Gas exchanges and water use efficiency in the selection of tomato genotypes tolerant to water stress

Abstract

Water stress can affect the yield in tomato crops and, despite this, there are few types of research aiming to select tomato genotypes resistant to the water stress using physiological parameters. This experiment aimed to study the variables that are related to the gas exchanges and the efficiency in water use, in the selection of tomato genotypes tolerant to water stress. It was done in a greenhouse, measuring 7 x 21 m, in a randomized complete block design, with four replications (blocks), being five genotypes in the F2BC1 generation, which were previously obtained from an interspecific cross between Solanum pennellii versus S. lycopersicum and three check treatments, two susceptible [UFU-22 (pre-commercial line) and cultivar Santa Clara] and one resistant (S. pennellii). At the beginning of flowering, the plants were submitted to a water stress condition, through irrigation suspension. After that CO2 assimilation, internal CO2, stomatal conductance, transpiration, leaf temperature, instantaneous water use efficiency, intrinsic efficiency of water use, instantaneous carboxylation efficiency, chlorophyll a and b, and the potential leaf water (Ψf) were observed. Almost all variables that were analyzed, except CO2 assimilation and instantaneous carboxylation efficiency, demonstrated the superiority of the wild accession, S. pennellii, concerning the susceptible check treatments. The high photosynthetic rate and the low stomatal conductance and transpiration, presented by the UFU22/ F2BC1#2 population, allowed a better water use efficiency. Because of that, these physiological characteristics are promising in the selection of tomato genotypes tolerant to water stress.

Full text

Genetics and Molecular Research 16 (2): gmr16029685
Gas exchanges and water use efciency in the
selection of tomato genotypes tolerant to water
stress
M.E.A. Borba, G.M. Maciel, E.F. Fraga Júnior, C.S. Machado Júnior,
G.R. Marquez, I.G. Silva and R.S. Almeida
Instituto de Ciências Agrárias, Universidade Federal de Uberlândia,
Campus Monte Carmelo, Uberlândia, MG, Brasil
Corresponding author: G.R. Marquez
E-mail: grepeza@gmail.com
Genet. Mol. Res. 16 (2): gmr16029685
Received March 24, 2017
Accepted May 5, 2017
Published June 20, 2017
DOI http://dx.doi.org/10.4238/gmr16029685
Copyright © 2017 The Authors. This is an open-access article distributed under the terms of
the Creative Commons Attribution ShareAlike (CC BY-SA) 4.0 License.
ABSTRACT. Water stress can affect the yield in tomato crops and,
despite this, there are few types of research aiming to select tomato
genotypes resistant to the water stress using physiological parameters.
This experiment aimed to study the variables that are related to the
gas exchanges and the efciency in water use, in the selection of
tomato genotypes tolerant to water stress. It was done in a greenhouse,
measuring 7 x 21 m, in a randomized complete block design, with four
replications (blocks), being ve genotypes in the F2BC1 generation,
which were previously obtained from an interspecic cross between
Solanum pennellii versus S. lycopersicum and three check treatments,
two susceptible [UFU-22 (pre-commercial line) and cultivar Santa
Clara] and one resistant (S. pennellii). At the beginning of owering,
the plants were submitted to a water stress condition, through irrigation
suspension. After that CO2 assimilation, internal CO2, stomatal
conductance, transpiration, leaf temperature, instantaneous water use
efciency, intrinsic efciency of water use, instantaneous carboxylation

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M.E.A. Borba et al.
Genetics and Molecular Research 16 (2): gmr16029685
efciency, chlorophyll a and b, and the potential leaf water (Ψf)
were observed. Almost all variables that were analyzed, except CO2
assimilation and instantaneous carboxylation efciency, demonstrated
the superiority of the wild accession, S. pennellii, concerning the
susceptible check treatments. The high photosynthetic rate and the
low stomatal conductance and transpiration, presented by the UFU22/
F2BC1#2 population, allowed a better water use efciency. Because of
that, these physiological characteristics are promising in the selection
of tomato genotypes tolerant to water stress.
Key words: Abiotic stress; Drought tolerance; Tomato; Wild accession
INTRODUCTION
The tomato cropping has an economical and social relevance in Brazil and all over the
world. The South American country is among the top 10 tomato producers (AGRIANUAL,
2016), evidencing the relevance of the crop. The tomato [Solanum lycopersicum (L.)] adapts to
different climatic conditions (Neves et al., 2013), and despite this, the abiotic stress occurrence,
like water stress, is one of the biggest problems that the crop presents (Morales et al., 2015b).
Predominantly, in the arid and semi-arid regions, the conicts for the water use are
a limiting factor. However, it is also observed in regions that have abundant water resources
but are incapable to supply the high demand (Telles and Costa, 2010). Beyond that, crops
that require a large irrigation system present elevated energy costs. In this sense, developing
genotypes that are tolerant to water stress has become a low-cost and more efcient strategy
in regions with water decit (Girotto et al., 2012).
The process of tomato domestication caused a shortage of genotypes that are tolerant
to water stress (Alvarenga, 2013). However, there are wild species that have a better water use
efciency than the commercial cultivars. The wild accession LA-716 (S. pennellii) is one of
them and, although it does not present good agronomic characteristics, the wild species uses
water more efciently than the cultivated plants (Rocha et al., 2016). With the plant breeding,
it is possible the rescue of important agronomic characteristics, through an interspecic cross
followed by backcrossings with a background of interest (Maciel et al., 2011).
Direct methods, used for selection genotypes tolerant to water stress, are expensive and
demand more time, which makes the indirect methods more viable. Many types of research have
proven the efciency of genotype selection from different techniques, like visual symptoms,
soil water balance, the ow of sap, and gas exchange analyses (Aksic et al., 2011; Celebi, 2014;
Begun et al., 2012; Rocha et al., 2016). However, the use of these techniques is scarce.
Because of that, this experiment aimed to study the variables related to gas exchange
and the water use efciency in the selection of tomato genotypes tolerant to water stress.
MATERIAL AND METHODS
The experiment was conducted at the Experimental Horticulture Station
(18°42.0'43.19''S and 47°29.0'55.8''W, 873 m above sea level) and at the Genetic Resources
Laboratory of the Federal University of Uberlândia - UFU, Monte Carmelo Campus, in the
period of May 2013 to August 2016.

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Tomato genotypes tolerant to water stress
Genetics and Molecular Research 16 (2): gmr16029685
The genetic material utilized was originally from the germplasm bank of UFU. The
segregating populations evaluated were, previously, obtained from an interspecic cross
between UFU-222 (S. lycopersicum) versus wild accession LA-716 (S. pennellii). UFU-222 is
a pre-commercial line that has small fruits (>18 g), 9° Brix and is susceptible to water stress.
The wild genotype also presents small fruits (15 g) but is tolerant to water decit (Morales et
al., 2015b; Rocha et al., 2016).
To select genotypes tolerant to water stress, ve F2BC1 populations, originated from
self-fertilization of the rst backcrossing, were utilized: UFU22/F2BC1#8, UFU22/F2BC1#9,
UFU47/F2BC1#11, UFU22/F2BC1#2, UFU22/F2BC1# and three check treatments, being two
susceptible (cv. Santa Clara and UFU-222) and one resistant (S. pennellii).
The genotypes were sown on May 5, 2016, in polystyrene trays of 200 cells, lled
with commercial coconut ber substrate. Thirty-ve days after sowing, the plants were
transplanted into pots (5-L volume), containing the same substrate that was used for seedling’s
production. The tomato plants were cultivated in a bow-type greenhouse, measuring 7 x 21 m
and ceiling 4 m, covered with transparent polyethylene lm of 150 microns, protected against
ultraviolet rays, and curtain side of white and anti-aphid scream. The cultural dealings were
made according to the tomato crop recommendations (Alvarenga, 2013). The experiment was
set up in a randomized complete block design, with three replications (blocks), totalizing 24
plots (8 genotypes x 3 blocks). Each plot consisted of one plant.
The climatic conditions, in the greenhouse, were monitored with the aid of an
automatic weather station. The ux density (W/m-2) of the global solar radiation was estimated
through the equipment Silicon Photodiode Pyranometer (NRLITE - Campbell Sci.), and the
temperature (°C) and relative humidity (%) were estimated using the Vaisala Sensor (HMP45C
- Campbell Sci.). The equipment was installed in the center of the greenhouse and above the
plants, and it was connected to a data acquisition system (Datalogger CR1000 - Campbell
Sci.). The data were stored every 15 min.
Tensiometers were installed in each plot, 20 days before the evaluation (98 days after
sowing, DAS), to evaluate the water tension on the substrate. The irrigation was done in a
controlled manner, keeping the substrate humidity in great conditions (-10 kPa). On the beginning
of the owering (116 DAS), the tomato plants were submitted to a water stress condition, where
the water potential was putten closed to -30 kPa through irrigation suspension. The evaluations of
the gas exchange, chlorophyll index, and leaf water potential were made on August 31 (118 DAS).
The parameters of the gas exchange were checked using three leaets of the middle
third of each plant, during the morning (approaching 8 h), using the equipment InfraRed Gas
Analyzer (IRGA, model LCA-4, analytical development Co., Hoddesdon, England). The
parameters obtained were: CO2 assimilation (A), internal CO2 (Ci), stomatal conductance
(Gs), transpiration (E), and leaf temperature (T). The instantaneous water use efciency was
calculated from the ratio between CO2 assimilation and leaf transpiration (A/E), the intrinsic
efciency of water use was calculated from the ratio between CO2 assimilation and the stomatal
conductance (A/Gs), and the instantaneous carboxylation efciency was estimated from the
ratio between the CO2 assimilation and internal CO2 (A/Ci).
The chlorophyll (a and b) levels were estimated through two leaets of each plant
of the middle third, using a chlorophyll meter (CFL-1030, Falker). The equipment estimates
the concentration of photosynthetic pigments with high precision, without needing chemical
reagents that are commonly used for this and saving time (Rigon et al., 2012). The values were
summed, and the mean of each plot was obtained.

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Genetics and Molecular Research 16 (2): gmr16029685
The potential leaf water (Ψf) was measured before sunrise (±5:00 am), with the aid
of a pressure chamber, type Scholander (Soil Moisture Model 3000), and ve leaets of the
middle third of each plant were used for it. The values were also summed and the mean of each
plot obtained. The statistical analyses were done with the aid of the GENES computational
program (Cruz, 2013).
RESULTS AND DISCUSSION
The water availability of the substrate was not restrictive for the water demand of the
plants (-10 kPa), until the decit beginning. After 2 days of the water decit imposition, during
the physiologic evaluations, the substrate was submitted to a moderate matricidal potential
(below -25 kPa) (Figure 1). Guida et al. (2017), when submitting tomato plants to a water
stress, observed that the soil matricidal potential below -28 kPa promotes a reduction in the
stomatal conductance, affecting the crop yield.
Figure 1. Water matricidal potential (kPa) of the substrate from August 18, 2016 to August 31, 2016.
Analyzing the climatic conditions in the greenhouse, it was found that the average
temperature was 23.4°C, which is close to the ideal temperature (18°-24°C), for the tomato
crop, according to Duarte et al. (2011). The mean relative humidity of the period was 60.3%,
presenting a variation of 33.5-86.4%. The average of the global solar radiation and vapor
pressure decit were, respectively, 167 W/m2 and 1.38 kPa (Figure 2).
Figure 2. Temperature (°C), relative humidity (RH, %), global solar radiation (Qg, W/m2), and vapor pressure
decit (kPa), monitored on August 31, 2016.

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Genetics and Molecular Research 16 (2): gmr16029685
Analyzing the leaf temperature, it was found that the cv. Santa Clara showed a
temperature 20% higher than the wild accession (S. pennellii). The genotypes of the UFU22/
F2BC1#9 and UFU47/F2BC1#11 populations and the pre-commercial line UFU-222 showed a
similar behavior to the cv. Santa Clara, while the genotypes of the UFU22/F2BC1#8 population
were similar to the wild accession (Table 1).
Table 1. Averages of leaf temperature (T leaf) (°C), stomatal conductance (Gs) (mol⋅m-2⋅s-1), transpiration
(E) (mmol⋅m-2⋅s-1), internal CO2 (Ci) (mmol⋅m-2⋅s-1), CO2 assimilation (A) (µmol⋅m-2⋅s-1), instantaneous water
use efciency (A/E) (mmol CO2/mol H2O), intrinsic efciency of water use (A/Gs) (mmol⋅m-2⋅s-1 H2O), and
instantaneous carboxylation efciency (A/Ci), in tomato genotypes, F2BC1, submitted to a water stress.
*Means followed by the same letter in the column do not differ, signicantly, from each other by the Tukey test (P < 0.05).
Genotype
T leaf
Gs
E
Ci
A
A/E
A/Gs
A/Ci
T1
UFU22/F2BC1#8
25.60
a
0.05
b
2.18
d
370.44
a
1.57
e
0.72
e
40.90
cd
0.001
E
T2
UFU22/F2BC1#9
30.37
d
0.09
c
1.25
c
354.22
a
4.84
c
3.84
c
49.00
c
0.013
Bc
T3
UFU47/F2BC1#11
30.03
cd
0.09
c
0.76
ab
299.11
bc
1.72
e
2.31
d
20.15
d
0.007
De
T4
UFU22/F2BC1#2
28.82
c
0.03
a
0.60
a
251.88
d
3.08
d
5.30
b
119.55
a
0.010
Cd
T5
UFU22/F2BC1#3
27.25
b
0.05
b
0.80
b
351.00
a
2.56
de
3.22
cd
58.12
c
0.010
Cd
T6
UFU-222
29.68
cd
0.14
d
2.26
d
258.88
cd
6.51
ab
2.89
cd
45.50
c
0.027
A
T7
cv. Santa Clara
30.75
d
0.13
d
2.22
d
234.11
d
6.95
a
3.13
cd
55.01
c
0.031
A
T8
Solanum pennellii
25.66
a
0.06
b
0.82
b
338.00
ab
5.62
bc
6.76
a
90.36
b
0.018
B
CV
2.87
15.57
7.70
9.42
19.27
22.88
27.48
28.260
Overall average
28.52
0.08
1.36
307.20
4.11
3.52
59.82
0.014
Morales et al. (2015a) afrmed that the variations in the leaf temperature could be used
to estimate the water stress of plants. The reduction of water soil availability causes an increase
in the leaf temperature, due to a reduction in the transpiration that is the main mechanism
responsible for the plant cooling (Morales et al., 2015a), even though this relation was not
observed in the present experiment probably due to a disturbance in the plant physiology.
One of the rst reactions that plants show in response to water stress is the stomatal
closure (Wang et al., 2012). The pre-commercial line UFU-222 presented the higher stomatal
conductance (Gs) and did not differ, signicantly, from the cv. Santa Clara. On the other
hand, the wild accession, S. pennellii, showed Gs lower (54%) than the cv. Santa Clara.
The genotypes of the UFU22/F2BC1#8, UFU22/F2BC1#2, and UFU22/F2BC1#3 populations
presented Gs similar or higher than the wild species.
The reduction in the stomatal conductance, due to a stomatal closure, reduces the waste
of water by transpiration. Even though this strategy affects the photosynthetic rate because it
complicates the CO2 diffusion, which is extremely vital for the process of photosynthesis.
Therefore, an effective regulation of the stomatal opening is primordial for the plant’s
development under water stress conditions (Santos et al., 2013).
The mean transpiration (E) observed was 1.36 mmol⋅m-2⋅s-1, and the highest were
observed in the genotypes Santa Clara, UFU22/F2BC1#8, and UFU-222. The genotypes of
the UFU22/F2BC1#2 and UFU47/F2BC1#11populations presented the lowest transpiration
rate, and the last one did not differ, signicantly, from the genotypes of the UFU22/F2BC1#3
population and the wild accession S. pennellii.
The lowest transpiration rate of the wild accession, S. pennellii, makes it capable of
tolerating the water stress. Because of this, the wild genotype can develop in arid regions
(Easlon and Richards, 2009), being, therefore, a promising material for the tomato breeding
that aims to nd genotypes tolerant to water stress. Low transpiration (E) might be associated
with nonstomatal factors, as variations on the leaf cuticle allow the water loss by transpiration.

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M.E.A. Borba et al.
Genetics and Molecular Research 16 (2): gmr16029685
The increase in thickness and mesophyll’s density are described as adaptive responses to water
stress (Poorter et al., 2009).
Evaluating the internal CO2, it was found that the genotypes of UFU22/F2BC1#8,
UFU22/F2BC1#9, and UFU22/F2BC1#3 populations did not differ from each other and had
similar behavior to the wild accession S. pennellii, presenting an average higher than the
overall of the experiment. A signicant difference was found between the wild genotype and
cv. Santa Clara, even though the genotypes UFU22/F2BC1#2 and UFU-222 did not differ from
cv. Santa Clara being all in the worst group.
High values of internal CO2 indicate that the genotype can provide more CO2 for the
ribulose-1,5-bisphosphate carboxylase-oxygenase (rubisco). However, this did not happen in
the present study, probably due to a decrease in the activity of enzymes involved in the CO2
xation process (Table 1).
Analyzing the CO2 assimilation (A) of the genotypes, it was found that the cv. Santa
Clara was 24% higher than the wild accession S. pennellii. However, both check treatments
were better than the rest of the genotypes. Furthermore, the other check treatment UFU-222 was
also better than the rest of the genotypes and showed a similar behavior to both susceptible and
resistant checks. Besides this, the genotype UFU22/F2BC1#9 highlighted showing a similar
behavior to the wild accession (Table 1).
The mean value of CO2 assimilation that was observed in the present experiment is
lower (4.11 µmol⋅m-2⋅s-1) than the results in other experiments under excellent water conditions
(15.6 µmol⋅m-2⋅s-1) (Galmés et al., 2012). However, it is similar to results found by Morales
et al. (2015b), when the photosynthesis decreased from 8.61 to 4.18 µmol⋅m-2⋅s-1, when the
water stress was submitted. A low CO2 assimilation proves that the water stress might affect
physiological and biochemical processes of the vegetables, such photosynthesis (Farooq et al.,
2009; Jaleel et al., 2009).
A higher instantaneous water use efciency, observed in the wild genotype (S.
pennellii), about the other treatments, is related to its low transpiration (E). Among the
evaluated genotypes, UFU22/F2BC1#2 highlighted, being the one that most resembled the
wild accession. On the other hand, S. pennellii was 2.2 more efcient in water use than the cv.
Santa Clara, which is possible the main result, proving its resistance to water stress. Similar
results were found by Machado et al. (2010), correlating the amount of transpiration water
with the dry matter production.
Analyzing the intrinsic efciency of water use, it was found that the overall average
was 59.82 mmol⋅m-2⋅s-1 and this high value was found in the UFU22/F2BC1#2 genotype
followed by the wild accession S. pennellii. On the other hand, the lowest intrinsic efciency
of water use was found in the UFU22/F2BC1#8 and UFU47/F2BC1#11genotypes. It was
also found that the wild accession was 64% more efcient than the cv. Santa Clara. High
photosynthetic rate associated with low stomatal conductance (Gs) are characteristics nd in
plants with tolerance to water stress, which also has a higher intrinsic efciency of water use
(Pazzagli et al., 2016). In environments with limited water resources, the positive magnitude
of these parameters becomes essential for the adequate functioning of plants and also for
demonstrating the physiological plasticity of the species to abiotic factors (Funk and Vitousek,
2007; Li et al., 2008; Silva et al., 2008).
Evaluating the instantaneous carboxylation efciency, the check treatment cv. Santa
Clara highlighted presenting the high value, i.e., 72% higher than the wild accession S. pennellii.
On the other hand, the lowest instantaneous carboxylation efciency was observed for the

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Tomato genotypes tolerant to water stress
Genetics and Molecular Research 16 (2): gmr16029685
UFU22/F2BC1#8 and UFU47/F2BC1#11 genotypes. According Silva et al. (2015), the relation
between internal CO2 (Ci) and CO2 assimilation (A), which is the instantaneous carboxylation
efciency, is a characteristic that makes possible to estimate non-stomatal factors that could be
affecting the photosynthetic process and, consequently, the biological yield.
The potential leaf water (Ψf) indicates the energy status of plants, where values
closely to zero indicate that the plant is not under water stress. However, high negative values
indicate an intense water stress (Morando et al., 2014). The mean value of leaf water potential,
observed in the experiment, was -0.635 mPa (Table 2). The lowest value was found in the cv.
Santa Clara, being 7.7 times smaller than the wild accession S. pennellii, which makes the
commercial genotype to be considered susceptible to water stress. The genotypes UFU22/
F2BC1#8, UFU22/F2BC1#9, UFU47/F2BC1#11, UFU22/F2BC1#2, and UFU-222 were the ones
that most approached to the S. pennellii, for the potential leaf water, even though they were
four times inferior compared to the wild genotype.
Table 2. Potential leaf water (Ψf, mPa), chlorophyll a, chlorophyll b, and total chlorophyll in tomato genotypes
F2BC1 submitted to water stress.
*Means followed by the same letter in the column do not differ, signicantly, from each other by the Tukey test (P < 0.05).
Genotype
Ψf (mPa)
Chlorophyll a
Chlorophyll b
Total chlorophyll
T1
UFU22/F2BC1#8
-0.536
b
37.68
abc
15.23
ab
52.91
ab
T2
UFU22/F2BC1#9
-0.463
b
41.41
a
15.10
ab
56.43
a
T3
UFU47/F2BC1#11
-0.560
b
40.70
ab
14.31
ab
55.01
a
T4
UFU22/F2BC1#2
-0.626
b
38.43
abc
14.76
ab
53.20
ab
T5
UFU22/F2BC1#3
-0.900
c
36.38
bc
13.15
ab
49.53
ab
T6
UFU-222
-0.576
b
30.50
d
8.35
c
38.85
c
T7
cv. Santa Clara
-1.253
d
35.48
c
11.50
bc
46.98
b
T8
Solanum pennellii
-0.163
a
39.66
abc
17.38
a
57.04
a
CV
10.11
4.29
10.88
5.36
Overall
average
-0.635
37.53
13.71
51.24
A relation between transpiration and the stomatal opening is related by Firmano et
al. (2009), where the higher is the stomatal conductance, stronger is the transpiration. Under
moderate stress conditions, plants remain with the stomata closed, to maintain greater water
potential and ideal turgor pressure, which is an important indication of tolerance to water
stress (Lawlor and Tezara, 2009).
During the experiment, the average chlorophyll a content was 37.53 mg/L, with a
coefcient of variation relatively low (4.29%) (Table 2). The genotypes UFU22/F2BC1#8,
UFU22/F2BC1#9, UFU47/F2BC1#11, and UFU22/F2BC1#2 highlighted presenting the highest
values and not differing, signicantly, from the wild check treatment, S. pennellii. The
lowest chlorophyll a content was observed in the plants of the pre-commercial line, UFU-
222. According to Rego and Possamai (2011), the chlorophyll content, in a plant, varies in
function of climatic conditions and an increase in this pigment reects in an increase in the
photosynthetic rate.
Almost all the genotypes that were evaluated showed a similar behavior related to the
specic chlorophyll b content. The lowest value was found in the genotype UFU-222, which
did not differ, signicantly, from the cv. Santa Clara. Furthermore, the genotypes UFU22/
F2BC1#8, UFU22/F2BC1#9, UFU47/F2BC1#11, UFU22/F2BC1#2, and UFU22/F2BC1#3 did
not differ, signicantly, from the check treatments cv. Santa Clara and S. pennellii (Table 2).
Analyzing the total chlorophyll, it was found an overall average of 51.24 mg/L. The
lowest content was observed in the pre-commercial line UFU-222, which differed signicantly

8
M.E.A. Borba et al.
Genetics and Molecular Research 16 (2): gmr16029685
from all the genotypes that were evaluated. On the other hand, the highest values were found
in the genotypes S. pennellii, UFU22/F2BC1#9, and UFU47/F2BC1#11. Besides that, these
three genotypes did not differ, signicantly, from the genotypes UFU22/F2BC1#8, UFU22/
F2BC1#2, and UFU22/F2BC1#3 (Table 2).
Low chlorophyll content might be associated with climatic conditions, mainly
related to high leaf temperature. Plants with a C3 metabolism, like tomato, have a better
photosynthetic behavior under temperatures between 20° and 25°C. When submitted to a water
stress conditions, the plants may present, in addition to stomatal closure, alterations in the
chlorophyll synthesis, functional and structural alterations in chloroplasts, and disturbances in
the processes of accumulation, transport, and distribution of assimilates.
CONCLUSIONS
All the variables analyzed, except CO2 assimilation and instantaneous carboxylation
efciency, evidenced a superiority of the wild accession S. pennellii, regarding the susceptible
check treatments.
The high photosynthetic rate, low stomatal conductance, and transpiration are
promising physiological characteristics that could be used in the selection of tomato genotypes
tolerant to water stress.
Conicts of interest
The authors declare no conicts of interest.
ACKNOWLEDGMENTS
The authors would like to thank the Federal University of Uberlândia, CNPq, and
FAPEMIG for the nancial support of this research project.
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