INTRODUCTION
Drought can be simply defined as a period of below normal
precipitation that limits plant productivity in a natural or agricultural
system. In the field, drought can cause a number of plant stresses including
temperature, light and nutrient stresses. However, the stress component
that defines drought is a decrease in the availability of soil water.
This decreased water availability can be quantified as a decrease in water
potential (Kramer and Boyer, 1995). Decreased water potential (decreased
free energy of the water) makes it more difficult for the plant to take
up water and this in turn elicits a range of responses that allow the
plant to avoid water loss, allow water uptake to continue at reduced water
potential or allow the plant to tolerate a reduced tissue water content.
The physiological mechanisms involved in cellular and whole plant responses
to water stress therefore generate considerable interest and are frequently
reviewed (Smith and Griffiths, 1993; Kramer and Boyer, 1995;
Neumann, 1995, 1997; Turner, 1997).
Shoot growth slowed. Plant leaves grow by increasing
cell numbers through division and then expanding these cells using water
in much the same way as air is used to blow up a balloon. When water is
limiting cell expansion is reduced.
Seed germination is usually the most critical stage in
seedling establishment, determining successful crop production (Almansouri
et al., 2001). Crop establishment depends on an interaction between
seedbed environment and seed quality (Khajeh-Hosseini et al., 2003).
Factors adversely affecting seed germination may include sensitivity to
drought stress (Wilson et al., 1985) and salt tolerance (Sadeghian
and Yavari, 2004). Seeds sown in seedbeds having unfavorable moisture
because of limited rainfall at sowing time yield in poor and unsynchronized
seedling emergence (Mwale et al., 2003), affecting the uniformity
of plant density with negative effects on yield. Salinity may also affect
the germination of seeds by creating an external osmotic potential that
prevents water uptake or due to the toxic effects of sodium and chloride
ions on the germinating seed (Khajeh-Hosseini et al., 2003).
Water availability and movement into the seeds are very
important to promote germination, initial root growing and shoot elongation
(Bewley and Black, 1994). Only, highly negative water potential, especially
in early germination, may influence the seeds water absorption, making
germination not possible (Braccini et al., 1996).
To allow germination, there is minimum moisture that
the seed should get and it depends on its chemical composition and of
the permeability of the tegument.
With water absorption, tissues will be rehydrated and
consequently starting intensification of breathing and of all the other
metabolic activities, that culminate in the necessary supply of energy
and nutrients to restart the embryonic axis growth (Carvalho and Nakagawa,
1988). In order for the process of germination to start, it is necessary
for the seed to reach an adequate level of hydration, which will permit
a reactivation of the metabolic processes (Cordoba et al., 1995).
Induced water deficit by polyethylene glycol showed similar
values to that observed in the fields (Thill et al., 1979), permitting
also vigour evaluation. In similar potential ranges, germination patterns
may be different between species or even between varieties of the same
species (Therios, 1982). Some species, as maize are sensitive to sodium
chloride during germination.
Polyethylene glycol of high molecular weight range (6000
or above) can not enter the pores of plant cells (Oertli, 1985) and thus
causes cytorrhysis rather than plasmolysis. Polyethylene glycol is also
a better choice for imposing low water potential than the often used solute
mannitol because mannitol has been shown to be taken up by plant cells
and can cause specific toxic effects on growth (Hohl and Schopfer, 1991;
Verslues et al., 1998).
Polyethylene glycol is the best solute that we are aware
of for imposing a low water stress that is reflective of the type of stress
imposed by a drying soil (Verslues and Bray, 2004; Verslues et al.,
1998; Van der Weele et al., 2000).
In this study, the effect of induced water deficit, either
by polyethylene glycol or by sodium chloride, was evaluated observing
the germination of two maize cultivars. We wanted to know which of these
materials had higher inhibitory effects in growth and germination of maize
plants.
MATERIALS AND METHODS
Plant materials and growth conditions: This study was conducted
at biochemistry laboratory, Department of Biology, Urmia University, Iran,
during the spring of 2007. Two genotypes of maize (Zea mays L.)
var. 704 and var. 301 were used. The seeds of both cultivars were germinated
in Petri dishes on two layers of filter paper at 25°C in an incubator.
After three days, the seedlings transferred to plastic pots (15 cm diameter,
20 cm depth) filled with sand and irrigated with half strength of Hoagland
nutrient solution. Six days seedlings were transferred to hydroponics
culture of aerated test tubes containing polyethylene glycol (PEG) 6000
or sodium chloride (NaCl) solutions with osmotic potential -0.15,
-0.49, -1.03 and -1.76 MPa, respectively (Table 1) (Burlyn
and Mirrill, 1973; Steuter et al., 1981; Nicholas, 1989) as treatments
and aerated test tubes containing half strength Hoagland nutrient solution
which served as control. Stress was applied for 24 h. Then, roots and
shoots length and dry weight of both varieties were measured.
Roots and shoots length and dry weight measurements: The length
of roots and shoots were measured and after that plant were dried at 105°C
until reached constant weight for the determination of dry weight (Fletcher,
1988).
Germination test: Seeds were submitted to germination, using include
osmotic potentials (0, -0.15, -0.49, -1.03, -1.76 MPa) of sodium chloride
and polyethylene glycol 6000 (Braccini et al., 1996). Germination
test was conducted with four replications per treatment with 25 seeds
each. Seeds were put in Petri dishes included three paper towels, moistened
to 2.25 its weight with one of the solutions mentioned before (Krzyzanowski,
1991). Germination was evaluated at day 3 after cultivation. The number
of seeds germinated was counted regularly and after final germination
the germination percentage was estimated. Only normal seedlings were counted
to determine germination percentage (Brasil, 1992).
Statistical analysis: Mean values were taken from measurements
of four replicates and Standard Error of the means was calculated. Differences
between means were determined by One-way ANOVA and Turkey`s multiple range
tests (p<0.05). Analyses were done using the SPSS (version 13.0).
RESULTS AND DISCUSSION
Five water potential was used: 0, -0.15, -0.49, -1.03 and -1.76 MPa.
Results of shoot and root length (Table 1, 2)
were similarly affected by sodium chloride and polyethylene glycol induced
water deficit. At zero potential, both shoot and root lengths reached
their highest values. All other treatments gradually reduced the seedling
growing. Kramer (1974) reported that the first
| Table 2: |
Effects of osmotic potential induced
by PEG 6000 and NaCl on roots length (mm) of two maize cultivars* |
 |
| *: Results are shown as mean?standard
error (p<0.05), obtained from four replicates |
| Table 3: |
Effects of osmotic potential induced
by PEG 6000 and NaCl on shoots length (mm) of two maize cultivars* |
 |
| *: Results are shown as mean?standard
error (p<0.05), obtained from four replicates |
| Table 4: |
Effects of osmotic potential induced
by PEG 6000 and NaCl on dry weight of roots (g/10 seedlings) of
two maize cultivars* |
 |
| *: Results are shown as mean?standard
error p<0.05), obtained from four replicates |
effect measurable due to water deficit was the growth reduction, caused
by the declining in the cellular expansion. The cellular elongation
process and the carbohydrates wall synthesis were very susceptible to
water deficit (Wenkert et al., 1978) and the growing decrease was a
consequence of the turgescence laying down of those cells (Shalhevet
et al., 1995).
Dry weight of shoot and root were affected by water deficit (Table
3, 4), but the roots affected higher than shoots
and sodium chloride has higher effect than PEG 6000. Water deficit at
-0.15 and -0.49 MPa showed higher root dry weight, at -1.03 and -1.76
MPa there were a significant reduction in root dry weight for both cultivars
at both treatments. According to Marur et al. (1994), water restriction
acted slowing physiological and biochemical processes and seedlings
at low water deficit showed a weak growing leading to a lower accumulation
of dry matter.
Braccini et al. (1996) reported that soybean roots exposed to water
deficit were well developed than the roots that grew without water deficit.
But, in our experiment, roots length decreased with increasing PEG and
NaCl concentrations. It seems that increase of root length is because
of uptake water from water resource and in our experiment we had drought
stress in water solutions of NaCl or PEG (physiological drought), therefore,
roots of both varieties decreased.
Results showed that the germination is inversely proportional to the
NaCl and PEG concentrations, it means that 704 and 301 cultivars of
maize showed a reduction in germination with an increasing in NaCl or
PEG concentrations induced water deficit (Table 5,
6), but this reduction in NaCl treatment were higher
than PEG treatment. At treatment by PEG, the germination was severely
decreased at -1.03 MPa, no germination occurred at -1.76 MPa. At treatment
by NaCl no germination occurred at -1.03 and -1.76 MPa in 301 var. and
germination was very low at -1.03 MPa and no germination occurred at
-1.76 MPa in 704 var. According Mayer and Poljakoff-Mayber (1989) results
like this could be attributed to absence of energy to start the germination
process, as energy was obtained by increments in the respiratory pathway
after the imbibition and in low levels of water potential tax water
absorption was processed slowly.
At control plants (Zero potentia L.), 301 var. presented a low germination
than 704 var. In water stress, germination decreased at both treatments
in both
| Table 5: |
Effects of osmotic potential induced
by PEG 6000 and NaCl on dry weight of shoots (g/10 seedlings) of
two maize cultivars* |
 |
| *: Results are shown as mean?standard
error p<0.05), obtained from four replicates |
| Table 6: |
Effects of osmotic potential induced
by PEG 6000 and NaCl on seed germination (%) of two maize cultivars* |
 |
| *: Results are shown as mean?standard
error (p<0.05), obtained from four replicates |
varieties. At treatment by PEG, in 704 var. germination
were 94.5% in control plants (Zero potentia L.) and decreased to
24.75% in PEG 30% (-1.03 MPa). In 301 var. germination were 82.75% in
control plants (Zero potentia L.) and decreased to 9.75% in PEG
30% (-1.03 MPa), but the germination decreased to zero in PEG 40% (-1.76
MPa) in both varieties. At treatment by NaCl, in 704 var. germination
were 92.25% in control plants (Zero potentia L.) and decreased
to 4.25% in -1.03 MPa. In 301 var. germination were 88% in control plants
(Zero potentia L.) and decreased to 0% in -1.03 MPa, the germination
decreased to zero in -1.76 MPa in both varieties.
The percentage results presented drastic decrease as
the potential went from -0.49 to -1.03 MPa in PEG and NaCl solutions.
Analysing the results of percentage in PEG and NaCl solutions, it was
seen that in low water potential (under -0.5 MPa), the largest reduction
found with PEG solutions compared to NaCl treatment, was also observed
by Nassiff and Perez (1997) in seeds of Pterogyne nitens.
When the potential was sufficiently low, such as -0.5
MPa, the seeds could contain sufficient water to start the germination
process (Phase I and II) without however, passing to root cell growth
(Phase III). The process of elongation and the cellular wall synthesis
are highly sensitive to water deficiency (Wenkert et
al., 1978) and reduction in growth could be due to the decrease
of turgor of these cells.
Water deficit worked decreasing velocity and seed germination
percentage and for each species there was water potential, that under
it germination did not occur (Adegbuyi et al., 1981). Germination
patterns could be different between species and between different varieties
in the same species (Therios, 1982).
These results could be attributed to the reduction of
the osmotic potential. Van der Mozel and Bell (1987) related that NaCl
could affect germination, as by the ionic effect, as by ion cell reaching
toxic levels or for combination of both. At -1.76 MPa both cultivars failed
to germinate. Santos et al. (1992) reported that soybean seed germination,
with high vigour, was null, when germinated in solutions of NaCl at -1.5
MPa.
There was significance in the interactions between cultivars
and NaCl induced water deficit. Shoot dry weight gradually decreased with
water deficit increase. Similar results were obtained by Santos et
al. (1992). There was significance in the interaction between cultivars
and NaCl induced water deficit, which led to reduction in shoot and root
length, especially at -1.03 and -1.76 MPa. Both cultivars showed very
good germination at osmotic potential zero, they were more sensible than
PEG to salt imposed water deficit. However, the results obtained with
NaCl did not match with the one observed for polyethylene glycol and NaCl
has higher effect than PEG, meaning that there was another kind of factor
acting in this case. It suggesting that NaCl and PEG acted through different
mechanisms.
Sodium chloride can be a strong osmotic agent, but it
affects the development just by increasing the sodium concentration in
the growing medium.
Sodium is a small ion that can pass easily throughout
cellular membranes and cells must pump it out expending energy to do that,
otherwise the water activity decrease and all the metabolic pathways can
be disturbed or disrupted, causing some misbalance in the energy production-consumption.
According Braga et al. (1999), potentials between -0.4 and -0.6
MPa declined all parameters (germination percentage, size and seedling
weight), in common bean plants and seed with low physiological quality
showed higher decrease when submitted to lower water potentials.
Germination, in strongly negative water potentials, especially
at the beginning of the imbibition could influenciate water absorption
by the seeds and this event could turn not viable the germination process
(Bansal et al., 1980).
Water deficit, induced by sodium chloride or polyethylene
glycol, affected germination and seedling development. Germination was
severely affected at -1.76 MPa. Parameters that evaluated seedling development
were more affected by water deficit than germination. Beginning at -0.15
MPa seedling started shoot and root growth reduction. Dry weight shoot
and root affected highly by water stress at both treatments.
For shoot length, cultivars showed the best result at
no water deficit, decaying highly at -1.76 MPa, what was according to
obtained data of Torres et al. (2000), where the increase in water
deficit represented a reduction in seedlings.
We conclude that NaCl and PEG adversely affected the
germination and seedling growth of maize. In low water stress PEG had
a greater inhibitory effect in germination than did NaCl. Our results
agree with those given by Murillo-Amador et al. (2002), who observed
that NaCl had a lesser effect on the germination of cowpea than did PEG
and Sadeghian and Yavari (2004), who stated that seedling growth was severely
diminished by water stress in sugar beet.
Moreover, distinct genetic differences were found among
the cultivars with respect to germination and seedling growth subjected
to NaCl and PEG.
Root and shoot length and seedling fresh and dry weight
were decreased by increasing NaCl and PEG concentrations. Consequently,
seedling growth was inhibited in maize. Differences between NaCl and PEG
were significant. Growth inhibition of NaCl was higher than that of PEG.
However, our findings showed that NaCl had greater inhibitory effects
on seedling growth than on germination. Higher germination in NaCl than
in PEG in low water stress could be explained by more rapid water uptake
in NaCl solutions and achievement of a moisture content that allowed germination.
Khajeh-Hosseini et al. (2003) suggested that the achievement in
NaCl solutions was due to rapid imbibition in soybean seeds.
Stress inhibition of germination could not be attributed
to an inhibition of mobilisation of reserves and that the main effect
of PEG occurred via an inhibition of water uptake while detrimental effects
of NaCl may be linked to effects of accumulated toxic ions.
In conclusion, in the germination and early seedling
growth stages the investigated cultivars showed different responses to
water stress induced by PEG and NaCl. However, seedling growth was more
sensitive to NaCl than was germination.
We suggest that effect of water stress induced by mannitol,
sucrose or other materials on growth and germination study in maize cultivars
and compare with our results.
ACKNOWLEDGMENTS
This work supported by Biology Department of Urmia University,
Iran. We greatly acknowledge Dr. M. Ilkhanipour for her support in providing
facilities.
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