00221589.1993.11516443

Journal of Horticultural Science
ISSN: 0022-1589 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/thsb19
Effects of salinity on nitrate, total nitrogen, soluble
protein and free amino add levels in tomato plants
F. Perez-Alfocea, M. T. Estan, A. Santa Cruz & Maria C. Bolarin
To cite this article: F. Perez-Alfocea, M. T. Estan, A. Santa Cruz & Maria C. Bolarin (1993) Effects
of salinity on nitrate, total nitrogen, soluble protein and free amino add levels in tomato plants,
Journal of Horticultural Science, 68:6, 1021-1027, DOI: 10.1080/00221589.1993.11516443
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Date: 26 October 2017, At: 21:19
Journal of Horticultural Science (1993) 68 (6) 1021-1027
Effects of salinity on nitrate, total nitrogen, soluble
protein and free amino acid levels in tomato plants
By F. PEREZ-ALFOCEA, M. T. ESTAN, A. SANTA CRUZ and MARIA C. BOLARIN
Centro de Edafologia y Biologia Aplicada del Segura, CSIC, Apdo. 4195, E-30080 Murcia, Spain
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SUMMARY
The effects of salinity on nitrogen compounds were studied in three tomato (L. esculentum Mill.) genotypes of different salt tolerance. The plants were grown under controlled conditions, and the salt treatments (0, 70 and 140 mM NaCI) were applied for
three (Harvest 1) and ten (Harvest 2) weeks. The effects of salinity on total Nand particularly N0 3 concentrations depended partly on the NaCllevel and duration of the stress,
but mainly on the different degrees of salt tolerance of the genotypes. In Harvest 1, the
most tolerant genotype (GC-72) showed the highest N0 3 increase in the roots and no
decreases in stem and leaf with increasing salinity; the intermediate-tolerant genotype
(P-73) showed a similar response to that of GC-72 only at 70 mM NaCI. However, the
most sensitive genotype (Volgogradskij) showed the greatest reductions in stem and leaf
N0 3 concentrations with salinity. With longer durations of stress the different responses
between the more tolerant genotypes were less evident; only Volgogradskij continued to
show the highest decreases in stem and leaf N0 3 concentrations. A restriction of N0 3
transport from the root to the shoot was noted in the plants of the more tolerant genotypes treated for three weeks. This capacity for retention disappeared when the salinity
induced reductions in the total N0 3 contents in the plants, as in the harvest 2. There was
an inverse relation between N0 3 and Cl accumulations in shoots of all genotypes and its
slope decreased with the salt sensitivity of the genotype. There was no relation between
the leaf protein content and the salt tolerance of the tomato genotypes. In Harvest 1, a
higher accumulation of amino acids, especially proline, was found in the leaves of the
more tolerant genotypes at 140 mM NaCI. However, in Harvest 2, a similar accumulation
of leaf proline was found in all genotypes, independent of their salt tolerances, and the
other amino acid contents remained similar or else decreased with salinity.
EXPOSURE to NaCl may affect plant growth
through osmotic effects, by a specific ion effect
or by inadequate uptake of an essential nutrient
(Greenway and Munns, 1980). Nitrogen, a substrate required for cell growth, is severely
affected under salt stress (Frota and Tucker,
1978; Luque and Bingham, 1981). Wallace and
Berry (1981) suggested that wheat (Triticum
aestivum, L.) yield reduction due to increased
salinity might not be due entirely to Cl toxicity,
but partly to induced deficiency of N0 3 caused
by the high external Cl concentration. However, in other cases, leaf nitrogen concentrations were unaffected or increased by salinity
(Bhivare and Nimbolkar, 1984; Seemann and
Critchley, 1985). Contradictory results have
also been found in tomato; total N and/or N0 3
contents have been reported to increase (Pessarakly and Tucker, 1988), to remain the same
(Hernando eta/., 1967) or to decrease (Papadopoulos eta!., 1985). Thus, effects of salt stress on
N and/or N0 3 contents in the plant are inconsistent and unpredictable.
Salinity affects the metabolism of nitrogen
containing compounds, e.g. protein synthesis
and free amino acid pool composition (Poljakoff-Mayber, 1982). In general, rates of protein synthesis are lower in NaCl treated plants
(Aspinall, 1986) but there is no clear evidence
that this causes growth reductions; in many
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1022
Salinity and tomato plants
cases, salinity induces an increase in total amino
acids and the accumulation of proline, as in
tomato (Shannon et al., 1987).
Different degrees of salt tolerance were
found in the three tomato genotypes used in
this study (Caro et al., 1991). However, the
mechanisms of salt tolerance were similar,
because they excluded Na and Cl from the
shoots when plants were salt treated in the
short-term, and they accumulated in the roots.
In treatments of longer duration, the ability to
Na and Cl regulate was only absent in the most
salt sensitive genotype (Volgogradskij), resulting in a massive influx of both ions into the
shoot system (Perez-Alfocea et at., 1993). To
complete this study of the physiological
responses of these tomato genotypes to salinity,
the effects of NaCl-induced salinity on total N
and different nitrogen compounds (N0 3, soluble proteins and free amino acids) were determined in different plant parts of these
genotypes.
MATERIALS AND METHODS
Plant material and culture conditions
Seeds of three genotypes of tomato (Lycopersicon esculentum Mill) Volgogradskij, P-73
and GC-72, were germinated and plants grown
in washed silica sand under controlled conditions, as described previously (Perez-Alfocea et
al., 1993). Briefly, a half-strength Hoagland's
solution was applied from the apperance of the
first leaf, and increased to full-strength two
weeks after sowing until the end of the experiment. NaCI treatments (0, 70 and 140 roM)
were applied from the appearance of sixth to
seventh leaves (3 weeks). The NaCI was added
to the nutrient solution in increments of around
25 mmoll- 1•
Three replicate plants per treatment were
harvested three and ten weeks after the salt
treatment was completed. The plants were cut
into roots, stems (including petioles) and leaves
(lamina) only, rinsed with distilled water and
then analyzed.
Analyses
Nitrate and total N contents of roots, stems
and leaves were determined and expressed as
concentrations of dry material. N0 3 content
was analyzed by the method of Lambert and
Dubois (1971). Total N was determined by the
micro-Kjeldahl method.
Analyses of soluble proteins, total free amino
acids and proline were made on lyophilized leaf
tissue. A 100 mg leaf sample was homogenized
with 5 ml of 50 mM phosphate buffer, at pH 7.8
in the presence of 1% (w/v) PVP and 0.01%
(w/v) Triton X-100, and centrifuged at 8000 g
for 15 min. Aliquots of the supernatant were
used to determine the soluble proteins following the method of Bradford (1976). Total free
amino acids (Moore and Stein, 1948) and proline (Bates et at., 1973) were measured using
ninhydrin.
Analysis of variance and regression were
performed according to SYSTAT standard
methods (Wilkinson, 1990).
RESULTS
Nitrate and total nitrogen concentrations
Concentrations of N0 3 were higher in stems
than in roots and leaves for all treatments and
all genotypes (Figure 1). In Harvest-1, N0 3
concentration increased significantly in roots of
salt treated plants compared with the controls
with GC-72 showing the highest increases. In
stems and leaves, the highest N0 3 decreases
with salinity were found in 'Volgogradskij'
treated with 70 mM NaCI. In P-73, leaf N0 3
concentration decreased only at 140 mM,
whereas in GC-72, there were no decreases in
N0 3 concentration in stem or leaf as salinity
increased.
The tendency for root N0 3 concentrations to
increase with salinity was lower in Harvest 2
than in Harvest 1. However, greater decreases
of stem and leaf N0 3 concentrations were
found in all genotypes when salt treatments
were applied for ten weeks.
The total N concentration was less affected
by salinity than that of N0 3 (Figure 2). Thus,
only 'Volgogradskij' significantly increased the
root total N concentration at 140 roM NaCl in
Harvest 2. Similar variations between N0 3 and
total N were shown by stem tissues. In leaves,
only 'Volgogradskij' and P-73 showed significantly decreases in total-N concentration in
Harvest 1, whereas in Harvest 2, all genotypes
had lower total-N concentrations, especially at
140 roM NaCl, similar to the variations of N0 3
with salinity, although the reductions with salinity were lower.
Total nitrate contents and relationship between
N01 and Cl accumulation
Mean values for total amounts of N0 3 in the
F. PEREZ-ALFOCEA, M. T. EsTAN, A. SANTA CRuz and MARIA C. BOLARIN
Harvest-1
0.5
Harvest-2
Root
genotypes (P-73 and GC-72) and decreased
slightly in their shoots. In 'Volgogradskij', shoot
N0 3 decreased about 80% compared with the
control plants at 70 mM NaCI. In Harvest 2,
Harvest-2
Harvest -1
0.4
Root
I
0.3
0.2
1023
2.0
1.5
0.1
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1.0
'j'
0
0
E
0
Stem
0 mM
~ 70 mM
!Ill 11.0 mM
2.5
2.0
0.5
"";"
~
0
0
_;
1.5
0
c:
0
....0
1.0
c:
cu
u
c:
0.5
0
Stem
"'
E
.!:
E
E
4.0
c:
3.0
0 mM
~ 70 mM
0] 140 mM
0
....0
....c:
0
L.
u
...
0
2.0
cu
u
c:
z
0
Leaf
0.7
u
1.0
z
0.6
....
0
0
0
0.5
1-
0.4
Leaf
4. 0
0.3
3.0
0.2
0.1
2. 0
v
p
GC
v
p
GC
FIG. 1
The effect of sahmty on NO, concentrations m different
plant parts of three tomato genotypes (V, Volgogradskij; P,
P-73: GC, GC-72) grown at three NaCIIevels for three (Harvest I) and ten (Harvest 2) weeks. Vertical bars are theSE of
the mean.
1.0
n
v
p
GC
v
P
GC
FIG. 2
plants were determined from dry weight data
and N0 3 concentration in roots and shoots
(Figure 3). In Harvest 1, N0 3 accumulation
increased only in the roots of the more tolerant
The effect of salimty on total N concentrations m different
plant parts of three tomato genotypes (V, Volgogradskij; P,
P-73; GC. GC-72) grown at three NaCtlevels for three (Harvest I) and ten (Harvest 2) weeks. Vertical bars are theSE of
the mean.
1024
Salinity and tomato plants
Harvest -2
Harvest -1
Root
60
0.12
0
0 mM
~
ITJl
70 m M
HO mM
45
0.08
30
0.04
, 5
0
E
E
c
0
0
~
0
Shoot
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~
E
:>
E
3.0
30
0
2.5
25
"'
2.0
20
1.5
15
1.0
10
:>
u
u
0
z
0.5
0
~v
5
0
p
GC
l
v
~ m
p
GC
FIG. 3
The effect of salinity on total amounts of N0 3 in root and
shoot of three tomato genotypes (V, Volgogradskij; P, P-73;
GC. GC-72) grown at three NaCllevels for three (Harvest 1)
and ten (Harvest 2) weeks.
N0 3 accumulation by roots increased slightly at
70 mM NaCI and decreased at 140 mM in all
genotypes except 'Volgogradskij', which had
drastically reduced N0 3 for both levels,
TABLE
I
Lmear regresswn equatwns (y =a + bx) between N0 3 and
Cl accumulations in root and shoot of three tomato genotypes
grown at different NaCI levels (0, 70, and 140 mM) for three
(Harvest I) and ten (Harvest 2) weeks
root
Harvest Genotype
2
Vol go
P-73
02-72
Volgo
P-73
02-72
a
b
shoot
a
b
n.s.
4.9 -3.77 0.93**•
0.05 0.17 0.68*
2.3 -0.38 0.72*
0.04 0.21 0.92*** 2.0 -0.18 0.71*
n.s.
36.4 -2.91 0.87**
n.s.
37.1 -1.46 0.84**
n.s.
15.2 -0.29 0.84**
n.s .. not significant; *, **,***,significant at P=O.OS, 0.01 and
0.001, respectively.
although it had the highest N0 3 accumulation
· in control plants. At this stage of growth, the
shoots reduced their N0 3 uptake with salinity,
these decreases being higher than those
observed in the first harvest.
In the more tolerant genotypes P-73 and
GC-72, N0 3 accumulation was directly correlated with Cl accumulation in root and inversely correlated with Cl accumulation in shoot in
Harvest 1 (Table I). In the most salt-sensitive
genotype (Volgogradskij), a highly significant
relationship between N0 3 and Cl accumulation
was found only in the shoot. The slopes of the
regression decreased in relation to the salt-sensitivity of the genotypes, with 'Volgogradskij'
showing the lowest value. At Harvest 2, no
relationship between N0 3 and Cl was found in
roots, whereas in shoots the N0 3 was inversely
correlated with Cl in all cultivars and the slopes
also decreased according to salt-sensitivity.
Leaf soluble protein, total free amino acid and
proline
Results of the protein and total free amino
acid analyses in leaves are shown in Table II. In
Harvest 1, the protein content decreased significantly with salinity in all genotypes, whereas
the total amino acid contents increased significantly with salinity, mainly in 'GC-72'. In 'Volgogradskij', total free amino acid increase was
due only to proline concentration; in 'P-73' and
'GC-72', in addition to the proline, the remaining total free amino acids also increased, especially in 'GC-72'. At 140 mM NaCI, proline
contents increased according to the salt-tolerance of the genotypes.
In Harvest 2, protein concentrations
decreased significantly only in 'P-73' and
remained constant in other genotypes. Total
free amino acids increased with salinity in the
three genotypes, but these increases were
exclusively due to proline; 'in P-73' the remaining amino acids decreased with salinity.
DISCUSSION
Pessarakly and Tucker (1988) indicated that
total N concentration in different tomato plant
parts increased markedly with increasing salinity. In this study, both total Nand N0 3 concentrations increased only in roots, but generally
decreased in stems and leaves. The decrease of
leaf N0 3 content with salinity has been
F. PEREZ-ALFOCEA, M. T. ESTAN, A. SANTA CRUZ and MARIA C. BOLARIN
1025
TABLE II
Concentrations (mg gDW- 1) of soluble protems, total free amino acids and proline in leaves of three tomato genotypes grown at
different NaCllevels for three (Harvest 1) and ten (Harvest2) weeks
Harvest 1
Genotype
NaCl mM
Volgo
0
70
140
P-73
0
70
140
0
70
140
GC-72
Proteins
1
99.3b
72.7a
78.3a
121.1b
90.0a
99.9a
101.8b
72.6a
90.lab
Harvest 2
Total Aas
Pro
Proteins
Total Aas
Pro
15.0a
18.8b
22.9c
10.9a
14.7b
22.9c
8.7a
16.5b
27.0c
O.la
3.lb
7.8c
O.la
2.0b
9.7c
O.la
3.5b
11.5c
38.7a
36.6a
38.0a
73.6b
66.3ab
54.9a
33.la
34.6a
34.2a
15.0a
20.3b
31.3c
16.1a
16.2a
27.1b
15.0a
18.6b
30.4c
0.2a
4.8b
14.8c
O.la
4.9b
14.0c
O.la
6.6b
14.5c
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'Mean separation by Dunnet test, for each genotype values not sharing a common letter differ sigmficantly at P=0.05.
reported not only in tomato cultivars but also in
wild tomato species (Rush and Epstein, 1976;
Papadopoulos and Rending, 1983).
Salinity effects on N0 3 and N concentrations
depend on the salinity level and the duration of
stress (Sinha et at., 1986; Al-Rawahy et at.,
1992). In roots, N0 3 increases with salinity were
higher when the salt treatments were applied
for short durations (three weeks), whereas the
N0 3 decreases in leaves were higher when the
salt treatments were applied for longer (ten
weeks). However, effects of salinity on N0 3
concentrations in different plant parts were
mainly due to the different degrees of salt tolerance of the genotypes. Thus, the most tolerant
genotype (GC-72) showed the highest increase
in N0 3 in roots, and no significant differences in
N0 3 concentrations of the stem and leaf were
found between the control and the high-salt
treatment in Harvest 1. At this time, stem and,
especially leaf N0 3 concentrations, increased at
70 mM NaCl with respect to the control. On the
contrary, the most sensitive genotype (Volgogradskij) showed greatest reductions in stem
and leaf N0 3 concentrations with salinity.
Substrate salinity inhibits N0 3 uptake by
roots (Sanchez Conde, 1976; Papadopoulos and
Rending, 1983). The results of this study suggest that genotypes with higher salt-sensitivity
will show a greater inhibition of N0 3 uptake by
roots. Moreover, the more tolerant genotypes
showed restricted transport of NaCl to the
shoot in the salt-treated plants in the shortterm. The capacity to retain N0 3 in the root was
lost when the salinity induced large reductions
in the total N0 3 content in the plant, as
occurred in Harvest 2 with the more tolerant
cultivars. At this time, N0 3 absorption in the
most salt-sensitive cultivar decreased with salinity both in the shoot and in the root.
The reduction in N0 3 content in salt-treated
plants was probably due to the inhibition of
N0 3 uptake by Cl (Aslam et at., 1984; McClure
et at., 1986), which might occur by the interaction between these ions at sites for ion transport
(Cram, 1983). If this is the case, an inverse
relationship between N0 3 and Cl accumulations might be expected, as reported by Kafkafi
et at. (1982). The inverse relationship between
N0 3 and Cl accumulations in shoots of all genotypes has been clearly demonstrated. In both
harvests, the more sensitive the genotype, the
lower the slope of the regression, which suggests that the slopes could be used as an index
of salt tolerance. Moreover, a direct relationship between N0 3 and Cl concentrations was
found in the roots of the more tolerant genotypes, as both ions were accumulated in roots
when the plants were salt-treated for three
weeks.
Decreasing leaf protein concentrations, a
typical response to NaCl-stress (Cusido eta/.,
1987) were found in all genotypes in the Harvest 1 and in 'P-73' in Harvest 2, but no relation
between protein contents and the salt tolerances of tomato genotypes was observed (Helal
and Mengel, 1979). A decrease in protein concentrations might reflect a retardation of protein synthesis or an acceleration of their
degradation, leading to an increase in the free
amino acids and/or an inhibition of amino acid
incorporation into proteins (Poljakoff-Mayber,
1982). In our experiments the increase in free
amino acids was mainly due to proline. The
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1026
Salinity and tomato plants
higher proline accumulation found in the more
tolerant genotypes, when the plants were
grown at 140 mM NaCI for three weeks, suggests a direct relationship between proline
accumulation and salt tolerance in tomato genotypes grown at high salt levels in the shortterm. Because the higher salt tolerances of
these genotypes were related to their higher
capacities for excluding CI and Na from the
shoots (Perez-Aifocea et al., 1993), an inverse
relationship between Na and proline accumulations was found, as also reported by Aspinall
and Paleg (1981) in leaves of barley. Consequently, proline accumulation would help
remove excess H+ before the low Na-accumuIation capacity of these genotypes (Bellinger
and Larher, 1987).
Although the total free amino acid contents
also increased in Harvest 2, they were not
related to the decreasing protein contents and
were exclusively due to the increase of proline,
as the remaining amino acid contents were
maintained or decreased with salinity. Moreover, similar proline accumulation was found in
all genotypes, irrespective of their salt tolerance. Consequently, proline accumulation in
Harvest 2 seems not to be due to the same
metabolic processes that occurred in Harvest 1.
Proline accumulation has been reported to
occur not only during water and saline stress,
but also during other stress conditions, e.g.
nutrient deficiencies (Stewart and Larher,
1980). A common denominator amongst these
stress conditions is the provision of a mechanism for detoxifying leaf tissue of excess
ammonia (Rabe, 1990). It could explain the
lack of relationship between proline accumulation and the salt tolerance of the tomato
genotypes.
The authors are grateful to Mrs M. Rojo for
her assistance. The study was supported by Project CICYT AGF92-0260.
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(Accepted 18 April1993)