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Research Article
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Effect of Salt Stress on Chlorophyll Content, Fluorescence, Na+
and K+ Ions Content in Rape Plants (Brassica napus L.) |
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V. Atlassi Pak,
M. Nabipour
and
M. Meskarbashee
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ABSTRACT
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In order to investigate the effect of salt stress on chlorophyll content and fluorescence, sodium (Na+) and potassium (K+) ions content of rape (Brassica napus L.) plants, ten genotypes were subjected to salinity levels (control [2.5], 6, 10, 14 and 18 dS m-1) for 30 days in hydroponics. Salt treatments were imposed to genotypes in root establishment stage (4 leaves). Results showed that quantum yield of photosystem II from light adapted (ΦPSII) and dark-adapted leaf (Fv/Fm), photochemical quenching (qP) and minimal fluorescence from dark-adapted leaf (Fo) were affected by salinity. Genotypes MHA4921 and Hyola 401 had highest shoot dry weight at the two higher salt treatments (14 and 18 dS m-1) and resulted the most tolerant to salinity among the tested genotypes. Chlorophyll (chl) fluorescence attributes was generally affected by salinity stress, except in the two salt tolerant genotypes and thus could be used as a tool for screening for salinity tolerance. Chlorophyll content (SPAD units) changed significantly in all genotypes, except in salt tolerant ones. Shoots Na+ content increased, by increasing salinity levels, but in MHA4921 this increase was higher than the other genotypes and may be relation to decline in the osmotic potential of cellular contents. Rape ability to accumulate sodium in response to salinity is one of the major criteria of salt tolerance. K+ content in shoots, at the different levels of salinity in MHA4921 and Hyola 401 were higher than the other genotypes.
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INTRODUCTION
Salt stress is a major abiotic stress problem in arid and semi-arid regions
(Sudhir and Murthy, 2004) and agriculture productivity
in these areas of the world is very low (Ashraf, 2004).
Hence, considerable improvement in salinity tolerance is essential in crop species
through conventional selection and breeding techniques (Ashraf
and Harris, 2004). Many biochemical and physiological criteria or traits
have been proposed for screening (Ashraf and Harris, 2004).
Classical methods of screening for salt tolerance are based on yield response
(Netondo et al., 2004) but the underlying genetic
mechanisms for yield are complex with considerable environmental influence (Ashraf,
2004). Chlorophyll (chl) fluorescence could be used for screening for salt
tolerance varieties and modified by salinity stress (Baker
and Rosenqvist, 2004). Chl fluorescence provides non-invasive and rapid
method for estimates of photosynthetic performance of plants (Kao
et al., 2003; Baker and Rosenqvist, 2004;
Zlatev and Yordanov, 2004). Effect of sodium chloride
(NaCl) stress on chl fluorescence has been studied in different plants. Application
of chl fluorescence for salinity tolerance was investigated in barely (Belkhodja
et al., 1994), sorghum (Netondo et al.,
2004), naked oat (Zhao et al., 2007), arabidopsis
and thellungiella (Stepien and Johnson, 2009), rice
(Moradi and Ismail, 2007) and wheat (Zair
et al., 2003). A significant decline in quantum yield of photosystem
II from dark-adapted leaf (Fv/Fm) accompanied by increase of non photochemical
quenching (NPQ) occurred in sorghum varieties with 250 mM NaCl (Netondo
et al., 2004). Moradi and Ismail (2007) reported
that no significant differences in quantum yield of PSII (ΦPSII) were observed
with increasing salinity level at vegetative stages in rice, but concluded that
NPQ increased significantly. They suggested that no reduction in ΦPSII
were observed in the tolerate lines but in the sensitive one it did. Zair
et al. (2003) noted that the Fv/Fm ratio decreased significantly
in salt sensitive line and remained unchanged in tolerant one in wheat. In wild
soybean species, as increasing salinity, no significant differences were found
in Fv/Fm and ΦPSII (Kao et al., 2003). Salinity
stress significantly reduced chlorophyll content, photochemical quenching (qP)
and Fv/Fm in naked oat (Zhao et al., 2007). The
effect of salt stress on chl fluorescence attributes was examined in arabidopsis
(Arabidopsis thaliana L.) and thellungiella (Thellungiella halophila
L.) by Stepien and Johnson (2009). They concluded
that increasing salinity resulted in a substantial increase in NPQ in arabidopsis
(salt sensitive) while in thellungiella (salt tolerance) this parameter remained
close to control levels at all salt concentrations. A considerable decrease
was observed in Fv/Fm and ΦPSII in arabidopsis while in thellungiella no
change occurred in Fv/Fm. In thellungiella ΦPSII did not changes in intermediate
salinity.
In conclusion, results show that in some experiments (Misra
et al., 2001; Zair et al., 2003; Stepien
and Johnson, 2009) quantum yield of PSII was an early indicator of salt
stress and provide important information on photosynthetic activity, but in
studies of Belkhodja et al. (1994) and Kao
et al. (2003) this parameter was not useful indicator for salt stress.
The present study was conducted to investigate the effect of salt stress on
chl fluorescence attributes in rapes (Brassica napus L.) genotypes known
to differ in their salinity tolerance.
MATERIALS AND METHODS This study was conducted in 2008-09 years in the growth chamber of Department of Agronomy and Plant Breeding, College of Agriculture, Shahid Chamran University, Ahvaz, Iran. The environmental conditions in the growth chamber were: Photosynthetic Active Radiation (PAR) 450 μmol/m/sec, day/night temperature 24/18°C, relative air humidity 55-70%. The tested rape genotypes were Sarigol, Ahatrol, Hyola401, Hyola308, RGS003, MHA8725, MHA4921, MHA4026, MHA9716 and MHA8716. All seeds samples were surface sterilized with 1% sodium hypochlorite solution for 20 min and washed with distilled water. Plants were germinated in germinator for a week. After a week, seedlings were transplanted in aerated Hoaglands solution. After the establishment of seedlings, genotypes were subjected to five level of salinity for 30 days. The experiment was designed as split plots on the basis of randomized complete design (RCD) with three replications. Salinity as main plot factor had five levels (control [2.5], 6, 10, 14 and 18 dS m-1). The genotypes were used as sub plot. NaCl (Merck) was used as a source of salt.
Chlorophyll fluorescence was measured with intact plants in the growth chamber.
Measurements of chlorophyll fluorescence (with chlorophyll fluorometer, PAM-2000,
Walz Germany) and chlorophyll content (SPAD units with a chlorophyll meters
SPAD-502, Minolta Japan) were made on fully expanded youngest leaf (Ashraf,
2001). Potassium and sodium were determined with flame photometer (Ashraf
and Ali, 2008). Analysis of variance was performed by SAS (version 6.12)
(Moradi and Ismail, 2007) and MSTATC (Atlassi
et al., 2008) programs. The mean values were compared by Duncans
test.
RESULTS
Salinity and genotypes had significant effect on shoot dry matter. Shoot dry
weight of ten genotypes decreased significantly with increasing NaCl concentrations
in Hoaglands solution. MHA4921 and Hyola401 had highest shoot dry weight
among all genotypes at two higher salt concentrations (14 and 18 dS m-1)
(Fig. 1). Comparison of different genotypes shows that MHA4026
and MHA9716 had lowest shoot dry weight (Fig. 1). Chlorophyll
content (SPAD units) did not change significantly in MHA4921 and Hyola401 (Fig.
1). Quantum yield of PSII (ΦPSII and Fv/Fm) was not affected dramatically
by salinity in two salt tolerant genotypes (Fig. 1, 2).
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Fig. 1: |
Shoot dry weight, Chlorophyll content (SPAD unites) and Quantum
yield of PSII from light adapted leaf (ΦPSII) of ten genotypes of rape
(B. napus). Three-week-old plants were exposed to salt for 30 days |
A trend of decrease in the SPAD values of genotypes can be observed by increasing
NaCl concentration, but it was not in MHA4921 and Hyola401 (Fig.
1). Fv/Fm had a significant decrease at 14 and 18 dS m-1 NaCl
concentrations (Fig. 2). Photochemical quenching (qP) showed
a similar trend as Fv/Fm and at the two higher NaCl concentrations dramatically
impaired (Fig. 2). ΦPSII and qP in MHA 4921 and Hyola
401 genotypes had a little decline at 18 dS m-1 NaCl (Fig.
1, 2).
In all genotypes increasing salinity resulted in increased non-photochemical quenching (NPQ) (Fig. 2). Minimal chlorophyll fluorescence (Fo) was increased by increasing salt concentration, but MHA4921 and Hyola401 were not affected (Fig. 3).
NaCl treatment resulted in accumulation of Na+ in leaves of genotypes.
Salt treatmentxgenotypes interaction was not significant for Na+
content (Table 1). The data shown in Fig. 3
suggest that, by increasing salinity, in all genotypes Na+ content
increased similarly. The Na+ content was the same for all genotypes
at the highest NaCl concentration but in MHA4921 genotype, Na+ content
was higher at the two highest salt concentrations (Fig. 3).
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Fig. 2: |
Photochemical quenching (qP), Non-photochemical quenching
(NPQ) and Quantum yield of PSII from dark-adapted leaf (Fv/Fm) of ten genotypes
of rape (B. napus). Three-week-old plants were exposed to salt for
30 days |
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Fig. 3: |
Minimal fluorescence from dark-adapted leaf (Fo), K+
and Na+ content of ten genotypes of rape (B. napus). Three-week-old
plants were exposed to salt for 30 days |
Table 1: |
Mean squares from analysis of variance of data for shoot dry
weight, quantum yield of photosystem II from light adapted (ΦPSII)
and dark-adapted leaf (Fv/Fm), SPAD unites, Na+ and K+ ions
content, photochemical quenching (qP), non-photochemical quenching (NPQ)
and minimal fluorescence from dark adapted leaf (Fo) |
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*,**: Significant at 0.05 and 0.01 levels, respectively, Duncans
test, ns: Non significant |
Shoot K+ concentration significantly reduced in all genotypes by
increasing salt treatment. As shown in Fig. 3, in two salt
tolerant genotypes, K+ content was the highest at all level of salt
concentrations.
DISCUSSION
Two genotypes (MHA4921 and Hyola401) had highest shoot dry weight at maximum
NaCl concentration while other two (MHA4026 and MHA9716) had the lowest shoot
dry weight: therefore MHA4921 and Hyola401 were ranked as salt tolerant and
MHA4026 and MHA9716 were salt sensitive. By increasing of salinity, sodium ions
changes the ratio of K:Na, which seems to affect the bioenergetic processes
of photosynthesis (Sudhir and Murthy, 2004). Effect of
salinity on plant growth may result from impairment of supply of photosynthetic
assimilates (Ashraf, 2004) and cell expansion in leaves
can be inhibited by salt stress (Chaves et al., 2009).
Each of these parameters may results in a decline in shoot dry weight. Conservation
of shoot dry weight is one of the selection criteria for salinity tolerance
(Shannon, 1998). In many plant species, screening methods
and the physiological works for salinity are based on the young plant stages
(Dasgan et al., 2002).
Some researchers suggest that decreasing dry mass production in shoots at early
vegetative growth stages is associated with decreased seed yield in rapes (Qasim
et al., 2003). Shoot biomass measuring at early vegetative growth
stages was introduced as a scale for show salt tolerant and sensitive variety
of rapes and a positive association between shoot biomass and seed yield was
found (Ashraf, 2001; Ashraf and
Ali, 2008). Other authors concluded that early vegetative growth responses
could be used as a trait for rapid selection of salt-resistant varieties in
rice (Aslam et al., 1993). Ashraf
(2001) suggested that shoot dry weight at the vegetative growth in Brassica
spp. was related to the ultimate of tolerance of these species.
Chlorophyll fluorescence and analyses for Fv/Fm and ΦPSII were useful
for the monitoring of salt stress in our experiment. The decrease of quantum
yield of PSII (ΦPSII) at increasing NaCl concentrations, except in salt
tolerant genotypes is in line with results reported by some authors on different
species and cultivars (Misra et al., 2001; Zair
et al., 2003). In sorghum (Netondo et al.,
2004) and rice (Moradi and Ismail, 2007) no changes
in this parameter under salt stress were observed. These authors noted that
these plant species PSII is highly resistant to salinity stress. In the present
study Fv/Fm and ΦPSII decreased significantly by increasing NaCl concentration
in salt sensitive genotypes. Salinity stress is though to cause lesions in the
reaction center of PSII (Yang et al., 1996; Baker
and Rosenqvist, 2004), either directly or via an accelerated senescence
(Netondo et al., 2004), which plays a critical
role in the response of photosynthesis (Yang et al.,
1996); hence ΦPSII has been widely used for the measurement of stress
condition of crops (Khan et al., 2006) to detected
stress-induced perturbations in photosynthetic apparatus (Baker
and Rosenqvist, 2004).
Reduction in ΦPSII is on of the major factor responsible for the drastic
reduction in photosynthetic rate under salt stress and differences in Fv/Fm
is often used as characteristics of cultivars or species differences in soybean
(Kao et al., 2003).
Photochemical quenching (qP) is a measure of the proportion of PSII reaction
centers capable of photochemistry (Thioyapong et al.,
2004) and changes in ΦPSII are similar to the changes in qP (Baker
and Rosenqvist, 2004). These changes have been seen in our experiment where
qP showed a significant decline at the highest salt concentration level, also
in two salt tolerant genotypes.
The negative impact of NaCl on photosynthesis rate in our experiment, results
in an increase in NPQ in all genotypes. Sudhir and Murthy
(2004) noted that salt stress enhances the oxigenase activity of RUBPco
and can cause a decline in Co2 fixation. Increase in NPQ may represent
the decreased demand for product of electron transport, which has been using
for assimilation and thus results in heat dissipation of light energy (Netondo
et al., 2004; Moradi and Ismail, 2007). Increase
in NPQ, might result from changes in protective high-energy-state (Stepien
and Johnson, 2009) and photoinhibition (Moradi and Ismail,
2007; Stepien and Johnson, 2009). NPQ and qP has
been used for screening of salt tolerant wheat genotypes (Zair
et al., 2003). In our experiment, NPQ was less informative for screening
of salt tolerant. In sorghum, qP significantly decreased but NPQ increased under
salinity conditions (Netondo et al., 2004). Shabala
et al. (1998) concluded that fluorescence quenching in leaves are
the most sensitive photosynthetic characteristics for measuring salinity tolerance
in maize. In this study, the high rates of qP in salt tolerant genotypes represent
the high efficiency of light use for electron transport by PSII and/or ability
of Hyola 401 and MHA 4921 to maintain QA partially oxidized.
Any excess of absorbed light energy can sensitize the formation of reactive
oxygen species (ROS) (Thioyapong et al., 2004).
Greater ΦPSII and therefore, qP in salt tolerant plants, in our experiment,
compared to the other genotypes likely contributed to the greater leaf chlorophyll
content. Some authors found that higher rates of ROS in salt sensitive rape
genotypes can cause damage to photosynthetic pigments (Ashraf
and Ali, 2008). Present results indicated that decrease in chlorophyll content,
in salt sensitive genotypes, resulted in a decrease in Fv/Fm and ΦPSII.
Leaf chlorophyll content was affected by salinity in tetraploid wheat (Munns
and James, 2003), rice (Sultana et al., 1999),
Brassica oleracea (Bhattacharya et al., 2004),
Brassica juncea (Qasim, 1998). Salinity can affect
chlorophyll content through inhibition of chlorophyll synthesis or an acceleration
of its degradation (Zhao et al., 2007). Thipyapong
et al. (2004) found that the chlorophyll losses due to salinity stress
is consistent with possible differences in ROS production among the genotypes
and suggested that, in salt sensitive genotypes ROS scavenging systems were
unable to detoxify ROS generated.
The chlorophyll loss in leaf results in an increase in Fo (Minimal fluorescence
from dark adapted leaf), in all genotypes studied. Fo ensures that the PSII
reaction centers are in the open state (Baker and Rosenqvist,
2004). Salt stress did not have any significant effect on Fo, in salt tolerant
genotypes. The increase in Fo at highest salt concentration is characteristic
of PSII inactivation and occurred concomitantly to the decrease in Fv/Fm (Zlatev
and Yordanov, 2004). The increased of Fo may be due to reduce in plastoquinon
acceptor (QA) and its ability to maintain oxidized completely (Zlatev
and Yordanov, 2004).
Other researchers suggested that more accumulation of sodium (Na+)
ions in shoots of salt sensitive genotypes (compared to the salt tolerant ones),
is one of the most important factors affecting chlorophyll losses (Dingkuhn
et al., 1992).
In our experiment, Na+ ions of each genotype were significantly
increased under salinity stress, with higher values recorded in the salt tolerant
genotype at highest NaCl concentration with no dramatic differences genotypes.
It seems that increase in Na+ content in this genotype results in
a decline in the osmotic potential of cellular contents and higher level of
water uptake (Zhang et al., 2001).
Mokhamed et al. (2006) noted that Na+
content in leaves of salt tolerant of rape genotype, was higher than salt sensitive
one.
On the contrary, Ashraf and McNeilly (2004) suggested
that the salt tolerant plants of rape was the lowest and the salt sensitive
one was the highest in shoot Na+ concentration when plants were subjected
to saline conditions. Some authors pointed out that salt tolerance in some plant
species has negative correlation with Na+ in plant shoots (Dasgan
et al., 2002). Other researchers suggested that the ability of plants
to accumulate inorganic ions (Na+ and Cl¯), in response to salinity,
may be one of the major criteria of salt tolerance, rape in particular (Zhang
et al., 2001).
Present results indicated that sodium influx from plant cells results in increase
in resistant to salt stress as reported by Mokhamed et
al. (2006) and Siddiqui et al. (2008).
Ashraf and Ali (2008) in their experiments did not observe
remarkable differences in Na+ content of shoots in rape genotypes
under saline conditions (150 mM). Dasgan et al. (2002)
reported that some salt tolerant genotypes of tomato show an inclusion mechanism
for Na+ while others show exclusion mechanism. Sodium transport into
the vacuole can remove toxic effect of sodium from the cytosol; these ions act
as an osmoprotectant within the vacuole (Zhu, 2003).
The role of sodium compartmentation in plant salt tolerance has been demonstrated
in transgenic Brassica napus (Zhang et al.,
2001).
Another indicator of plant salt tolerance is their ability to maintain potassium
(K+) ions at high level of salinity (Blumwald
et al., 2000) and high preservation of K+ content as increasing
NaCl concentration, as indicated in our experiment, results in ionic homeostasis
in the salt tolerant genotypes (Mokhamed et al.,
2006). Protein synthesis in plant cells depends on physiological K+
concentration (Blumwald et al., 2000) and its
conservation in plant cells is essential for cell metabolism.
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