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Journal of Horticulture
Research Article

A New Technique to Realize a Drastic Acceleration of Crop Growth in the
DFT Hydroponic Cultivation with Hyper-oxygenated Nutrient Solution
Hiroaki Tsutsumi1*, Risa Higashi2, Takahiro Kinugasa3
Department of Environmental Resources, Prefectural University of Kumamoto, Kumamoto, Japan; 2Department of Environmental and Symbiotic
Sciences, Graduate School of Prefectural University of Kumamoto, Japan; 3Kinugasa Co., Ltd., Kumamoto, Japan
1

ABSTRACT
Aeration of the nutrient solution has been tried in order to enhance the growth of leaf vegetables
such as lettuce, rape, spinach etc, in the hydroponic cultivation with the DFT hydroponic cultivation
system in greenhouses. In this study, we chose mini-bok choy as a test crop, and examined the dailychanges of oxygen intake by the root in the hyper-oxygenated nutrient solution of the DFT hydroponic
cultivation system, and revealed the presence of an unknown capacity of the root for oxygen intake fromthe nutrient solution, responding to the changes of light intensity, as it can take in oxygen from thehyper-oxygenated nutrient solution as an alternative oxygen source, but the oxygen intake does not run
in the light conditions enough for photosynthesis. Mini-bok choy cultivated with the hyper-oxygenatednutrient solution with DO between 8.85 and 22.06 mgO2 L -1 exhibited approximately 2.0 and 1.7 holdsof the growth acceleration of its aerial part and root in the period of the experiment over 23 days.
We presented a micro-bubble technique to create a hyper-oxygenated nutrient solution for realizing thedramatic acceleration of the crop growth with a minimum cost applicable to the actual hydroponic
cultivation of the crop in a greenhouse.

Keywords: Hydroponics; Hyper-oxygenation; Micro-bubble; Mini-bok choy; Oxygen intake

INTRODUCTION

from the shoot [7].

Greenhouse cultivation of horticultural crops has recently evolved
as one of the most advanced styles of agriculture [1]. In this facility,
the environmental conditions that affect the growth of the crop
are well controlled, being monitored at all times. The adjustmentof the concentration of carbon dioxide gas in the air has beentreated as one of the primary issues to sustain or enhance thephotosynthetic activity of the crop [2,3]. Frequent ventilationof the air or addition of carbon dioxide gas is often required tocreate enriched conditions of carbon dioxide gas of the air [2-6].
This technique focuses on the biosynthetic activity of the cropduring the daytime, when the crop can self-produce oxygen gas byphotosynthesis, using it partly for keeping its metabolic activity.
However, the plant cannot undergo photosynthesis during thenighttime. Although the shoot is exposed to the air, the root
aeration is limited and partly relies on the oxygen transmission

The diurnal fluctuation of oxygen transmission from the shoot to
the root may bring oxygen stress to the root during the nighttime[7]. In particular, in the case of hydroponic cultivation of the crop,
the root is always submerged in water, where oxygen is much lessdiffusible and soluble than in the air [8], and subject to oxygen
stress throughout the nighttime. The endodermis of the root is
known as an organ that has an ability to actively transport ionsin one direction, resisting their diffusion back into the cortex [9].
Since this function requires energy, its activity is depressed underoxygen-stressed conditions that lead the crop to a restriction of
glycolysis and respiration which decreases the adenylate energy
status, a widespread decrease in biosynthetic activity, and aswitch to pathways that consume less ATP [10,11]. Therefore, theshortage of oxygen supply to the root restricts the nutrient intake
and various biosynthetic activities until the oxygen transmission

Correspondence to: Hiroaki Tsutsumi, Department of Environmental Resources, Prefectural University of Kumamoto, Kumamoto, Japan, Tel: +8196-321-6765; E-mail: hiro@pu-kumamoto.ac.jp
Received: November 20, 2020; Accepted: December 04, 2020; Published: December 11, 2020
Citation: Tsutsumi H, Higashi R, Kinugasa T (2020) A New Technique to Realize a Drastic Acceleration of Crop Growth in the DFT Hydroponic
Cultivation with Hyper-oxygenated Nutrient Solution. J Hortic 20:07.
Copyright: © Tsutsumi H. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

J Hortic, an open access journal
ISSN: 2376-0354

Volume 7 • Issue 4

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from the shoot to the root resumes the next morning.
Recently, new types of water aerators such as the oxygen enricher[12] and micro/nano-bubble generator [13-16] have beendeveloped. They release tiny air bubbles of less than 100 µm of thediameter, and these bubbles have a drastically larger capacity fordissolving oxygen gas compared to the water to the macro-bubbles
released from conventional aquarium aeration devices [13,16]. In
the research of horticulture, the new aeration devices have been
applied to Deep Flow Technique (DFT) hydroponic cultivationof vegetables [12,17-19], aiming at creating dissolved oxygen (DO)saturated conditions or hyper-oxygenated conditions over 20 to
30 mgO2 L -1 in the nutrient solution efficiently, enhancing their
growth by the increase of oxygen transmission from the treatednutrient solution to the root of the cultivated crop. A small scaleof the laboratory experiment with a DFT hydroponic cultivation
system demonstrated that the fresh leaf weight of lettuce (Lactuca
sativa L.) increased to 2.3 times heavier with hyper-oxygenatednutrient solution containing DO over 20 mgO2 L -1 to the control
with untreated one 14 days later [12]. The fresh leaf weight ofrape (Brassica campestris) increased about 1.3 times heavier tothat of the control 28 days later, creating max. 1.2 mgO2 L -1 ofhigher DO conditions in the nutrient solution [17], and theaerial part of spinach (Spinacia oleracea L.) also grew to about
1.4 times heavier in fresh weight to that of the control 21 days
later, creating a max. 1.0 mgO2 L -1 of higher DO conditions in
the nutrient solution [18] by the aeration of the nutrient solution
with air micro-bubbles in the DFT hydroponic cultivation
experiment. Although the detailed mechanisms of how thegrowth of the vegetables was promoted significantly are still not
clear, these studies give us an empirical idea as aeration of thenutrient solution can awake the metabolic activity of the crops
during the nighttime in the same way as the daytime, bringing a
big promotion of their growth.
 ne of the authors of this paper, Hiroaki Tsutsumi, has developedOa micro-bubble generator, eco-Bubble®, in collaboration with
Takohgiken Co., Ltd. (Kumamoto, Japan) [15] to apply this new
aeration device to aquaculture and plant cultivation in the fields,
aiming at increasing DO supply drastically to the cultured aquatic
animals [14,15] and plants when phytoplankton in the water andcultivated plants themselves cannot undergo photosynthesis
during the nighttime. In this study, we chose mini-bok choy(Brassica rapa var. chinensis) as a test crop for the hydroponic
cultivation experiment with the DFT hydroponic cultivation
system, and conducted an experiment with a hyper-oxygenatednutrient solution created by releasing pure oxygen micro-bubbles
in the pool actually used for the horticultural cultivation of

J Hortic, Vol. 7 Iss. 4

vegetables by an agricultural corporation. We monitored the
DO fluctuations of the nutrient solution of the hydroponic
cultivation system and examined the impact of the elevationof DO of the nutrient solution on the respiratory activity fromthe roots of the mini-bok choy and its growth. We report what
benefits the use of the hyper-oxygenated nutrient solution canbring to the DFT hydroponic cultivation of the crop with theresults of the cultivation experiment, and discuss the mechanismsof the physiological responses of the crop to the hyper-oxygenatednutrient solution, including how the micro-bubble technique forcreating the hyper-oxygenated nutrient solution can be applied tothe acceleration of growth of the crop with a reasonable cost forpractical horticultural cultivation.

MATERIALS AND METHODS
The cultivation experiment of mini-bok choy was conductedwith the DFT hydroponic cultivation system for leaf vegetables
in a green house, which is owned by an agricultural corporation,
Kinugasa Co., Ltd. (located in Kikuyo-machi, Kikuchi gun,Kumamoto Prefecture, Japan). We prepared two pools for thecultivation experiment of mini-bok choy. Its seedlings wereplanted in sponges, which were fixed in every other hole of 120
in a Styrofoam planting panel (80 x 55 cm). 32 sets of this panel
were prepared for the experiment, and 16 panels were put on the
nutrient solution with a depth of 15 cm in each of the two pools(640 x 130 x 20 cm). The nutrient solution was gently circulatedwith a circulation pump.
The concentrations of the nutrients of the water in thesetwo pools were adjusted to around 0.7 mS cm-1 in electric
conductivity throughout the period of the experiment, using
a powder nutrient which contained mixed Rocket Power No.
2 and No. 3 manufactured by Nittofc Co., Ltd., Japan, at theweight ratio of 2:3. At one of these two pools, which is referred
to as “Experiment”, two sets of the micro-bubble generator (ecoBubble®-S1 (Input of power 395W in 60Hz), Taikohgiken Co.,
Ltd., Kumamoto, Japan) were installed at the brim of the middle
part of the pool facing each other (Figure 1). The air intake ofthe nozzle of each micro-bubble generator was connected to thedischarge port of a gas cylinder with 7 m3 of pure oxygen gas(99.99%) with a plastic tube. The oxygen gas released as microbubbles with the mean diameter of about 15 µm from the microbubble generators will dissolve in about 1.25 m3 of the nutrient
solution in the pool with a dramatically higher efficiency thanthat of milli-sizes of bubbles released from the conventional airstone (diameter 50 mm) [14,15]. Another pool with no aeration
system is referred to as “Control”.

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DO of the nutrient solution in “Experiment” was kept in hyperoxygenated conditions with 16.7 to 21.7 mgO2 L -1 due to theaeration of pure oxygen micro-bubbles, while that in “Control”
with no aeration system was restricted in the unsaturated
range between 1.7 and 4.2 mgO2 L -1 throughout the period of
the experiment (Figure 2). During the nighttime, a dynamic
fluctuation of DO of the nutrient solution was observed inExperiment. The observation of DO of the nutrient solution
between 14:00 on November 14 and 15:00 on November 15 in2018 shows a typical example (Figure 3).

Figure 1: The DFT hydroponic cultivation system for mini-bok choywith two sets of micro-bubble generators for releasing pure oxygen microbubbles in the nutrient solution stored at a pool (640 x 130 x 20 cm) in
Experiment.

The cultivation experiment of mini-bok choy was carried out
at the two pools used for the DFT hydroponic cultivation
system in the greenhouse. The experiment was conducted for
23 days between 8 November and 30 November in 2018. The
temperature and DO of the nutrient solution were measured in
the afternoon (13:00 to 15:00) in both Experiment and Control
between November 13 and 30, using a multi-probe (ProODO,
YSI/Nanotech Inc., U.S.). The temperature of the nutrient
solution in Experiment, which had decreased from 18.6 °C to13.9 °C, was slightly lower than that in Control (17.6 °C and13.5 °C) during the period of the experiment. As for Experiment,each of the micro-bubble generators was run, and released 0.7 Lof pure oxygen micro-bubbles per minute for four hours between
23:30 and 3:30 every day to dissolve oxygen gas to the nutrient
solution. The changes of DO of the nutrient solution in both ofExperiment and Control were monitored with a probe every 30
min. between 14:00 on 14 November and 15:00 on 15 November
2018. In each of Experiment and Control, five and ten roots ofthe cultured mini-bok choy were collected randomly at the start (8November 2018) and at the end of the experiment (30 November2018), respectively. Their fresh weights of the aerial part and root
were weighed.
The statistical significances of the correlations between theelapsed time and the DO of the nutrient solution of the poolin Experiment in five different periods between 14:00 on 14
November and 15:00 on 15 November 2018 were tested by
Pearson’s correlation coefficient, and the differences of the
fresh weights of the aerial part and the root of bok choy between
Experiment and Control were tested by Mann-Whitney’s U-test,
respectively, with a computer software for Macintosh (Apple Inc.,
U.S.), StatView® (Ver. 4.5, Abacus Concepts, Inc., U.S.).

RESULTSDO Fluctuations of the Nutrient Solution
We conducted a cultivation experiment of mini-bok choy with
the DFT hydroponic cultivation system using a hyper-oxygenatednutrient solution, in a greenhouse (Figure 1). In the daytime, theJ Hortic, Vol. 7 Iss. 4

Figure 2: The changes of DO of the nutrient solution of the pools
measured in the daytime (13:00 to 15:00) in the DFT hydroponic
cultivation system of mini-bok choy used for the experiment during theperiod of the experiment between 13 November and 30 November 2018.

Figure 3: A typical example of the diurnal DO fluctuations monitored
every thirty minutes between 14:00 on 14 November and 15:00 on 15
November 2018. They were divided into five different periods (Period 1
to Period 5), and a regression line was interpolated to the data plots ineach of the five periods.

The DO of the nutrient solution fluctuated between 8.85 and22.06 mgO2 L -1 during this period, and was characterized by
five different periods (Period 1 to Period 5) (Figure 3, Table 1).
In Period 1 and Period 5, the DO of the nutrient solution had
decreased gently at the ratios of -0.09 mgO2 L -1 h-1 and -0.17

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mgO2 L -1 h-1, respectively. This phenomenon can be regarded
partly due to a natural loss of oxygen gas to the ambient air. In
contrast, a rapid decrease of DO of the nutrient solution wasobserved between 17:00 and 23:00 in Period 2 (-0.96 mgO2 L -1h-1) and between 03:30 and 8:00 in Period 4 (-0.78 mgO2 L -1 h-1),
respectively. In Period 3 between 23:30 and 03:30, the nutrient
solution was aerated with pure oxygen micro-bubbles released by
the two sets of micro-bubble generators at the gas flow rate of 1.4
L min-1 (in total) to keep the hyper-oxygenated conditions. The
DO of the nutrient solution increased at the rate of +3.59 mgO2
L -1 h-1. If additional DO was not supplied to the nutrient solution
in this period, its DO seemed to have decreased to almost thesame level as Control until 8:00 on November 15, judging fromthe decreasing DO rates in Period 2’ and Period 4. In Control,
the nutrient solution was under hypoxic conditions (1.50 to2.55 mg L -1) throughout this period. During the nighttime, noalternative source of oxygen was available for the root of the crop.
Line
No.

Time

Temper- Slope
ature
(mgO2
(°C)
L -1 h-1)

R2

P

1

14:00 to
16:30

18.1 to
17.9

-0.09

0.023

0.793

2

17:00 to
23:30

17.9 to
16.0

-0.96

0.955

<0.00012’

19:00 to 17.2 to
23:30
16.0

-0.77

0.992

<0.00013

23:30 to 16.0 to
03:30
15.6

3.59

0.98

<0.00014

03:30 to 15.6 to
08:00
14.7

-0.78

0.951

<0.00015

08:00 to 14.7 to
15:00
17.2

-0.17

0.128

0.194

The regression line 2’ was calculated with the DO data of thenutrient solution between 19:00 and 23:30 on 14 November 2018
in Period 2.
Table 1: The effect of hyper-oxygenation of the nutrient solution on
the growth of mini-bok choy in Experiment and requirements for theexperiment over 23 days.

Timing of Critical Points of DO Fluctuations
These diurnal changes of the DO of the hyper-oxygenated nutrient
olution in Experiment indicate the presence of critical points inoxygen transmission from the shoot to the root and intake of DO
of the solution through the root at 17:00 in November 14 and8:00 on November 15. According to the weather report at thenearest local meteorological observatory, the critical points hadcome when the total solar radiation decreased to 0.47 MJ m-2 at
17:00, and then increased to 0.36 MJ m-2 at 8:00, respectively[20](Figure 4). The timing of these critical points was just coincidental

J Hortic, Vol. 7 Iss. 4

with the dim periods at twilight and dawn, respectively. Hence,
we are able to explain this as the cultivated crop potentiallypossesses a function to take in DO of the solution through theroot if ample DO is contained in it, and its switch is turned onwhen the solar radiation has decreased to the levels unsuitablefor photosynthesis. It is turned off when it has increased to theintensity strong enough for photosynthesis even if the nutrient
solution is hyper-oxygenated.

Figure 4: Diurnal changes of the total solar radiation observed at thenearest meteorological observatory (Kumamoto Local MeteorologicalObservatory) to the experimental site, which is about 10 km apart,
between 14:00 on 14 November and 15:00 on 15 November 201820.
The darks area indicates the period that enough light intensity was notavailable for photosynthesis by the mini-bok choy cultivated in this study.

Growth of Mini-bok Choy
The differences of the DO conditions of the nutrient solution
between Experiment and Control were reflected as those of thecrop growth for 23 days in the experiment (Figure 5). The fresh
weights of the seedlings in Experiment and Control were 2.14 ±0.31 g and 2.17 ± 0.68 g in the aerial parts (mean ± S.D., n=5),
and 0.45 ± 0.03 g and 0.46 ± 0.14 g in the root (mean ± S.D.,
n=5), respectively. Both fresh weights of the aerial part and root
showed no significant difference between them (Mann-Whitney’s
U-test, aerial parts: P=0.917, n=5; roots: P=0.602, n=5). After
cultivation for 23 days, the fresh weights of the aerial parts androot grew to 20.34 ± 8.09 g and 3.10 ± 1.16 g (mean ± S.D., n=10)
in Experiment, while growth in Control was restricted to 10.23± 3.79 g and 1.82 ± 0.55 g (mean ± S.D., n=10), respectively.
The differences of the fresh weights of both of the aerial partsand root were statistically significant between Experiment andControl (Mann-Whitney’s U-test, aerial parts: P<0.001, n=10;
roots: P=0.019, n=10). Figure 6 compares the whole images of
the individuals which are closest o the mean fresh weight of theaerial parts and root in Experiment and Control at the end of
the experiment. It is obvious that the increase of oxygen intake
through the root during the nighttime brings a remarkablepromotion of crop growth (approximately 2.0 and 1.7 folds ofthe growth acceleration of the aerial parts and root to Control,
respectively).

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the water under the present atmosphere of 1 atm that contains
about 21% of oxygen.
Is the Micro-Bubble Technique for Aerating the Nutrient
Solution Cost-Beneficial?

Figure 5: Fresh weights of the mini-bok choy cultured in the DFT
hydroponic cultivation system in the experiment. (a) aerial parts, (b)root. Bars indicate the standard deviations. The asterisks indicate thestatistically significances of the values of Experiment to those of Control.

Figure 6: Images of the mini-bok choy most close to the mean fresh
weight of the aerial parts and root in (a) Control (aerial parts: 10.45g(mean:10.23 g), root: 1.88 g (mean: 1.82 g), and (b) Experiment (aerial
parts: 20.71 g (mean: 20.34 g), root: 2.60 g (mean 3.10 g)) after 23 days
from the start of the cultivation experiment.

DISCUSSION
The Mechanisms of the Physiological Responses of the Crop to
the Hyper-Oxygenated Nutrient Solution
In this study, the results of the cultivation experiment of minibok choy in the DFT hydroponic cultivation system demonstratethat oxygen intake through the stomata on the leaves hardly
contributes to the oxygen transmission to the root during thenighttime when the internal oxygen production by photosynthesis
has stopped, but it can rely on the DO of the nutrient solution as
an alternative oxygen source to the root if they DO of the nutrient
solution is in the range between 8.85 and 22.05 mgO2 L -1 (DOsaturated or hyper-oxygenated conditions). However, the oxygen
intake of the root stops when the solar radiation has increased tointensity strong enough for photosynthesis even if the nutrient
solution is hyper-oxygenated. (Figures 3 and 4). These facts reveal
an unknown capacity of the root of mini-bok choy with two waysof oxygen intake, responding to the light intensity. This capacity
seems to be applicable for a wide variety of crops, but it tends tobe concealed since oxygen is much less diffusible and soluble to

J Hortic, Vol. 7 Iss. 4

The previous DFT hydroponic cultivation experiments, with thenutrient solution aerated by air micro/nano-bubbles, created DO
saturated levels slightly higher than those of the control (max.
0.3 to 1.2 mgO2 L -1), and have already demonstrated a significant
promotion of increase in the fresh weight of the aerial part of rape(about 1.3 times greater, 28 days) [17], spinach (about 1.4 times
greater, 21 days) [18], and Cos lettuce (Romaine lettuce, Lactuca
sativa L. var. longifolia) (about 2.1 times greater, 14 days later) [19],respectively, although they did not present the mechanisms ofgrowth promotion. However, all experiments were conducted insmall tanks or containers with only 70 to 125 L of the nutrient
solution [17-19]. If these aeration systems that use ambient air
are introduced to the DFT hydroponic cultivation of the crops
in actual greenhouses for horticulture, it will be hard to sustain
a large amount of oxygen demand of the crops or a large-size
aeration device and a large-capacity of power will be required tobalance with the DO demand of the crops during the nighttime.
In the present study, we conducted the cultivation experimentof mini-bok choy in a pool for the DFT hydroponic cultivation
actually used for the horticultural activity, aerating about 1.25
m3 of the nutrient solution with pure oxygen micro-bubbles
released from two sets of small micro-bubble generators for fourhours in a day, and realized approximately 2.0 and 1.7 folds ofthe growth acceleration of the aerial part and root to those ofControl, respectively (Figure 5). Nevertheless, this experiment
was characterized by extremely small power consumption for
aeration, 3.16 kWh (395W x 2 x 4 (h)), and only 360 L of pureoxygen gas consumption (0.75 L (min.-1) x 2 x 60 (min.) x 4 (h))
per night, due to an extremely high dissolution capacity of pureoxygen micro-bubbles to the nutrient solution. 360 L of pureoxygen gas weighs 489.4 g. In the aeration of the nutrient solution
in Experiment for four hours between 23:30 on 14 November
and 3:30 on 15 November 2018 (Figure 3), 221.5 g O2 stock of
the nutrient solution increased as DO. Therefore, 45.3% of 360
L of pure oxygen gas released as micro-bubbles dissolved into thenutrient solution in the pool of the DFT cultivation system inExperiment in this period.
However, this efficient aeration method for the nutrient
solution with pure oxygen micro-bubble still does not seem tobe cost-effective enough to introduce it to the actual hydroponic
cultivation of the crop in a greenhouse, although it realized
approximately 2-folds of accelerated growth of the crop in
Experiment in this study (Figure 5). Table 2 concludes the effectof hyper-oxygenation of the nutrient solution on the growth ofmini-bok choy in Experiment and displays requirements for theexperiment over 23 days. We estimated that the total weightof the crop in Experiment increased by 9,734 g to Control,
but the increase of the sales amount of the crop by the growth
acceleration seems to be almost in balance with the total cost of5

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power consumption (72.68 kWh) and 8,280 L of pure oxygen gas
spent for aeration of the nutrient solution.
Control

Experiment2.17 ± 0.68*

2.14 ± 0.31*

Weight of aerialpart (g root -1)

10.23 ± 3.79*

20.34 ± 8.09*

Mean weightincrease of aerialpart (g root -1)

8.06

18.2

Total weightincrease of 960
roots (g) in a pool

7,738

17,472[Growth]Weight of seedling(g root -1)(after 23 days)

Accelerationof growth inExperiment (g per
960 roots)

9,734[Power and gas
consumption]
Consumption of
power for aeration(kWh d -1)

none

3.16

Total consumption nonefor 23 days (kWh)72.68

Consumption ofpure oxygen gas(L d -1)

360

none

Total consumption nonefor 23 days (L)
*

8,280

mean ± S.D., n=10

Table 2: The effect of hyper-oxygenation of the nutrient solution on
the growth of mini-bok choy in Experiment and requirements for theexperiment over 23 days. * mean ± S.D., n=10.

A Solution to Minimize the Cost for Hyper-Oxygenation of the
Nutrient Solution
The nutrient solution can be hyper-oxygenated by replacing
pure oxygen gas with 90% oxygen gas produced by an “oxygen
concentrator”, which has been used for oxygen therapy forhumans. For example, one set of the small one that runs with
290W of power can produce 5 L min.-1 of 90% oxygen gas withcost-free ambient air, and can provide gas at the same gas flowrate (0.75 L min.-1) as the experiment in this study to at leastsix sets of the micro-bubble generators used in the experiment.

J Hortic, Vol. 7 Iss. 4

Furthermore, oxygen concentrators have been disposed of after
patient use for several years at least in Japan (and presumably invarious countries with advanced medical care systems), but it canstill produce useful oxygen gas with a concentration of 90% orslightly less. In our preliminary tests, another type of micro-bubble
generator, which is made up of a submersible pump with 250W
of power consumption and nozzle for releasing micro-bubbles,
could make 1 m3 of the hyper-oxygenated water containing about
30 mgO2 L -1 of DO from tap water in a tank within one hour,
being supplied with about 90% oxygen gas from “an used oxygen
concentrator” at a gas flow rate of 0.75 L min-1. Therefore, thecombined use of the micro-bubble generator along with the usedoxygen concentrator enables us to minimize the installation cost
to use 90% oxygen gas and the running cost for preparing thehyper-oxygenated nutrient solution for hydroponic cultivation of
the crop.
We have already tested this new aeration method with thenutrient solution for the DFT hydroponic cultivation of otherleafy vegetables such as lettuce, spinach, basil, and parsley,and cultivation of flowers in pots with soil such as cyclamen
and phalaenopsis orchid, and we have commonly confirmed asignificant acceleration of growth. The results of the experiments
will be reported elsewhere. This method seems to be applicableto the cultivation of a wide variety of crops and plants forhorticulture, and brings a remarkable acceleration of growthand/or increase of production just by suppling hyper-oxygenatednutrient solution or water as if it was done in grain production bybreed improvement in the 1940s to 1960s referred to as “Green
Revolution” [21,22].

CONCLUSION
The results of the cultivation experiment of a test crop forhydroponic cultivation with the DFT hydroponic cultivation
system, mini-bok choy, revealed its unknown basic capacity foroxygen intake of the root, responding the changes of light intensity.
It can rely on the DO of the nutrient solution as an alternativeoxygen source to the root when the internal production of oxygen
production by photosynthesis has stopped, if ample oxygen is
dissolved in the nutrient solution. However, the oxygen intakeof the root does not run in the light conditions enough for thephotosynthesis even if the nutrient solution is hyper-oxygenated.
Mini-bok choy, cultivated in the pool with hyper-oxygenatednutrient solution aerated by oxygen micro-bubbles, exhibited
approximately 2.0 and 1.7 folds of the growth acceleration of
its aerial part and root in the period of the experiment over 23days, respectively, to those of Control without any aeration of
the nutrient solution. The micro-bubble generator used for theexperiment realized an extremely high dissolution rate of oxygengas, 45.3%, to the nutrient solution, and its combined use withan oxygen generator enables us to minimize the cost for creating
hyper-oxygenation of the nutrient solution, and to realize adramatic acceleration of the growth of the crop.

6

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ACKNOWLEDGMENTS
We thank Mr. T. Nishi and I. Takase for his technical support
on the installation and use of the micro-bubble generators at agreenhouse for the experiment. Dr. Jeffery Stewart Morrow for
his critical reading and correction of the English text of the draft
paper.

SOURCES OF FUNDING
The experiment was conducted with the annual research budget
provided from Prefectural University of Kumamoto to thelaboratory of Hiroaki Tsutsumi.

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