UV Lighting in Horticulture: A Sustainable Tool for Improving Production Quality and Food Safety

Abstract

Ultraviolet (UV) is a component of solar radiation that can be divided into three types
defined by waveband: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (<280 nm). UV light can influence the physiological responses of plants. Wavelength, intensity, and exposure have a great impact on plant growth and quality. Interaction between plants and UV light is regulated by photoreceptors such as UV Resistance Locus 8 (UVR8) that enables acclimation to UV-B stress. Although UV in high doses is known to damage quality and production parameters, some studies show that UV in low doses may stimulate biomass accumulation and the synthesis of healthy compounds that mainly absorb UV. UV exposure is known to induce variations in plant architecture, important in ornamental crops, increasing their economic value. Abiotic stress induced by UV exposure increases resistance to insects and pathogens, and reduce postharvest quality depletion. This review highlights the role that UV may play in plant growth, quality, photomorphogenesis, and abiotic/biotic stress resistance.

Full text

horticulturae
Review
UV Lighting in Horticulture: A Sustainable Tool for Improving
Production Quality and Food Safety
Danilo Loconsole * and Pietro Santamaria


Citation: Loconsole, D.; Santamaria,
P. UV Lighting in Horticulture: A
Sustainable Tool for Improving
Production Quality and Food Safety.
Horticulturae 2021,7, 9. https://
doi.org/10.3390/horticulturae7010009
Received: 16 December 2020
Accepted: 15 January 2021
Published: 17 January 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional clai-
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Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
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ditions of the Creative Commons At-
tribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Department of Agricultural and Environmental Science, University of Bari Aldo Moro, Via Amendola 165/A,
70120 Bari, Italy; pietro.santamaria@uniba.it
*Correspondence: danilo.loconsole@uniba.it; Tel.: +39-080-544-30-39
Abstract:
Ultraviolet (UV) is a component of solar radiation that can be divided into three types
defined by waveband: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (<280 nm). UV light
can influence the physiological responses of plants. Wavelength, intensity, and exposure have a
great impact on plant growth and quality. Interaction between plants and UV light is regulated
by photoreceptors such as UV Resistance Locus 8 (UVR8) that enables acclimation to UV-B stress.
Although UV in high doses is known to damage quality and production parameters, some studies
show that UV in low doses may stimulate biomass accumulation and the synthesis of healthy
compounds that mainly absorb UV. UV exposure is known to induce variations in plant architecture,
important in ornamental crops, increasing their economic value. Abiotic stress induced by UV
exposure increases resistance to insects and pathogens, and reduce postharvest quality depletion.
This review highlights the role that UV may play in plant growth, quality, photomorphogenesis, and
abiotic/biotic stress resistance.
Keywords: UV-A; UV-B; UV-C; ROS; LED; photomorphogenesis; phenols; postharvest
1. Introduction
Climate change, global water scarcity, and salinization have negatively affected arable
land [
1
]. Global warming decreases production [
2
], while the demand for fresh high-
quality vegetables is increasing. Efficient artificial growing systems are essential to feed
a human population in constant increase [
3
]. Greenhouse and indoor plant production
systems produce clean fresh food, increasing yield and quality by controlling plant growth
conditions. The latest technology in light emitting diode (LED) lighting is being adopted by
farmers and allows broad control of spectra in the case of urban farming with fully artificial
lighting. Spectrum modulation makes it possible to control growth and quality, including
plant architecture, nutrient content, and phytochemical levels [
4
–
7
]. LED lighting can be
used to complement natural daylight. Ultraviolet (UV) is part of the light spectrum, and
can be divided into three types by waveband: UV-A (315–400 nm), UV-B (280–315 nm),
and UV-C (<280 nm). UV-C and most UV-B is absorbed by the stratospheric ozone layer,
whereas UV-A and a small part of UV-B reach the earth’s surface and can be absorbed by
plants. Although UV-B is the smallest component of sunlight (<0.5% of the total energy
reaching the surface), it has the highest energy and has a considerable impact on animals
and plants [8].
UV light has a great impact on plant growth and quality, in relation to wavelength,
intensity and exposure. Several studies have shown that high UV-A doses have a negative
effect on PSII elaboration, decreasing maximum quantum efficiency, electron transport rate
and photosynthesis due to a reduction in ribulose-1,5-bisphosphate carboxylase-oxygenase
(RuBisCO) (C3 species) or PEP carboxylase (C4 species) and an increase in stomatal re-
sistance and reactive oxygen species (ROS) production [
9
–
13
]. UV-B can induce abiotic
stress and photomorphogenic changes. High UV-B exposure induces physiological stress,
Horticulturae 2021,7, 9. https://doi.org/10.3390/horticulturae7010009 https://www.mdpi.com/journal/horticulturae

Horticulturae 2021,7, 9 2 of 13
including ROS production, DNA deterioration and damage, and impairment of cell pro-
cesses [
14
]. Low UV-B exposure promotes photomorphogenic changes, such as cotyledon
expansion, biosynthesis of anthocyanins and flavonoids, inhibition of hypocotyl growth
and stomatal opening [
8
,
15
,
16
]. Interaction between plants and UV light is regulated by
photoreceptors, such as UV Resistance Locus 8 (UVR8), that allows acclimation to UV-B
stress [
8
,
16
,
17
]. UVR8 is associated with the chromatin of UV-B-responsive genes, such as
the promoter region of elongated hypocotyl 5 (HY5). UVR8 activity is also associated with the
E3 ubiquitin ligase, known as constitutively photomorphogenic 1 (COP1). UVR8 interacts
with COP1, and both are associated with photomorphogenic response [18].
Like UV-B, UV-C triggers stress responses in plants and may damage DNA and modify
the functioning of chloroplasts and mitochondria [
19
–
21
]. Stressed plants may react by
producing ROS. ROS signaling modifies physiological processes and produces secondary
metabolites [21–23].
Exposure to UV also has a great impact on human health: the effects of UV radiation
can manifest in the short and long term. Sunburn, tanning, and photosensitivity are
the most common short-term effects [
24
]. Long term effects range from lentigines to
melanocytic naevi, actinic keratosis and skin cancer [
24
]. Eyes can also be affected by
UV exposure, manifesting as photokeratitis, cancer of the eye and cataract. Occupational
exposure to UV should be kept to a minimum and protection should be always be worn.
The risk can be drastically reduced by wearing appropriate closely woven clothing and a
brimmed hat to reduce face and neck exposure. Sunscreen and eye protection (in relation
to light type and intensity) are also advisable [24].
2. UV Influence on Growth and Inhibition
Although UV light is known to damage quality and production parameters, some
studies show stimulatory effects of UV-A on biomass accumulation (roots and shoots) in
certain species. The response of plants to UV radiation may be species-specific, ranging
from stimulation to inhibition [
25
–
29
]. Among the studies that have shown species-specific
effects of UV-A, promotion of biomass accumulation in roots was found in four Mediter-
ranean species, including Laurus nobilis, grown with little irrigation [
28
,
29
]. This was
attributed to improved efficiency of water use and increased photosynthesis rates, indicat-
ing that growth improvement was the result of UV and environmental factors. Changes
in biomass allocation have also been linked to different allocation of resources [
30
]. This
was reported in four cultivars of cucumber (Cucumis sativus) which showed a decrease in
shoot biomass accumulation with no effects on roots after UV-A exposure [
31
]. Similar
results were observed in Triticum sativum [
32
] and in four Arabidopsis thaliana ecotypes
studied by Cooley et al. [
26
]. Negative effects of UV-A on shoot biomass were attributed
to direct exposure of aerial parts to sunlight and it was argued that promotion of root
growth was associated with UV-A photoreceptors in shoots, implicated in the transmission
of long-distance signals that regulate root growth. Promotion of root growth by UV-A
with no effects on shoots was also found in two cultivars of Glycine max under greenhouse
conditions [33].
UV-B has also been found to affect growth. Several authors have reported a stim-
ulatory effect of 4.15
µ
mol
·
m
−2·
s
−1
UV-B [
34
,
35
] and 0.86
µ
mol
·
m
−2·
s
−1
UV-B [
35
] on
photosynthesis in cucumber seedlings. Other studies showed inhibition of hypocotyl
elongation in cucumber [
36
,
37
], as well as promotion of stem diameter growth at medium
intensity (3.33
µ
mol
·
m
−2·
s
−1
UV-B), without any damage, and a decrease in net photo-
synthesis at high intensity (5.54 or 6.67
µ
mol
·
m
−2·
s
−1
) [
37
]. In studies by Liu et al. [
37
],
3.33 µmol·m−2·s−1
UV-B promoted stem diameter growth, soluble sugar content, total
ascorbic acid, and superoxide dismutase, peroxidase and catalase activities in cucumber
leaves by 13.6–22.3%, 22.7–56.7%, 16.9–23.2%, 23.8–25.9%, 34.1–50.4% and 27.4–36.4%,
respectively, without compromising net photosynthesis rate.
Recent studies have evaluated the effects of UV-C on plant growth and photosynthe-
sis. A positive effect of UV-C was found by Darras et al. [
38
] on ‘Belladona F1’ tomato

Horticulturae 2021,7, 9 3 of 13
(
Solanum lycopersicum
) plants. Irradiated with UV-C at 1.0 kJ m
−2
, the plants showed a 25%
increase in total fruit number and a 36% increase in fruit weight compared to non-irradiated
controls. A smaller increase was found with 2.5 kJ
·
m
−2
(24% and 31%, respectively). In
the same experiment, UV-C was found to decrease plant height and net CO
2
assimilation.
Other evidence from the same experiment showed that pulsed UV-C irradiation induced
changes in growth and fruit set in ‘Belladonna F1’ tomato plants. This higher yield of
tomatoes may be due to shared UV-B photoreceptors also absorbing UV-C light [
21
], as
occurs in some photomorphogenic reactions, promoting an increase in biomass [38].
Earlier experiments showed that pre-harvest UV-C irradiation induced flowering
in certain ornamental plants [
39
–
42
]. In most cases there was no decrease in net CO
2
assimilation between UV-C-irradiated plants and controls, which means no damage to PSII.
Similar results were found in other species: chlorophyll fluorescence (Fv/Fm) of lettuce
(Lactuca sativa) plants exposed to UV-C light was not reduced by doses up to 1.70 kJ
·
m
−2
,
indicating no damage to PSII [
43
]. On the contrary, there was a significant decrease in net
CO
2
assimilation, stomatal conductance, Fv/Fm and yield in strawberry (Fragaria x ananassa
Duch.) plants exposed to 1.5 kJ
·
m
−2
every four days [
44
]. This means that effects may be
species-specific.
3. UV-Mediated Production of Healthy Compounds
3.1. Total Phenol Content
Phenol compounds are secondary metabolites that are usually related to plant defense
responses. Light plays an important role in several kinds of biotic and abiotic stimulation,
triggering production of various phytochemicals (Figure 1). Several studies have shown
that UV-A and UV-B signaling can both increase phenol content [
45
–
47
]. Studies on UV
signaling have shown that UV-A and UV-B interact with different photoreceptors and/or
mechanisms. The increase in phenol content mediated by UV-A was delayed in lettuce and
Ixeris dentata with respect to the same response mediated by UV-B [
30
,
46
,
47
]. UV-B stress
can increase the content of secondary metabolites useful for human health. For example,
abiotic stress induced by exposure of mung bean sprouts to UV-B leads to significant
accumulation of vitamin C, phenols and flavonoids, improving nutritional value [
48
]. These
findings are in line with the results of experiments on buckwheat (Fagopyrum esculentum)
sprouts [
49
], tomato [
50
], and Hypericum spp. [
51
]. Production of vitamin C, phenols and
flavonoids in mung bean (Vigna radiata) sprouts may be promoted by low-dose irradiation
with UV-B [48].
3.2. Flavonoid Content
Flavonoids are the major group of phenols related to responses to UV, and have a main
antioxidant function [
52
]. Previous studies have established that flavonoids have UV-A and
UV-B-absorbing properties. Most flavonoids absorb in the
315–400 nm
UV-A range [
53
], play-
ing an important role as antioxidants and protective compounds [
54
,
55
]. Several studies have
shown that UV-A promotes accumulation of flavonols in
Mesembryanthemum crystallinum
[
56
].
Morales et al. [
57
] and Götz et al. [
45
] found that production was dose-dependent in
Betula pendula
and Arabidopsis thaliana, respectively. Besides light, other environmental factors
modulate UV-A interactions with plants. For example, flavonoid levels in Pinus sylvestris,
modulated by UV-A exposure, were enhanced under high, but not low nutrient availability.
UV-A exposure decreased leaf content of quercetin derivatives in Arbutus unedo under low
precipitation [58,59].
3.3. Light-Absorbing Phenol Compounds
Phenol compounds are an important group of phenylalanine-derived secondary
metabolites with a wide range of properties, making them suitable for the food, pesticide,
pharmaceutical and cosmetic industries [
60
]. Phenol compound biosynthesis in plants is
regulated by UV-A [
30
] (Figure 1) and UV-B [
60
] radiation. Lighting dose may be mod-
ulated to maximize plant performance and accumulation of secondary metabolites [
61
].

Horticulturae 2021,7, 9 4 of 13
Based on studies by Murthy et al. [
62
], UV-B stimulation of phenol compounds may be
exploited in two steps: (i) plants are cultivated under optimal conditions until a determined
growth stage; (ii) secondary metabolism is stimulated by UV-B to enhance phenol produc-
tion. Species selection is essential as certain species may be naturally rich in secondary
metabolites. For example, sweet basil (Ocimum basilicum L.), a member of the Lamiaceae
family, accumulates high levels of different phenols, especially rosmarinic acid. These
compounds can be used in the cosmetic, food and pesticide industries [
63
–
65
]. Several
studies have shown that phenol production increased in sweet basil plants exposed to UV-B
light [
66
,
67
]. Phenylalanine ammonia lyase (PAL) is the key enzyme of phenylpropanoid
metabolism, triggering the biosynthesis of phenol compounds; several studies have shown
that UV-B stress modulates an increase in PAL [
68
]. Mosadegh et al. [
69
] showed that plants
treated with 34, 68 and 102 kJ
·
m
−2·
day
−1
UV-B have enhanced production of phenols
compared to controls, reaching the highest values at 102 kJ
·
m
−2·
day
−1
. This could be due
to a dose-dependent effect.
Horticulturae 2021, 7, x FOR PEER REVIEW 4 of 12
pharmaceutical and cosmetic industries [60]. Phenol compound biosynthesis in plants is
regulated by UV-A [30] (Figure 1) and UV-B [60] radiation. Lighting dose may be modu-
lated to maximize plant performance and accumulation of secondary metabolites [61].
Based on studies by Murthy et al. [62], UV-B stimulation of phenol compounds may be
exploited in two steps: (i) plants are cultivated under optimal conditions until a deter-
mined growth stage; (ii) secondary metabolism is stimulated by UV-B to enhance phenol
production. Species selection is essential as certain species may be naturally rich in sec-
ondary metabolites. For example, sweet basil (Ocimum basilicum L.), a member of the La-
miaceae family, accumulates high levels of different phenols, especially rosmarinic acid.
These compounds can be used in the cosmetic, food and pesticide industries [63–65]. Sev-
eral studies have shown that phenol production increased in sweet basil plants exposed
to UV-B light [66,67]. Phenylalanine ammonia lyase (PAL) is the key enzyme of phe-
nylpropanoid metabolism, triggering the biosynthesis of phenol compounds; several
studies have shown that UV-B stress modulates an increase in PAL [68]. Mosadegh et al.
[69] showed that plants treated with 34, 68 and 102 kJ·m−2·day−1 UV-B have enhanced pro-
duction of phenols compared to controls, reaching the highest values at 102 kJ·m−2·day−1.
This could be due to a dose-dependent effect.
Figure 1. UV-A effects on photosynthesis. A plus (+) sign indicates enhanced response (either positive or negative); a green
arrow indicates a positive response, and a red arrow indicates a negative response. (A) High intensity UV-A exposure
enhanced photoinhibition in isolated photosynthetic structures (chloroplasts and thylakoids), whereas leaves exposed to
UV-A activate photoprotective mechanisms (leaf phenols and antioxidant enzymes), reducing photoinhibition. (B) Under
non-saturating light exposure, direct (a) or indirect (b) UV-A absorption by photosynthetic pigments induced blue-green
fluorescence by phenols, and/or (c) increased stomatal opening, leading to enhanced photosynthesis rates [30].
3.4. Carotenoid Production
Different factors influence carotenoid production. Light interacts with phytochrome
and/or UV-B receptors, regulating carotenoid accumulation [70], while ethylene modu-
lates abiotic stress, including light stress [71]. Several studies report that UV-B radiation
stimulates ethylene production in green tissues of many species, such as pear (Pyrus spp.)
shoots, Arabidopsis, tobacco (Nicotiana spp.), and tomato [72–74], mediated by the ROS
pathway. UV-B-mediated carotenogenesis is also influenced by genotype: indeed, differ-
ent responses to UV-B exposure, related to endogenous ripening factors, have been de-
scribed in different tomato genotypes [75,76]. Other studies found that carotenoid biosyn-
thesis genes are generally down-regulated after chronic exposure to UV-B, but up-regu-
lated after acute exposure to UV-B [77].
Figure 1.
UV-A effects on photosynthesis. A plus (+) sign indicates enhanced response (either positive or negative); a green
arrow indicates a positive response, and a red arrow indicates a negative response. (
A
) High intensity UV-A exposure
enhanced photoinhibition in isolated photosynthetic structures (chloroplasts and thylakoids), whereas leaves exposed to
UV-A activate photoprotective mechanisms (leaf phenols and antioxidant enzymes), reducing photoinhibition. (
B
) Under
non-saturating light exposure, direct (a) or indirect (b) UV-A absorption by photosynthetic pigments induced blue-green
fluorescence by phenols, and/or (c) increased stomatal opening, leading to enhanced photosynthesis rates [30].
3.4. Carotenoid Production
Different factors influence carotenoid production. Light interacts with phytochrome
and/or UV-B receptors, regulating carotenoid accumulation [
70
], while ethylene modu-
lates abiotic stress, including light stress [
71
]. Several studies report that UV-B radiation
stimulates ethylene production in green tissues of many species, such as pear (Pyrus spp.)
shoots, Arabidopsis, tobacco (Nicotiana spp.), and tomato [
72
–
74
], mediated by the ROS
pathway. UV-B-mediated carotenogenesis is also influenced by genotype: indeed, different
responses to UV-B exposure, related to endogenous ripening factors, have been described
in different tomato genotypes [
75
,
76
]. Other studies found that carotenoid biosynthesis
genes are generally down-regulated after chronic exposure to UV-B, but up-regulated after
acute exposure to UV-B [77].

Horticulturae 2021,7, 9 5 of 13
3.5. Anthocyanin Content
Anthocyanins are a major class of phenols with multiple health benefits. Light-
modulated anthocyanin production in plants has been studied by many authors, and UV
light is known to play an important role in the anthocyanin biosynthesis pathway [
78
].
An experiment carried out by Su et al. [
79
] showed that UV-B at 5 W m
−2
induced an-
thocyanin biosynthesis, with a time-dependent increase. This is in line with the find-
ings of Solovchenko et al. [
80
] in apple (Malus domestica) skin, Wang et al. [
78
] in turnip
(
Brassica rapa
) seedlings, and Tsurunaga et al. [
49
] in Triticum sprouts. Several studies
have shown that PAL plays an important role in anthocyanin accumulation under UV-B
irradiation [
81
,
82
]. When plants are exposed to a stressing environment, stored informa-
tion is used to develop “memory” for future resistance [
83
]. This phenomenon was also
observed for UV-B exposure: indeed, when light exposure was stopped, production of
anthocyanin did not stop at the same time but continued to increase for more than six
hours; the same was observed for PAL activity [
79
]. The above findings suggest that UV
light enrichment can improve the nutritional quality of crops by enhancing production of
healthy compounds. However, more research into the effects of UV treatment under closed
conditions, such as in plant nurseries, is needed [47].
4. UV Signaling for Photomorphogenesis
Plant architecture is a key aspect for quality and production in horticulture. Dwarfed
plant architecture is important for ornamental plants, increasing their economic value [
84
],
especially at higher latitudes characterized by low light quality and quantity, where plants
tend to elongate and the quality of production is low. Dwarfed architecture has advantages
during transport, production and transplant and is less susceptible to damage [
85
]. More
compact plants take up less space during transport and in transplant nurseries. Dwarfed
plants, for example cucumber, produce the same number and size of fruit as non-dwarfed
plants [85]. Saving space and reducing damage enables more sustainable production.
Traditionally, plant architecture was regulated using plant growth regulators (PGR),
mechanical stimulation [
86
,
87
] or irrigation deficits [
88
]. Instead, light can play an impor-
tant role in dwarfing. Much work has been done on the influence of UV light on plant
morphology, and plant response is not always the same but is species-dependent [
89
].
UV-B exposure has been found to lead to a decrease in foliar area, shoot dry mass and shoot
length in some species [
33
,
90
–
94
], whereas in others, it leads to an increase in leaf area, fresh
weight, and dry weight [
95
]. Although the effect of UV-B is variable and species-dependent,
exposure generally increases the number of stems per tiller [93].
Comparisons of the effects of UV-A and UV-B exposure on cucumber show that plants
exposed to UV-A and UV-B are both dwarfed, but plants exposed to UV-B lack a sturdy
stem [
85
]. Stimulatory and inhibitory effects of UV-A and UV-B, respectively, were found
in Arabidopsis. Supplementary exposure to UV-A led to a 30–150% increase in rosette
diameter in eight accessions of A. thaliana grown indoors under low PAR conditions [
96
].
Additional supplementation with UV-B enhanced this phenotype. The same effect was
observed under outdoor conditions. More studies showed that the UV-B dwarfing effect
was higher than the stimulatory effects of UV-A on leaf size [
96
]. These findings suggest that
UV light may be a sustainable alternative to chemical growth regulators for the production
of dwarfed plants in horticulture [85].
5. Biotic Resistance
Plant exposure to solar UV-B radiation increases plant resistance to herbivorous insects
and microbial pathogens [
97
]. The defense mechanism is regulated by JA-dependent and
JA-independent pathways. This is supported by findings in Arabidopsis mutants with a
deficiency in JA-signaling or insensitivity to JA, where UV-B had positive effects against
Botrytis cinerea [
97
]. On the contrary, some plants such as a Nicotiana attenuata mutant
silenced for a lipoxygenase (JA signaling pathway) gene, lost its defenses [
98
]. Besides jas-
monate signaling, the photoreceptor UVR8 is also implicated in plant defense mechanisms

Horticulturae 2021,7, 9 6 of 13
mediated by UV-B light [
99
], promoting accumulation of sinapates [
97
], which are precur-
sors of syringyl-type (defense) lignin synthesis. This mechanism prevents penetration of
fungal hyphae by strengthening cell walls [
8
,
100
]. UV-B exposure also triggers indirect
defense systems against insects by UVR8-controlled accumulation of flavonoids [8,101].
Due to its strong biological effect, UV radiation was observed to be effective in salt
stress mitigation. Seed priming is a pre-sowing treatment which enables seeds to germinate
more efficiently. Priming with UV-C light improved development of lettuce under stress
conditions, increasing root and leaf dry weight in saline conditions [
102
]. Studies by
Farhoudi et al. showed that the best result was reached at 0.85 kJ
·
m
−2
UV-C. UV-C priming
allows more uniform growth under saline conditions [
102
]. It also reduces Na absorption
and toxicity. UV-C presumably plays a role in the antagonistic relationship between K and
Na [
103
]. Plants treated with a moderate dose of UV-C (Figure 2) showed higher phenol
content and higher antiradical activity in the presence of NaCl [104].
Horticulturae 2021, 7, x FOR PEER REVIEW 6 of 12
against Botrytis cinerea [97]. On the contrary, some plants such as a Nicotiana attenuata mu-
tant silenced for a lipoxygenase (JA signaling pathway) gene, lost its defenses [98]. Besides
jasmonate signaling, the photoreceptor UVR8 is also implicated in plant defense mecha-
nisms mediated by UV-B light [99], promoting accumulation of sinapates [97], which are
precursors of syringyl-type (defense) lignin synthesis. This mechanism prevents penetra-
tion of fungal hyphae by strengthening cell walls [8,100]. UV-B exposure also triggers in-
direct defense systems against insects by UVR8-controlled accumulation of flavonoids
[8,101].
Due to its strong biological effect, UV radiation was observed to be effective in salt
stress mitigation. Seed priming is a pre-sowing treatment which enables seeds to germi-
nate more efficiently. Priming with UV-C light improved development of lettuce under
stress conditions, increasing root and leaf dry weight in saline conditions [102]. Studies by
Farhoudi et al. showed that the best result was reached at 0.85 kJ·m−2 UV-C. UV-C priming
allows more uniform growth under saline conditions [102]. It also reduces Na absorption
and toxicity. UV-C presumably plays a role in the antagonistic relationship between K
and Na [125]. Plants treated with a moderate dose of UV-C (Figure 2) showed higher phe-
nol content and higher antiradical activity in the presence of NaCl [103].
Figure 2. Plant morphological and biomass accumulation responses to UV. UV-A exposure has positive and negative
effects on biomass accumulation and morphology, while UV-B effects are primarily negative. Changes in plant architec-
ture and biomass allocation may influence resource uptake (light, water and nutrients). Changes mediated by UV-A may
also depend on genetic and/or environmental factors [30].
6. Postharvest and Storage
6.1. Postharvest Metabolic Processes Regulation
In recent years, consumer concern for quality, nutraceutical characteristics and food
security has grown. Globalization has led to trade on an international instead of a local
scale. In this scenario, postharvest techniques become more important and sustainable
cultivation is preferred to conventional methods. Modulation of light intensity and/or
spectrum is promising due to its importance for metabolic processes such as biosynthesis
of phytochemicals [50]. UV-B radiation may trigger accumulation of certain antioxidant
molecules with a role in UV protection [104]. The results of treatment are plant species
and dose-dependent. Plants treated with UV radiation develop mechanisms that protect
them from stress. This protective stress response triggers changes in secondary metabo-
lism, leading to accumulation of certain phytochemicals [105]. Research results indicate
that UV-B radiation can be considered a sustainable new non-chemical way of slowing
quality depletion in the postharvest storage, due to its capacity to enhance antioxidant
activity, for example in tomato [106]. Similar results were found with UV-C in tomato,
Figure 2.
Plant morphological and biomass accumulation responses to UV. UV-A exposure has positive and negative effects
on biomass accumulation and morphology, while UV-B effects are primarily negative. Changes in plant architecture and
biomass allocation may influence resource uptake (light, water and nutrients). Changes mediated by UV-A may also depend
on genetic and/or environmental factors [30].
6. Postharvest and Storage
6.1. Postharvest Metabolic Processes Regulation
In recent years, consumer concern for quality, nutraceutical characteristics and food
security has grown. Globalization has led to trade on an international instead of a local
scale. In this scenario, postharvest techniques become more important and sustainable
cultivation is preferred to conventional methods. Modulation of light intensity and/or
spectrum is promising due to its importance for metabolic processes such as biosynthesis
of phytochemicals [
50
]. UV-B radiation may trigger accumulation of certain antioxidant
molecules with a role in UV protection [
105
]. The results of treatment are plant species
and dose-dependent. Plants treated with UV radiation develop mechanisms that protect
them from stress. This protective stress response triggers changes in secondary metabolism,
leading to accumulation of certain phytochemicals [
106
]. Research results indicate that
UV-B radiation can be considered a sustainable new non-chemical way of slowing quality
depletion in the postharvest storage, due to its capacity to enhance antioxidant activity, for
example in tomato [
107
]. Similar results were found with UV-C in tomato, namely delay of
ripening, improved firmness, and extended shelf life [
108
]. An experiment by Park and
Kim [
109
] showed that 2.0 kJ
·
m
−2
UV-C triggered the antioxidant pathway, increasing
quercetin by 25% as well as flavonoid content. The highest quercetin level was found after
15 days of storage at 0 ◦C.

Horticulturae 2021,7, 9 7 of 13
6.2. Reduction of Microbial Population
It has been demonstrated that postharvest UV application can be useful to reduce
the microbial population, e.g., disinfection with non-ionizing UV-C. The main benefits are
health security (absence of chemical residues) and low cost [
110
]. UV-C doses ranging from
0.5 to 20 kJ
·
m
−2
were reported to limit microbial activity, inducing formation of pyrimidine
dimers that alter the DNA helix, inhibiting microbial replication. Previous studies indicate
that the effect of UV-C treatment was the same in the temperature range 5–37
◦
C but may
have been influenced by product surface and irradiation angle [
111
,
112
]. The microbial
population may also be indirectly limited by plant defense mechanisms triggered by UV-C
stress, delaying product deterioration. Abiotic stress conditions induced by treatment
lead to activation of secondary metabolism, increasing production of phytochemicals
with nutraceutical activity [
113
]. These effects have been observed for zucchini squash
(
Cucurbita pepo
) [
114
], tomatoes [
115
], sweet potato (Ipomoea batatas) [
116
] and watermelon
(Citrullus lanatus) [
117
]. In the same experiment carried out by Park and Kim [
109
], mi-
crobial population development was tested: garlic (Allium sativum) stored at 0
◦
C treated
with 2 kJ
·
m
−2
UV-C showed a reduction in microbial growth (
3.82 log CFU
) compared to
control (4.86 log CFU) after 30 days storage. Other authors obtained similar results on
fresh cut fruit and vegetables [
117
,
118
]. Several studies showed that UV-C light has a direct
germicidal effect [117] and indirectly triggers the defensive phenol pathway [119].
6.3. The Problem of Browning and PPO Inhibition
Another postharvest problem is the browning of freshly cut vegetables [
120
]. Enzymic
browning of fruit and vegetables takes place in the presence of oxygen when polyphenol
substrates are exposed to polyphenol oxidase (PPO) and/or phenol peroxidase, as a
consequence of brushing, peeling, cutting, and crushing during postharvest handling,
which lead to a breakdown of cell structure [
121
]. Several studies have confirmed that
UV-C can be useful as a PPO inhibitor in model systems, apple derivatives [
122
], fresh
apple juice [
123
] and mushroom extracts [
124
]. Unfortunately, the potential for UV-C
treatment is limited by possible adverse effects on food, including alteration of sensory
quality attributes such as color [
125
]. Besides UV-C, UV-A has been found effective in PPO
inhibition in several fruit [
123
,
125
]. The results showed that PPO was inactivated in fresh
apple and pear juice by UV–visible treatment using a 400 W high-pressure mercury lamp
for 120 min. The lamps were placed 22.5 cm from the surface and had a spectrum between
250 and 740 nm and an incident energy of 3.88
·
10
−7
E
·
min
−1
. UV-A LED treatment was
also found effective in limiting color degradation of freshly cut apples by approximately
60% after 60 min exposure. The lamps emitted 9
×
10
−3
kJ
·
m
−2
. Unlike UV-C, UV-A
controls browning without decreasing organoleptic and nutraceutical quality [120].
7. Conclusions
A lot of research has been conducted on the effects of ultraviolet radiation on plants
and plant tissues. UV light has been found to be effective on plant growth, product quality,
and photomorphogenesis. UV-A stimulated biomass accumulation improving efficiency of
water use and photosynthesis rates, and UV-B promoted growth, photosynthesis, and stem
diameter at medium intensity. UV-A and UV-B lead to significant accumulation of several
secondary compounds such as vitamin C, phenols, and flavonoids with UV-protective
properties, as well as chlorophyll, carotenoids, and anthocyanins. Plant architecture is
a key aspect for quality and production in horticulture, and light can play an important
role in dwarfing. Plant exposure to solar UV-B radiation increased plant resistance to
herbivorous insects and microbial pathogens. In the globalized market of recent years,
the importance of nutraceutical characteristics and food security is growing. Modulation
of light intensity and/or spectrum is promising given its importance for postharvest
metabolic processes. Research results indicated that UV-B radiation can be considered a
sustainable new non-chemical way of slowing quality depletion in the postharvest storage.
Moreover, UV application can be useful in reducing the microbial population. By contrast,

Horticulturae 2021,7, 9 8 of 13
little is known about interactions between light and other abiotic and biotic factors in
relation to plant physiology and genetics. An important target for future studies may
be to explain interactions between UV and other climatic factors in the search for new
agricultural practices to improve the quality and healthiness of plant products, especially
in greenhouses and urban plant factories.
Author Contributions:
D.L.: drafting, bibliographic research and critical revision of the manuscript;
P.S.: coordination of the work, drafting, and critical revision of the manuscript. All authors have read
and agreed to the published version of the manuscript.
Funding:
This review was funded by the Rural Development Programme of the Apulia Region
(Italy) 2014–2020, Submeasure 16.2 (Support for pilot projects and development of new products,
practices, processes and technologies, and transfer and dissemination of results obtained by Opera-
tional Groups), in the framework of the SOILLESS GO project (www.soilless.it), project code (CUP)
B97H20000990009, paper n. 4.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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