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agronomy
Article
Effects of NaCl on Hydroponic Cultivation of
Reichardia picroides (L.) Roth
Rita Maggini 1, * , Stefano Benvenuti 1, Federico Leoni 2, Luca Incrocci 1and Alberto Pardossi 1
Citation: Maggini, R.; Benvenuti, S.;
Leoni, F.; Incrocci, L.; Pardossi, A.
Effects of NaCl on Hydroponic
Cultivation of Reichardia picroides (L.)
Roth. Agronomy 2021,11, 2352.
https://doi.org/10.3390/
agronomy11112352
Academic Editor: Bruce Schaffer
Received: 2 October 2021
Accepted: 18 November 2021
Published: 20 November 2021
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1
Department of Agriculture, Food and Environment, University of Pisa, Via del Borghetto 80, 56124 Pisa, Italy;
stefano.benvenuti@unipi.it (S.B.); luca.incrocci@unipi.it (L.I.); alberto.pardossi@unipi.it (A.P.)
2Institute of Life Sciences, Scuola Superiore Sant’Anna, Piazza Martiri della Libertà33, 56127 Pisa, Italy;
federico.leoni@santannapisa.it
*Correspondence: rita.maggini@unipi.it
Abstract:
Wild edible plant species are often more tolerant to salinity than many crop plants. Consid-
ering the salinization of irrigation water that is progressively affecting the Mediterranean region and
the market demand for new plant foods, the cultivation of wild edible species could represent a valid
alternative to conventional vegetable crops. In this study, Reichardia picroides (L.) Roth, a widespread
spontaneous herb of ethnobotanical tradition, was grown for four or six weeks under a greenhouse
in a floating system for the production of baby leaves. In order to improve the nutraceutical quality
of the tissues, the plants were exposed to the following NaCl concentrations in the nutrient solution:
1.7 (control), 25, 50, and 100 mM. The results showed that a 4-week growing period in a floating
system with 50 mM NaCl in the nutrient solution increased the content of bioactive molecules without
affecting the fresh yield. After six weeks of cultivation, despite a decrease in biomass production
as compared with the control, the leaves of salt-treated plants contained higher levels of bioactive
molecules along with lower amounts of nitrate ion.
Keywords: antioxidant; domestication; nutraceutical; plant stress; salinity
1. Introduction
Progressive salinization of irrigation water is an issue of concern in the Mediterranean
region and is becoming a limiting factor for the productivity of vegetable crops, which
generally show low tolerance toward continuous application of saline water [
1
,
2
]. Wild
edible species that are adapted to severe environmental conditions could represent a valid
alternative to less tolerant vegetables [
3
]. On the other hand, it has been reported that wild
food plants may contain high nitrate levels [
4
]. Plant response to saline conditions involves
complex mechanisms that differ among species [
5
,
6
]; however, a general effect of salinity
by NaCl is the competition between chloride and nitrate for root uptake, which decreases
nitrate accumulation in leafy vegetables including edible greens [5,7].
At present, the market demand is encouraging the introduction of new horticultural
crops and the exploitation of local foods that can meet the consumers’ favour [
8
]. Wild
plant species from the ethnobotanical tradition can satisfy both these requirements and,
furthermore, can be regarded as potential functional foods [
9
], as they often possess a
higher content of bioactive molecules than many vegetables [
4
]. These natural health-
promoting phytochemical compounds are the result of plant adaptation to the natural
environment and are generally produced as defense molecules against biotic or abiotic
stress conditions [
10
]. Often these substances are antioxidant compounds belonging to
the class of polyphenols that, in species with edible leaves, are commonly associated
with a bitter taste [
11
]. Despite a general tendency by the consumers to avoid bitter
foods, the connection between dietary intake of healthy antioxidants and bitter taste could
positively influence the acceptance of wild edible species [
12
,
13
], especially if they are used
as ingredients in a vegetable mix. The market demand for baby greens for the production of
Agronomy 2021,11, 2352. https://doi.org/10.3390/agronomy11112352 https://www.mdpi.com/journal/agronomy
Agronomy 2021,11, 2352 2 of 12
ready-to-eat mixed salads is in continuous expansion [
4
] and has promoted the cultivation
of several leafy species that were traditionally collected at the spontaneous state (e.g., rocket
salad [
14
]), thus preserving the natural environment, and contributing to the sustainable
maintenance of agrobiodiversity [
15
]. The hydroponic technique, particularly the floating
system, is typically used for the production of ready-to-eat baby leaves and could represent
a suitable choice also for the cultivation of wild herbs [
16
]. Unfortunately, domestication
often causes a decline in beneficial properties, as cultivated plants are less exposed to stress
factors than those in the original environment [
17
], in particular when plants are grown
in hydroponic systems, where root uptake of both water and nutrients is facilitated [
18
].
On the other hand, the nutraceutical properties of hydroponically grown plants could
be modulated in dependence of the composition of the nutrient solution; thus, a proper
formulation of the nutrient recipe could help to partially recover the typical properties of
the plants at the spontaneous state.
Although the cultivation of wild edible plants is receiving increasing attention, the best
growing practices to optimize the production and ensure both high yield and high quality
of these crops remain largely unknown. Among relatively unexploited edible species,
Reichardia picroides (L.) Roth (in the Asteraceae family), generally called common brighteyes
and known in Italy as terracrepolo, is a widespread spontaneous herb in the Mediterranean
region traditionally used as a food herb and as a medicinal plant. This species shows a high
adaptability to different unfavorable environmental conditions and can grow in different
habitats, either inland or in coastal areas subjected to the deposition of marine salts [
19
].
Moreover, in a recent work hydroponically grown R. picroides plants grown in hydroponic
systems exhibited salt tolerance [
20
]. Based on the above considerations, the aim of the
present study was to evaluate the effect of NaCl salinity on crop yield and produce quality
of R. picroides plants cultivated in a floating system under greenhouse conditions.
2. Materials and Methods
2.1. Plant Material and Growing Conditions
The seeds were obtained from spontaneous plants collected in Tuscany (Agnano, Pisa,
Italy; 43
◦
73
0
N, 10
◦
48
0
E). They were sown on rockwool plugs hosted in alveolar polystyrene
containers that were placed in a greenhouse with mist irrigation. Germination (approxi-
mately 85%) occurred within 20 days, and the seedlings were transplanted into a floating
system 73 days after sowing, when about five-six leaves were completely developed, and
the rootlets emerged at the bottom of the rockwool plugs (Figure 1). The experimental
setup consisted of polystyrene trays floating on 60-L plastic tanks with aerated nutrient
solution. Each tank hosted one tray with six plants and two tanks were arranged for each
treatment, with a plant density of 36 plants/m
2
. During the experiment, which was carried
out in spring from 30 March to 10 May 2017, the average values of air temperature, global
radiation, and relative humidity inside the greenhouse were 19.6
◦
C, 8.4 MJ/m
2·
day, and
65%, respectively. The nutrient solution was prepared using tap water and the proper
amounts of inorganic salts to obtain the following composition (mM): 10.0 N-NO
3−
, 1.0 P-
H
2
PO
4−
, 8.0 K, 4.5 Ca, 2.0 Mg, 1.7 Na, 4.0 S-SO
42−
, 3.0 Cl, 0.040 Fe, 0.025 B, 0.003 Cu,
0.010 Zn, 0.010 Mn, 0.001 Mo. The values of pH and EC were 5.5 and 2.4 dS/m, respectively.
Along with the control (1.7 mM NaCl), three saline treatments were applied by NaCl
addition to the nutrient solution up to the following concentrations: 25, 50, and 100 mM,
which corresponded to EC values of 4.6, 6.9, and 11.7 dS/m, respectively. To avoid osmotic
shock to the plants, the final NaCl concentration in the two latter treatments was reached
gradually, through a daily increase of 25 mM in NaCl concentration. During the growing
cycle, the nutrient solution was checked every 1–2 days for pH and EC; possible variations
were compensated by the addition of nitric acid or fresh nutrient solution.
Agronomy 2021,11, 2352 3 of 12
Agronomy 2021, 11, x FOR PEER REVIEW 3 of 13
the growing cycle, the nutrient solution was checked every 1–2 days for pH and EC; pos-
sible variations were compensated by the addition of nitric acid or fresh nutrient solution.
Figure 1. Seedlings of Reichardia picroides after 10 weeks from sowing, ready for transplanting into
floating system.
2.2. Growth Analysis
Four plants for each treatment (two plants from each tank) were sampled four and
six weeks after transplanting for the determination of the number of leaves and the fresh
(FW) and dry (DW) weight of both roots and leaves. For the latter parameter, fresh sam-
ples were dried in a ventilated oven at 70 °C until constant weight.
2.3. Leaf Sampling and Extraction
Three leaves were detached from each plant collected for the growth analysis. The
leaves were chosen among the first completely developed ones from the inner of the ro-
sette, were cut into pieces, and mixed to obtain one sample of about 1 g fresh weight (FW),
which was stored at −80 °C until analysis. Pure methanol was used as the extraction sol-
vent in all the determinations except those of total anthocyanins and flavonol glycosides,
which employed 80% methanol containing 1% hydrochloric acid. The leaf samples were
extracted twice with 5 mL aliquots of extraction solvent, using mortar and pestle. At each
extraction step, the tubes containing the extraction solvent and the pellet were sonicated
four times in an ice bath for 15 min, stored overnight at −20 °C, and centrifuged for 5 min
at 2700× g. For each sample, the supernatant aliquots were pooled and used for the spec-
trophotometric determination of the antioxidant capacity and the content of chlorophylls,
anthocyanins, flavonol glycosides, and total phenols [19]. A Lambda35 UV-vis spectro-
photometer (Perkin Elmer, Waltham, MA, USA) was used for all the absorbance readings,
and the results were expressed on a FW basis.
Figure 1.
Seedlings of Reichardia picroides after 10 weeks from sowing, ready for transplanting into
floating system.
2.2. Growth Analysis
Four plants for each treatment (two plants from each tank) were sampled four and six
weeks after transplanting for the determination of the number of leaves and the fresh (FW)
and dry (DW) weight of both roots and leaves. For the latter parameter, fresh samples were
dried in a ventilated oven at 70 ◦C until constant weight.
2.3. Leaf Sampling and Extraction
Three leaves were detached from each plant collected for the growth analysis. The
leaves were chosen among the first completely developed ones from the inner of the rosette,
were cut into pieces, and mixed to obtain one sample of about 1 g fresh weight (FW), which
was stored at
−
80
◦
C until analysis. Pure methanol was used as the extraction solvent in
all the determinations except those of total anthocyanins and flavonol glycosides, which
employed 80% methanol containing 1% hydrochloric acid. The leaf samples were extracted
twice with 5 mL aliquots of extraction solvent, using mortar and pestle. At each extraction
step, the tubes containing the extraction solvent and the pellet were sonicated four times in
an ice bath for 15 min, stored overnight at
−
20
◦
C, and centrifuged for 5 min at 2700
×
g.
For each sample, the supernatant aliquots were pooled and used for the spectrophotometric
determination of the antioxidant capacity and the content of chlorophylls, anthocyanins,
flavonol glycosides, and total phenols [
19
]. A Lambda35 UV-vis spectrophotometer (Perkin
Elmer, Waltham, MA, USA) was used for all the absorbance readings, and the results were
expressed on a FW basis.
2.4. Chlorophylls and Carotenoids
The extracts were diluted 1:10 with methanol, and the concentrations of the pigments
(g/kg FW) were calculated from absorbance readings at 665.2, 652.4, and 470 nm according
to Lichtentahler and Buschmann [21].
Agronomy 2021,11, 2352 4 of 12
2.5. Antocyanins and Flavonol Glycosides
The determination was performed according to Hrazdina et al. [
22
], after dilution
of the acidic extract as needed. The content of total anthocyanins was assessed by ab-
sorbance readings at 530 nm and expressed as mg cyanidin-3-glucoside/kg FW, using
the value 38,000 L/mol
·
cm for the molar absorptivity. The content of flavonol glyco-
sides was assessed at 360 nm and expressed as mg quercetin-3-glucoside/kg FW, using
20,000 L/mol·cm as the value of the molar absorptivity.
2.6. Total Phenols
The absorbance of 1:100 diluted methanol extract was read at 320 nm [
23
]. The results
were expressed as absorbance units of the pure extract per gram leaf tissue, A (320 nm)/g
FW. In addition, the Folin-Ciocalteu assay was carried out by mixing 100
µ
L methanol
extract with 2.0 mL distilled water, 300
µ
L Folin-Ciocalteu phenol reagent, and, after four
minutes, 1.6 mL of 7.5% sodium carbonate. The solutions were kept 2 h at room temperature
and the absorbance was measured at 765 nm [
23
]. Gallic acid standard solutions were used
for calibration, and the results were expressed as mg gallic acid/kg FW.
2.7. Antioxidant Capacity
Two distinct assays were used to determine the antioxidant capacity both as ferric
reducing antioxidant power (FRAP) [
24
] and as 2,2-diphenyl-1-picrylhydrazyl radical
scavenging activity (DPPH) [
25
]. In the former assay, acetate buffer at pH 3.6 (2.0 mL) was
mixed in a spectrophotometric cuvette with 900
µ
L FRAP reagent containing 2 mM ferric
chloride and 1 mM TPTZ (2,4,6-tris(2-pyridyl)-s-triazine), and 100
µ
L diluted 1:4 methanol
extract. The absorbance was read at 593 nm and compared with a calibration curve obtained
with standard solutions of ferrous ammonium sulphate. The results were expressed as
mmol Fe(II)/kg FW. For the DPPH assay, 30
µ
L methanol (blank) or methanolic extract
(sample) were added to 2.97 mL of 20 mg/L DPPH solution. After 45 min in the dark at
room temperature, the absorbance (A) was read at 515 nm, and the percentage inhibition
of the DPPH radical per gram fresh tissue was calculated as follows:
% Inhibition/g FW = 100 + [(Ablank−Asample )/Ablank]/g FW (1)
2.8. Nitrates
The spectrophotometric determination of nitrates was performed following
Cataldo et al. [
26
]. The assay was carried out on samples of dried powdered leaf tis-
sue (100 mg) that were extracted with 10 mL deionized water on an orbital shaker at
room temperature for two hours. The aqueous extract (70
µ
L) as mixed with 300
µ
L of
concentrated sulphuric acid containing 5% salicylic acid. After 20 min, 1.5 M NaOH (10 mL)
was added, and the solution was allowed to cool at room temperature for 20 min. The
absorbance was read at 410 nm, and the nitrate concentration was determined through a
standard calibration curve. The results were expressed as mg NO3−/kg FW.
2.9. Statistical Analysis
The data were subjected to two-way ANOVA, with the treatment (NaCl concentration)
and the sampling date as the sources of variation, and the Bonferroni post-test was used
for means separation. Both the linear regression analysis and the Principal Component
Analysis (PCA) were applied to the water content, the nutraceutical parameters, and the
nitrate content of the leaves. The Statgraphics Centurion Version 17 software (Statpoint
Technologies, Warrenton, VA, USA) was used for the statistical analyses.
3. Results
3.1. Plant Growth and Crop Yield
After four weeks from transplanting, the growth parameters decreased significantly
only in plants grown at 100 mM NaCl concentration, except the root biomass fresh and
Agronomy 2021,11, 2352 5 of 12
dry matter, which remained unchanged (Table 1). In contrast, two weeks later all the salt
treatments caused a strong reduction in both leaf area and leaf biomass (Table 1). With
100 mM NaCl, the root biomass was also affected, along with the dry matter content of the
leaves (Table 1). For all the growth parameters, the two-way ANOVA showed no significant
interaction between NaCl concentration in the nutrient solution and the sampling time
(Table 1). Obviously, the latter had a strong effect on biomass production, which was
significantly higher in older plants. Nevertheless, plant age did not affect leaf number
(Table 1). With NaCl concentrations up to 50 mM, the fresh yield of younger plants did not
significantly change and averaged 1.86 kg/m
2
(Figure 2, left); in contrast, crop yield was
significantly lower (
−
63% as compared with the control) at 100 mM NaCl concentration.
After six weeks from transplanting, the yield of control plants was 3.25 kg/m
2
and was
significantly lower at all the tested NaCl concentrations (Figure 2, left); the loss of leaf fresh
weight was approximately 26%, 37%, and 68% at 25, 50, and 100 mM NaCl, respectively.
Table 1.
Growth parameters for Reichardia picroides plants grown in floating system with different NaCl concentrations in
the nutrient solution and sampled at four and six weeks after transplanting.
Sampling
Time NaCl (mM) Leaf
Number
Leaf Biomass Root Biomass
FW (g) DW (g) FW (g) DW (g)
Four weeks
1.7 (Control) 76 ±18 a 60.79 ±18.99 a 4.932 ±0.857 a 10.84 ±1.35 a 0.910 ±0.113 a
25 68 ±16 a 47.94 ±18.12 a 4.541 ±0.956 a 13.13 ±1.26 a 1.030 ±0.229 a
50 77 ±11 a 46.14 ±5.73 ab 4.164 ±0.817 a 10.69 ±1.89 a 0.893 ±0.172 a
100 40 ±8 b 22.38 ±5.75 b 2.545 ±0.270 b 9.10 ±2.68 a 0.780 ±0.262 a
Six weeks
1.7 (Control) 85 ±18 a 84.43 ±14.70 a 8.697 ±1.549 a 15.64 ±2.76 a 1.560 ±0.165 a
25 79 ±14 ab 62.23 ±10.43 b
6.590
±
0.970 ab
17.06 ±3.47 a 1.570 ±0.300 a
50 71 ±8 ab 53.53 ±16.82 b 6.355 ±1.004 b 19.79 ±5.63 a 2.063 ±0.267 a
100 57 ±10 b 26.70 ±12.02 c 2.841 ±0.900 c 7.94 ±3.89 b 0.747 ±0.278 b
Main effects
Four weeks 65 a 44.31 b 4.046 b 10.94 b 0.903 b
Six weeks 73 a 56.72 a 5.977 a 15.11 a 1.395 a
1.7 (Control) 81 a 72.61 a 6.568 a 13.24 a 1.235 a
25 74 a 55.08 a 5.566 a 15.10 a 1.300 a
50 74 a 49.83 ab 5.259 a 15.24 a 1.299 a
100 49 b 24.54 b 2.693 b 8.52 b 0.763 b
2-way ANOVA
NaCl concentration ** *** *** *** *
Time ns * *** *** **
Interaction ns ns ns ns Ns
Mean values
±
standard deviation of four replicates. For each parameter and sampling time, different letters among the treatments indicate
significant difference at p< 0.05. FW: fresh weight; DW: dry weight. Asterisks denote statistical significance according to 2-way ANOVA:
***, significant at p< 0.001; **, significant at p< 0.01 *, significant at p< 0.05; ns, not significant.
3.2. Leaf Nitrate Content
The level of nitrates in the leaves was unaffected by sampling time, despite the
significant influence of salinity (Figure 2, right). On the other hand, a strong interaction
between the two factors was found for the nitrate content, similarly to the content of
carotenoids and unlike all the other parameters determined in this work. Moreover, an
extremely low linear correlation coefficient (0.05) was obtained between NaCl concentration
and the content of nitrate, which was in the range 1500–3200 mg NO
3−
/kg FW. Compared
with the control, the leaf nitrate content was significantly lower with the 25 mM treatment at
both sampling times and with 100 mM NaCl after six weeks of cultivation (
Figure 2
, right).
Agronomy 2021,11, 2352 6 of 12
Agronomy 2021, 11, x FOR PEER REVIEW 8 of 13
Four weeks Six weeks
0
1
2
3
4
a
a
a
b
ab
b
b
c
A***
B*
A x B ns
kg FW /m
2
Four weeks Six weeks
0
1000
2000
3000
4000
a
b
a
b
aa
a
b
A***
B***
A x B **
mg NO
3
/kg FW
100 mM NaCl50 mM NaCl25 mM NaClControl
Time after transplanting
Figure 2. Crop yield (left) and leaf content of nitrates (right) in Reichardia picroides plants grown in floating system with different
NaCl concentrations in the nutrient solution and sampled four and six weeks after transplanting. Mean values with standard devia-
tion of four replicates. For each sampling date, different letters indicate significant differences at p < 0.05. The results of two-way
ANOVA are reported in the right box: ***, significant at p < 0.001; **, significant at p < 0.01 *, significant at p < 0.05; ns, not significant.
FW: fresh weight.
Figure 3. Pearson’s coefficients for quality parameters of the fresh leaf tissues of Reichardia picroides plants grown in float-
ing system with different NaCl concentrations in the nutrient solution (1.7, control; 25; 50; 100 mM) and sampled four and
six weeks after transplanting. Four replicates were collected for each treatment and sampling time. Wat: water content;
NO
3−
: nitrates; Chl: total chlorophylls; Car: carotenoids; An: anthocyanins; FG: flavonol glycosides; TP: total phenols; PI:
phenol index; FRAP: ferric reducing antioxidant power; DPPH: 2,2‒diphenyl‒1‒picrylhydrazyl radical scavenging activ-
ity. * denotes statistical significance at p < 0.05.
Figure 2.
Crop yield (
left
) and leaf content of nitrates (
right
) in Reichardia picroides plants grown in floating system with
different NaCl concentrations in the nutrient solution and sampled four and six weeks after transplanting. Mean values with
standard deviation of four replicates. For each sampling date, different letters indicate significant differences at
p< 0.05
. The
results of two-way ANOVA are reported in the right box: ***, significant at p< 0.001; **, significant at p< 0.01 *, significant
at p< 0.05; ns, not significant. FW: fresh weight.
3.3. Antioxidant Capacity and Compounds
Among all the nutraceutical properties measured in the fresh leaf tissues, a significant
interaction between NaCl salinity and harvest time was found only for the content of
carotenoids (Table 2). Both NaCl concentration and harvest time did not influence the leaf
content of chlorophylls and carotenoids, while the antioxidant capacity and the contents
of total phenols, flavonol glycosides, and anthocyanins generally increased with salinity
at both sampling times (Table 2). The results for the content of total phenols showed that
the differences among the treatments were more pronounced for the Folin-Ciocalteu assay
than the phenol index, especially at the first sampling time (Table 2); however, the same
trend was observed in both parameters. Likewise, the results of the determination of the
antioxidant capacity showed a similar trend, although with the FRAP assay the differences
among the treatments were more evident than those observed with the DPPH assay,
particularly in younger plants (Table 2). The linear regression analysis showed significant
(p< 0.05) Pearson’s coefficients (>0.95) between the results of the FRAP and DPPH assays
that were used for the determination of the antioxidant capacity, and between the results
of the Folin-Ciocalteu assay and the phenol index that were used for the determination
of the content of total phenols (Figure 3). Moreover, both the antioxidant capacity and
the content of total phenols were significantly correlated with the content of flavonol
glycosides (Pearson’s coefficient > 0.86) and anthocyanins (Pearson’s coefficient > 0.67)
(Figure 3). According to the PCA, the principal components (PC1 and PC2) associated with
the highest eigenvalues in the scree plot (Figure 4A) explained 60.4% and 15.3% of the
total variance, respectively. The plot of component weights (Figure 4B) showed a strong
co-variance among total phenols, FRAP, flavonol glycosides, and phenol index, which were
correlated also with anthocyanins and DPPH. The six parameters contributed strongly
to PC1, while chlorophylls and carotenoids mainly contributed to PC2. Moreover, water
content was negatively correlated to PC1, and nitrate content was negatively correlated to
PC2. However, the scatterplot (Figure 4C) showed that the clusters that could be identified
across data points were not sufficiently separated for an effective characterization among
the treatments and sampling times.
Agronomy 2021,11, 2352 7 of 12
Table 2.
Water content, amounts of total chlorophylls, flavonol glycosides and total phenols, and antioxidant capacity of leaf tissues from Reichardia picroides plants grown in floating
system with different NaCl concentrations in the nutrient solution and sampled four and six weeks after transplanting.
Sampling
Time NaCl (m) Water Content
Total
Chlorophylls
(mg/kg FW)
Carotenoids
(mg/kg FW)
Anthocyanins
(mg Cy-3-glu/kg
FW)
Flavonol
Glycosides
(mg Qu-3-glu/
kg FW)
Total Phenols Antioxidant Capacity
Folin-Ciocalteu
(mg GAE/
kg FW)
Phenol Index
(A320/g FW)
FRAP
(mmol Fe(II)/
kg FW)
DPPH (%
Inhibition/
g FW)
Four weeks
1.7 (Control) 0.922 ±0.010 a 390 ±130 a 86.8 ±26.6 b 21.69 ±2.46 a 698 ±81 b 1167 ±0.301 b 7.22 ±1.29 a 7.28 ±1.93 c 24.57 ±3.48 a
25 0.916 ±0.012 a 383 ±102 a 92.6 ±34.7 b 22.28 ±4.42 a 703 ±185 b 1314 ±0.174 ab 7.28 ±1.50 a 9.14 ±1.49 bc 25.22 ±2.48 a
50 0.922 ±0.007 a 427 ±39 a 139.7 ±83.6 a 27.01 ±3.64 a 1240 ±248 a 1823 ±0.308 a 12.04 ±1.50 a 12.75 ±1.79 ab 33.56 ±8.00 a
100 0.914 ±0.012 a 325 ±70 a 103.6 ±14.0 ab 23.78 ±2.91 a 1182 ±230 a 1836 ±0.329 a 10.27 ±1.92 a 13.50 ±2.70 a 35.84 ±6.55 a
Six weeks
1.7 (Control) 0.897 ±0.003 a 394 ±91 a 109.2 ±22.3 ab 24.71 ±5.21 b 870 ±208 a 1419 ±213 b 8.53 ±1.87 b 10.38 ±1.82 b 30.06 ±4.82 b
25 0.894 ±0.004 a 346 ±36 a 127.4 ±34.3 a 40.15 ±8.42 a 1342 ±277 a 1995 ±420 ab 12.66 ±3.53 ab 14.68 ±3.22 a 42.58 ±8.12 ab
50 0.891 ±0.019 a 395 ±111 a 98.0 ±22.8 ab 39.67 ±6.75 a 1275 ±615 a 1959 ±685 ab 12.88 ±4.56 ab 14.45 ±3.93 a 42.80 ±11.58 a
100 0.873 ±0.015 a 367 ±77 a 78.7 ±6.3 b 38.84 ±5.83 a 1664 ±633 a 2296 ±664 a 15.28 ±5.10 a 16.25 ±3.94 a 47.82 ±10.08 a
Main effects
Four weeks 0.919 a 381 a 23.69 b 956 b 1535 b 9.20 b 10.67 b 29.80 b
Six weeks 0.889 b 376 a 35.84 a 1288 a 1917 a 12.34 a 13.94 a 40.82 a
1.7 (Control) 0.909 a 392 a 23.20 b 784 b 1293 b 7.87 b 8.83 b 27.32 b
25 0.905 a 365 a 31.21 ab 1022 ab 1655 ab 9.97 ab 11.91 ab 33.90 ab
50 0.907 a 411 a 33.34 ab 1258 ab 1891 ab 12.46 ab 13.60 a 38.18 ab
100 0.894 a 346 a 31.31 a 1423 a 2066 a 12.78 a 14.87 a 41.83 a
2-way ANOVA
NaCl concentration ns ns ns ** * ** * ** **
Time *** ns ns *** * * ** ** ***
Interaction ns ns * ns Ns ns ns ns ns
Mean values
±
standard deviation of four replicates. For each parameter and sampling time, different letters among the treatments indicate significant difference at p< 0.05. Cy-3-glu: cyanidine-3-gucoside;
Qu-3-glu: quercetin-3-glucoside; GAE: gallic acid equivalents; FW: fresh weight. Asterisks denote statistical significance according to 2-way ANOVA: ***, significant at p< 0.001; **, significant at p< 0.01 *,
significant at p< 0.05; ns, not significant.
Agronomy 2021,11, 2352 8 of 12
Agronomy 2021, 11, x FOR PEER REVIEW 8 of 13
Four weeks Six weeks
0
1
2
3
4
a
a
a
b
ab
b
b
c
A***
B*
A x B ns
kg FW /m
2
Four weeks Six weeks
0
1000
2000
3000
4000
a
b
a
b
aa
a
b
A***
B***
A x B **
mg NO
3
/kg FW
100 mM NaCl50 mM NaCl25 mM NaClControl
Time after transplanting
Figure 2. Crop yield (left) and leaf content of nitrates (right) in Reichardia picroides plants grown in floating system with different
NaCl concentrations in the nutrient solution and sampled four and six weeks after transplanting. Mean values with standard devia-
tion of four replicates. For each sampling date, different letters indicate significant differences at p < 0.05. The results of two-way
ANOVA are reported in the right box: ***, significant at p < 0.001; **, significant at p < 0.01 *, significant at p < 0.05; ns, not significant.
FW: fresh weight.
Figure 3. Pearson’s coefficients for quality parameters of the fresh leaf tissues of Reichardia picroides plants grown in float-
ing system with different NaCl concentrations in the nutrient solution (1.7, control; 25; 50; 100 mM) and sampled four and
six weeks after transplanting. Four replicates were collected for each treatment and sampling time. Wat: water content;
NO
3−
: nitrates; Chl: total chlorophylls; Car: carotenoids; An: anthocyanins; FG: flavonol glycosides; TP: total phenols; PI:
phenol index; FRAP: ferric reducing antioxidant power; DPPH: 2,2‒diphenyl‒1‒picrylhydrazyl radical scavenging activ-
ity. * denotes statistical significance at p < 0.05.
Figure 3.
Pearson’s coefficients for quality parameters of the fresh leaf tissues of Reichardia picroides
plants grown in floating system with different NaCl concentrations in the nutrient solution (1.7,
control; 25; 50; 100 mM) and sampled four and six weeks after transplanting. Four replicates were
collected for each treatment and sampling time. Wat: water content; NO
3−
: nitrates; Chl: total
chlorophylls; Car: carotenoids; An: anthocyanins; FG: flavonol glycosides; TP: total phenols; PI:
phenol index; FRAP: ferric reducing antioxidant power; DPPH: 2,2-diphenyl-1-picrylhydrazyl radical
scavenging activity. * denotes statistical significance at p< 0.05.
Agronomy 2021, 11, x FOR PEER REVIEW 9 of 13
0 5 10
0
2
4
6
A
Principal Component
Eigenvalue
-0.4 -0.2 0.0 0.2 0.4 0.6
-0.5
0.0
0.5
1.0
An
FG
Chl
Car
TP
PI
FRAP
DPPH
Wat
NO3
B
PC1
PC2
-5 0 5 10
-2
0
2
4
Control
25 mM NaCl
50 mM NaCl
100 mM NaCl
C
PC1
PC2
Figure 4. Principal Component Analysis (PCA) for quality parameters of fresh leaf tissues of Reichardia picroides plants
grown in floating system with different NaCl concentrations in the nutrient solution (1.7, control; 25; 50; 100 mM) and
sampled four and six weeks after transplanting. (A): scree plot; (B): plot of component weights (water content, Wat; total
chlorophylls, Chl; carotenoids, Car; flavonol glycosides, FG; total phenols, TP; phenol index, PI; ferric reducing antioxidant
power, FRAP; 2,2‒diphenyl‒1‒picrylhydrazyl radical scavenging activity, DPPH; anthocyanins, An; nitrates, NO3); (C):
scatterplot of data obtained after the first (large symbols) and second (small symbols) sampling.
4. Discussion
4.1. Plant Growth and Crop Yield
Salt stress can limit the root uptake of both water and nutrients and impair plant
water relations and leaf photosynthesis [5]. Plant response to salinity depends on plant
genotype, developmental stage, growing conditions, the level of salinity in the root zone,
and the duration of the exposure to stress conditions [27,28]. In our study, the detrimental
effect of salinity was more severe in th e leaves tha n in the roots, and in s ix-week-old plants
than in younger ones. In fact, after four weeks from transplanting, only 100 mM NaCl
caused a significant decrease in the leaf biomass production, whereas root growth was
unaffected. In contrast, in older plants, a significant growth reduction was already ob-
served with 25 mM NaCl, and this outcome became more evident at higher salt concen-
trations; with 100 mM NaCl the root tissues were also affected, suggesting that, in the
conditions tested in this work, R. picroides is sensitive to salinity after six weeks treatment.
However, the response of this species to salinity may depend on environmental and phys-
iological factors, including growing period and plant age. For example, after nine weeks
of hydroponic culture during winter (January-March), Alexopoulos et al. [20] reported for
R. picroides a fresh yield of 1.12 kg FW/m2 yield; this value was lower compared with our
results (2.19 and 3.25 kg FW/m2 for four- and six-week-old control plants, respectively;
Figure 2) and did not change significantly when the plants were grown at moderate salin-
ity (6 dS/m). Even lower yield values (1.01 kg FW/m2) were found by the same authors in
control plants of hydroponically grown Taraxacum officinale [20], which is another herb in
Figure 4.
Principal Component Analysis (PCA) for quality parameters of fresh leaf tissues of Reichardia picroides plants grown
in floating system with different NaCl concentrations in the nutrient solution (1.7, control; 25; 50; 100 mM) and sampled four
and six weeks after transplanting. (
A
): scree plot; (
B
): plot of component weights (water content, Wat; total chlorophylls,
Chl; carotenoids, Car; flavonol glycosides, FG; total phenols, TP; phenol index, PI; ferric reducing antioxidant power, FRAP;
2,2-diphenyl-1-picrylhydrazyl radical scavenging activity, DPPH; anthocyanins, An; nitrates, NO
3
); (
C
): scatterplot of data
obtained after the first (large symbols) and second (small symbols) sampling.
Agronomy 2021,11, 2352 9 of 12
4. Discussion
4.1. Plant Growth and Crop Yield
Salt stress can limit the root uptake of both water and nutrients and impair plant water
relations and leaf photosynthesis [
5
]. Plant response to salinity depends on plant genotype,
developmental stage, growing conditions, the level of salinity in the root zone, and the
duration of the exposure to stress conditions [
27
,
28
]. In our study, the detrimental effect of
salinity was more severe in the leaves than in the roots, and in six-week-old plants than in
younger ones. In fact, after four weeks from transplanting, only 100 mM NaCl caused a
significant decrease in the leaf biomass production, whereas root growth was unaffected.
In contrast, in older plants, a significant growth reduction was already observed with
25 mM NaCl, and this outcome became more evident at higher salt concentrations; with
100 mM NaCl the root tissues were also affected, suggesting that, in the conditions tested
in this work, R. picroides is sensitive to salinity after six weeks treatment. However, the
response of this species to salinity may depend on environmental and physiological factors,
including growing period and plant age. For example, after nine weeks of hydroponic
culture during winter (January-March), Alexopoulos et al. [
20
] reported for R. picroides a
fresh yield of 1.12 kg FW/m
2
yield; this value was lower compared with our results (2.19
and 3.25 kg FW/m
2
for four- and six-week-old control plants, respectively; Figure 2) and
did not change significantly when the plants were grown at moderate salinity (6 dS/m).
Even lower yield values (1.01 kg FW/m
2
) were found by the same authors in control plants
of hydroponically grown Taraxacum officinale [
20
], which is another herb in the Asteraceae
with great adaptation to hydroponic cultivation [
29
–
31
]. The high yield that was obtained
in this work for R. picroides (Figure 2) showed that plant growth in the floating system was
very fast and demonstrated the suitability of this species for the hydroponic production of
baby leaves. Despite a severe biomass reduction at high salinity, the yield obtained with
100 mM NaCl (0.81 and 0.96 kg FW/m
2
for four- and six-week-old plants, respectively;
Figure 2) was comparable to that of unstressed hydroponically grown T. officinale [20].
4.2. Leaf Nitrate Content
Generally, salinity causes a decrease in the nitrate content of the leaves, which is
attributed to impaired root functionality and reduced capacity to absorb water and nutri-
ents and, in the case of salinity by NaCl, also to the antagonistic uptake of chloride and
nitrate [
32
]. In our work, this effect was evidenced under mild salinity conditions (25 mM
NaCl) at both sampling times, while at higher salinity a significant decrease of nitrate
content was observed only with 100 mM NaCl after six weeks of cultivation (Figure 2). On
the other hand, at both sampling times the leaf nitrate content in plants grown with 50 mM
NaCl did not significantly differ from the levels found in the control, despite a significant
decrease of leaf biomass (Figure 2), indicating that, at this salinity level, the root uptake
of nitrate ion was reduced and, at the same time, the mobilization of vacuolar nitrate as
a nitrogen source was not effective in sustaining plant growth. In contrast, in T. officinale
the decrease of fresh yield caused by salinity was associated with a significant decrease
in nitrate content, which was lower than 1000 mg/kg FW in salt-treated plants [
20
]. This
suggests that the ability to absorb nitrate ions could be impaired by salinity to a greater
extent in salt-sensitive species than in more tolerant ones. High nitrate content in leafy
vegetables is known to represent a risk for human health [
33
], being associated with gastric
cancer [
34
] and, in order to preserve food safety and public health, the European Union
has set limits to the nitrate content of leafy vegetables. For example, the maximum level
allowed is 3500 mg NO
3
/kg FW for fresh spinach, 4000 or 5000 mg NO
3
/kg FW, depending
on the season, for greenhouse lettuce [
35
]. These limits are much higher than those found
in this work for R. picroides; however, the issue of nitrate reduction should be considered
as a key factor in the development of suitable growing protocols for edible greens. In
that sense, nitrate accumulation in hydroponically grown crops can be decreased by an
increase of the NH
4+
/NO
3−
ratio in the nutrient solution, or by eliminating nitrates from
the nutrient solution a few days prior to harvest [36].
Agronomy 2021,11, 2352 10 of 12
4.3. Antioxidant Capacity and Compounds
Both the antioxidant capacity and the content of phenolic compounds in R. picroides
grown in a floating system were lower than in spontaneous or pot-grown plants [
19
];
particularly, the values of the two parameters reported for wild plants were 34.5 mmol
Fe(II)/kg FW and 4320 mg GAE/kg FW, respectively. In addition, the average content of
total phenols obtained by Savo et al. [
37
] for R. picroides was 22.4 mg GAE/g DW against
14.4 mg GAE/g DW found in the control plants in our experiment. However, in this study,
the addition of NaCl to the nutrient solution was effective in stimulating the synthesis
and accumulation of phenolic compounds with antioxidant activity. In contrast, previous
work from our laboratory had evidenced that a significant increase in the content of
phenolic compounds could not be obtained in R. picroides by application of a NaCl solution
through the foliar spray to simulate marine aerosol [
19
]. Therefore, the present study
shows that the occurrence of chronic stress through the root system, rather than sudden
stress through foliar treatment, is necessary to elicit an adequate physiological response
in this species. This outcome also shows the suitability of the hydroponic technique for
the growth of baby leaves of improved nutraceutical quality, due to the possibility of
optimal management of the salinity of the nutrient solution. A different behaviour has
been reported for hydroponically grown T. officinale; in this species, a salt-induced growth
reduction was not accompanied by an increase in the content of phenolic compounds,
which remained below 1000 mg GAE/kg FW [20].
The antioxidant capacity, as assessed through two distinct assays, and the con-
tent of total phenols showed a similar increasing trend and were strongly correlated
(
Figures 3and 4B
) in agreement with previous findings [
37
]. This suggests a direct involve-
ment of phenolic compounds in the antioxidant response of R. picroides to salinity stress.
Moreover, the high correlation of both parameters with the content of flavonol glycosides
and, albeit to a lesser extent, anthocyanins (Figure 3), indicated the importance of both
classes of bioactive molecules within the pool of phenolic compounds of this species. The
synthesis of bioactive molecules such as phenolic compounds is commonly involved in
plant adaptation to stress conditions [
32
]. In R. picroides grown in hydroponics, the leaf
content of total phenolics, chlorophylls (a, b, and total), and carotenoids increased when
the pH of the nutrient solution was kept at 4.0 instead of 5.5 or 7.0 [
38
]. In our study,
NaCl salinity did not influence the leaf content of photosynthetic pigments (Table 2), and
the content of carotenoids was not correlated to NaCl concentration under the conditions
tested in this work (Figure 3).
As several quality parameters were investigated in this study, the data were subjected
to both regression analysis and PCA (Figures 3and 4). The results showed that the latter
was not effective in the identification of clusters among data points, which were not
sufficiently separated to provide adequate discrimination across treatments and sampling
times. However, both statistical analyses showed the relationships among the variables,
highlighting positive correlations that involved different classes of bioactive molecules
5. Conclusions
The data supported our hypothesis that abiotic stress such as salinity could enhance
the nutraceutical qualities of the leaf tissues in R. picroides. However, although with the
floating system abiotic stress could be easily applied through a simple modification in
the composition of the nutrient solution, the observed effect was only a limited increase
in the antioxidant capacity and the level of phenolic compounds. At the same time, a
significant decrease in the nitrate content was not observed in all the saline treatments. On
the other hand, although the bioactive properties typical of the plants at the spontaneous
state were only partially retained in cultivation, the hydroponically grown plants could
still ensure satisfactory levels of beneficial compounds, especially in comparison with other
medicinal plants in the Asteraceae family, such as T. officinale. In this study, a four-week
growing period in a floating system with 50 mM NaCl in the nutrient solution increased the
leaf content of bioactive molecules without affecting biomass production, while a severe
Agronomy 2021,11, 2352 11 of 12
yield reduction caused by salinity was observed with 100 mM NaCl after six weeks from
transplanting. However, in these conditions, the quality of the edible parts was improved
in comparison with the control, as the leaf tissues contained higher levels of bioactive
molecules along with lower amounts of nitrate ions. Despite further studies are necessary
to make the production of spontaneous species sustainable and cost-effective, the findings
of this work can open a perspective for short-cycle (four weeks) production of baby leaves
for ready-to-eat mixtures, expanding the market offer, favouring agrobiodiversity, and
contributing at the same time to the recovery of ethnobotanical traditions.
Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10
.3390/agronomy11112352/s1.
Author Contributions:
Conceptualization, S.B., L.I. and A.P.; Data curation, F.L.; Formal analysis,
R.M.; Investigation, R.M. and F.L.; Methodology, R.M. and L.I.; Project administration, A.P.; Resources,
S.B. and A.P.; Supervision, A.P.; Validation, S.B. and L.I.; Visualization, R.M.; Writing—original draft,
R.M.; Writing—review & editing, R.M., S.B., F.L., L.I. and A.P. All authors have read and agreed to
the published version of the manuscript.
Funding:
This research was co-founded by the ERBAVOLANT project (Rural Development policy
2014–2020-Measure 16.2: Support to the Operational Groups of agricultural European Innovation
Partnership (EIP-AGRI)).
Data Availability Statement:
The data presented in this study are available as Supplementary Material.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Saysel, A.K.; Barlas, Y. A dynamic model of salinization on irrigated lands. Ecol. Model. 2001,139, 177–199. [CrossRef]
2.
Machado, R.M.A.; Serralheiro, R.P. Soil salinity: Effect on vegetable crop growth. Management practices to prevent and mitigate
soil salinization. Horticulturae 2017,3, 30. [CrossRef]
3.
Reis, M.; Coelho, L.; Santos, G.; Kienle, U.; Beltrão, J. Yield response of stevia (Stevia rebaudiana Bertoni) to the salinity of irrigation
water. Agric. Water Manag. 2015,152, 217–221. [CrossRef]
4.
Lenzi, A.; Orlandini, A.; Bulgari, R.; Ferrante, A.; Bruschi, P. Antioxidant and mineral composition of three wild leafy species: A
comparison between microgreens and baby greens. Foods 2019,8, 487. [CrossRef]
5.
Isayenkov, S.V.; Maathuis, F.J.M. Plant salinity stress: Many unanswered questions remain. Front. Plant Sci.
2019
,10, 80.
[CrossRef] [PubMed]
6.
Mishra, P.; Mishra, J.; Arora, N.K. Plant growth promoting bacteria for combating salinity stress in plants—recent developments
and prospects: A review. Microbiol. Res. 2021,252, 126861. [CrossRef] [PubMed]
7.
Rouphael, Y.; Kyriacou, M.C. Enhancing quality of fresh vegetables through salinity eustress and biofortification applications
facilitated by soilless cultivation. Front. Plant Sci. 2018,9, 1–6. [CrossRef]
8.
Prohens, J.; Burruezo, A.R.; Nuez, F. New crops: An alternative for the development of horticulture. J. Food Agric. Env.
2003
,1(1),
75–79.
9.
Garcia-Oliveira, P.; Barral, M.; Carpena, M.; Gullón, P.; Fraga-Corral, M.; Otero, P.; Prieto, M.A.; Simal-Gandara, J. Traditional
plants from Asteraceae family as potential candidates for functional food industry. Food Funct.
2021
,12, 2850–2873. [CrossRef]
[PubMed]
10.
Matsuura, H.N.; Rau, M.R.; Fett-Neto, A.G. Oxidative stress and production of bioactive monoterpene indole alkaloids: Biotech-
nological implications. Biotechnol. Lett. 2014,36, 191–200. [CrossRef]
11.
Soares, S.; Kohl, S.; Thalmann, S.; Mateus, N.; Meyerhof, W.; De Freitas, V. Different phenolic compounds activate distinct human
bitter taste receptors. J. Agric. Food Chem. 2013,61, 1525–1533. [CrossRef]
12.
Ghirardini, M.P.; Carli, M.; del Vecchio, N.; Rovati, A.; Cova, O.; Valigi, F.; Agnetti, G.; Macconi, M.; Adamo, D.; Traina, M.; et al.
The importance of a taste. A comparative study on wild food plant consumption in twenty-one local communities in Italy. J.
Ethnobiol. Ethnomed. 2007,3, 1–14. [CrossRef] [PubMed]
13.
Vecchio, R.; Cavallo, C.; Cicia, G.; Del Giudice, T. Are (All) Consumers Averse to Bitter Taste? Nutrients
2019
,11, 323. [CrossRef]
[PubMed]
14.
Hall, M.; Jobling, J.; Rogers, G. Some perspectives on rocket as a vegetable crop: A review. Veg. Crop. Res. Bull.
2012
,76, 21–41.
[CrossRef]
15.
Long, C.L.; Li, H.; Ouyang, Z.; Yang, X.; Li, Q.; Trangmar, B. Strategies for agrobiodiversity conservation and promotion: A case
from Yunnan, China. Biodivers. Conserv. 2003,12, 1145–1156. [CrossRef]
16.
Giménez, A.; Fernández, J.A.; Pascual, J.A.; Ros, M.; López-Serrano, M.; Egea-Gilabert, C. An agroindustrial compost as alternative
to peat for production of baby leaf red lettuce in a floating system. Sci. Hortic. 2019,246, 907–915. [CrossRef]
Agronomy 2021,11, 2352 12 of 12
17.
Ceccanti, C.; Landi, M.; Incrocci, L.; Pardossi, A.; Venturi, F.; Taglieri, I.; Ferroni, G.; Guidi, L. Comparison of three domestications
and wild-harvested plants for nutraceutical properties and sensory profiles in five wild edible herbs: Is domestication possible?
Foods 2020,9, 1065. [CrossRef]
18.
Sharma, N.; Acharya, S.; Kumar, K.; Singh, N.; Chaurasia, O.P. Hydroponics as an advanced technique for vegetable production:
An overview. J. Soil Water Conserv. 2018,17, 364. [CrossRef]
19.
Maggini, R.; Benvenuti, S.; Leoni, F.; Pardossi, A. Terracrepolo (Reichardia picroides (L.) Roth.): Wild food or new horticultural
crop? Sci. Hortic. 2018,240, 224–231. [CrossRef]
20.
Alexopoulos, A.A.; Assimakopoulou, A.; Panagopoulos, P.; Bakea, M.; Vidalis, N.; Karapanos, I.C.; Petropoulos, S.A. Impact of
salinity on the growth and chemical composition of two underutilized wild edible greens: Taraxacum officinale and Reichardia
picroides.Horticulturae 2021,7, 160. [CrossRef]
21.
Lichtentahler, H.K.; Buschmann, C. Chlorophylls and carotenoids; Measurement and characterization by UV-VIS spectroscopy.
Curr. Protoc. Food Anal. Chem. 2001,1, F4-3. [CrossRef]
22.
Hrazdina, G.; Marx, G.A.; Hoch, H.C. Distribution of secondary plant metabolites and their biosynthetic enzymes in pea (Pisum
sativum L.) leaves—anthocyanins and flavonol glycosides. Plant Physiol. 1982,70, 745–748. [CrossRef]
23.
Kang, H.M.; Saltveit, M.E. Antioxidant capacity of lettuce leaf tissue increases after wounding. J. Agr. Food Chem.
2002
,50,
7536–7541. [CrossRef] [PubMed]
24.
Benzie, I.F.F.; Strain, J.J. The ferric reducing ability of plasma (FRAP) as a measure of “Antioxidant power”: The FRAP assay. Anal.
Biochem. 1996,239, 70–76. [CrossRef]
25.
Dudonné, S.; Vitrac, X.; Coutière, P.; Woillez, M.; Mérillon, J.M. Comparative study of antioxidant properties and total phenolic
content of 30 plant extracts of industrial interest using DPPH, ABTS, FRAP, SOD, and ORAC assays. J. Agric. Food Chem.
2009
,57,
1768–1774. [CrossRef] [PubMed]
26.
Cataldo, D.A.; Haroon, M.H.; Schrader, L.E.; Youngs, V.L. Rapid colorimetric determination of nitrate in plant tissue by nitration
of salicylic acid. Commun. Soil Sci. Plant Anal. 1975,6, 71–80. [CrossRef]
27.
Gupta, B.; Huang, B. Mechanism of Salinity Tolerance in Plants: Physiological, Biochemical, and Molecular Characterization. Int.
J. Genom. 2014,2014, 701596. [CrossRef] [PubMed]
28.
Ma, Y.; Dias, M.C.; Freitas, H. Drought and Salinity Stress Responses and Microbe-Induced Tolerance in Plants. Front. Plant Sci.
2020,11, 1750. [CrossRef]
29.
Letchamo, W.; Gosselin, A. Root and shoot growth and chlorophyll content of Taraxacum officinale provenances as affected by
defoliation and debudding under organic and hydroponic cultivation. J. Hortic. Sci. 1995,70, 279–285. [CrossRef]
30.
Dorais, M.; Papadopoulos, A.P.; Luo, X.; Leonhart, S.; Gosselin, A.; Pedneault, K.; Angers, P.; Gaudreau, L. Soilless greenhouse
production of medicinal plants in north Eastern Canada. Acta Hortic. 2001,554, 297–303. [CrossRef]
31.
Léonhart, S.; Pedneault, K.; Gosselin, A.; Angers, P.; Papadopoulos, A.P.; Dorais, M. Diversification of greenhouse crop production
under supplemental lighting by the use of new cultures with high economic potential. Acta Hortic. 2002,580. [CrossRef]
32.
Rouphael, Y.; Petropoulos, S.A.; Cardarelli, M.; Colla, G. Salinity as eustressor for enhancing quality of vegetables. Sci. Hortic.
2018,234, 361–369. [CrossRef]
33.
Karwowska, M.; Kononiuk, A. Nitrates/nitrites in food—risk for nitrosative stress and benefits. Antioxidants
2020
,9, 241.
[CrossRef]
34.
Ahluwalia, A.; Gladwin, M.; Coleman, G.D.; Hord, N.; Howard, G.; Kim-Shapiro, D.B.; Lajous, M.; Larsen, F.J.; Lefer, D.J.;
McClure, L.A.; et al. Dietary Nitrate and the Epidemiology of Cardiovascular Disease: Report from a National Heart, Lung, and
Blood Institute Workshop. J. Am. Heart Assoc. 2016,5, e003402. [CrossRef] [PubMed]
35. The European Commission. Commission Regulation (EU) No. 1258/2011 amending Regulation (EC) No. 1881/2006 as regards
maximum levels for nitrates in food stuffs. O. J. Eur. Union 2011,L320, 15–17.
36.
Santamaria, P.; Gonnella, M.; Elia, A.; Parente, A.; Serio, F. Ways of reducing rocket salad nitrate content. Acta Hortic.
2001
,548,
529–536. [CrossRef]
37.
Savo, V.; Salomone, F.; Mattoni, E.; Tofani, D.; Caneva, G. Traditional Salads and Soups with Wild Plants as a Source of
Antioxidants: A Comparative Chemical Analysis of Five Species Growing in Central Italy. Evid.-Based Complementary Altern. Med.
2019,2019, 1–12. [CrossRef]
38.
Alexopoulos, A.A.; Marandos, E.; Assimakopoulou, A.; Vidalis, N.; Petropoulos, S.A.; Karapanos, I.C. Effect of nutrient solution
pH on the growth, yield and quality of Taraxacum officinale and Reichardia picroides in a floating hydroponic system. Agronomy
2021,11, 1118. [CrossRef]
