Getting to the roots of aeroponic indoor farming

Abstract

Vertical farming is a type of indoor agriculture where plants are cultivated in stacked systems. It forms a rapidly growing sector with new emerging technologies. Indoor farms often use soil‐free techniques such as hydroponics and aeroponics. Aeroponics involves the application to roots of a nutrient aerosol, which can lead to greater plant productivity than hydroponic cultivation. Aeroponics is thought to resolve a variety of plant physiological constraints that occur within hydroponic systems. We synthesize existing studies of the physiology and development of crops cultivated under aeroponic conditions and identify key knowledge gaps. We identify future research areas to accelerate the sustainable intensification of vertical farming using aeroponic systems.

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Getting to the roots of aeroponic
indoor farming
Summary
Vertical farming is a type of indoor agriculture where plants are
cultivated in stacked systems. It forms a rapidly growing sector with
new emerging technologies. Indoor farms often use soil-free
techniques such as hydroponics and aeroponics. Aeroponics
involves the application to roots of a nutrient aerosol, which can
lead to greater plant productivity than hydroponic cultivation.
Aeroponics is thought to resolve a variety of plant physiological
constraints that occur within hydroponic systems. We synthesize
existing studies of the physiology and development of crops
cultivated under aeroponic conditions and identify key knowledge
gaps. We identify future research areas to accelerate the sustainable
intensification of vertical farming using aeroponic systems.
Introduction
A period of rapid development in agricultural technology is
underway, with precision dosing, machine learning, process
automation, robotics, gene editing, and indoor farming paving
the way for a revolution in agricultural productivity (Rose &
Chilvers, 2018; Klerkx & Rose, 2020). Indoor farming has
expanded quickly within the horticultural sector due to yield
consistency and environmental control capabilities (Benke &
Tomkins, 2017). Indoor farming divides into two broad sectors:
greenhouse and vertical farming. Vertical farming has emerged as
an increasingly economic strategy within horticulture, enabling
improvements in resource- and land-use efficiency.
Vertical farming involves plant cultivation in vertically stacked
irrigation systems, using artificial or natural light (Fig. 1). This
commonly uses soil-free growing environments and hydroponic or
aeroponic irrigation technology (Benke & Tomkins, 2017).
Benefits include urban food production, fewer food miles, seasonal
independence of crop production, price stabilization, product
consistency, isolation from pathogen pressures, cultivation at
latitudes incompatible with certain crops (e.g. desert and arctic
areas), and utilization of space including disused buildings or
tunnels (Despommier, 2011; Specht et al., 2014; Benke &
Tomkins, 2017). Further benefits include crop production without
impacting soil health, and nutrient recapture and recycling (Benke
& Tomkins, 2017). This makes vertical farming land- and water-
use efficient (Despommier, 2011). One commercial forecast
suggests that the vertical farming industry will have annual
compound growth of 21.3% to reach an estimated value of $9.96
billion by 2025 (Grand View Research, 2019). The potential
benefits and value of indoor vertical farming has caused the
proliferation of cultivation technologies (Benke & Tomkins, 2017;
Shamshiri et al., 2018).
A driver of technological innovation for vertical farms is
minimizing operational costs whilst maximising productivity.
One such expanding technology is aeroponics (Fig. 1). For
example, the number of ‘aeroponic’ patents filed increased
from 320 between 1975 and 2010 to over 1000 in the last
decade (Google Patents, 2020). Aeroponics is thought to
resolve several plant physiological constraints occurring during
hydroponic cultivation. This can include greater oxygen
availability within the root bed and enhanced water use
efficiency (Jackson, 1985; Mobini et al., 2015). However, the
variety of aeroponic technologies, species cultivated, and
growth conditions makes systematic comparisons of technolo-
gies and growth conditions challenging. Whilst aeroponics can
provide advantages for plant performance, it also requires more
extensive farm infrastructure and control technology compared
with the more mature technologies of hydroponics. Therefore,
aeroponics might be less compatible with certain economics,
crops, or locations with intermittent electricity supply. To
refine the commercial implementation of aeroponic horticul-
ture, we examine the effects of aeroponic cultivation upon
several aspects of plant physiology, development and produc-
tivity. We identify knowledge gaps and areas for future plant
sciences research to advance this field.
What is aeroponic cultivation?
Aeroponics exposes plant roots to nutrient-containing aerosol
droplets (Fig. 1). This is in contrast to hydroponics, which includes
partial or complete root immersion in a nutrient solution, and drip
irrigation involving application of nutrient solution to the
rhizosphere (Fig. 1) (Keeratiurai, 2013; Benke & Tomkins, 2017;
Lakhiar et al., 2018). Within the context of aeroponics, an aerosol is
an ensemble of solid particles or liquid droplets suspended in a gas
phase (Hinds, 1999). In nature, plants including epiphytic orchids
and bromeliads absorb naturally occurring aerosols such as mist
through leaves and aerial roots (Zotz & Winkler, 2013). In
horticulture, the most commonly used aerosol generation technol-
ogy is high pressure atomization, where high pressure liquids are
forced through a small orifice, breaking the liquid stream into
droplets. This typically generates aerosol droplets of 10–100 µm
(Lakhiar et al., 2018). Other atomization methods include inkjet
printer droplet on-demand generators, low pressure atomization,
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Drip irrigation
Deep water culture
Nutrient film technique
Flood and drain (ebb and flow)
High pressure atomization
Aero-hydro system
Hydroponics
Aeroponics
Solution
pump
Air pump
Solution
outlet Overflow
drain
Aerosol-generating
nozzles
Solution pump
Overflow
drain
Aerosol-generating
nozzles with pump
Solution outlet
Solution pump
Solution
pump
Solution pump
Fig. 1 Hydroponic irrigation methods include drip irrigation, deep water culture, nutrient film technique and flood and drain. In drip irrigation systems,
a nutrient solution is fed into a variable growing medium that supports the root system. Deep water culture submerges roots in nutrient solution, with
plants supported by a membrane preventing aerial tissue immersion. Nutrient film method exposes the bottom of the root bed to a flowing nutrient
solution whilst the top of the root bed remains exposed to air. Flood and drain systems immerse the root system with a nutrient solution for a period
of time. Subsequently, this is drained and collected into a reservoir to aerate the root bed. Aeroponics atomizes the nutrient solution, which deposit onto
the root surface. Aero-hydro systems atomize nutrient solution whilst exposing the lower root bed to recirculated nutrient solution. Air pumps are
common during deep water culture and can be added to other systems to increase root zone oxygen.
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and ultrasonic atomization, which generate varied droplet size
distributions (Reis et al., 2005; Lakhiar et al., 2018).
Aerosol deposition, capture and nutrient uptake on the
root surface
We propose that aeroponic cultivation involves a cycle of aerosol
deposition and capture (Fig. 2a,b). We reason that aerosol droplets
become deposited on the root surface, and coalesce to form a thin,
nutrient-dense aqueous film (Fig. 2a). The mechanisms of nutrient
and water uptake during hydroponic and aeroponic cultivation
might be similar, because both involve interaction between an
aqueous nutrient solution and plant root. We predict that root
surface thin-film formation is likely governed by aerosol
composition, plant root architecture and environmental properties
(Table 1).
The thickness of known biological thin-films, such as bacterial
biofilms and alveolar surfactants, range from micrometres to
millimetres (Murga et al., 1995; Adams & McLean, 1999; Siebert
& Rugonyi, 2008). Therefore, we reason that root surface thin-
films might occupy this range. However, root surface aerosol
droplet capture and thin-film formation is likely to be dynamic and
to exhibit spatiotemporal heterogeneity (Fig. 2a,b). Mathematical
modelling and experimentation with Artemisia annua hairy root
cultures predicts that aerosol droplet size, root architectural
properties and root hairs influence droplet deposition and aerosol
capture efficiency (Wyslouzil et al., 1997). Aerosol droplets <2lm
are thought unlikely to deposit on the root surface, whilst the
(1) Deposition
(3) Decay (2) Retention
Root hairs
(a)
(b)
CO
2
O
2
O
2
O
2
CO
2
CO
2
O
2
CO
2
O
2
O2
O2CO2
Nutrients
Nutrients
Nutrients
O2
CO2
O2CO2
Nutrients
O2
CO2
O2
CO2O2
O2
O
2
ts
(1) Deposition (2) Retention (3) Decay
Time
Fig. 2 Models for irrigation cycle and nutrient exchange during aeroponic horticulture. (a) Proposed aeroponic thin-film replenishment cycle. During the
deposition phase, aerosol droplets deposit onto the root surface. Smaller aerosol droplets might access spaces between root hairs. Droplets might also collide,
gain volume and exit the aerosol, landing on roots or collecting into the nutrient solution at the bottom of the bed. Retention refers to the accumulation of thin-
films over areas of the root surface that persist for a period of time. These are likely to be heterogeneous, leading to heterogeneous gas exchange and nutrient
uptake. During the decay phase, thin-films will be removed by evaporation and gravity in a manner dependent upon root architecture, surface tension and
relative humidity. Thin-films are replenished by generation of further aerosol. (b) Model for nutrient uptake and gas exchange within an aeroponic system. As
aerosol droplets become deposited, the quantity of gas exchange between the root and the environment will decrease and nutrient availability will increase.
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deposition efficiency of droplets >2lm increases with greater
droplet size (Wyslouzil et al., 1997). Root hairs increase droplet
capture efficiency compared with hairless roots (Wyslouzil et al.,
1997).
Investigation of the formation, thickness, composition and
residency times of aeroponically-produced root surface thin-films
could allow aeroponic cultivation systems to be tuned for the
optimal performance of specific crops (Table 1). It would be
informative to assess the interplay between these parameters during
root surface thin-film formation and retention for different crops.
This might inform aerosol delivery regimes and characteristics for
specific crops at defined developmental stages to ensure water,
nutrient and oxygen uptake supports optimal plant performance.
Productivity within aeroponic cultivation
Yields from aeroponic cultivation can exceed compost or hydro-
ponic cultivation for certain crops (Wyslouzil et al., 1997; Souret &
Weathers, 2000; Ritter et al., 2001; Hayden et al., 2004; Kratsch
et al., 2006; Chandra et al., 2014). One study reported that yield s of
aeroponically cultivated basil, parsley, cherry tomato, squash, bell
pepper and red kale increased by 19%, 21%, 35%, 50%, 53% and
65% compared to soil culture, respectively (Chandra et al., 2014).
Greater saffron bulb growth and unaltered saffron yield has also
been reported under aeroponic horticulture (Souret & Weathers,
2000). Aeroponic cultivation was also reported to achieve greater
tomato fruit mass when aeroponic and hydroponic cultivation were
compared directly (1.95 g per fruit from aeroponics; 1.56 g per
fruit from hydroponics) (Wang et al., 2019).
The effectiveness of root crop cultivation by aeroponics depends
upon crop variety and method of cultivation. One study reported a
mean root storage increase of more than 20 g dry weight for cassava
cultivated aeroponically compared with drip hydroponic cultiva-
tion (Selvaraj et al., 2019). Another investigation reported that
potato tuberization occurred 6–8 d earlier in aeroponic cultivation
than in hydroponic cultivation (Chang et al., 2012). However, a
Table 1 A variety of factors will influence root thin-film thickness and retention during aeroponic cultivation.
Property Characteristics Outcomes References
Aerosol
phase
Aerosol
particle size
distribution
Most atomization techniques will not generate a
monodisperse ensemble of aerosol. Aerosol droplet
size may change after generation
Increases in size distribution introduce variation in
deposition efficiency across the root system. Larger
droplets are more likely to deposit on roots close to
the point of aerosol generation
Shum et al. (1993);
Nuyttens et al.
(2007)
Aerosol
particle
velocity
After generation, aerosol droplet velocity is
generally likely to decrease
Aerosol particle velocity will impact the aerosol
distribution throughout the root system, impacting
uniformity of aerosol capture efficiency
Shum et al. (1993)
Hygroscopicity The chemical composition of an aerosol will
determine its reaction to changes in the relative
humidity of the surrounding gas phase. Water will
evaporate out of, or condense into, the droplet in
response to imbalances between the water activity
of the droplet and root chamber environmental
conditions
Changes in droplet size distribution. Mitchem et al.
(2006)
Changes to nutrient solution electrical conductivity
and pH
Odum et al. (1996);
Topping et al.
(2005)
Electrostatic
effects
Some atomization processes can induce electrostatic
charges in aerosol
Given that both the root and aerosol phase can have
charge effects, aerosol droplets might be repelled
or attracted to the root system
Xi et al. (2014)
Thin-
film
phase
Evaporation
rate
Rate of water evaporation from the thin-film to the
gas phase
We predict that evaporation of water from the thin-
film will alter pH and electrical conductivity of thin-
film nutrient solution
Sultan et al. (2005)
Gravity We speculate that at a certain volume, the thin-film
will accumulate sufficient mass that gravity will
cause it to drip from the root
We speculate that gravity effects will produce crop-
specific and developmental stage variation in the
refresh rate of the nutrient solution on the plant
root
This is a testable
hypothesis
Root system
architecture
Spatial configuration of all roots (primary, lateral,
accessory roots) in three dimensions, which
changes during plant development
Root system density and configuration is predicted
to affect aerosol droplet capture efficiency, thin-
film thickness, and thin-film residency- and
replenishment rate
Wyslouzil et al.
(1997); Osmont
et al. (2007)
Root hair
density and
length
Root hairs are tubular epidermal protrusions from
the root surface. Root hair properties such as
density and length affect the root surface area
available for absorption of water and nutrients
Increased root hair density and length is predicted to
capture droplets more effectively than glabrous
roots or roots with shorter/fewer hairs, which will
affect thin-film formation and residence time
Wyslouzil et al.
(1997); Grierson
et al. (2014)
Root surface
properties
and root
exudation
Topological features of root surface, and variety of
compounds that roots exude by passive and active
processes
We predict that root surface characteristics and the
root exudate mixture will affect the formation and
residency of thin-films by altering adherence/
coherence of aqueous droplets on the root surface
Badri & Vivanco
(2009); Galloway
et al. (2018)
The aerosol phase describes factors that influence airborne aerosol properties, and the thin-film phase refers to factors that influence the deposition, retention
and decay of root-surface aqueous films. In addition to aerosol physics and chemistry, thin-film thickness and retention will depend upon crop type.
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separate study identified that relative to hydroponics, aeroponic
cultivation increased potato minituber yield by 70% but the mean
tuber weight was 33% lower (Ritter et al., 2001). In that study, delayed
tuberization only allowed one productive cycle over a year, compared
with two productive cycles for hydroponically grown potatoes (Ritter
et al., 2001). Furthermore, whilst aeroponic-cultivated burdock was
reported to accumulate 49% more aerial biomass compared with soil
cultivation, the harvestable root biomass was unaltered (Hayden et al.,
2004). We speculate that differences between these studies might arise
from differing cultivation platforms, and environmental and geno-
typic variability. For example, Ritter et al. (2001) attributed delayed
tuber formation to enhanced vegetative growth caused by an unlimited
nitrogen supply, whilst studies by Chang et al. (2012) and Tokunaga
et al. (2020) identified variation between tuber yield of distinct potato
and cassava cultivars during aeroponic cultivation. Therefore, it would
be informative in future to compare and understand the performance
of different varieties of specific crops cultivated aeroponically, under
various environmental conditions, to identify traits compatible with
aeroponic cultivation in particular climates.
Root zone oxygen, plant productivity and aeroponic
cultivation
Root zone aeration supports plant productivity by allowing root
respiration (Fig. 3a) (Armstrong, 1980; Soffer et al., 1991).
Reduced root zone oxygen decreases yield, growth rates, and
mineral and water uptake (Rosen & Carlson, 1984; Tachibana,
1988; Soffer et al., 1991). In closed growing systems, aeration also
prevents the release of gaseous hormones such as ethylene that can
inhibit growth (Weathers & Zobel, 1992; Raviv et al., 2008).
Aeroponic systems provide the advantage that roots can, theoret-
ically, access all available root zone oxygen, whereas in hydroponic
culture, the low water solubility of oxygen means that dissolved
oxygen concentrations may need to be closely monitored when
cultivating certain plant species, to ensure that dissolved oxygen
concentrations do not become limiting for plant growth (Jackson,
1985; Goto et al., 1996; Ritter et al., 2001; Wang & Qi, 2010;
Mobini et al., 2015; Gopinath et al., 2017). This can be optimized
during hydroponics through regular nutrient solution cycling, or
by bubbling oxygen into the nutrient solution (Fig. 1).
Aeroponics allows artificial elevation of root zone O
2
to enhance
yield. One study identified in tomato and cucumber a positive linear
relationship between root zone O
2
concentration and growth rates,
when root zone gaseous O
2
increased between 5% (v/v) and 30% (v/
v), plateauing ab ove c.35%O
2
(v/v) (Nichols et al., 2002).However,
to evaluate the viability of this strategy, it would be helpful to gain a
better understanding of the relationship between O
2
concentration
and growth rate for other aeroponic-cultivated species.
Relationship between root zone temperature and CO
2
within aeroponic cultivation
In vertical farms, high root zone temperatures inhibit root growth
and cause nutrient deficiency, reducing photosynthetic efficiency
(Tan et al., 2002; He et al., 2007, 2010, 2013; Choong et al., 2016).
This inhibition can be reversed in aeroponic horticulture by root
zone cooling or CO
2
supplementation (Tan et al., 2002; He et al.,
2010, 2013). For example, cooling the root zone of aeroponic-
cultivated lettuce to 20°C increased root surface area and root/
shoot mineral content compared with plants grown at tropical
temperatures (23–38°C) (Tan et al., 2002), and similar root-zone
cooling in tropical greenhouses increased lettuce shoot yields
(Choong et al., 2016). Furthermore, root zone CO
2
supplemen-
tation of aeroponically grown lettuce, with root zone temperatures
of 20–38°C, increased the Rubisco concentration and protected
plants against photoinhibition, potentially due to increased NO
3

uptake (He et al., 2013). This increased the dry weight of lettuce
shoots and roots by 1.8 and 2.5-fold, respectively, but decreased the
shoot : root ratio at CO
2
≥10 000 ppm (He et al., 2010).
Therefore, adjusting the root zone temperature and CO
2
concen-
tration can improve growth, mineral uptake and nutritional
content.
Root exudation and microbial interactions during
aeroponic cultivation
Plants release an estimated 20% of assimilated carbon as root
exudates, which includes high and low molecular weight com-
pounds that can inhibit or benefit growth (Kuzyakov & Domanski,
2000; Badri & Vivanco, 2009; Baetz & Martinoia, 2014; Delory
et al., 2016; Mommer et al., 2016; Huang et al., 2019). It is
important to understand the effects of root exudation during
aeroponic cultivation because the nutrient solution is recycled for
some time within closed systems (Fig. 3a). For example, plant
autotoxicity can arise from exuded organic acids within recycled
nutrient solutions (Yu & Matsui, 1993, 1994; Asao et al., 2003;
Hosseinzadeh et al., 2017). However, little is known about the
types, concentrations and variation in recycled root exudates for
distinct crop species grown using aeroponic systems, or their
consequences for plant performance. Because the physical and
chemical properties of nutrient solutions change when atomized
into aerosols (Hinds, 1999), root exudates might change chemi-
cally or precipitate, changing the effects of exudates on plant and/or
microbial growth. Plants also release volatile organic compounds
(VOCs) into the root zone (Dudareva et al., 2006; Widhalm et al.,
2015; Delory et al., 2016; Pickett & Khan, 2016; Vivaldo et al.,
2017) that might partition into the aerosol phase (Odum et al.,
1996; Sander, 2015) and, therefore, incorporate autotoxic com-
pounds into aerosol droplets (Fig. 3a). Incorporation of VOCs into
aerosol droplets will change the aerosol vapour pressure, potentially
altering the concentrations of nutrients delivered to the roots. Since
root exudate compounds such as the polysaccharide xyloglucan
increase substrate cohesion (Galloway et al., 2018), exudate
compound(s) might alter thin-film retention and nutrient uptake
by changing cohesion and adhesion characteristics at the interface
between thin-films and root surfaces (Figs 2b, 3a).
Root exudates are important for microbial growth and shaping
rhizosphere microbial communities (De-la-Pe~na et al., 2008;
Chaparro et al., 2014; Hugoni et al., 2018; Sasse et al., 2018). There
are relatively few studies of root microbiome development during
aeroponic cultivation (Fig. 3a). One recent study found that the
root-associated microbial community of aeroponically grown
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lettuce was dominated by proteobacteria and was distinct from
microbial communities present on the germination trays or
nutrient solution (Edmonds et al., 2020). Given that some bacterial
species are unculturable after aerosol dispersion (Reponen et al.,
1997; Dabisch et al., 2012; Zhen et al., 2013) and that each
atomization method affects bacterial membrane integrity and cell
survival differently (Fernandez et al., 2019), more extensive
characterization of microbial communities at the root–aerosol
interface and within the nutrient solution will be informative. This
might identify beneficial or inhibitory effects of these microbial
communities on aeroponic productivity for a variety of crops
throughout their development. This could inform the develop-
ment of probiotic microbial treatments to support biofertilization
and biocontrol, including protection of the crop and aeroponic
system from invasion by human and plant pathogens. One method
to introduce such probiotics could be to inoculate the seeds at the
point when they are moistened to break dormancy and induce
germination.
H
H
O
H
H
O
H
H
O
H
H
O
5)
H
H
O
H
H
O
H
H
O
HH
O
HH
O
HH
O
Day Night
Epidermis
Exodermis (Hypodermis)
Cortex
Endodermis
Casparian strips
Vascular tissue
Root hairs
Aeroponics Hydroponics
Root
exudates
Microbes
Microbial
exudates
Root
exudates
VOCs
(a)
(b)
Aero- Hydro-
Root hairs
Fig. 3 Interactions between aeroponically grown plants and their environment. (a) Interactions between the aerial and root phases and their environment.
Light/dark conditions and diel nutrient supply cycles might be optimized to enhance plant productivity. The root zone CO
2
and O
2
concentrations affect plant
productivity and have potential for manipulation to enhance productivity. Volatile organic compounds (VOCs) released into the root zone might alter the
aerosol properties and nutrient availability. Interactions between root exudate compounds and nutrient solution ions will affect thin-film development and
retention. Root exudates will shape the aeroponic microbial community and microbial exudates might, in turn, affect crop productivity and protection. (b) Root
architecture and anatomy can differ between hydroponic and aeroponic cultivation, with aeroponically-cultivated roots having increased root hair abundance
and hydrophobic barriers in the exodermis (shown in red) compared with hydroponic cultivation.
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Root morphology and anatomy in aeroponic
cultivation
Root morphology and architecture affects aerosol capture and thin-
film formation. For example, aeroponically grown roots can have
increased root hair abundance compared with hydroponically
grown roots (Kratsch et al., 2006), which will in turn influence
aerosol capture (Fig. 3b). Given that root hair development is both
dynamic and influenced by environmental heterogeneity and the
nutrient or water status of plants (Gilroy & Jones, 2000; Vissenberg
et al., 2020), it will be valuable to assess how root hairs develop on
aeroponically-grown plant species at a variety of developmental
stages. More research is required to establish which microscale and/
or macroscale root traits are important for aerosol capture at various
developmental stages, considering differences between crops. This
knowledge might influence the aerosol properties (e.g. droplet size)
and nutrient dosing regimen that are administered at each
developmental stage to optimize aerosol capture, and nutrient
and water uptake.
Because the anatomy of root cell layers influences nutrient and
water uptake (Enstone et al., 2002), it is important to understand
how root anatomy might be influenced by aeroponic cultivation.
For example, the exodermal hydrophobic barriers differ between
the maize root hypodermis following aeroponic and hydroponic
culture (Fig. 3b) (Zimmermann & Steudle, 1998; Freundl et al.,
2000; Meyer et al., 2009; Redjala et al., 2011). Hydroponically
grown maize roots lacked exodermal hydrophobic barriers, whilst
hydrophobic barriers were present in the exodermis, 30–70 mm
from the root tips, following aeroponic cultivation (Fig. 3b)
(Zimmermann & Steudle, 1998). Greater depth of knowledge of
root anatomical specializations during aeroponic culture would be
informative across a wider range of crops, at various developmental
stages. We speculate that species with thicker hydrophobic barriers
might require longer aerosol atomization periods, using droplets
containing greater nutrient concentrations.
Diel cycles and photoperiod in aeroponic cultivation
The light conditions in indoor farms can be tuned to crop
requirements. For example, the photoperiod influences the growth
and development of many plant species (Turner et al., 2005; Song
et al., 2015). Since the light spectrum influences the morphology
and metabolite content, altering the spectrum can adjust the shape,
flavour, fragrance or nutrient content of vertically farmed crops
(Darko et al., 2014; Dou et al., 2017; Fraser et al., 2017;
Holopainen et al., 2018).
The specificity in the timing and intensity of aeroponic nutrient
dosing provides opportunities to align daily aeroponic and lighting
regimes for optimal growth (Fig. 3a). Daily fluctuations in
fertilization could be applied, such as day- and night-specific
nutrient mixes. This strategy has been proposed to manipulate the
nutrient composition of salad crops (Albornoz et al., 2014),
capitalizing on diurnal stomatal opening and transpiration stream
activity. By providing greater nitrogen concentrations to the roots
during the dark period and lower concentrations during the light
period, nitrogen over-accumulation within leaves can be prevented
(Albornoz et al., 2014). Diel fluctuations in nutrient concentra-
tions also appear to increase the yield of some tomato varieties
(Santamaria et al., 2004).
The relationship between the light/dark cycle and the endoge-
nous circadian rhythm influences plant growth and development.
Laboratory experiments with Arabidopsis found that a mismatch
between the endogenous circadian period and the period of the day/
night cycle reduces growth and causes mismanagement of transi-
tory starch reserves (Dodd et al., 2005; Graf et al., 2010). This
relationship between circadian rhythms and light conditions is
important for vertical farms. For example, lettuce growth rates can
be estimated from circadian rhythm parameters of the seedlings,
and this information can be used to transfer the best-performing
seedlings from the nursery to the farm (Moriyuki & Fukuda,
2016). This can maximise the number of individual plants meeting
certain growth criteria (Moriyuki & Fukuda, 2016). Similarly, the
timing of artificial light and dark cycles during tomato cultivation
influences tomato growth and survival (Highkin & Hanson, 1954).
This might explain why humans selected for a longer circadian
period and later circadian phase during tomato domestication at
higher latitudes with longer photoperiods (M€uller et al., 2016,
2018). Therefore, knowledge of circadian biology can be
exploited to optimize daily lighting regimes in vertical farms to
maximise productivity. In future, it might be possible to exploit
integrated plant growth models that incorporate knowledge of
circadian rhythms (Chew et al., 2014) to optimize photo- and
thermoperiodic conditions for specific vertically farmed crop
varieties.
Conclusions and recommendations for future work
We conclude by suggesting strategic areas of future research to
underpin increased productivity and sustainability of aeroponic
vertical farms.
Understand why aeroponic cultivation can be more productive
than hydroponic or soil cultivation, to inform crop breeding and
farm engineering. Potential testable hypotheses concern altered
photosynthetic performance, oxygen availability, stomatal physi-
ology and water relations, nutrient supply, carbohydrate partition-
ing, and resource competition within the root and aerial phases of
plants in growing trays. This also involves the investigation of why
certain genotypes are better suited to aeroponic cultivation, because
this might allow the breeding of varieties with enhanced perfor-
mance during aeroponic cultivation or extension of the range of
crops that can be cultivated with aeroponics.
Understand root developmental architecture under standardized
aeroponic conditions for a key range of crops at a variety of
developmental stages, and how this differs from hydroponic- and
soil-based cultivation. Growing conditions reflect the local envi-
ronment, technologies and crop varieties, so comparing model
crops under standardised conditions might provide insights to
inform cultivation conditions.
Understand the relationship between aeroponic droplet size,
nutrient content, droplet deposition and plant per formance. This is
important to identify aerosol generation technology or regimes that
Ó2020 The Authors
New Phytologist Ó2020 New Phytologist Foundation
New Phytologist (2020) 228: 1183–1192
www.newphytologist.com
New
Phytologist Viewpoints Forum 1189

are appropriate and most profitable for each crop at a variety of
developmental stages. It will also inform optimization of crop
quality and nutrition within aeroponic systems.
Understand the relationship between aeroponic fertilization and
daily (24 h) cycles, and how this relationship affects crop perfor-
mance. The relationship between daily cycles of environmental
conditions (e.g. lighting, airflow, temperature, humidity), aerosol
supply and composition, and crop metabolism presents opportu-
nities to adjust crop performance, appearance, nutrient composi-
tion and flavour.
Establish experimental and analytical frameworks for compar-
ison of vertical farming technologies for a range of crops.
Frameworks should collate productivity metrics and resource
consumption to allow assessment of the environmental and
economic sustainability of each technology. This could underpin
more rapid technological development and collaboration towards
improved food security.
Understand the nature and recycling of root exudates within the
nutrient solutions of closed aeroponic systems. This includes
identification of recirculated compound types, their crop species-
dependency, chemical and physical changes in exudates caused by
aerosol generation, and crop performance impacts. This is
important for greenhouse and vertical farm engineering, and
pairing crops with optimum cultivation technologies.
Understand how different aeroponic atomization methods affect
microbial community structure at the root–aerosol interface, and
the consequences for crop productivity, crop protection, food
safety and farm engineering.
Acknowledgements
AND is grateful to The Royal Society for awarding an Industry
Fellowship (SIF\R2\182028). This work was funded by the UK
Biotechnology and Biological Sciences Research Council (SWBIO
DTP awards BB/J014400/1 and BB/M009122/1, and Institute
Strategic Programme GEN BB/P013511/1) and Innovate UK
(Industrial Strategy Challenge Fund). BME is grateful to Claire
Grierson, Jill Harrison and Emily Larson for their insightful
comments during the drafting of this manuscript.
Author contributions
The article was conceived by BME, JRF and AND. CAG designed
and produced the figures. BME, LRM, CAG, BR, JRF and AND
wrote the article.
ORCID
Antony N. Dodd https://orcid.org/0000-0001-6859-0105
Bethany M. Eldridge https://orcid.org/0000-0002-6598-3701
Calum A. Graham https://orcid.org/0000-0002-4450-9111
Bethany M. Eldridge
1
, Lillian R. Manzoni
2
, Calum A.
Graham
1,3
, Billy Rodgers
2
, Jack R. Farmer
2
*and
Antony N. Dodd
3
*
1
School of Biological Sciences, University of Bristol,
Bristol, BS8 1TQ, UK;
2
LettUs Grow, Chapel Way, Bristol, BS4 4EU, UK;
3
John Innes Centre, Norwich Research Park,
Norwich, NR4 7UH, UK
(*Authors for correspondence: tel +44 (0)117 290 0015,
email jack@lettusgrow.org (JRF); tel +44 (0)1603 450015,
email antony.dodd@jic.ac.uk (AND))
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Key words: aeroponics, plant factory, plant–microbe interactions, plant
physiology, surface interactions, vertical farming.
Received, 24 April 2020; accepted, 18 June 2020.
New Phytologist (2020) 228: 1183–1192 Ó2020 The Authors
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