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EFFECTS OF SOIL SALINITY IN THE
GROWTH OF AMBROSIA ARTEMISIIFOLIA
BIOTYPES COLLECTED FROM ROADSIDE
AND AGRICULTURAL FIELD
a
b
Seok Hyun Eom , Antonio DiTommaso & Leslie A. Weston
c
a
Department of Horticultural Biotechnology , Kyung Hee University ,
Yongin , Republic of Korea
b
Department of Crop and Soil Sciences , Cornell University , Ithaca ,
New York , USA
c
Plant Biology and Weed Science, School of Agricultural and Wine
Sciences , Charles Sturt University , Wagga Wagga , Australia
Published online: 25 Oct 2013.
To cite this article: Seok Hyun Eom , Antonio DiTommaso & Leslie A. Weston (2013) EFFECTS
OF SOIL SALINITY IN THE GROWTH OF AMBROSIA ARTEMISIIFOLIA BIOTYPES COLLECTED FROM
ROADSIDE AND AGRICULTURAL FIELD, Journal of Plant Nutrition, 36:14, 2191-2204, DOI:
10.1080/01904167.2013.836226
To link to this article: http://dx.doi.org/10.1080/01904167.2013.836226
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Journal of Plant Nutrition, 36:2191–2204, 2013
C Taylor & Francis Group, LLC
Copyright ISSN: 0190-4167 print / 1532-4087 online
DOI: 10.1080/01904167.2013.836226
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EFFECTS OF SOIL SALINITY IN THE GROWTH OF AMBROSIA
ARTEMISIIFOLIA BIOTYPES COLLECTED FROM ROADSIDE
AND AGRICULTURAL FIELD
Seok Hyun Eom,1 Antonio DiTommaso,2 and Leslie A. Weston3
1
Department of Horticultural Biotechnology, Kyung Hee University, Yongin, Republic of
Korea
2
Department of Crop and Soil Sciences, Cornell University, Ithaca, New York, USA
3
Plant Biology and Weed Science, School of Agricultural and Wine Sciences, Charles Sturt
University, Wagga Wagga, Australia
2
Morphological differences were observed between roadside (R) and agricultural field (F) biotypes of Ambrosia artemisiifolia, in which R-type seedlings were shorter and produced larger and
heavier seeds under greenhouse grown conditions. Previous findings indicated that A. artemisiifolia R-types exhibited greater salt tolerance with respect to germination. However, the impact of
biotype and salt tolerance on morphological variation has not been investigated in A. artemisiifolia plants. After performing replicated greenhouse experiments with both biotypes, it was shown
that salinity level was a critical factor influencing both seedling and mature plant size and this
response was dependent upon biotype. The R-type exhibited slight but significant increases in growth
at low/mild salinity levels (50–100 mM) compared with non-saline conditions, while the F-type
exhibited significantly reduced growth at the low/mild salinity levels. The reductions in growth of
F-type plants in low/mild salinity were similar to those reductions of R-types observed in non-saline
conditions. As both biotypes produced seeds at low/mild salinity levels, we conclude that low/mild
salinity affects A. artemisiifolia plant size and overall growth rate, and secondly, certain F-type
plants may acclimate to the roadside environment over time by reducing their size while producing
larger seed under saline conditions. It is possible that this species may exhibit changes in morphology
after several generations of exposure to saline roadside conditions. Toxicity due to salt treatment at
high salinity (400 mM) was observed in both biotypes, whereas the R-type was more tolerant to both
low and high salinity levels with respect to seed germination. Differential A. artemisiifolia growth
responses which occur from seed germination to plant maturity may be partially attributed to its
ability to tolerate saline soil conditions both under greenhouse and field conditions. This ability to
tolerate saline conditions may be especially important in early spring when roadside soils experience
increased salinity, caused by de-icing salt treatments applied during the winter season.
Received 23 February 2011; accepted 13 September 2011.
Address correspondence to Seok Hyun Eom, Department of Horticultural Biotechnology, College
of Life Sciences, Kyung Hee University, Yongin 446-701, Republic of Korea. E-mail: [email protected]
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S. H. Eom et al.
Keywords: common ragweed, germination, ion content, salt tolerance, water
transpiration
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INTRODUCTION
Plant distribution in complex ecosystems often occurs in response to various environmental factors that influence rhizospheric conditions (including soil type and texture, nutrient availability, microorganism colonization,
and water/gas content), atmospheric conditions (atmospheric composition,
light availability and temperature), and plant-plant interactions (Hoffman
and Blows, 1994; Klironomos, 2002). As a plant distributional barrier, soil
salinity has played a significant role in determining the composition of plant
communities in several regions (Greipsson et al., 1997; Houle et al., 2001;
Brauer and Geber 2002; Rothfels et al., 2002). Soil salinity can negatively
affect plant growth resulting in water deficits, nutritional imbalances and
ion toxicity (Munns, 1993; Liu and Zhu, 1997; Davenport and Tester, 2000;
Cuin et al., 2003). Thus, salt tolerant plant species are abundant in certain
saline soil areas including salted or de-iced roadsides (Scott and Davison,
1985).
Roadside soils experience different salinity levels depending on the region of the world and season. Differential salinity levels encountered along
roadside areas may produce unique plant assemblages and communities
(Brauer and Geber, 2002; Rothfels et al., 2002; DiTommaso, 2004). Roadsides subjected to frequent applications of deicing salts during the winter
months can experience high saline conditions. In the U.S., an estimated 9.1
billion kg of deicing salt are applied to roadways during the winter, especially in large metropolitan areas (D’Itri, 1992). The accumulated salts in
soils can negatively affect seed germination and seedling growth of annual
plant species as well as perennial growth in the early spring (Greub et al.,
1985; Thompson and Rutter, 1986; Eom et al., 2007). Moreover, high salinity
levels can negatively affect plant growth by persisting in roadside soils for the
entire growing season (Hutchinson and Olson, 1967; DiTommaso, 2004).
Thus, relatively salt-tolerant plant species can often be observed growing in
roadside areas with native and/or weedy species predominating.
Salt tolerance is not only important for early physiological plant processes such as seed germination and seedling growth (Houle et al., 2001;
DiTommaso, 2004), but also has a significant influence on mature plant
growth and reproduction. However, the growth response of mature plants
to the saline soil conditions often found along roadsides has not received
much attention in the literature.
Ambrosia artemisiifolia L. (common ragweed) is a widely distributed annual herb in the Asteraceae, native to eastern North America (Basset and
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Salt Response in Ragweed Biotypes
2193
Crompton, 1975). It commonly colonizes highly disturbed areas such as agricultural fields and roadsides (Rentch et al., 2005). Although many studies
have documented the allergenic potential of common ragweed pollen (e.g.
Wayne et al., 2002; Makra et al., 2004), few studies have investigated the
degree of morphological and ecological differences in A. artemisiifolia biotypes from contrasting salinity habitats. In one study, DiTommaso (2004)
reported that seed germination of A. artemisiifolia biotypes collected from
old field versus roadside habitats differed significantly in response to a range
of salinity concentrations. In general, seeds collected from roadside populations had greater and more rapid germination than seeds from agricultural
populations (DiTommaso, 2004). The research reported here was designed
to evaluate whether salt tolerant characteristics that may be observed at earlier growth stages (e.g. seed germination and seedling stages) for some A.
artemisiifolia biotypes are maintained as plants mature.
MATERIALS AND METHODS
Seed Collection
A. artemisiifolia seeds were collected in three locations in southwestern
Quebec, Canada (45◦ 25 N, 73◦ 56 W); one agricultural field biotype (F-type)
was collected from a cropping field that had been in fallow for 2 years,
and two roadside biotypes (R1-type and R2-type) collected directly alongside two major Quebec highways. The R1-type was more evenly distributed
along the roadway than was the R2-type. The detectable concentration of
soluble sodium chloride (NaCl) in the field (<2 mg kg−1 soil) and roadside
(250–350 mg kg−1 soil) soils differed significantly among sample collection
areas (DiTommaso, 2004).
Test of Inheritance in Seed Size and Height
Plants originating from seed collected from either field or roadside locations were grown under greenhouse conditions to determine whether
morphological differences in biotypes are potentially heritable traits. The
growth conditions in the greenhouse included a 25–28◦ C day (14 h) and
18–20◦ C night (10 h) temperature regimen, with supplemental lighting of
200 μmol·m−2·s−1 during the 14-h day period. Shorter A. artemisiifolia individuals ranging from 25-to-30 cm in height were selected from both F-type
and R-type seedlings during a first screening trial in the greenhouse when
R
seedlings were produced in flats using artificial growth media (Metro-mix
510 growth media, Scotts Co., Marysville, Ohio, USA). Taller individuals
ranging from 50-to-75 cm in height were also selected from both biotypes at
the same time. Seed from each of these mature individuals were collected,
germinated following a 6-wk moist stratification period at 4◦ C, and plants
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S. H. Eom et al.
grown under similar growth conditions as parental types. Plant height and
seed weight after harvest was recorded for three generations of individuals.
All experiments were performed under non-saline growth conditions.
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Salt Treatments at Mature Plant Stages
Seeds of A. artemisiifolia biotypes (F, R1, and R2) were germinated in
Petri dishes in a 28◦ C light (150 μmol·m−2·s−1 Photosynthetic photon Flux)
conditioned growth chamber for a period of six days.
In the first experiment, plants were grown in soil media without NaCl
treatment for 30 d and their growth response was then evaluated under
saline conditions. For salinity evaluation in a soil media, seedlings were
transplanted in 14-cm diameter pots with four seedlings planted per pot. A
mixture (v/v) of 50% Hudson silt clay loam soil, 25% sand, and 25% MetroR
510 growth media (Scotts Co., Marysville Ohio, USA) was used for this
mix
experiment. The seedlings were grown in a greenhouse having a 14 h light:
10 h dark daily cycle and temperature cycle of 25–28◦ C light: 18–20◦ C dark
for 30 d, and received 200 μmol·m−2·s−1 additional light intensity during the
day. Full strength Hoagland’s solution was supplied biweekly (200–300 mL
pot−1). Five NaCl concentrations (0, 50, 100, 200, and 400 mM) were applied.
Pots were arranged using a completely randomized block design with 5
replications. NaCl solutions (300–500 ml) were added to the soil surface of
pots on a daily basis by irrigation. Pot weights were measured to determine
daily water transpiration of A. artemisiifolia two times a day; once before
irrigation (water in a pot was consumed by plants during the day) and once
1 h after salt treatment irrigation was performed such that approximate field
capacity of soil was achieved after irrigation (no water dripping after initial
drainage from pots). Five non-planted pots were used as a control to measure
ambient water evaporation rates. Plant water transpiration rates (TR) for
each salinity treatment were determined using the following equation:
TR = [{(Wt(f – u) −We(f – u) ) / Wc(f – u) } × 100]/4
where, W refers to water consumption in a pot, t refers to plants treated
with NaCl solutions, c refers to plants in non-NaCl treated controls, e refers
to unplanted pots, f refers to pot weight fully saturated with water, and u
refers to pot weight after daily water loss. After 21 d of NaCl treatment,
aboveground plant tissue was harvested from each pot and oven-dried at
65◦ C for 24 h and weighed to determine aboveground biomass.
In the second experiment, seedlings were grown under hydroponic conditions. Seedlings were directly transplanted from Petri dishes to 14-cm in
diameter pots where full strength Hoagland’s solutions (pH 6.8) with NaCl
concentrations of 0, 25, 200 mM were supplied. Air was bubbled into solution through a commercial silicone tube. Aqueous treatment solutions were
Salt Response in Ragweed Biotypes
2195
provided daily as consumed. This hydroponic culture experiment was carried
out in the same greenhouse and growing conditions where salinity testing
using a soil media was conducted. Plant height was measured at 7-to-14 d
after NaCl treatments were initiated (corresponding to 37-to-44 d after transplanting in soil media and 20-to-30 d after the hydroponic experiment was
initiated).
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Ion Content (Na, K, Ca) Determination by Inductively-coupled
Plasma Spectrometry Analysis
Foliar samples from both the controls and 200 mM NaCl-treated individuals were taken at 21 d after the start of the soil pot experiment. The foliar
samples were collected from individual pots (n = 5) per treatment. Thus,
five samples per treatment were analyzed. The surface of collected leaves was
rinsed with distilled water and tissue was oven-dried at 40◦ C for 7 d. Samples
(0.5 g dry weight/treatment) were digested using A MARS 5 Nitric Acid Microwave and analyzed using an inductively coupled plasma (ICP) emission
spectrometer (SPECTRO CIROS CCD– ICP Spectrophotometer, ATL Inc.,
CA, USA) at the Cornell Nutrient Analysis Laboratory, Ithaca, NY, USA.
Statistical Analysis
Biomass and daily water use data are presented as means with standard
errors for each treatment. Means of all data were subjected to standard
ANOVA procedures using the SAS software (SAS version 8.02, SAS Institute Inc., Cary, NC, USA). Significant differences among treatment means
were determined at the 5% level using Fisher’s protected least significant
difference (LSD) tests.
RESULTS
Distribution of Each Biotype and Seed Weights by Generation
Seedlings produced from seeds collected from each habitat were grown
in non-saline soils and measured to determine plant height (Figure 1). Seeds
collected from the R-1 roadside area produced approximately 90% shorter,
low-growing seedlings (25–49 cm height). In comparison, approximately
67% of seedlings originating from seeds collected along the second roadside
(R-2) exhibited this short growth habit. In contrast, seed collected from
the agricultural field site produced taller seedlings (50–75 cm), only 5% of
which exhibited a short growth habit. When harvested seeds from subsequent
generations were planted under similar soil and growing conditions, the
larger seeds typically produced seedlings exhibiting the low-growth habit
(i.e. short plants and reduced biomass), whereas the smaller seeds generally
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Agricultural field area
Roadside area
R2
R1
L: 90%
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H: 95%
L: 5%
H : L = 33 : 67%
H: 10%
FIGURE 1 Distribution of ragweed biotypes between roadside and agricultural field conditions. L indicates shorter plants ranging from 25 to 40 cm in height. H indicates taller plants ranging from 50 to
70 cm in height.
produced taller, larger seedlings (Table 1). Differences in the size of seeds
produced from seed collected in the agricultural field or roadside setting
may actually be genetically determined because seed size differences were
maintained through the third generation of greenhouse-grown plants after
field seed collection (Table 1).
Effect of Salinity on Seed Germination
Seeds collected from the two roadside populations (R-type) exhibited
higher germination rates in both the non-NaCl treatment controls and NaCl
treatments relative to seeds collected from the agricultural field population
TABLE 1 Weight (n = 3, ±SE) for Ambrosia artemisiifolia seeds produced from roadside (R1 and R2)
and field (F) biotypes from two height classes grown in non-saline soil media under greenhouse
conditions for three generations
Seed weight (g/100 seeds)
Biotype
R1
R2
F
Height1
1st generation
2nd generation
3rd generation
L
H
L
H
L
H
0.545 ± 0.011 a2
0.385 ± 0.012 a
0.535 ± 0.034 a
0.362 ± 0.011 a
0.531 ± 0.016 a
0.348 ± 0.007 a
0.585 ± 0.008 a
0.364 ± 0.006 a
0.558 ± 0.043 a
0.349 ± 0.011 a
0.567 ± 0.015 ab
0.349 ± 0.010 a
0.706 ± 0.011 b
0.347 ± 0.008 a
0.599 ± 0.034 a
0.351 ± 0.011 a
0.608 ± 0.030 b
0.350 ± 0.006 a
1L indicates shorter plants ranging from 25-to-40 cm in height. H indicates taller plants ranging from
50-to-70 cm in height.
2For each biotype and height category, means for the three generations with the same letter are not
significantly different (P < 0.05) according to a Fisher’s protected LSD test.
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Salt Response in Ragweed Biotypes
FIGURE 2 Effect of salinity on percentage seed germination in R1, R2 and F Ambrosia artemisiifolia
biotypes. Total ninety seeds per a biotype, each of thirty seeds in a Petri dish, were tested were tested in
a time. The seed germination test was performed in triplications.
(F-type) (Figure 2). Also, seed germination was delayed in all 200 mM NaCl
treatments relative to the lower salinity levels with no differences in response
observed between the biotypes. No seed from the three A. artemisiifolia populations germinated when subjected to the 400 mM salt treatment. However,
seeds subjected to the lower salinity levels (50 and 100 mM NaCl) did not
generally exhibit delayed germination.
Effect of Salinity on Plant Growth
In the non-NaCl treatment control, plants of the F-type (4.11g) produced
62 and 20% more aboveground biomass than plants from the R1 (2.53g) and
R2 (3.43g)-types, respectively (Table 2). Shoot dry weight of F-type tended
to decrease with increasing NaCl treatment concentration, with significant
differences among treatments. In direct contrast, aboveground dry biomass
of R-types was not significantly different among treatments ranging from
0 to 100 mM NaCl concentrations, but was significantly decreased only at
higher NaCl concentrations. Interestingly, the aboveground dry biomass of
TABLE 2 Mean aboveground biomass (±SE) of Ambrosia artemisiifolia roadside biotypes (R1, R2) and
field (F) biotype 21 d after NaCl treatments were initiated
Aboveground biomass (g ·plant−1)
NaCl (mM)
0
50
100
200
400
LSD
R1
R2
F
2.53 ± 0.22 a
2.60 ± 0.12 a
2.24 ± 0.26 ab
1.82 ± 0.19 bc
1.49 ± 0.12 c
0.43
3.42 ± 0.45 a
3.66 ± 0.34 a
3.26 ± 0.30 a
2.25 ± 0.24 b
2.01 ± 0.16 b
0.77
4.11 ± 0.28 a
3.46 ± 0.21 b
3.11 ± 0.19 bc
2.95 ± 0.13 cd
2.58 ± 0.21 d
0.47
Means for each biotype having the same letter across NaCl concentrations are not significantly different
(P < 0.05) according to a Fisher’s protected LSD test.
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FIGURE 3 Height response of R1, R2, and F Ambrosia artemisiifolia biotypes to salt treatments in soil
media (above graphs) and in hydroponic (bottom graphs) experiments.
R-types was slightly increased at 50 mM NaCl treatments although there were
no significant statistical differences between the control and 50 mM NaCl
treatments.
Under hydroponic conditions, plant height of R-type plants was significantly increased in 25 mM NaCl treatments compared with the untreated
controls, whereas height of F-type was significantly decreased at these low
NaCl concentrations (Figure 3). However, 200 mM NaCl concentrations affected the height of all biotypes with severe reduction observed in all treated
seedlings. R1-type was more severely reduced in height (42% of control) at
14 d after NaCl treatment, whereas F-type was reduced to 51% of the untreated control. In soil pot experiments, R biotypes did not exhibit growth
reductions in height at lower NaCl concentrations (0–100 mM), but exhibited significant height decreases at higher NaCl concentrations (≥200 mM).
When considering the mortality of different biotypes, R-types did not survive
the 400 mM NaCl treatment after 14 d, whereas F-type did survive somewhat
longer, based on our visual observations. All biotypes survived at less than
200 mM NaCl concentrations, although visual symptoms of damage differed
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FIGURE 4 Water transpiration patterns among R1, R2 and F Ambrosia artemisiifolia biotypes for 7 d after
NaCl treatment initiation. Error bars are standard error of mean (n = 5).
among biotypes. Shoot dry weight accumulations of all biotypes followed a
similar pattern to that of height (Table 2).
Water Transpiration
Water transpiration among biotypes was measured in the soil pot experiment. The average daily water transpiration of each biotype in NaCl
treatments is shown in Figure 4. Water transpiration rate of R-types was not
significantly different at 50 mM NaCl treatment compared to non-NaCl treatment control, while that of F-type was significantly decreased (statistical data
not presented in Figure 4). However, water transpiration observed in R1
biotype was severely decreased only at higher NaCl treatments (≥200 mM),
while that of F-type decreased from 50 to 200 mM NaCl concentrations.
Water usage within R-type plants that were NaCl treated was stabilized
after two days, with severe reduction of water availability observed only at
higher NaCl treatments. In comparison, plants of the F-type treated with
400 mM NaCl exhibited a more gradual pattern of decreasing water usage
until 5 d after initial salt treatment.
K+, Na+, and Ca++ Concentrations in Leaf Tissues
Although roadside seed germination appeared more tolerant to high
salinity levels, mature R biotype plants exhibited severe damage to high
NaCl treatments, similar to F-type plants, in terms of growth and water availability. Roadside soils where significant levels of de-icing salt had been applied in winter season contained significantly higher levels of NaCl than did
general agricultural field soils. In this experiment, we found that roadside
ragweed biotypes accumulated more sodium ions in their shoot tissues after greenhouse exposure to high NaCl treatments. Sodium ion content in
foliar tissues was higher in R1-type than in F-type (58% of R1-type) under
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S. H. Eom et al.
TABLE 3 Mean leaf ion content (± SE) of three Ambrosia artemisiifolia biotypes receiving either 0 or
200 mM NaCl treatment
Leaf ion content (mg/g d.w.)
Biotype
R1
R2
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F
NaCl (mM)
0
200
0
200
0
200
K+
14.82 ± 0.47 a
17.48 ± 2.11 a
13.26 ± 2.69 a
20.30 ± 4.15 a
14.65 ± 0.47 b
24.42 ± 1.93 a
Na+
0.72 ± 0.18 b
19.95 ± 2.77 a
0.68 ± 0.09 b
19.19 ± 3.58 a
0.42 ± 0.05 b
16.16 ± 4.47 a
Ca++
41.13 ± 6.88 a
40.49 ± 4.28 a
28.65 ± 5.93 a
43.23 ± 4.98 a
30.98 ± 3.86 a
45.65 ± 5.56 a
Values are means of n = 5. Means for each biotype at the two NaCl concentrations having the same
letter are not significantly different (P < 0.05) according to a Fisher’s protected LSD test.
control conditions as well (Table 3). In the 200 mM NaCl treatment, sodium
content was significantly increased in all biotypes, with 28 fold higher concentrations than the control in R-types and 38 fold higher concentrations in
F-type plants. However, total sodium content per gram dry weight at 200 mM
NaCl treatment was not significantly different among all biotypes. Potassium
content of all biotypes increased with 200 mM NaCl treatment. The potassium level was higher in F-type than R1-type foliage although potassium
contents were not significantly different among all biotypes in untreated controls. Calcium content in the control treatment was significantly higher in
R1-type than in R2 and F biotypes but calcium content in both R2-type and
F-type was significantly increased at the 200 mM NaCl treatment, exhibiting about a 1.5 fold increase in calcium in comparison to the untreated
control.
DISCUSSION
The relationship between seed size and initial seedling growth is thought
to be positively correlated and related to ability to tolerate saline conditions
(Main and Nafziger, 1992; Beaton and Dudley, 2004). When seed selection experiments were performed for evaluation of salt tolerance in wheat
cultivars, larger seeds produced greater yields and shoot biomass in both
saline and non-saline conditions (Grieve and Francois, 1992). In the case
of A. artemisiifolia, our data showed that initial seed size of this species was
negatively correlated with final plant size, in terms of height and above
ground biomass. Based upon the random seed collection of this species
in either saline roadside or non-saline areas, plants producing larger and
heavier seeds with fewer seed numbers were mainly distributed in the roadside area, whereas plants generating smaller/lighter seeds with greater seed
numbers were present in the agricultural field setting. Differential seed size
among ragweed biotypes may be a strategy allowing greater adaptation to a
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Salt Response in Ragweed Biotypes
2201
changing environment with increasing abiotic stress; thus a ragweed biotype
distributed in an agricultural field territory may need to produce many seed
of potentially smaller size to and disperse these smaller seeds to remain competitive in this environment, in contrast to a biotype achieving tolerance to
a critical abiotic stress factor, such as salt concentration. In this case, a larger
seed may facilitate germination under already challenging saline conditions
(Beaton and Dudley, 2004).
Roadside biotypes exhibited greater tolerance to saline conditions when
examining seed germination under high NaCl concentrations (DiTommaso,
2004). Salinity also caused delayed germination in Phaseolus species (BayueloJiménez et al., 2002). However, lower salinity levels (50 and 100 mM NaCl)
did not appear to delay germination in either biotype. Thus, low saline
conditions in soil may have a greater effect upon whole plant growth in
contrast to the specific process of germination.
It was reported that salt tolerance in seed germination processes and
in mature tomato plants was not correlated (Foolad and Lin, 1997). In this
study, we found that the evaluation of salt tolerance in mature plants of various biotypes should provide additional evidence as to why certain ragweed
biotypes have become localized along roadside areas and demonstrate variable seed germination results. In the preliminary stages of seed germination,
differences among biotypes were not observed in low salt concentrations
(100 mM NaCl). However, our results indicate that significant differences
between biotypes were observed in whole plants when low and high salinity production was assessed in R and F biotypes. Interestingly, we observed
that height of F-type seedlings in 25 mM NaCl in hydroponic conditions
was similar to that of R-type in the control or at 25 mM NaCl (Figure 3).
We suspect that decreased height of F-type ragweed in low salinity may be
explained by acclimation or adaptation of this biotype to roadside environments experiencing roadside salt application. We observed that over 90%
of ragweed seed collected from the roadside was the low growing biotype
(Figure 1). Two reasons the predominant distribution of a low growing biotype in roadside are suspected are the response over time due to acclimation
to salinity and additionally, the response to selection by mowing. It is known
that mowing for roadside vegetation management is performed three to four
times per year in both USA and Canada. Thus, low growing plant species
may exhibit strategic advantages associated with seed distribution along
roadsides.
Various plant species and biotypes may have different requirements of
ion concentrations for enhanced water uptake. Plant uptake of soil water
occurs from root zone to foliage by passage of water through the xylem vascular system. Water containing mild concentrations (5–20 mM) of salts such
as KCl, NaCl, and calcium chloride (CaCl2 ) improved plant water uptake
in xylem flow compared to de-ionized water itself (Zwieniecki et al., 2001).
Although the ability to take up and translocate water is variable among plant
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S. H. Eom et al.
species, salt tolerant species generally exhibited increases in growth as well
as water uptake at mild (or low) salt concentrations in past research (Suárez
and Medina, 2005). In comparison, salt sensitive species exhibited decreases
in growth and water uptake in response to increasing salt concentrations
(Chartzoulakis et al., 1995). Our results are similar to those symptoms reported above in that salt tolerant species increased their growth, while salt
sensitive species exhibit decreased growth in low salinity. In R-1 salt tolerant
seedlings, shoot dry weight and water availability were increased 3% and
9%, respectively in 50 mM NaCl treatment at 21 days after experimental
initiation compared to non-NaCl treated controls. In F-type salt sensitive
seedlings, shoot dry weight and water availability were decreased 16% and
12%, respectively in 50 mM NaCl treatments compared to untreated control
treatments.
In conclusion, we found that salt resistant ragweed biotypes exhibited
differential responses to salt stress at seed germination and in mature stages
of growth. R-type seeds were more tolerant to salt stress either at low or high
salt concentrations compared to F-type seeds. When considering the mature
plant, R-types exhibited greater salt tolerance in terms of plant height, water
availability and biomass at lower salt concentrations, but were rapidly and
severely damaged under high salt stress, similar to salt sensitive plants of
F biotype. With higher salt exposure, roadside biotypes quickly responded
and did not utilize salt response mechanisms such as water re-uptake during
continuous high salt retention in soils. Exposure to high saline conditions
resulted in a lack of seed production in both biotypes. However, stimulation in growth of R-type seedlings possessing low growth habits was clearly
observed in mild salt exposure conditions, whereas decreased growth of the
F biotype was observed under the same low salt conditions. Therefore, we
conclude that low growing ragweed biotypes which exhibit salt tolerance at
low NaCl concentrations tend to dominantly distribute in saline roadside
conditions, with possible selection caused by these existing saline conditions
or repeated roadside mowing.
Over time a ragweed plant could experience reductions in salt concentrations along roadside soils as NaCl availability differs depending on time
of application and solubility or melting. Differential salt responses during a
plant’s growth cycle to saline conditions may be linked to actual saline conditions encountered in roadside soils where ragweed distributes, with high
salinity levels in early spring with spring melting and lower salinity levels
later in the growing season. Therefore, natural selection for a lower growing
biotype of A. artemisiifolia that is tolerant to low salinity has likely occurred in
roadside environments, where mild saline soil conditions persist during the
entire growing season. In order to tolerate roadside saline conditions, it appears that several strategies have been developed in common ragweed; first,
morphological changes exhibiting larger/heaver seed size and smaller plant
size, and secondly, efficient water availability and usage within the plant in
Salt Response in Ragweed Biotypes
2203
response to mild salinity experienced during growth and development and
seed maturation periods.
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