Modern plant cultivation technologies in agriculture under controlled environment: a review on aeroponics

Abstract

This review paper describes a novel approach to plant cultivation under soil-less culture. At present, global climate change is expected to raise the risk of frequent drought. Agriculture is in a phase of major change around the world and dealing with serious problems. In future, it would be difficult task to provide a fresh and clean food supply for the fast-growing population using traditional agriculture. Under such circumstances, the soil-less cultivation is the alternative technology to adapt effectively. The soil-less system associated with the Hydroponic and Aeroponics system. In the aeroponics system, plant roots are hanging in the artificially provided plastic holder and foam material replacement of the soil under controlled conditions. The roots are allowed to dangle freely and openly in the air. However, the nutrient rich-water deliver with atomization nozzles. The nozzles create a fine spray mist of different droplet size at intermittently or continuously. This review concludes that aeroponics system is considered the best plant growing method for food security and sustainable development. The system has shown some promising returns in various countries and recommended as the most efficient, useful, significant, economical and convenient plant growing system then soil and other soil-less methods.

Full text

REVIEW ARTICLE
Modern plant cultivation technologies in agriculture under controlled environment:
a review on aeroponics
Imran Ali Lakhiar, Jianmin Gao, Tabinda Naz Syed, Farman Ali Chandio and Noman Ali Buttar
Key Laboratory of Modern Agricultural Equipment and Technology, Ministry of Education, Institute of Agricultural Engineering, Jiangsu University,
Zhenjiang, Jiangsu, People’s Republic of China
ABSTRACT
This review paper describes a novel approach to plant cultivation under soil-less culture. At present,
global climate change is expected to raise the risk of frequent drought. Agriculture is in a phase of
major change around the world and dealing with serious problems. In future, it would be difficult
task to provide a fresh and clean food supply for the fast-growing population using traditional
agriculture. Under such circumstances, the soil-less cultivation is the alternative technology to
adapt effectively. The soil-less system associated with the Hydroponic and Aeroponics system. In
the aeroponics system, plant roots are hanging in the artificially provided plastic holder and foam
material replacement of the soil under controlled conditions. The roots are allowed to dangle freely
and openly in the air. However, the nutrient rich-water deliver with atomization nozzles. The
nozzles create a fine spray mist of different droplet size at intermittently or continuously. This
review concludes that aeroponics system is considered the best plant growing method for food
security and sustainable development. The system has shown some promising returns in various
countries and recommended as the most efficient, useful, significant, economical and convenient
plant growing system then soil and other soil-less methods.
ARTICLE HISTORY
Received 10 December 2017
Accepted 30 April 2018
KEYWORDS
Food sustainability;
controlled environment;
aeroponics; ultrasonic
atomization; nutrient
solution; plant production
Introduction
One of the greatest challenges of today is to end hunger
and poverty while making agriculture and food systems
sustainable. However, providing clean and fresh food for
next generation is our main concerns especially for growing
global population (Alexandratrs and Bruinsma 2012). The
world food production is rising faster than population
and per capita consumption increase. Studies reported
that in 2050, the world population expects to surpass ten
billion people, 34% higher than now. Nearly much of popu-
lation increase will occur in the developing countries
(Cohen 2002;UN2010). According to FAO (2009,2011)
report, the high concentration of people has major socio-
economic ramifications, food production, supply and secur-
ity issues which require closer examination. Also, it will
highlight the several problems, challenges and cause to
increase the number of hungry and malnourished people.
In 2050, additional 60% to 70% global food production
will need to feed the more urban and larger population
(Foote 2015).
In the future, the additional pressure will be working on
how we more efficiently utilize the natural resources to pro-
duce food. The natural resources include soil, water, air and
how to use them in sustainability. However, around the quar-
ter of arable land has been declared unproductive, unfertile
and unsuitable to perform agriculture activates. The reasons
behind these issues are inadequate soil management, soil
degradation, fast regional climate changes, rapid urbaniz-
ation, industrialization, fewer recovery chances of natural fer-
tility, continuous cropping, the frequent drought, less water
management, water pollution and the decrease in ground-
water (Popp et al. 2014).
Bliesner et al. (2005) reported water is another critical
resource. The scarcity of water is the most important and
crucial issue to perform agriculture activities and inflicting
insecurity on the social problems. The problems include
minimum crop production with high population: (i) Highly
dependent on climatic conditions and poor growing season
due to starving in different parts of the world, (ii) Higher
demand for biofuels could further influence on inputs,
prices of farm produce, land, water, and endanger a global
food security. As mentioned above, resource constraints
for agricultural production have become more stringent
than in the past while the growth of yields is slowing
down. It is a primary reason why people express fears that
there are growing risks that world food production may
not be enough to feed a growing population and ensure
food security for all. However, it could be challenging to
provide supplemental food products to feed the entire popu-
lation using traditional/open field cultivation system.
Although, the open field cultivation associated with enor-
mous risks and uncertainties from biotic and abiotic stres-
ses, such as pest attacks, droughts, floods and high winds.
As it required the larger area for cultivation, higher land
preparation cost, number of labors and the excess amount
of water.
Under such circumstances, researchers search out the new
farming technologies and suggested that the proposed sol-
ution is to implement the currently accessible technologies
under a controlled environment. In recognition of this, the
soil-less system is one of them. Butler and Oebker (2006)
© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
CONTACT Jianmin Gao gaojianminujs@163.com
JOURNAL OF PLANT INTERACTIONS
2018, VOL. 13, NO. 1, 338–352
https://doi.org/10.1080/17429145.2018.1472308

reported that soil-less is the method of plant cultivation with-
out the use of soil within substrate culture or water culture.
The technique facilitates many socio-economic benefits
including the ability to deal with the increasing global food
challenges, environmental changes for the mitigating, malnu-
trition, management and efficient utilization of natural
resources. Furthermore, the soil-less technique can provide
continues, enough, fresh, clean and hygienic vegetable supply
throughout the year without any interval. The system uses
minimum input and facilitates to multiple plant harvesting
with maximum output. The concept of the soil-less culture
seeks to offer an innovative solution to ensure the environ-
mental and economic sustainability of food supplies with
high nutritional quality. It is a highly recommended plant
growing technique for all countries having less arable land,
rapid environmental changes and increasing food challenges
with the indigenous population (Pual 2000; Sardare and
Admane 2013). Naville (1913) reported that several ancient
civilizations practiced the technique. While the first symbol
of container-grown plants documented paintings were
found on the Walls of ancient temple Deir el Bahari.
Although, many hundred years ago in B.C., Aztecs and Egyp-
tian hieroglyphics also used water culture to grow the specific
plants and the hanging garden of Babylon is also an excellent
example. In 1627, Francis Bacon published the book named
Sylva Sylvarum and John Woodward published work in
1699 and discussed the soil-less culture.
Singh et al. (2010) said that presently several soil-less tech-
niques had been practiced to grow the plant under controlled
conditions. Although, it primarily associated with the method
of Hydroponic and Aeroponics. According to Farran and
Mingo-Castel (2006) the term Hydroponic and Aeroponics
have been taken from the Greek and Latina terms Hydro
and Aero which means water and air whereas Ponic means
(labor) respectively. In both systems, the plant grows without
soil by providing the artificial supporting structure under
controlled conditions. The plant roots survive in the air
under mist environment or wholly immersed in nutrient
rich-water (Beibel 1960; Reyesa et al. 2012).
Overview and concept of the aeroponics system
Aeroponics is the science of plant cultivation without incor-
poration of the soil or a substrate culture. Where plant
grows in the air with the assistance of an artificial support
and no soil or substrate is required to support the plant as
shown in Figures 1 and 2(Osvald et al. 2001). Basically, it
is an air water culture cultivation system, the roots of plant
are hanged inside a sealed container under darkness and
openly exposed in the air to get water nutrient-rich spray
through atomizers. The upper portion of the plant leaves
and crown extend above the wet zone. The root and canopy
of the plant are separated by the artificially provided struc-
ture. The system uses the nutrient-enriched spray in the air
with the help of pressure nozzles or foggers to sustain
hyper growth under controlled conditions (Nir 1982; Engen-
hart 1984; Zsoldos et al. 1987; Barak et al. 1996; Mbiyu et al.
2012). US Patent Publication No 1999/5937575A defined that
Aeroponics system provides many advantages for agricultural
research and production as a modern research tool. However,
the concept and idea of plant cultivation in the air by provid-
ing artificial support and environment is not much old. The
researchers acquired the idea to see the most of the growing
plants on rocks near the waterfalls. They frequently observed
that plants were successfully grown on rocks near the water-
falls. Although, the roots of plants are openly hanged in the
air. It was the logical extension for plant growing in air
under spray mist condition. Rains (1941) reported that gen-
erally in nature these conditions happen on tropical islands
like Hawaii. Throughout the literature survey on the histori-
cal overview of the aeroponics system. It was found that in the
early 1921s, Barker (1922)first developed primitive air plant
growing system and used for laboratory work to investigate
the plant root structure. He reported that air plant growing
technique is the natural and simple practice to grow plants
without incorporation of soil. The absence of soil made
study much easier: roots of plant hanged in midair while
the stems detained in artificial place (Peterson and Krueger
1988). In 1940, the technique was frequently used by many
researchers as a modern research tool in plant root studies
(Christi and Nichols 2004; Chang et al. 2008). Carter
(1942) studied the air-growing culture and cultivated the
pineapple plant. He concluded that air-growing culture is
useful technique for plant roots studies. The air-growing cul-
ture reduced the mechanical injuries and interferences with
significant growth compared with soil, sand, or even aerated
water culture. The discrete nature of atomization spray inter-
val and duration allows for measuring the nutrient uptake
concentrations within the plant over time under different
conditions. The atomization spray provides the intermittent
mist of nutrients to plant root on different periodic intervals
for the specific duration rather than a permanent misting.
Klotz (1944) was the first researcher to discover vapor misted
citrus plants. He did the work to facilitate his research studies
of diseases of citrus and avocado roots. Vyvyan and Travell
(1953) successfully grew apple plants under mist environ-
ment. Went (1957) at the Earhart Laboratories in Pasadena,
California grew tomato and coffee plants in a water-tight con-
tainer with fine nutrient mist propelled by atomization injec-
tor with pressure. He named the method as ‘Aeroponics plant
growing system’(Stoner 1983). Peterson and Krueger (1988)
stated that in the present scenario, only aeroponics system is
the most efficient plant cultivation system to grow the plant
without the interference of soil comparing with other soil-
less techniques. The nutrient-mist system uses a minimum
amount of water and provides an excellent environment for
plant growth (Buer et al. 1996). Hessel et al. (1993) and Claw-
son et al. (2000) examined the usefulness of the aeroponics
system for spaceflight and revealed that system contributes
to advances in several areas of plant root studies. The studies
include root micro-organisms (Hung and Sylvia 1988; Sylvia
and Jarstfer 1992; Wagner and Wilkinson 1992) root
response to drought (Hubick et al. 1982)effects of oxygen
concentrations on root growth (Shtrausberg and Rakitina
1970;Soffer and Burger 1988); legume-rhizobia interaction
(Zobel et al. 1976); arbuscular mycorrhizal fungi production
(Sylvia and Hubbel 1986) and plant cultivar differences in
root growth. The system achieved the performance for saving
the water up to 99%, nutrients 50% and 45% less time than
soil-based cultivation (NASA 2006). The adaptability of the
system could help the researchers to make the application
to spaceflight plant growth systems appealing. Zobel (1989)
and Mirza et al. (1998) said in aeroponics system plant
roots quickly nourish the available nutrients and grows
under controlled conditions. The controlled conditions
include uniform nutrients concentration, EC and pH values,
JOURNAL OF PLANT INTERACTIONS 339

temperature, humidity, light intensity, atomization fre-
quency, atomization spray time, atomization interval time,
and oxygen availability. However, the plant grows speedily
in the system due to the sterile environment and abundant
oxygen availability in the growth chamber. However, several
research studies practiced the modern plant growing technol-
ogy for the cultivation of horticultural ornamental, the root of
herbs and root based medicinal plants production (Clayton
and Lamberton 1964; Cho et al. 1996; Park and Chiang
1997; Burgess et al. 1998; Garrido et al. 1998a; Garrido
et al. 1998b; Scoggins and Mills 1998; Molitor et al. 1999;
Kamies et al. 2010).
Stoner (1983) reported that before 1966s the aeroponics
system was performed only as a laboratory analysis to inves-
tigate the plant root structure. However, in 1966, the system
exits the laboratories to move into the field to cultivate the
plants for commercial purposes. In 1982 the system got pub-
lic attention when ‘The Land’pavilion at Disney’s Epcot Cen-
ter opened. Finally, in 1983 GTi successfully introduced the
first commercial aeroponics system known as Genesis Root-
ing system, commonly called the Genesis Machine. The
machine was operated and controlled with a different type
of microchips and simply joined within an electrical outlet
and a water atomizer nozzle. Whereas, NASA also took inter-
est to push aeroponics for further studies and applied the
technique to grow the plant in space. Several academics
research studies concluded and evidenced to support the sys-
tem and cultivated plant with significant yield. The research-
ers pointed out that the soil-less technique made the
aeroponics cultivated crops easier for harvest (Cho et al.
1996; Park and Chiang 1997; Biddinger et al. 1998; Scoggins
and Mills 1998). Hoffman and Kolb (1997) grew winter wheat
seedlings in Aeroponics system and determined the effect of
barley yellow dwarf on plant root and shoot. The results indi-
cated that barley yellow dwarf severely reduced root length,
distance from seminal root tip to the nearest lateral root,
and the root to shoot ratio. In some cultivars, the shoot per-
cent dry matter and number of adventitious roots were
increased by barley yellow dwarf. A study conducted in
1991–92 in Sardinia, Italy, by growing certain hybrid tomato
varieties. The result stated that certain hybrid tomato varieties
produced abundant and excellent commercial quality fruit in
a very short time (Leoni et al. 2008). In 2001, a University of
Arizona study aimed to determine the effect of aeroponics on
plants prized for the medicinal properties of their roots by
growing echinacea and burdock. The researchers got some
startling results. The echinacea plant suffered fungal and
insect outbreaks and the yield was still comparable to those
found in natural field condition. While the burdock greatly
outperformed its outdoor counterparts: the harvested roots
were highly yielded than those found in natural field con-
ditions. Chang et al. (2012) said aeroponics could be an
appropriate system for producing potato minitubers. He
found that an interruption of nutrient supply at stolon growth
stage significantly increases root activity, restricts stolon
growth, and finally induces tuber initiation. Therefore, non-
tuberizing conditions such as hot temperatures and late sea-
son cultivars favor the use of this nutrient interruption tech-
nique. He and Lee (1998) found that the growth of shoot, root
and photosynthetic responses of three cultivars of Lettuce
(Lactuca sativa L.) confined to different root zone tempera-
tures and growth irradiances under tropical aerial conditions
was better to the aeroponically grown crops compared to the
control. Luo et al. (2009) developed a very successful method
of producing hearted lettuce in the tropics using aeroponics
and root cooling. It was also found that the effects of elevated
root zone CO
2
and air temperature on photosynthetic gas
exchange, nitrate uptake and total reduced nitrogen content
in aeroponically grown lettuce plants (Johnstone et al.
2011). A comparison of the product yield, total phenolics,
total flavonoids, and antioxidant properties was done in
different leafy vegetables/herbs and fruit crops grown in aero-
ponics growing systems and in the field. The antioxidant
properties of those crops were evaluated using 2,2-diphe-
nyl-1-picrylhydrazyl (DDPH) and cellular antioxidant
(CAA) assays. In general, the study shows that the plants
grown in the aeroponics system had a higher yield and com-
parable phenolics, flavonoids, and antioxidant properties as
compared to those grown in the soil (Chandra et al. 2014).
The vitamin C content was highest in all herbs cultivated in
aeroponics whereas the essential content oil was highest in
Holy Basil and Perilla cultivated in the substrate (Bohme
Figure 1. Aeroponics plant growing system with computer controlled techniques.
340 I. A. LAKHIAR ET AL.

and Pinker 2014). In Aeroponics system, the seedlings of Aca-
cia mangium were cultivated and result showed it is the well-
known technique for furnishing a very rich air environment
around the plant root, enhance the performance and induce
rapid growth of seedlings under greenhouse conditions (Mar-
tin-Laurent et al. 2000). Thus, the aeroponics system has
potential to increase income and reduce the cost of pro-
duction of quality seed, thereby, making it more accessible
to growers in developing countries.
The present study aims to describe a novel approach of
plant cultivation under the soil-less system and provides the
brief literature survey about the aeroponics system. We
reviewed the literature survey and found that several research
studies have been concluded and recommend that in future
the world increasing food demands will highlight several
challenges not only for human as well as for animals.
Under such circumstances, the aeroponics system is con-
sidered as safe and environmentally friendly plant cultivation
method. The system can conserve water and energy. It uses
nutrient rich-water recirculation hence; system offers lower
water and energy inputs per unit growing area (Ritter et al.
2001; Farran and Mingo-Castel 2006; Lester 2014) and offer
a means of controlling the atmosphere in the root zone and
provide a method for measuring of cell division and physio-
logical responses of plant (Smucker and Erickson 1976). The
adoption of the system would be economical, beneficial and
valuable for local farmers to perform agricultural activities.
It would be the best solution to provide the clean, fresh veg-
etable and sustenance to rapidly booming population.
World interest towards aeroponics system
Humans necessitate needs the fresh and clean food for survi-
val. The fresh and clean food comes from many resources,
one of them is a plant. However, the world population is
rapidly growing and putting pressure on available natural
resources to produce food. The increasing pressure is causing
several serious problems such as global warming and
reduction in natural resources. Pinstrup-Andersen (2017)
reported that at the same time the water scarcity, drawdown
of groundwater level, water mismanagement, waterlogging
and salination of soils could effect on the food production
and cause a severe problem in many countries. Besides, the
researchers are conducting experimental studies to search
out the alternative and appropriate technologies in agricul-
ture. One aspect of their investigations is the plant, and
their ability to survive and flourish under distinct situations.
In fact, the soil, water, and surrounding environmental con-
ditions are three essential factors to grow a healthy plant.
NASA (2006) study reported that what happens if the soil
Figure 2. Aeroponics system. 1. Fog transmission pipeline; 2. Nutrient solution reflux line; 3. Tube-axial fan; 4. Atomizing chamber; 5. Ultrasonic atomizer; 6. Cultiva-
tion box; 7. Control line of Ultrasonic atomizer; 8. Batter board of ultrasonic atomizer.
Figure 3. Aeroponics and NASA (Hydroponics gardening 2014).
JOURNAL OF PLANT INTERACTIONS 341

is completely taken out of the equation still plant will grow.
Their study concluded that yes plant could grow under
such condition by providing nutrient spray mist in the air
as shown in Figure 3. The High-performance crop production
technology is rapidly gaining the intention of the people as a
modern-day agriculture activity. Nowadays, it has been sig-
nificantly used in agriculture around the world (Hoehn
1998). While Decade ago, the utilization of the system was
limited almost around the world aspect some countries like
China and Korea. They used an aeroponics system for com-
mercial application to produce good qualities of potato
seeds (Kim et al. 1999). The use of aeroponics for plant culti-
vation is very recent in Europe. Currently, it is practiced in
South America (Mateus-Rodríguez et al. 2013), and the
attempts have been made to present the system in African
countries (Otazú 2014). The system is highly accepted and
recommended for plant cultivation in following countries
include: Taiwan (Wu et al. 1997); Australia, (Mohammad
et al. 2000); Singapore (Wilson 2000); France (Souret and
Weathers 2000); Spain (Ritter et al. 2001); Thailand (Sriha-
jong et al. 2006); New Zealand (Johnstone et al. 2011);
Japan (Kitya et al. 2008); Russia (Akopyan et al. 2009); Malay-
sia (Tik et al. 2009); Germany (Germer et al. 2011); South
Korea (Chang et al. 2012); India (Buckseth et al. 2016); Indo-
nesia (Idris and Sani 2012); Bolivia, Colombia, Ecuador,
Ethiopia, Mongolia, Peru and Uzbekistan (CIP 2012); Sri
lanka (Weerahewa 2013); Italy (Nijs 2013); Kenya (Mbiyu
et al. 2012); Korea (Yeo et al. 2013); African counties like
Uganda, Tanzania, Mozambique, Malawi, Ghana, Ethiopia
(Niyi 2013; Kumari et al. 2016; Tessema et al. 2017; Oteng-
Darko et al. 2017); Slovakia (Pala et al. 2014); Vietnam
(ICAA 2014); Bhutan (Wangchuk 2014); Canada (Oraby
et al. 2015); Greece (Salachas et al. 2015); Nigeria (Ogbole
2016); Philippine (Philippines 2016) Iran (Shahbani et al.
2013); Poland (Rykaczewska 2016); Egypt (Singer et al.
2012) and Abu Dhabi (Alshrouf 2017).
Components and atomizers required for
aeroponics system
Spray misters and droplet size
Aeroponics is a modern-day plant growing technique in the
agriculture. It does not require any single soil particle to
provide essential nutrient for plant growth. However, in
aeroponics plant roots receive nutrient spray mist eject
from the atomization nozzles. The atomization is the
method of breaking up liquid molecules into fine droplets
(Avvaru et al. 2006). US Patent Publication No 2011/
0023359 A1 described that the most common aeroponics
system uses pressurized water that is sprayed onto the
roots using simple garden sprinkler-type fluid nozzles.
However, different atomizers are developed in several disci-
plines along with different spray patterns and orifices to
provide tiny liquid droplets down to 1 micron. The atomi-
zers are categorized as high, medium and low-frequency
atomization as shown in Figure 4 (Rajan and Pandit 2001;
Lu et al. 2009). The atomization nozzles with small orifices
could create the problems such as nozzle clogging and cause
to stop the water nutrient supply. In order, to avoid the
nozzle orifice clogging the mesh filters are used to prevent
the nozzle clogging. While the larger nozzles with bigger
orifice decrease the chances of nozzle blockage but require
high-pressure to operate. It is essential to select the suitable
atomization nozzles to produce required droplet size. The
droplet size varied from sub-microns to thousands of
microns and characterized in a different classification. For
high-pressure atomization nozzles, the droplet size is
classified fine atomization mist of 10 to 100 microns. The
jet spray nozzles with 0.000635 m (0.025-inch) and
0.0004064 m (0.016-inch) orifice under the operating
pressure pump at 551580.5832 and 689475.729 Pa (80 to
100 psi) deliver the drop size of 5–50 microns and 5–25
microns respectively. However, in the aeroponics system,
the ideal droplet size range for most of the plant species
is in-between 30 and 100 microns. Within this range the
smaller droplets saturate the air, maintaining humidity
levels within the growth chamber. The conventional wisdom
is that droplets below 30 microns tend to remain in the air
like a fog and fail to achieve continuous plant growth.
While droplets size more than 100 microns tend to fall
out of the air before containing on the plant root, and
too large droplet means less oxygen is present in the growth
chamber. Selection of atomization nozzles should be based
on the requirements of the growers. Ultrasonic foggers,
whose working frequencies are between 1 and 3 MHz, are
expensive and difficult to maintain. The foggers require
special electrical circuits to drive them, so their structures
are very complex. Also, they affect the chemical properties
of the nutrient solution. The foggers are suitable for indoor
gardening. The low-pressure nozzles are more convenient to
use than foggers. These nozzles are cheap and easy to main-
tain, but their atomization quantities are small. Low
pressure atomization nozzles are recommended for the
small-scale gardening. The high-pressure atomization noz-
zles are operated with air-compressor. The compacted air
delivers energy to break down liquid into very fine droplets.
These nozzles have large atomization quantity, but the
noises produced by them are very high. These type of ato-
mization nozzles are recommended for large scale
gardening.
Ultrasonic atomization fogger
The ultrasonic foggers are used to mimic ideal artificial
humidity in the air with a little mist found rainforests.
The atomization foggers are low-cost and resulted in high
yield for plant development. It is a relatively small metallic
covered device which comprises a plastic shell, built-in
AC/DC adapter, and a small piezoelectric ultrasonic trans-
ducer. The piezoelectric transducer is the core component
which generates the high-energy vibrations at frequencies
from 0.5 to 3.0 MHz. The atomization foggers are placed
in the center of the container under one to four inches of
liquid solution. The foggers produce a very light solution
spray that floats around the container in the air and sprays
as thick fog like clouds. The fogger generates a fine mist hav-
ing a particle diameter of only a few microns (http://www.
farnell.com/datasheets/81205.pdf). These fine particles are
cable of carrying nutrients from the standing water nutrient
from the reservoir towards the plant root. Benefits include
humidification and exponentially improved root exposure
to oxygen. These improve oxygen flow to the roots while
creating a suitable humid atmosphere for the plant to thrive
in and for successful growth (https://en.wikipedia,.org/wiki/
Ultrasonic)_hydroponic_fogger). The fog is cold and slightly
wet but possesses no threat to the user (http://www.tech-faq.
342 I. A. LAKHIAR ET AL.

com/ultrasonic-fogger.html). Although, the horticulturalists
might be purchase commercial ultrasonic foggers for plant
cultivation and to layout the indoor garden with a small
ultrasonic fogger as shown in Figure 5.
Pressure (Airless and Air spray) atomization nozzles
The report was published (www.pnramerica.com/pdfs/p49_
53.pdf) on Air-atomization nozzles express that several
industrial practices are implementing nozzles to atomize the
liquids into the fine particle. However, the standard atomiza-
tion nozzles are used to atomize the liquid from the small
orifice with low and high-pressure to get the fine particles.
Pressure atomization is a method to create a fine spray and
that uses air to atomize the liquid. In most instances when
small droplets are required, the air-atomization nozzles are
operated by providing the air with high-pressure through
the air-compressed container. The compacted air delivers
energy to break down the larger liquid particles into very
fine particles. The liquid is distributed through the orifice
into the atomization nozzle, where high-pressure air stream
produces the shearing force to break up the large liquid mol-
ecules. The air stream carrying the fine particle collides with
resonator located in front of the nozzle and generates a field
of high-frequency ultrasonic sound waves. Their exposure
causes the larger droplets to break up into a fine mist spray.
The droplet size dependent on the operating frequency. The
audible noise produced by high-pressure nozzles is extremely
high. The nozzle structure is designed as an internal mix or
external mix set-up. The pressure atomization is a method
of fine spray application that does not use compressed air
to atomize the liquid. The discharge of the nozzle depends
on the liquid flow, size of the spray head and pressure
pump. However, the atomization nozzles are placed in the
center of the growth chamber and connected with pressure
pump to provide water nutrient mist. While the roots are sus-
pended above or inside the chamber and directly precipitate
Figure 4. Atomization nozzles used in aeroponics system: (a) Ultrasonic atomization fogger; (b) High-pressure (Air) atomization nozzle; (c) Pressure (airless) atomiza-
tion nozzle.
Figure 5. Lettuce cultivated in aeroponics ultrasonic atomization fogger system: (a) Plant leaves; (b) Plant root.
JOURNAL OF PLANT INTERACTIONS 343

with uniform water nutrient mist. The aeroponics system
designed with pressure nozzle is more convenient, simple to
operate and maintain. The main advantage of the quick adop-
tion of this system is the low operational cost. Figure 6 shows
the lettuce plant cultivated in an aeroponics system with air
and airless atomization nozzle.
pH and EC (electrical conductivity) meter
The pH is defined as measure the degree of acidity or alka-
linity of a liquid solution. Although, the EC is a measure of
all of the salts dissolved in water, including those added to
the fertilizer. The unit of EC is ds m
−1
.Different methods
are used to determine the pH and EC value of the nutrient
solution. However, the most common, simple and easy way
to measure the value is pH and EC meter. In the aeroponics
system, where water and nutrient solution is recycled repeat-
edly. Therefore, it is important to regularly measure the pH
and EC value of the nutrient solution for successful plant
growth. If the readings are not at the proper level, the grower
needs to adjust them. The ideal pH and EC range for each
plant depend on available environmental conditions (Sonne-
veld and Voogt 2009). Although, the pH and EC values of the
prepared nutrient solution could not exceed than 7 and
2.5ds m
−1
. The optimum EC and pH values of nutrient sol-
ution in aeroponics system lie between 1.5 to 2.5 ds m
−1
and 5.5 to 6.5 and 5.0 as shown in Table 1 (Resh 2004; Cha-
dirin et al. 2007).
Light and temperature
Temperature is the primary environmental factor that influ-
ences the frequency of plant growth and development. It
influences not only on the initial growing stage but also on
harvesting period (Hatfield and Prueger 2015). In the aeropo-
nics system, both air and nutrient solution temperature
should be controlled for quick plant maturation. As tempera-
tures rise, the chemical processes proceed at faster rates and
deteriorate the enzyme activities. The optimum temperature
range for all plants is 15–25°C. However, the temperature
of growth chamber should be not higher than 30°C and less
than 4°C (Otazú 2014). Naoya et al. (2008) reported that in
controlled cultivation light energy is the significant ecological
factor required for the plant. The light intensity and quality,
not only delivers the energy as well provide numerous mor-
phogenesis and physiological responses for plant growth
(Rajapakse et al. 1992; Fukuda et al. 2008; Li and Kubota
2009). To adjust the various light environments, recently a
light emitting diode (LED) has spread as a new light source
in the aeroponics system. A study by Mori and co-workers
(2002) reported that high photosynthesis and growth rate
of the plant grown under LED were observed. The LED
could be considered as the best light producer for plant
growth in the aeroponics system. It provides multiple light
qualities and effects of light on the plant growth in a con-
trolled condition. Whereas, the light LED has smaller mass
and volume, good life, energy-saving, single wavelength and
narrow bandwidth (Bula et al. 1991; Brown et al. 1995).
Humidity and dissolved oxygen concentration
Aeroponics system is the application of plant growth with-
out soil by delivering water nutrient solution in the air.
The system is based on 100% available moisture in the
growth chamber. In addition, the humidity is the amount
of available water in the growth chamber as water vapor
content. In the aeroponics system, humidity is the main
component required for successful plant growth and devel-
opment. However, the plant growth is significantly affected
by increase and decrease in relative humidity (Ford and
Thorne 1974; Schussler 1992). It effects on plant physiologi-
cal functions and creates diseases problems (Gislerod and
Mortensen 1990). Therefore, it is important to regularly
maintain and control the required humidity concentration
of growth chamber based on plant need. The aeroponics sys-
tem provides best oxygenation environment for plant
growth. It allows plant roots to grow in the air with a plenti-
ful supply of oxygen. Hence, no any additional mechanism is
required.
Misting frequency and nutrient reservoir
In the aeroponics system, the atomization spray time and
interval time are the essential factors for successful plant cul-
tivation. As we discussed, the aeroponics system is per-
formed without soil. Therefore, it is essential for the
grower to fix the atomization spray time and interval time
based on the plant requirement. The wrong schedule could
create serious problems for plant growth because in the sys-
tem there is no any medium to support the plant. Many
research studies had been successfully cultivated plant
under different atomization spray time and interval time.
However, the potato had been cultivated under the atomiza-
tion spray time and interval time 20-sec on and 5-min off,3-
min on and 5-min off, 3-sec on and 10-min off, 30-sec on
and 5-min off, 10-sec on and 20-mint off, 15-min on and
15-min offduring day time and 15-min on and 1-h off
during night time, and 10-sec on and 3-min offduring day
time and 10-sec on 5-min offduring night time, respectively
(Farran and Mingo-Castal 2006; Ritter et al. 2001; Abdulla-
teef et al. 2012; Chang et al. 2012; Mateus-Rodríguez et al.
2012; Tsoka et al. 2012; Rykaczewska 2016). Biddinger
et al. (1998) and Osvald et al. (2001) successfully cultivated
the tomato under the atomization spray time and interval
time of 3-sec on and 10-min off, and 60-sec on and 5-min
offrespectively. Moreover, many studies cultivated several
plants under different atomization spray time and interval
time such as cucumber 7-sec on and 10-min off(Peterson
and Krueger (1988); lettuce1.5-min on and 5-min off(Lac-
tuca sativa L.) (Kacjan-Marsic and Osvald 2002); Saffron
(Crocus sativus L.) 1-min on and 1-min off(Souret and
Weathers 2000); 96 Echinacea (Echinacea purpurea) and
30 burdocks (Arctium lappa) 30-sec on and 60-sec off
(Pagliarulo and Hayden 2000); Anthurium andreanum 15-
sec on and 15 mints off(Fascella and Zizzo 2007); Acacia
Table 1. Represents the recommended pH and EC value for some plants (Kim
et al. 1999; Farran and Mingo-Castel 2006; Abdullateef et al. 2012; Irman and
Ikhsan 2012).
Plant pH EC
Onions 6.0–7.0 1.4–1.8
Cucumber 5.8–6.0 1.7–2.2
Carrots 5.8–6.4 1.6–2.0
Spanish 5.5–6.6 1.8–2.3
Lettuce 5.5–6.5 0.8–1.2
Tomato 5.5–6.5 2.0–5.0
Potato 5.0–6.0 2.0–2.5
344 I. A. LAKHIAR ET AL.

Mangium 40-sec on and 30-sec off(Weber et al. 2007);
Maize 1-sec on and 15-mints off(du Toit et al. 1997); Acacia
mangium 15-sec on and 1-min off(Martin-Laurent et al.
1997); Peas (Pisum sativum) 3-sec on and 10-mins off
(Rao et al. 1995) Onion 7-sec on and 90-sec off(Jarstfer
et al. 1998) and Whitei (Hook. F) Skeels, T. riparia (Hochst.)
Codd and C. speciosa (l.f) Hassk 2-sec on and 2-min off
(Kumari et al. 2016). In the aeroponics system, the nutrient
reservoir is designated as separate or outside and inside or
within the growth chamber. The purpose of the nutrient
reservoir is to store the solution. In the separate or outside
reservoir, the atomization nozzles are connected to the deliv-
ery line through pressure pump to supply the solution to the
growth chamber. The drain line is provided in the growth
chamber to recycle excess solution. Although, in inside
reservoir, the atomization nozzles directly get the nutrient
supply from the bottom of the growth chamber, where it
drips back down after misting on the root system.
Role of computer intelligent control techniques in
aeroponics system
At present time-controlled environment agriculture demands
extra efficiency for substantial returns. Humans also need
decision making and automatic sophisticated computer
monitoring and control technique to reduce intentions and
enhance the efficiency of the system (Tik et al. 2009). Akyildiz
et al. (2002) surveyed to adopt different computer intelligent
and advanced wireless network techniques in various fields.
He and coworkers reported that currently, agriculture is a
potential field of deployment with computer networking
techniques. The efficiency of the agriculture activities in a
controlled environment could be enhanced by adopting the
computer intelligent techniques. Tik et al. (2009) reported
that WSN in agriculture is mainly focused on two major
areas: (i) experimental or simulation work on various routing
protocols and network topologies to increase data transfer
rates whilst maintaining or reducing power consumption
(Galmes 2006; Camilli and Cugnasca 2007; Konstantinos
et al. 2007) and (ii) proof-of-concept applications to demon-
strate the efficiency and efficacy of using sensor (Figure 7)
networks to monitor and control agriculture management
strategies (Dinh et al. 2007; Narasimhan et al. 2007; Pierce
and Elliott 2008). However, the aeroponics system is the
science of plant cultivation by controlling the surrounding
environmental conditions without soil. Therefore, there is a
Figure 6. Lettuce cultivated: (a, b) High-pressure (Air spray) aeroponics atomization system; (c, d) Pressure (Airless spray) aeroponics atomization system.
Figure 7. pH and EC probe, Temperature and light sensor (Tik et al. 2009).
JOURNAL OF PLANT INTERACTIONS 345

maximum possibility to face many problems and challenges
associated with plant growth. The problems and challenges
include a water nutrient buffer solution, failure of the delivery
pump, spray time, time interval, and atomizer frequency.
Moreover, the environmental problems include root temp-
erature, humidity percentage, and light intensity. These pro-
blems require special care to escape damage or speedy death
of plants during cultivation period. Hence, it is important for
growers to control and maintain them on proper time as
exposed. There are different ways to deal with these chal-
lenges by accepting the modern techniques. Different
research studies recommended that these problems could
be monitored by adopting the automatic artificial intelligence
techniques in the aeroponics system.
Pala et al. (2014) worked on fault detecting by applying
computer programing methods (Figure 8) in the aeroponics
system. He and team concluded that the Neural-network sys-
tem is the efficient technique of detecting mechanical and bio-
logical faults in the deep trough from the system. The
detected defects are based on two separate fault conditions.
The first condition represents the fault detection information
in the system on behalf of working sensors: (water nutrient
solution; EC and pH value, humidity, air temperature and
light intensity). The second fault detection is based on the
biological faults occurring in the system (transpiration rate)
(Ferentinos et al. 2003a; Ferentinos and Albright 2003b).
The obtained real-time data from the system could apply to
maintain the growth chamber temperature, ventilation from
growth chamber, light intensity, and available water nutrients
solutions properties (Tik et al. 2009; Sahu and Mazumdar
2012; Song et al. 2012).
Mechanization and optimizing of root
environment in aeroponics cultivation system
Throughout history and literature review, man has endea-
vored to understand and manipulate his surrounding
environment. One aspect of his investigation was the plant,
and their ability to survive and flourish under distinct situ-
ations. The research results indicated that direct and adequate
supply of mineral nutrients is a significant factor in the cre-
ation of the root domain environment. Hayden (2006)
reported that plant root development and growth laid on sev-
eral factors include the initiation, elongation, and spreading
out of fresh root axes. Plant root structure responds to the
root zone environment by substituting in growth and branch-
ing systems as well the variations in plant hormone synthesis
and response probably mediate these plastic responses to the
root zone environment as well as contribute to genetic var-
iances in root architecture and plasticity. Jonathan et al.
(2012) revealed that plant root growth and development
depends on an adequate quantity of carbohydrates supply
from surrounding photosynthesis concentration and avail-
able environmental conditions. Therefore, many roots sur-
rounding environmental conditions that influence
photosynthesis, including water availability, light intensity,
temperature, and nutrient availability may impact on root
growth by affecting carbohydrate supply to the plant roots
(Kawasaki et al. 2014). Compared with the other traditional
cultivation methods, aerosol culture becomes the advantages
of free extension of the root system, (Vincenzoni 1977;
Sumarni et al. 2013) direct and sufficient oxygen uptake,
rapid and convenient water absorption, and full
mineralization with mist supply, creating the best root
environment for plant growth.
Aeroponics is defined as a system of soil-less culture where
roots are continuously or discontinuously bathed with fine
drops of nutrient-rich water. The method requires no any
substrate like rock wool, dirt, coir, vermiculite or perlite
and entails growing plants with their roots periodically wetted
with a fine mist of atomized nutrients. Excellent aeration is
the main advantage of aeroponics (Carter 1942). Plant root
structure response to the root zone environment via substi-
tuting in growth and branching systems. Thus, only aeropo-
nics provides numerous advantages include a free extension
of the root system, direct and sufficient oxygen uptake,
rapid and provision of uniform nutrient spray mist with
best root growth environment (Vincenzoni 1977; Sumarni
et al. 2013).
Plant growing system
As discussed above, the aeroponics system differs from both
Hydroponic and in vitro plant growing techniques. Unlike
hydroponic system, which uses water nutrient-rich solution
as a growing medium and provides essential nutrient for sus-
tain plant growth. However, it is conducted without any grow-
ing medium (Lakkireddy et al. 2012). In the system, the nursery
plants might be either raised as seedlings using specially
designed lattice pots or cuttings could be placed directly into
the system for rapid root formation. Lattice pots allow the
root system to develop down into the growth chamber where
it is regularly misted with nutrient under controlled conditions.
Zobel et al. (1976) reported that root zone environmental con-
ditions play a significant role in healthy plant growth and
attaining the excellent quality of seed production. Siddique
et al. (2015) revealed that only efficient root system provides
unobstructed growth space for the plant under atomization
conditions. Soffer and Burger (1988) reported that once plant
located in the atomization systemroots start to get most favor-
able root aeration system. The lower portion of the plant
entirely suspends in the mist air environment and provide
root organism to gain the required factors. The base of the cut-
ting is supplied with high levels of oxygen and moisture in a
humid environment and helps the plant to get 100% of the
fresh oxygen from the air to promote and support root metab-
olism and accelerates formation. The increasing metabolism
rate of plant growth was reported up to 10 times greater than
soil system (Stoner 1983; Chiipanthenga et al. 2012).
Nutrient solution management in aeroponics system
Aeroponics uses less water and nutrients because the plant
roots are sprayed at intervals using a precise droplet size
that could utilize most efficiently by osmosis to nourish the
plant. The nutrient solution is an aqueous solution mainly
containing soluble salts of necessary components for higher
plant yield (Steiner 1968). The essential inorganics ions
have important and clear physiological role and their absence
prevents the plant life growth stage (Taiz and Zeiger 1998).
However, the nutrient composition depends on the plant cul-
tivation method, the kind of medium, growth stage, weather,
method of applying nutrient solution, etc (Guang-jae et al.
2007). The plants need 17 essential inorganics nutrient for
maintaining optimum health and significant yield (Kochian
2000; Bailey and Nelson 2012). Rolot et al. (2002) reported
346 I. A. LAKHIAR ET AL.

the main necessary nutrient components required for vigor-
ous plant growth included phosphorus (P), sulfur (S), potass-
ium (K), nitrogen (N), and zinc (Zn). The carbon (C) and
oxygen (O) is directly supplied from the atmosphere. The
plant cannot exist without the deficiency of these elements,
and these elements cannot be exchanged with any other
nutrients. Therefore, in the aeroponics system, the plant is
grown without soil by providing nutrient mist through atomi-
zation nozzles. It is important to supply accurate essential
nutrient on proper time with required concentration. Up to
now, different researchers used different nutrients concen-
tration for preparing the nutrient-rich water. Dennis, Hoag-
land, and Daniel recognized several recipes for preparing
the mineral nutrient solution for water culture. Knop and
other plant physiologists revealed that K, Mg, Ca, Fe, and P
along with S, C, N, H, and O are all essential nutrient
elements for plant life (Lakkireddy et al. 2012). There are sev-
eral nutrient solutions recipes and Table 2 shows some of
them. These recipes are mostly used in the aeroponics system
until now. The success or failure of the system primarily
depends on the strict nutrient management. Therefore, it is
important to manipulate and adjust the EC and pH level of
water nutrient solution. Moreover, replace the nutrient sol-
ution after every 2 to 3 days or whenever necessary.
Aeroponics engineering and potential challenges
The aeroponics system is the modern technique of the plant
cultivation. Until now, it is not entirely implemented around
the world. The system has many potential challenges which
could be answered through research studies. Bucksetha
et al. (2016) and Stoner and Clawson (1998) revealed that
in the aeroponics system the main problem is related to
water nutrient droplet size. The larger droplets permit the
less supply of the oxygen availability in the root zone. The
smaller droplets produce too much root hair without devel-
oping a lateral root system for sustainable growth. Currently,
most of the studies on the aeroponics system are based on the
growth, yield, and quality of the plant. However, only limited
studies had been carried out to determine the influence of
various droplet sizes on plant yield and nutrient physio-
chemical properties of the nutrient solution. The main poten-
tial challenge and drawback of the system is constant power
supply throughout the plant growth. Any prolonged rupture
of power energy shut down the nutrient supply and contrib-
utes to permanent plant damage. The mineralization of the
ultrasonic transducers requires attention and may be prone
to potential components failure.
Figure 8. The proposed architecture of control and communication mechanism between human operator and Aeroponics system (Pala et al. 2014).
Table 2. Some essential mineral elements concentration by different authors.
Nutrient
Steiner
(1984)
Cooper
(1988)
Hewitt
(1966)
Hoagland and
Arnon (1938)
mg L
−1
N 168 200–236 168 210
P31604131
K 273 300 156 234
Ca 180 170–185 160 160
Mg 48 50 36 34
S 336 68 48 64
Fe 2–4 12 2.8 2.5
Cu 0.02 0.1 0.064 0.02
Zn 0.11 0.1 0.065 0.05
Mn 0.62 2.0 0.54 0.5
B 0.44 0.3 0.54 0.5
Mo Not
considered
0.2 0.04 0.01
JOURNAL OF PLANT INTERACTIONS 347

Advantages of aeroponics system
Martin-Laurent et al. (1999) suggested that Aeroponics tech-
nique is a current innovative and appropriate technology. It
has the potential to cultivate plants in large quantities, tree
saplings associated with soil micro-organisms, such as AM
fungi, for reforestation of degraded land in the humid regions.
Aeroponics is an indoor horticulture practice. It is the best to
adopt aeroponics in areas where the soil is not suitable for
plant growth. Aeroponics is an incredible amount of water
as little as compared with other plant growing systems. The
system reduces the labor cost, consumes less water usage by
98%, fertilizer usage by 60%, pesticide and herbicides usage
by 100% and maximize plant yield by 45% to 75% than either
hydroponics or geoponics system (Stoner 1983; NASA 2006).
The nutrient solution could be recycled easily for reuse. The
system allows for vertical farming, thus increasing the yield by
more space for the plant. The possibilities of multiple harvests
of a single perennial crop and accelerated cultivation cycle
due to the increased rate of growth and maturation. The
mature plant could be removed easily at any time without dis-
turbance another plant. The diseases could not expand
quickly because of clean root material free from soil, soil-
borne organisms and adulteration from foreign plant species
contaminants. While in other soil-less system plant diseases
could spread through nutrient distribution in growth
chamber from plant to plant. The plant receives 100% of
the available carbon dioxide and oxygen to the leaves,
stems, roots, and accelerating growth with reducing rooting
time. The system is not subjected to weather conditions.
The plants could be grown and harvest throughout the year
without any interference of soil, pesticides, and residue. It is
environmentally friendly and economically efficient plant
growing system.
Disadvantages of aeroponics system
.Expensive for long scale production
.The plant grower must need a specific level of proficiency
to operate the system.
.The grower must have the information about the appro-
priate quantity of required nutrient for plant growth in
the system.
.It is important to supply the required concentration of the
nutrients. There is no any solid culture to absorb the excess
nutrients if supply excess plant will die.
.The system design material is little expensive. As the well-
designed system requires advanced equipment. It mainly
constant high-pressure pumps, atomization nozzles, EC,
and pH measuring devices, temperature, light intensity
and humidity sensors and timer to control the system.
What we know and what remains to be known in
aeroponics system
Presently, aeroponics cultivation is implicated for growing
vegetable crops. It is a relatively new technology. Therefore,
enough material about the system is yet distant. Otazú
(2014) declared that in aeroponics system production is
essentially sensitive to climate and the vegetative period
increase by 1–2 months. It can substantially increase income
and decrease the seed production cost which represents the
system as more convenient to growers. The sequential plant
harvesting is required, and the initial cost could be obtained
quickly. The system must analyze with new cultivars and
the artificial conditions including extra lights, temperature
and humidity control system. The artificial conditions should
be equipped in the greenhouse for the plant cultivation at
different latitudes. The study of root temperature is not as
well documented. The interrelationships among root and
shoot temperature influence on growth are still unidentified.
The evaluation, assessment, and utilization of aeroponics sys-
tem for commercial plant developing purpose are not studied.
However, most of the studies are focused on root research, as
root microorganisms and root responses to drought. Each
plant needs a specific optimum supply of the nutrient sol-
ution at specific growth point. So, the optimum concentration
of nutrient solution for each plant should be investigated and
distinguished. The plant spacing should be figured out for
each cultivar. Different plant materials such as cuttings and
tuber sprouts should be compared in the system. In the aero-
ponics system, the traditional methods of pest/disease control
are not applicable thus the modern diseases control should be
developed. Based on weather and field production conditions,
the best plant production period should be determined for
each plant. The artificial lighting could be used to grow the
plant.
Future application prospects
Previous research works done on the aeroponics system are in
favor to adopt this cultivation system. In a relatively short
period, the aeroponics system has adopted in many situations
from outdoor field culture to indoor greenhouse culture. It is
also recommended as a highly specialized culture in the space
application and space-age technique. At the same time, the
system could be used in developing countries of the Third
World to accommodate intensive food production in the lim-
ited area. In future, aeroponics would effectively use in those
regions where fresh water and fertile soils will not be accessi-
ble. Thus, it could be the potential application for food pro-
duction in those regions having vast parts of the nonarable
land, small area and big population, and as well in desert
regions. The system could be highly practiced to grow veg-
etables in small countries whose chief industry is tourism.
The tourist facilities restaurants and hotels might grow
their own fresh vegetables and provide fresh food to the
tourist.
Conclusion
This review paper of the existing literature revealed as the
population increases. The demands for clean and fresh food
increases alarmingly with the population. People will turn
to new plant growing technologies to fill up increasing food
demands. Moreover, this review article concluded that aero-
ponics is the modern, innovative and informative technology
for plant cultivation without corporation of the soil. The sys-
tem is the best plant growing technology in many aspects
comparing with different cultivation system. The system is
quickly increasing momentum, popularity and fastest grow-
ing sector of modern agriculture. It would be effectively
employed in various countries for vegetable production
where natural resources are insufficient.
348 I. A. LAKHIAR ET AL.

Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
We acknowledged that this work was financially supported by grant
from the National Natural Science Foundation of China [grant number
51275214], Major projects of Natural Science Foundation of universities
in Jiangsu [grant number 17KJA416001] and the ‘Project Funded by the
Priority Academic Program Development of Jiangsu Higher Education
Institutions [grant number 37(2014)].’
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