Download Seed germination traits of two plant functional

Journal of
Plant Ecology
VOLUME 4, NUMBER 3,
PAGES 169–177
SEPTEMBER 2011
doi: 10.1093/jpe/rtq025
Seed germination traits of two plant
functional groups in the saline
deltaic ecosystems
Advanced Access published
on 8 September 2010
available online at
www.jpe.oxfordjournals.org
Xiao-dong Zhang, Wen-ting Xu, Bo Yang, Ming Nie and Bo Li*
Coastal Ecosystems Research Station of the Yangtze River Estuary, Ministry of Education Key Laboratory for Biodiversity Science
and Ecological Engineering, Institute of Biodiversity Science, Fudan University, 220 Handan Road, Shanghai 200433, People’s
Republic of China
*Correspondence address. Coastal Ecosystems Research Station of the Yangtze River Estuary, Ministry of
Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science,
Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China. Tel: +86-21-65642178;
Fax: +86-21-65642178; E-mail: [email protected] or [email protected]
Abstract
Aims
Salt stress resulting from soil salinization is one of the driving forces
of the land degradation throughout the world. The modern Yellow
River delta is one of the most saline areas in China. Phytoremediation
can be an effective way to restore the salinized ecosystems, which
requires selecting appropriate plant species. This study explored the
germination responses of common plant species from contrasting
habitats in the Yellow River delta to varying salinity, offering experimental information for ecosystem restoration in the Yellow River
delta.
Methods
In this study, 15 common plant species from the Yellow River delta
were divided into two groups (high-salinity and low-salinity groups)
by their natural habitats using Canonical Correlation Analysis. Seeds
of each species were treated with five salinity levels (0, 5, 10, 20 and
30 ppt), using a randomized complete block design, and germinated
seeds were counted and removed daily for 28 days to calculate the
final germination proportion and mean time to germination. The germination responses of seeds to salinity treatments were compared
between the two groups.
INTRODUCTION
Salt stress resulting from soil salinization is one of the driving
causes of the land degradation throughout the world. Estuarine wetlands occasionally experience seasonally high salinity
resulting from the high evaporation, and are often undergoing
secondary salinization due to their generally lower elevation,
which are exposed to saline groundwater (Jolly et al. 2008).
Nevertheless, a wide range of plant species, growing naturally
on the coastal saline areas, can survive high salinity that is
Important Findings
In relation to salinity, seed germination behavior of the test species
was closely related to the salinity level of the habitats over which they
were distributed. Species from the habitats with higher salinity had
generally higher final germination proportion but shorter mean time
to germination than those from the habitats with lower salinity in all
of five salinity treatments used. The final germination proportion and
mean time to germination of low-salinity group species were more
sensitive to salinity than those of high-salinity group species. Selecting the species with high final germination proportion and short
mean time to germination is important for restoration of salinized
land.
Keywords: deltaic ecosystems
traits d restoration d salinity
d
functional groups
germination
Received: 14 April 2010 Revised: 26 July 2010 Accepted: 1 August
2010
equal to or greater than that of seawater because of their particular physical mechanisms to accumulate ions in their
vacuoles and cytoplasm or excrete the salt out of the leaves
(Gorham 1995). Salt-tolerant species are often selected to reconstruct natural hydrologically-balanced communities. Here,
an important question is: how are the plant species selected for
revegetation? Aronson (1989) defines the species occurring
under naturally saline conditions as ‘halophytes’, and the opposite as ‘glycophytes’. Barrett-Lennard (2002) has summarized the effects of salinity on growth of salt-tolerant
Ó The Author 2010. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China.
All rights reserved. For permissions, please email: [email protected]
d
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Journal of Plant Ecology
species; and Chinnusamy et al. (2005) have reviewed the molecular basis of salt tolerance in plants. However, most previous
studies have focused on the responses of plants to salinity
within species (e.g., Houle et al. 2001; Megdiche et al. 2007;
Rumbaugh et al. 1993). A few studies have been conducted
to explore the general differences in plant traits between
halophytes and glycophytes in relation to salinity. Woodell
(1985) has systematically studied the relationship between
germination dynamics and natural distribution of plant
species. He used 27 coastal plant species, and grouped them
into three types according to their germination responses to
salinity treatments, finding that their natural distribution
could be explained partially by their germination traits. Leyer
and Pross (2009) have also revealed that species restricted to
adjacent habitats have distinctive seed traits and germination
behavior, although the habitats are distinguished by whether
the river is flooded or not. Moreover, the seeds of plants growing in coastal meadows periodically exposed to sea water are
more tolerant of salt stress than those of plants growing in
dunes less exposed to sea water (Necajeva and Levinsh 2008).
The composition, persistence and dynamics of natural plant
communities are greatly influenced by the seed germination
traits, which greatly determine the presence or absence of
one species in particular habitats (Bungard et al. 1997). In salinized lands, continuous evaporation of water deposits salt on
the soil surface, where many seeds are available (Tobe et al.
2001). Thus, germination traits play a major role in the distribution of species in saline environments (Ungar 1978). Most
studies addressing the functional ecology of plant communities
typically start with partitioning the total species pool into distinct clusters based on a set of a priori selected plant traits. Likewise, species in contrasting microhabitats can also be separated
into several functional groups by their growing environments,
and the distinction of some specific biological traits among the
environmentally-based groups could be observed, e.g., seed
traits and germination behavior strongly are related to flooding
status in different habitats (Leyer and Pross 2009). In order to
explore the differences of germination traits of species from salinity-varying environments and to offer experimental information for ecosystem restoration in the Yellow River delta,
we conducted a laboratory study with field calibration. Specifically, the aims of our study reported here are two-fold: (i) to
determine the effects of environmental variables on distribution
patterns of plant species in saline ecosystems and (ii) to compare
the germination traits of plant functional groups classified by environmental variables in relation to salinity.
MATERIALS AND METHODS
Seed collection
The modern Yellow River delta (from 37°35’ to 37°54’N and
118°43’ to 119°20’E), formed from the repeated diversions
of the river course and sediment deposition of the Yellow River
since 1955 (Gao et al. 1989), is one of the saline areas in China.
The hardly ameliorated zone, including salt marshes and the
terraced uplands with secondarily salinized soil, covers
>70% of the total area of the Yellow River delta, and the soil
salt content in this area ranges from 2 to 30 ppt (Guan et al.
2001). Fifteen common species in the Yellow River delta were
selected for germination test, whose major attributes are presented in Table 1, covering ;1600 km2. In mid-November
2006, seeds of 15 species were collected when they were mature in the salt marshes of the Yellow River delta. Bulk seeds of
each species were obtained from at least 50 individuals, but
Table 1: test species and their major attributes
Species
Family
Life forma
Halophyte or glycophyteb
Functional group
Seed mass (mg per seed)
Polygonum aviculare
Polygonaceae
A
Low salinity
2.89 6 0.15
Atriplex centralasiatica
Chenopdiaceae
A
+
High salinity
2.04 6 0.19
Suaeda glauca
Chenopdiaceae
A
+
High salinity
3.37 6 0.07
Suaeda salsa
Chenopdiaceae
A
+
High salinity
1.88 6 0.06
Salsola collina
Chenopdiaceae
A
+
Low salinity
1.11 6 0.09
Abutilon theophrasti
Malvaceae
A
High salinity
8.00 6 0.68
Cucumis bisexualis
Cucurbitaceae
A
+
Low salinity
5.46 6 0.11
Limonium bicolor
Plumbaginaceae
P
+
High salinity
0.46 6 0.03
Apocynum venetum
Apocynaceae
P
+
High salinity
0.37 6 0.01
Metaplexis japonica
Asclepiadaceae
P
High salinity
2.69 6 0.11
Rubia cordifolia
Rubiaceae
P
Low salinity
10.37 6 0.78
Leonurus artemisia
Labiatae
A or B
Low salinity
0.73 6 0.01
Tripolium vulgare
Compositae
A or B
Low salinity
0.74 6 0.11
Artemisia lavandulaefolia
Compositae
P
Low salinity
0.17 6 0.02
Artemisia scoparia
Compositae
A or P
Low salinity
0.08 6 0.02
a
b
+
‘A’ for annual; ‘P’ for perennial and ‘B’ for biennial.
Functional types classified by Zhao and Li (1999): ‘+’ for halophyte and ‘ ’ for glycophyte.
Zhang et al.
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Seed germination traits of plant functional groups
171
those of Cucumis bisexualis from a dozen of individuals as most
plants had shed their seeds. The seeds were stored in refrigerator at 4°C for 2 months to break their dormancy. The florets of
Polygonum aviculare and Limonium bicolor, the bracts of Atriplex
centralasiatica and the cactus-like perianth of Suaeda glauca
were removed to ensure the presence of seeds. Follicles and
seed hairs were removed from Apocynum venetum and Metaplexis japonica seeds. The pericarps of C. bisexualis and Rubia cordifolia were removed from the seeds. Seeds of the other species
could be directly obtained. Seeds were only used if they had
intact seed coats and appeared to have embryos. Seed mass
was obtained by weighing three lots of one-hundred seeds
for each species.
Germination test
Germination experiments were carried out from mid-April to
mid-May 2007. Fifteen species were tested for their germination responses to varying salinity. All seeds used in this study
were surface sterilized in 1% sodium hypochlorite for 1 min,
followed by repeated rinsing with distilled water twice. One
experimental unit consisted of 50 seeds of one species evenly
placed on five filter paper layers imbibed with 10 ml of NaCl
solution of constant concentration in 90-mm petri dishes with
covers. Totally 1000 seeds of each species were treated at five
salinity levels (0, 5, 10, 20 and 30 ppt), with four replicates per
treatment. The experiment used a randomized complete block
design, with the petri dishes blocked by growth chambers. Germination was carried out in the dark at 20°C, and seeds were
considered as germinated when their emerging radicals were
visible. Germinated seeds were counted and removed daily for
a period of 28 days, after which no more germination was observed. In order to avoid the variation in salinity due to evaporation, one empty petri dish with a five-filter-paper layer
imbibed with 10-ml deionized water was placed in the same
chamber as control check. The mass of the petri dishes was
weighed before and after being placed in the chamber for 1
week, and 2 g of mass loss was found, indicating that 2 g of
water was evaporated within 1 week. Thus, 2 ml of deionized
water was added to each dish to keep the moisture and salinity
of filter paper.
Species grouping based on field investigation
It is hard to identify whether one species is halophyte or
glycophyte. In order to determine if a species is adapted
to saline environments, a field investigation was carried
out in September 2007. Twelve sites (Fig. 1) were randomly
selected in the Yellow River delta to explore the relationship
between species distribution and environmental factors of
natural communities, with six 1 3 1 m quadrats for each site.
A total of 72 quadrats were sampled, and the abundance
of each species was recorded. Three evenly distributed
soil cores of 2.8 cm in diameter and 15 cm in depth were
taken and mixed within each quadrat. Soil pH, salinity
and water content of each quadrat with three replicates
were measured.
Figure 1: the sampling sites in the Yellow River delta. Circles stand for
sampled communities along the Yellow River and triangles for the
towns nearby the sampled sites.
The abundance of the 13 species (other two species
not included and explained below) were extracted and
transformed to frequency by the formula:
Aij
Qij =
13
;
+ Aij
j
1
where Qij stands for the frequency of species j in quadrat i, Aij is
the abundance of species j in quadrat i and j is from 1 to 13. The
frequency matrix of 13 species in 72 quadrats with environmental
variables, pH, salinity and water content was analyzed by Canonical Correlation Analysis (CCA) method using CANOCO V. 4.5.
Data analyses
The final germination proportion was calculated and the mean
time to germination was then estimated by the formula
(Brenchley and Probert 1998):
28
+ Nij 3 dj
MTG =
j
1
28
;
+ Nij
j
1
where MTGi is the mean time to germination of species i, and
Nij is the number of seeds of species i germinating on day j.
The means of final germination proportion and mean time
to germination for five treatments of high-salinity group and
low-salinity group species (based on the result of species
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Journal of Plant Ecology
grouping) were compared using independent sample t-test. In
order to avoid the interspecific difference in the trends of final
germination proportion and mean time to germination with
the increasing salinity, the data on final germination proportion were transformed to relative final germination proportion
as proportionF/proportionM, where proportionF is the final
germination proportion in each salinity treatment and proportionM is the mean proportion of final germination in control
treatment. The trends of relative final germination proportion
and mean time to germination of high- and low-salinity group
species were linearly regressed and an analysis of covariance
(ANCOVA) was performed to test the difference in slope of
two groups with salinity as covariate. All statistic analyses were
analyzed by using SPSS13.0.
RESULTS
Species grouping
The 15 species were separated into two groups (Fig. 2), highand low-salinity group species, which correspond to halophytes
and glycophytes, respectively. The mean salinity of the habitats
for the upper cluster was >3 ppt and that for the lower cluster of
seven species was all <2 ppt. However, two species, Leonurus artemisia and R. cordifolia, were not included in CCA (Fig. 2), because they did not occur in randomly selected quadrats but were
common in the Yellow River delta. Because these two species
are typical glycophytes (Zhao and Li 1999), they were classified
into low-salinity group. The high-salinity group species, including A. centralasiatica, S. glauca, Suaeda salsa, L. bicolor, A. venetum
and M. japonica, were mainly distributed in the environments
with the salinity ranging from 0.7 to 14.8 ppt with the mean
(6standard error (SE)) of 4.46 6 2.22 ppt, and the low-salinity
group species, including P. aviculare, Salsola collina, C. bisexualis,
R. cordifolia, L. artemisia, Tripolium vulgare, Artemisia lavandulaefolia, Abutilon theophrasti and Artemisia scoparia, were distributed
mainly in the environments with the salinity ranging from 0.5
to 3.1 ppt with the mean (6SE) of 1.51 6 0.34 ppt. The first two
axes of CCA of species–environment relations explained 89.5%
of the total variance.
Final germination proportion
The mean final germination proportion of high-salinity group
species was significantly higher than that of low-salinity group
species in all of five salinity treatments (Fig. 3 and Fig. 4A).
Final germination proportions of almost all high-salinity group
species were very high, being close to 1.0 in controls, while
those of only four of low-salinity group species including C.
bisexualis, S. collina , R. cordifolia and T. vulgare were slightly
>0.5 in controls. Five low-salinity group species including C.
bisexualis, L. artemisia, P. aviculare, R. cordifolia, A. scoparia
and A. lavandulaefolia failed to germinate at the salinity of
>20 ppt, while germination of all high-salinity group species
occurred in all salinity treatments (Fig. 3).
Final germination proportion of both high- and low-salinity
group species generally decreased with increasing salinity, but
Figure 2: thirteen species were classified into high- and low-salinity
groups by environmental factors. Closed symbols stand for halophytic
species and open symbols for glycospecies species according to Zhao
and Li (1999). Numbers in parentheses indicate the salinity of the habitats of the species. Species codes are 1, Atriplex centralasiatica; 2, Cucumis bisexualis; 3, Artemisia lavandulaefolia; 4, Limonium bicolor; 5,
Metaplexis japonica; 6, Salsola collina; 7, Tripolium vulgare; 8, Abutilon theophrasti; 9, Polygonum aviculare; 10, Suaeda glauca; 11, Artemisia scoparia;
12, Apocynum venetum and 13, Suaeda salsa.
high-salinity group species were obviously less sensitive to salinity than low-salinity group species. Though the relative germination proportion of both groups was negatively related to the
salinity (Table 2), ANCOVA results showed that the interaction
between salinity and functional group was significant (Table 3),
indicating that the relative germination proportion of low-salinity group species declined with increasing salinity faster than
that of high-salinity group species. Final germination proportion
of A. centralasiatica, A. venetum, S. salsa and M. japonica of highsalinity group even had a slight increase from 0 to 5 ppt of salinity, but no species in low-salinity group had similar responses
at low salinities. Final germination proportions of five of highsalinity group species were close to 1.0 at salinities of 0, 5 and 10
ppt, S. glauca and S. salsa of which showed consistently high germination proportion even at salinities of 20 and 30 ppt. Metaplexis
japonica, A. venetum and L. bicolor had a dramatic decline in final
germination proportion from >0.8 to <0.5 between 10 and 20
ppt. Final germination proportions of six species of low-salinity
group declined from 5 ppt (Fig. 3).
Mean time to germination
Mean time to germination of both functional groups increased
with increasing salinity, and that of low-salinity group was significantly longer than that of high-salinity group for all of five
Zhang et al.
|
Seed germination traits of plant functional groups
173
Figure 3: final germination proportion and mean time to germination (days) of 15 common species collected in the Yellow River delta.
salinity treatments (P < 0.05) (Fig. 4B). Mean time to germination of high-salinity group species was <3 days, while that of
the low-salinity group species steadily increased from 3.8 to 6.8
days when the salinity increased from 0 to 10 ppt. Both of the
two functional groups had a dramatically increasing mean
time to germination between 10 and 20 ppt, from 2.8 to 6.5
days for high-salinity group and from 6.8 to 14.4 days
for low-salinity group. Cucumis bisexualis, P. aviculare and
R. cordifolia failed to germinate at 20 and 30 ppt, and L. artemisia
and A. scoparia failed to germinate at 30 ppt (Fig. 3).
Salinity treatments generally increased the mean time to
germination of both groups (Table 2), but high-salinity group
species were obviously less sensitive to salinity than lowsalinity group species. The ANCOVA results showed that the
interaction between salinity and functional group was significant (Table 3), indicating that the mean time to germination of
174
Journal of Plant Ecology
study, we focused on the salinity effects on two germination
traits, final germination proportion and mean time to germination. Distinctive patterns of germination traits of high- and lowsalinity group species were observed in relation to salinity.
Classification of functional groups
Traditionally, four trait-based classification systems have been
used to divide plant species into different functional groups
(Lavorel et al. 1997), one of which is built on the relationship
between species distribution pattern and the environmental
factors (Deckers et al. 2004; Purdy et al. 2005). Here we divided
the 15 species into two groups based on their natural habitats
that are mainly different in soil salinity. Although the three
environmental factors, pH, salinity and water content, might
not be adequate to characterize the habitats, the community
composition in the Yellow River delta is determined primarily
by soil salinity (Zhang et al. 2007). Similar results can be observed in similar ecosystems, e.g., the upper intertidal ecosystems in south California (Callaway et al. 1990) and the brackish
riverbanks in New Zealand (Wilson et al. 1996). Our CCA
results demonstrated that the distribution pattern of highsalinity group species was determined primarily by soil salinity,
while that of low-salinity group species was affected by several
other factors. However, in this study some of the previously
defined halophytes were classified into glycophytes or vice
versa. Three species (S. collina, C. bisexualis and T. vulgare) that
are considered as halophytes by Zhao and Li (1999) were
sorted into low-salinity group, while two species (M. japonica
and A. theophrasti) that are not considered as halophytes were
sorted into high-salinity group in this study. This inconsistence
may to a certain degree reflect the complex responses of plants to
salt stress through tolerance or adaptation. The germination behavior of these above five species was similar to that of the other
species within their groups (Fig. 3). Therefore, the classification
of the 15 species by the seed germination traits basically reflected
their distributions that were observed in the field.
Figure 4: germination responses of high- and low-salinity group species to salinity (ppt). (A) Mean germination proportion; and (B) Mean
time to germination (days). Values are mean 6 SE. Significant differences (t-test, P < 0.05) are marked by asterisks.
low-salinity group species increased with increasing salinity
faster than that of high-salinity group species. Mean time to
germination of A. theophrasti, A. venetum, L. bicolor and M. japonica of high-salinity group increased dramatically from 10
to 20 ppt and that of others of high-salinity group only slightly
increased through all five treatments.
DISCUSSION
Species growing in similar habitats often have similar ways of
responding to the abiotic factors. The aim of this paper was to
explore the differences of germination traits between two
functional groups that are distributed over and might have adapted themselves to the environments of varying salinity. In this
Differential germination responses of two groups to
salinity
For individual species, it has been widely recognized that final
germination proportion decreases and mean time to germination increases along the salinity gradient (Egan and Ungar
1999; Huang et al. 2003; Nolasco et al. 1996), but how different
germination traits are among the functional groups remains
largely unclear.
This study provided the evidence that seed germination was
strongly correlated with the salinity under natural conditions
in which plants grow. Generally, the final germination proportion of high-salinity group species was higher than that of lowsalinity group species (Fig. 4A), and the high-salinity group
species germinated faster than low-salinity group species in
all of five salinity treatments (Fig. 4B) as observed in previous
studies (Ungar 1978). This implies that species growing in saline habitats might have adapted themselves to the high soil
salinity, possessing the ability of well germinating in saline
Zhang et al.
|
Seed germination traits of plant functional groups
175
Table 2: summary of linear regression analyses between salinity (x) and final germination proportion and mean time to germination (y) of
two functional groups
R2 (adjusted)
P
0.020
0.479
<0.001
0.034
0.715
<0.001
1.468
0.199
0.223
<0.001
3.581
0.358
0.327
<0.001
y
Groups
Intercept
Final germination
proportion
High salinity
1.078
Low salinity
0.965
Mean time to
germination
High salinity
Low salinity
Slope
Table 3: analysis of covariance testing the difference in germination response of two functional groups (main factor) to salinity (covariate)
Final germination proportion
Mean time to germination
Source of variation
Degrees of freedom (df)
F
P
df
F
P
Salinity
1, 296
269.49
<0.001
1, 296
18.88
<0.001
Functional group
1, 296
83.72
0.008
1, 296
13.38
0.001
Salinity 3 group
1, 296
2.13
<0.001
1, 296
7.12
0.008
environments. High germination proportion and short germination time could facilitate plants to get established faster in
the salt marshes when their seeds are deposited on a ‘safe site’
during their drift with the tidal water (Mariko et al. 1992). Final
germination proportion and mean time to germination of
high-salinity group species were almost unaffected by low salinities of 5 and 10 ppt (Fig. 3), which has also been observed in
other salt marsh plants (Mariko et al. 1992) and inland desert
halophytes (Yang et al. 2009). Two species (S. salsa and S.
glauca) had high final germination proportion and short mean
time to germination even at 30 ppt, at which the other species
hardly germinated. In the field, S. salsa is the pioneer species in
newly formed mudflats and most saline environments where
other species cannot grow well. S. glauca seemed not to be as
salt-tolerant as S. salsa as it is usually mixed with other species
in upland rather than intertidal areas. However, S. glauca may
be more competitive than S. salsa as the former always has
greater aboveground biomass and higher occurrence frequency in mildly saline communities (He et al. 2009).
The low-salinity group species with lower germination proportion and longer germination time (Fig. 4) may take the riskavoiding strategy through delaying germination, making their
seeds geminate in milder environments with lower salinity and
higher temperature so that they are able to become a stronger
competitor during the growing season (Purdy et al. 2005; Ungar
1998). Species from unstable and unpredictable environments
may have low germination proportion as well as germination
speed because seedling establishment is unpredictable as germination phase of a species lasting for a longer time can avoid
the risk of seedling mortality caused by environmental stochasticity like disastrous events (Mariko et al. 1992). Final germination proportions of all low-salinity group species were
close to zero when the salinity increased to 20 ppt (Fig. 3).
Although it is hard to ascertain whether or not the decrease
of final germination proportion at high salinity is adaptive,
the inhibition might be an advantage for plants to keep their
seed bank and wait for the coming of benign conditions.
Final germination proportion and mean time to germination
of low-salinity group species were more sensitive to salinity
than those of high-salinity group species (Table 2), which
basically explained the natural distribution of the study species
in the field. When the soil salinity was low, the low-salinity
group species dominated the communities as they were more
competitive during growing stage. On the other hand, when
the soil salinity was high, the high-salinity group species
dominated the communities in the field partially because they
had good germination performance under the saline
environments.
Implications for land restoration
Studying seed germination of functional groups in saline environments can provide important information for species selection in ecosystem restoration. Species in the same group have
similar patterns of responding to salinity. Although not all species in the saline deltaic ecosystems were tested in this study,
the patterns revealed from this study may also help us to predict the germination dynamics of untested species. An earlier
study conducted by Woodell (1985) grouped coastal plants into three types according to their germination behavior in response to salinity and explained their distribution patterns in
the field through using their germination traits. Our study
shared the similar aim to Woodell’s (1985), while it was verified in an opposite way. Woodell’s sites and ours were both
located close to the sea, and the ecosystems were largely affected by salinity. Thus, the relationship may be applicable
to similar coastal ecosystems, while the application in other
systems (e.g. arid regions) needs further research.
Halophytes’ seeds possess varying degree of salt tolerance
during germination (Ungar 1995). The salinity of the natural
habitats (<10 ppt for high-salinity group species and <2 ppt for
176
low-salinity group species) was much lower than that used in
this experiment. Final germination proportion and mean time
to germination of a particular species in a natural community
cannot be directly determined only by the salinity level in its
habitats, but should also be linked to other environmental conditions such as temperature and water availability at the time
of germination (Noe and Zedler 2000). The mean temperature
in the Yellow River delta in spring (April) when most of seeds
germinate was 13.4°C in 2007, which was much lower than
the temperature used in our experiment. Another possible reason for the high final germination proportion was that the
seeds were placed on filter paper in water-filled petri dishes,
while the actual soil water content was much lower in the
field. In addition, the soil salinity fluctuates seasonally as
the precipitation varies throughout the year (Sumner and
Belaineh 2005). In spring, the precipitation was 14.7 mm, being much lower than that in August (237.9 mm) (Zhao 2007),
and thus, the soil salinity may be much higher during the germinating season than during the growing season. This may indicate that many species in the Yellow River delta may be filtrated
during the germination stage at which the conditions are much
more severe than those during the growing season. This has also
proved that selecting the species with high final germination proportion and short mean time to germination is important for the
restoration of saline lands.
CONCLUSIONS
Our study demonstrated that seed germination behavior of
plant species, in relation to soil salinity, was closely related to
the salinity of the habitats over which they are distributed.
High-salinity group species had generally higher final germination proportion and shorter mean time to germination than
low-salinity group species in all of five salinity treatments. Final
germination proportion and mean time to germination of lowsalinity group species were more sensitive to salinity than those
of high-salinity group species. Our results suggested that some
species in Yellow River delta may be filtrated during the germination stage at which the conditions are much more severe than
those during the growing season, especially in the environments with high salinity. Therefore, selecting the species that
have greater germinating ability and germinate faster is critical
to the success of saline land restoration through revegetation.
FUNDING
National Basic Research Program of China (2006CB403305),
and a Research Award for Outstanding Doctoral Students of
Fudan University to Xiao-dong Zhang.
ACKNOWLEDGEMENTS
We thank Mr. Naishun Bu for field guidance and seed collection and
Ms. Tingting Zhang for drawing the map. Our appreciation also goes to
Prof. Fazeng Li for species identification.
Conflict of interest statement: None declared.
Journal of Plant Ecology
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