light irradiation, andthe difference Voff [Fig. 2(b)] as that between the largest potential decrease and the value
before light interruption. In a previous study, it was reported that the amplitude of thepotential variation Von obtained by starting the illumination has a strong correlation
with the photosynthetic rate, and that the Voff obtained by stopping the illumination
has a strong correlation with photochemical reactions and the respiration rate. These
results suggested that these parameters can be used to evaluate the various physiologicalactivities of the plant.(11,12)
In our experiments, first, we investigated the temperature dependence of bioelectric
potential response and photosynthetic rate. The bioelectric potential and photosynthetic
rate were measured from 15 to 35 °C at 5 °C intervals (the optimal temperature of golden
pothos plants is about 20 °C). Generally, the photosynthesis-temperature curve is often
symmetrical or bell-shaped.(15,16)
Next, we also observed the bioelectric potential response when the light interruption
time was gradually shortened. The light irradiation time was fixed at 1 h, and only thelight interruption time was changed (1, 3, 5, and 10 min).

Von

Illumination stop

Potential

Potential

Illumination start

Voff
Time

Time

(a)

(b)

Fig. 2. Typical bioelectric potential response to (a) light irradiation and (b) light interruption anddefinition of evaluation parameters Von and Voff.

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3.

Results and Discussion

3.1

Relationship between ambient temperature and bioelectric potential
response

First, we compared the characteristics of the bioelectric potential responses at various
ambient temperatures. Figure 3 shows the time series data of bioelectric potential
responses from 15 to 35 °C at 5 °C intervals, when the illumination was started andstopped repeatedly. In this figure, the potential responses changed immediately withlight irradiation and interruption at all ambient temperatures. The waveform of thepotential response differs depending on the ambient temperature, and the characteristics

OFFON(a) 15 oCOFFON(b) 20 oC

Potential(c) 25 oC(d) 30 oC(e) 35 oC

10 mV
0

30

60
90
Time (min)120

Fig. 3. Time series data of bioelectric potential response from 15 to 35 °C at 5 °C intervals.

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Sensors and Materials, Vol. 26, No. 7 (2014)of potential increases and decreases are reproducible at every temperature. In particular,
the amplitudes of the potential at 25 and 30 °C showed the largest responses, and thatat 15 °C showed the smallest response when the illumination was started. On the other
hand, the amplitudes of the potential gradually increased from a low temperature to ahigh temperature when the illumination was stopped.
We also measured the change in CO2 concentration in response to light irradiation andinterruption, and the relationship between the ambient temperature and photosynthetic
rate of the sample plants is shown in Fig. 4. This figure shows that the photosynthetic
rate differed with the ambient temperature. In this figure, the plotted marks are themean value from each set of eight data points, and each error bar indicates the standard
deviation. It was also shown that the photosynthesis-temperature curve had a peak
between 25 and 30 °C. Generally, the optimal temperature of golden pothos plants isabout 20 °C, but it was reported that the optimal temperature changes depending on thegrowth temperature. Plants grown at low temperatures show greater photosynthetic
capabilities at lower temperatures, whereas plants grown at high temperatures show
greater photosynthetic capabilities at higher temperatures.(17) Thus, it is thought thatthe optimal temperature of the sample plant was changed to 20 °C by our growthenvironment.
Figure 5 shows Von (a) and Voff (b) at various ambient temperatures. In this figure, theplotted marks are the mean value from each set of eight data points, and each error bar
indicates the standard deviation (at only 15 °C, the mean value and standard deviation
were calculated from five data points, because the potential variation was sometimes
too small to read). It was found from Fig. 5(a) that each Von data is variable but tends
to show a bell shape similarly to the photosynthesis-temperature curve (Fig. 4), and thatthe peak temperature is between 25 and 30 °C. Figure 5(b) indicates that Voff tended toincrease with ambient temperature. According to plant physiological studies, the dark

Photosynthetic rate (ppm/h)

30
25
20
15
10
5
0

15

20

25

30

35

40

Ambient temperature (°C)
Fig. 4. Photosynthetic rates of sample plants at various ambient temperatures.

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15

15

10

10

Voff (mV)Von (mV)

Sensors and Materials, Vol. 26, No. 7 (2014)

5

0

10

20

30

40

5

0

10

20

30

Ambient temperature (°C)

Ambient temperature (°C)

(a)

(b)

40

Fig. 5. (a) Von and (b) Voff at various ambient temperatures.

respiration and chloroplast electron transport rates increased with increasing ambient
temperature.(14,16,17) In contrast, the activation state of the plant enzyme, ribulose-1,5bisphosphate carboxylase/oxygenase (RuBisCO), decreased with increasing ambient
temperature. RuBisCO is the enzyme involved in the Calvin cycle that catalyzes thefirst major step of carbon fixation.(13) It was considered that the observed Voff was related
to these plant physiological activities. Therefore, a more detailed investigation of therelationship between the plant physiological activities and the bioelectric potential
responses when light illumination was stopped is required in our future work.

3.2

Light interruption time and bioelectric potential responses

To improve the photosynthetic rate and productivity in a plant factory, it is desirable
that the light interruption time is as short as possible. However, in our present evaluation
method, light has to be interrupted for at least 30 min to observe a significant potential
response when the illumination is started. The photosynthetic reaction is reasonably
disturbed during light interruption time. Therefore, it is necessary to study the shortest
optimal light interruption time for the evaluation of plant physiological activities using
the bioelectric potential response.
In this section, we observed the bioelectric potential response when the lightinterruption time was gradually shortened. Figure 6 shows the bioelectric potential
responses at various light interruption times. We observed very simple responses when
the interruption time was 1 min. The potential response decreased immediately after
light interruption was observed, and then the potential increased rapidly up to the initialvalues after light illumination. The potential responses at the interruption times of 3 and10 min had similar waveforms. First, the potential responses decreased and had the firstsmall peak about 1 min after the light interruption. Then, the response at 3 min decreased
gradually until the light illumination was started, and the response at 10 min also
decreased until the second peak was formed and then increased gradually until the light468

Sensors and Materials, Vol. 26, No. 7 (2014)

Light interruption(a) 1 minPotential(b) 3 min(c) 5 min(d) 10 min

5 mV
0

10

20
Time (min)30

40

Fig. 6. Bioelectric potential responses at various light interruption times.

illumination was started. After the light illumination was started, the potential slightly
increased, and then decreased soon and had a peak. Finally, we observed the distinctive
potential response when the interruption time was 5 min. The potential response did notinclude several small peaks; thus, it looks like a sine curve. All waveform trends wereobserved repeatedly in multiple plants. We obtained the difference in potential response
depending on the light interruption time. We also required the measurement of the CO2
concentration and the analysis of the relationships between plant physiological activities
and potential responses in these experiments.

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Conclusions

In this study, we investigated the effects of ambient temperature and light interruption
time on the evaluation of plant activities. We measured the potential difference ofthe two electrodes on the leaflet and leafstalk of the sample plant. To determine
the photosynthetic rate, an NDIR-type CO2 analyzer was used. The measurement
temperature was controlled from 15 to 35 °C at 5 °C intervals when necessary, andthe humidity was controlled at 60 to 70% RH using the growth chamber. First, thetemperature dependence of the bioelectric potential responses was investigated at various
ambient temperatures. The obtained results showed that the potential responses changed
immediately with light irradiation and interruption at all ambient temperatures. We also
indicated that plant physiological activities, such as the photosynthetic and respiration
rates, could be expressed by Von and Voff. Therefore, a more detailed investigation of therelationship between plant physiological activities and bioelectric potential responses
when light illumination is started and stopped should be carried out in our future work.
Finally, we also observed the bioelectric potential response when the light interruption
time was gradually shortened (1, 3, 5, and 10 min). The obtained results showed thatthe difference in potential response depended on the light interruption time. These
results contribute to the improvement of the precision of the evaluation system for plant
physiological activities using bioelectric potential responses.

Acknowledgements
This work was supported by a JSPS Grant-in-Aid for Young Scientists (A), Grant No.
24686049.

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About the Authors
Yuki Hasegawa received her Master of Education and her Doctor Degree in Engineeringfrom Saitama University in 2001 and 2005, respectively. She is now an associate
professor at the Department of Electrical and Electronics Systems, Saitama University,
since 2014. Her current research interests are environmental monitoring and controllingsystems using plant bioelectric potential, and taste sensors using several types of sensing
films. She is a member of the Institute of Electrical Engineers of Japan, the Japan
Society of Applied Physics, and the Japanese Society of Agricultural, Biological andEnvironmental Engineers and Scientists.
Genta Yamanaka received his B. Eng. Degree from Saitama University in 2013. He
is a master course student of the Division of Mathematics, Electronics and Informatics,
Graduate School of Science and Engineering, Saitama University. He is a student
member of the Institute of Electrical Engineers of Japan.
Ki Ando received his D. Eng. Degree from Graduate School of Science and Engineering,
Saitama University, in 2012. Since 2013, he is an assistant professor at Faculty ofEngineering of Tokyo Denki University. His research interests are plant bioelectric
potential responses to light and their applications. His research field is electronic
measurement technology enhancing the quality of human life. He is a member of theInstitute of Electrical Engineers of Japan and the Research Institute of Signal Processing
Japan.
Uchida Hidekazu received his Master Degree in Electronics Engineering and his Doctor
Degree in Engineering from Saitama University in 1990 and 1997, respectively. He isa professor at the Department of Electrical and Electronic Systems, Saitama University,
since 2014. He is a member of the Japan Society of Applied Physics and the Institute ofElectrical Engineers of Japan.