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Botany Department, University of Georgia, Athens, Georgia 30602
Received for publication September 1, 1998. Accepted for publication December 22, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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s stress. The study species, Chrysothamnus nauseosus ssp. consimilis and Sarcobatus vermiculatus (hereafter referred to by genus), differ in their distribution along salinity gradients, with Chrysothamnus inhabiting only less saline areas. In growth chamber studies, declining s (-0.82 to -2.71 MPa) inhibited germination of both species, and Chrysothamnus was less tolerant of s stress than Sarcobatus. Germination fell below 10% for Chrysothamnus at -1.64 MPa (NaCl and PEG), and for Sarcobatus at -2.4 MPa PEG. Neither species exhibited ion toxicity. There was substantial ion enhancement for Sarcobatus in lower s, allowing for 40% germination in -2.71 MPa NaCl. For seedling RGR, species were not different at -0.29 or -0.82 MPa (0 and 100 mmol/L NaCl, respectively), but Chrysothamnus RGR declined substantially at -1.3 MPa (200 mmol/L NaCl). The greater stress tolerance of Sarcobatus was not associated with a lower RGR under nonsaline conditions. Species differences in seed and seedling s stress tolerance probably contribute to the restricted distribution of Chrysothamnus to less saline areas. The Na uptake of Sarcobatus seedlings enhances its ability to deal with declining s and establish in more saline areas.
Key Words: Chrysothamnus nauseosus • plant distribution; relative growth rate; salinity • Sarcobatus vermiculatus • seed germination; stress tolerance
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ungar, Benner, and McGraw, 1979
; Snow and Vince, 1984
; Ungar, 1991
; Pennings and Callaway, 1992
; Ball and Pidsley, 1995
). There is contrasting evidence as to whether the differential germination and early growth responses to salinity contribute to species distributions and zonations in these habitats (Waisel, 1972
; Rozema, 1975
; Rabinowitz, 1978
; Huiskes et al., 1985
; Ungar, 1991
). We investigated seed germination and seedling growth responses to salinity for two desert shrubs that are differentially distributed along a steep salinity gradient at Mono Lake, CA.
Increasing salinity generally reduces germination in glycophytes and to a lesser degree in halophytes (Hayward and Bernstein, 1958
; Waisel, 1972
; Ungar, 1991,
1996
; Khan and Ungar, 1997
). Two processes mediate this reduction: osmotic effects due to declining soil solute potential (s), creating a water stress for the plant, and ionic effects due to seed or seedling ion uptake and/or accumulation (Hayward and Bernstein, 1958
; Waisel, 1972
; Ungar, 1991
). Under natural conditions, however, these basic effects additionally interact with soil moisture content (volume of water relative to soil). As soil moisture declines, soil total water potential (w) declines due to both soil matric potential (m) and covarying soil s, which reflects increased concentration of solutes (Caldwell, 1974
; Roundy, 1984
; Hardegree and Emmerich, 1990
). Thus it is a challenge to examine the interacting effects s and m may have on plant establishment in saline desert soils that routinely experience wide fluctuation in soil moisture at the surface.
Salinity-induced declines in germination are usually due to only osmotic (substrate s) effects for halophytes, whereas glycophytes are more likely to exhibit additional ion toxicity (Hayward and Bernstein, 1958
; Ungar and Hogan, 1970
; Macke and Ungar, 1971
; Cluff, Evans, and Young, 1983
; Romo and Haferkamp, 1987
; but see Hyder and Yasmin, 1972
). Ionic effects may be distinguished from osmotic effects by comparing the effects of salt solutions and isotonic (equal s) solutions of an inert osmotic medium such PEG (polyethylene glycol) that cannot penetrate into the cell wall. Inhibition of germination in PEG-treated seeds is attributed to osmotic effects, and any difference in germination of salt-treated relative to PEG-treated seeds is attributed to ionic effects. Although usually negative, ionic effects occasionally increase germination over the baseline s (Macke and Ungar, 1971
; Romo and Eddleman, 1985
). In this case, ion uptake assists in making the seed or seedling w more negative than soil w, thus helping the plant to overcome the s effects of the saline substrate (Eddleman and Romo, 1987
). The general lack of ion toxicity for halophytes has been alternatively confirmed by almost complete recovery of germination potential after salt-treated seeds are returned to fresh water (Ungar, 1996
; Egan, Ungar, and Meekins, 1997
).
Biotic factors can also play a role in determining species distributions in harsh habitats (Ungar, Benner, and McGraw, 1979
; Bertness and Ellison, 1987
; Ungar, 1992
; Bertness and Hacker, 1994
). Plant responses to abiotic and biotic factors may be related through relative growth rate (RGR). Adaptations to unproductive and adverse habitats (i.e., stress tolerance) often include an inherently low RGR (Grime and Hunt, 1975
; Grime, 1979
; Chapin, 1991
; Chapin, Autumn, and Pugnaire, 1993
; Ball and Pidsley, 1995
). Because low RGR is also thought to be correlated with poor competitive ability, there may be an apparent trade-off between stress tolerance and competitive ability. The upper limits for the distribution of halophytic species along salinity gradients may be limited by competition with less stress-tolerant species (Snow and Vince, 1984
; Kenkel et al., 1991
). Thus, it is worthwhile to determine whether greater stress tolerance is associated with inherently lower maximum RGR under optimal conditions.
At Mono Lake, California, declines in the lake level have created a steep abiotic stress gradient by exposing former lakebed substrates. At the Mono Dunes Ecosystem Research Site, the end of the gradient farthest from shoreline consists of marginally saline sand dunes (Toft, 1995
; Donovan, Richards, and Muller, 1996
). This site supports a diverse plant community dominated by two shrubs: Chrysothamnus nauseosus (Palla.) Britt. ssp. consimilis (E. Greene) H. M. Hall & Clements (Asteraceae) and Sarcobatus vermiculatus (Hook.) Torrey (Chenopodiaceae). Soil salinity increases with proximity to the Mono Lake shoreline and more saline substrates are populated only by Sarcobatus and Distichlis spicata (Schaber, 1994
; Donovan, Richards, and Schaber, 1997
). Although Chrysothamnus is generally characterized as a Na-excluding glycophyte, the ssp. consimilis has been described as somewhat salt tolerant (Roundy, Young, and Evans, 1981
). In contrast, Sarcobatus is a Na-accumulating halophyte that can survive in solutions up to 1 mol/L NaCl (McNulty, 1969
; Glenn and O'Leary, 1984
). Thus, species differences in salinity tolerance at various life history stages may drive differences in spatial distribution. Intermediate regions of the gradient, where Chrysothamnus does not occur, do receive high seed input and can support Chrysothamnus germination and growth when well watered (Schaber, 1994
; Fort and Richards, 1998
). This suggests that Na toxicity is not preventing Chrysothamnus recruitment in these soils. However, Na ion and s effects at lower soil w warrant further examination.
In this study, our first objective was to determine whether the study species differed in seed germination responses to declining s generated by NaCl or PEG. We predicted that s inhibition of germination would be greater for Chrysothamnus than for Sarcobatus and that NaCl would have both osmotic and ion toxicity effects on Chrysothamnus and osmotic and possibly ion enhancement effects on Sarcobatus. Our second objective was to determine whether the study species differed in seedling RGR. We predicted that declines in seedling RGR in response to salinity would be greater for Chrysothamnus than for Sarcobatus, but that Chrysothamnus would have a higher RGR than Sarcobatus in nonsaline substrates.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). At maturity, Sarcobatus seeds contain a fully formed chlorophyllous embryo, surrounded by a membranous pericarp (Eddleman, 1979
). The embryo uncoils within hours of imbibition when the pericarp is weak or ruptured. The intact pericarp can inhibit germination, but under natural conditions it is probably breached during overwintering by freeze–thaw cycles and sand abrasion. Sarcobatus has a 30–60 d after-ripening period, and cold stratification is not required, but after cold stratification optimal germination temperatures are 20°–30°C (Eddleman, 1979
). Chrysothamnus seeds have temperature-dependent dormancy that varies by population, with the temperature response generally appropriate to ensure early spring germination (Khan et al., 1987
; Meyer, McArthur, and Jorgensen, 1989
). For our experiment, seeds of Chrysothamnus and Sarcobatus were collected in the fall of 1994 from the Mono Dunes Ecosystem Research Site, near Mono Lake, California, USA (38°5' N, 118°58' W), and stored at 4°C until use. The study was conducted in early 1996 using Chrysothamnus seeds (from six plants) that were visually checked for presence of an embryo, and Sarcobatus seeds (from eight plants) that were scarified by hand using sandpaper to rupture the pericarp and to check for presence of an embryo.
Germination study
Four replicates of 25 randomly selected seeds each were used for each treatment. Treatment factors were two species (SP), two osmotica (OS), and six water potentials (s), applied in a randomized complete block design. Additionally, controls with no osmotica were included for each species in each block. Replicates consisted of 11-cm diameter covered petri dishes wrapped with ParafilmTM and contained 40 g of sterilized river sand brought to field capacity with solution. Solutions consisted of one-fourth-strength modified No. 2 Hoagland's solution (Epstein, 1972
) mixed with osmotica: NaCl or PEG (m.w. 8000). NaCl solutions were 100, 200, 300, 400, 500, or 600 mmol/L. Isotonic quantities of PEG were computed by the program of Michel and Radcliffe (1995)
. Soil s (approximated by w due to low m) was then measured with individually calibrated thermocouple psychrometers (series 83, J. R. D. Merrill, Logan, Utah, USA) (Brown and Bartos, 1982
). Final soil s treatments were -0.29 MPa (Hoagland's solution only), -0.82, -1.30, -1.65, -2.04, -2.40 and -2.72 MPa. T test comparisons of measured s confirmed that there were no significant differences for NaCl-enhanced sand and PEG-enhanced sand at each target s.
Seeds were dusted with N-[(trichloromethyl)-thio]-4-cyclohexene-1,2-dicarboximide (CaptanTM) prior to incubation to control fungi. Seeds were incubated in a growth chamber at 25°C, which is within the range of optimal temperature for germination of both species (Romo and Eddleman, 1985
; Khan et al., 1987
) with a photoperiod of 14-h:10-h (light : dark). Although temperature does affect germination of these species, the temperature optima are broad and decreasing temperature does not change the basic shape of the germination response to salinity (Eddleman, 1979
; Romo and Eddleman, 1985
; Khan et al., 1987
). Light was provided by compact fluorescent bulbs (15 µmol·m-2·s-1, 400–700 nm). Germination was recorded daily through 20 d. Chrysothamnus seeds were considered germinated upon emergence of the radicle. Sarcobatus seeds were considered germinated when the embryos uncoiled and root hairs were evident. We were unable test recovery of germination potential after returning salt-treated seeds to fresh water because of fungal infections on many of the ungerminated seeds by the end of the 20-d treatments.
Germination percentage values (Percentage of total number of seeds in each petri dish) on day 20 were arcsine transformed and analyzed in a three-way analysis of variance (ANOVA) procedure (PROC GLM; SAS, 1989
). When a significant three-way interaction was found, further analyses were made using a two-way ANOVA.
RGR study
Seeds were germinated on moistened filter paper and subsequently transplanted to 3.8 cm diameter x 19 cm long tube pots of washed river sand, 
72 h after radicle elongation. Plants were grown in a growth chamber at a thermoperiod of 25°C:15°C (day : night) and a 14-h photoperiod (700 µmol·m-2·s-1, 400–700 nm), and watered daily. The three treatments were 0, 100, and 200 mmol/L NaCl in one-fourth-strength modified Hoagland's solution. At field capacity, soils watered with these solutions had soil w of -0.29, -0.82, and -1.30 MPa, respectively. Half-strength salinity treatments began on day 11 (after transplant), and full-strength salinity treatments followed on day 14.
The experiment used a split-split plot design with three factors (species, salinity, and harvest date). Plants were harvested on days 19, 24, 30, 36, 42, and 48 after transplant, using five replicates of each species in each salt treatment at each harvest date. Leaf area was measured and analyzed with a scanner and Delta-T Scan software (Delta-T Devices Ltd., Cambridge, UK). Roots were excavated using water to gently wash away the sand. Plants were dried at 70°C to determine total dry biomass (leaf, stems, and roots). For plants harvested on days 19 and 36, ground leaves were ashed at 500°C and the ash was dissolved in dilute acid (10% nitric and 30% hydrochloric acid). Leaf extracts were analyzed for Na, K, Ca, and Mg with an Inductively Coupled Plasma Emission Spectrometer (965, Thermo Jarrell Ash Corp., Franklin, Massachusetts, USA) at the University of Georgia Chemical Analysis Laboratory.
Relative growth rates (RGR, g·g-1·d-1) were analyzed using the method of Poorter and Lewis (1986)
. In this ANOVA analysis, ln-transformed plant biomass (dependent variable) is modeled as a function of harvest date. Significant interaction terms that include harvest date (HD) indicate differences in RGR. Graphs of RGR were made using a modification of the classical interval method that derives RGR values for nonadjacent dates (skipping one harvest) to minimize auto-correlation (Poorter, 1989
). These RGR values were assigned to each harvest day in that interval and the resulting two to three RGR values for each harvest date were then averaged. This method smoothes out variation between harvests while retaining trends in RGR (Poorter, 1989
).
RGR is the product of the LAR (leaf area ratio, m2/g) and NAR (net assimilation rate, g·m-2·d-1). Instantaneous values of LAR were calculated using the equation of Hunt (1990,
p. 16). Differences among instantaneous values of LAR for each harvest were examined by an ANOVA, and significant differences among means were evaluated using the Student-Newman-Keuls range test. Instantaneous NAR was calculated using the equation of Hunt (1990
, p. 16), using non-adjacent intervals and averaging as was done for the RGR graphing. However, NAR values could not be statistically compared in this study design because (a) NAR values for each date were not independent of other dates and this ruled out the use of separate ANOVAs for each harvest, and (b) NAR cannot be represented by the slope of a line plotting two dependent variables (i.e., it is a compound growth rate parameter containing three variables [Hunt, 1990
]), and this rules out the RGR ANOVA technique. The relationships between LAR and RGR and between NAR and RGR were analyzed with regressions.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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s treatments, the results of a three-way ANOVA indicated that the main effects of species (SP), water potential (s), and osmotica (OS) were all highly significant (P < 0.001). However, all of the interaction terms were also highly significant (P < 0.001), so it was necessary to perform further analyses within each species in order to elucidate the nature of the differences.
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s and OS were significant, and interaction term of s x OS was not significant (Table 1). Germination declined with decreasing s in both NaCl and PEG and NaCl increased germination by a small percentage at s as compared to PEG (Fig. 1). Chrysothamnus germination fell below 10% at -1.65 MPa for both NaCl and PEG. The few seeds that did germinate in NaCl at -1.65 MPa (5%) and -2.03 MPa (1%) looked sickly and did not proceed with radicle elongation.
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s and OS were significant, and, unlike Chrysothamnus, the interaction term of s x OS was also highly significant (Table 1). Germination declined with s in both NaCl and PEG, but germination was greatly enhanced by the NaCl at lower s, as compared to PEG (Fig. 1). Sarcobatus seeds were able to germinate in all treatments except -2.72 MPa PEG. Seeds that germinated in -2.42 MPa PEG (5%) were barely able to uncoil, looked sickly, and did not proceed with radicle elongation. Sarcobatus maintained the ability to germinate in solutions of lower s than Chrysothamnus, regardless of whether the osmotica was NaCl or PEG.
RGR study
The split-split plot ANOVA for ln-transformed plant biomass (Table 2), which incorporated main effects of species (SP), salinity (SL), and harvest date (HD), used data from the first four harvests because Chrysothamnus seedlings did not survive the 200 mmol/L NaCl treatment after the fourth harvest. The significant second-order interaction in this analysis (SL x SP x HD) indicates that overall Chrysothamnus and Sarcobatus have different RGRs, but that the differential response to SL treatment needs to be taken into account to describe the species differences. RGR analysis of SP x HD for each salinity treatment indicated that the two species did not differ in RGR at 0 or 100 mmol/L NaCl (SP x HD not significant, Table 3), using data from all six harvests. At the 200 mmol/L NaCl, however, Sarcobatus did have a significantly higher RGR than Chrysothamnus (SP x HD significant, Table 3), using data from the first four harvests.
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900% (on average) higher than that for 200 mmol/L NaCl. For LAR, there were significant differences for Chrysothamnus on days 19, 36, and 48 and for Sarcobatus on days 30, 36, and 48. RGR was positively correlated with NAR (Fig. 3, P < 0.001) with r2 values of 0.88 and 0.76 for Chrysothamnus and Sarcobatus, respectively. In contrast, RGR was not significantly correlated with LAR for either species.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). We confirmed our prediction that seeds and seedlings of Sarcobatus would be more tolerant of salinity stress than those of Chrysothamnus. However, the results did not support our prediction of ion toxicity for Chrysothamnus or of a higher seedling RGR for Chrysothamnus than for Sarcobatus under well-hydrated nonsaline conditions.
Increasing NaCl had a strong s or osmotic effect that decreased germination of both species. This finding of highest germination at zero salinity is consistent with previous findings for Sarcobatus (Romo and Eddleman, 1985
) and for C. nauuseosus ssp. viridulus (Khan et al., 1987
), which has alternatively been classified as a variety of ssp. consimilis (Anderson, 1986
; Khan et al., 1987
). Highest germination in the least saline substrate is the norm for most glycophytes and halophytes (see Introduction), and optimal germination in saline substrate (as compared to no salt) has only been found for a few extreme halophytes (Ungar, 1991
).
A comparison of isotonic (same s) substrates generated by PEG or NaCl demonstrated that neither study species exhibited ion toxicity, but both species exhibited significant ion enhancement. For Chrysothamnus the enhancement was relatively small and increased germination by only 3–15% in the range of -0.82 to -1.65 MPa. For Sarcobatus, the ion enhancement effect was not apparent at less negative s but increased dramatically with more negative s. NaCl enhancement increased germination (relative to PEG) by 10% at -1.65 MPa and 40% at -2.72 MPa and allowed Sarcobatus germination to proceed at much lower s. This magnitude of ion enhancement for Sarcobatus for Mono Lake populations is consistent with that found for other populations in Montana (Romo and Eddleman, 1985
). The accumulation of Na by the imbibing embryo functions to promote a water potential gradient between the embryo and substrate, making germination conditions more favorable than possible on low s substrates lacking Na (Romo and Eddleman, 1985
; Eddleman and Romo, 1987
). However, the trait of ion enhancement was not found for Sarcobatus populations from Oregon (Romo and Haferkamp, 1987
), indicating substantial population differences (environmental and/or genetic) in this seed germination response. A substantial ion enhancement effect has also been found for NaCl and the halophyte Puccinellia nuttalliana at intermediate soil w (-1.2 MPa) (Macke and Ungar, 1971
), although in that case the NaCl enhancement of germination did not extend the lower limit of soil w for successful germination.
The study species differed in seedling RGR responses to decreasing s induced by NaCl. Sarcobatus seedling RGR did not decline from 0 to 200 mmol/L NaCl, whereas Chrysothamnus seedling RGR declined substantially in response to 200 mmol/L NaCl. The 200 mmol/L NaCl treatment also induced mortality for Chrysothamnus so that there were no seedlings left to harvest after 2.5 wk. Our results for young seedlings (treatments initiated at 14 d) are consistent with those found for older seedlings of these species (treatments initiated at 3 mo, Mono Lake seed source) for height growth and mortality responses to salinity (Richards, 1994
). In that study, after a 5-wk salinity treatment, Chrysothamnus survival was 36 and 8% in 205 and 343 mmol/L NaCl, respectively, whereas Sarcobatus survival was 100% in all treatments up to 592 mmol/L NaCl (Richards, 1994
).
Variation in RGR can be further explored by looking at its components of LAR and NAR. LAR is an index of the leafiness of the plant, i.e., ratio of photosynthetic surface area to respiratory mass. NAR can be interpreted as the combined physiological processes of photosynthetic carbon gain and carbon losses primarily from respiration (Poorter and Remkes, 1990
). Our study showed that intraspecific variation in RGR was correlated with NAR and not LAR for each species. This intraspecific pattern is consistent with the findings for other species' responses to salinity (Jansen, Pot, and Lambers, 1986
; Long and Baker, 1986
; Ball, 1988
; Cramer, Epstein, and Läuchli, 1990
; Ball and Pidsley, 1995
).
The species comparison of RGR under optimal conditions did not conform to our expectations. In the 0 mmol/L NaCl treatment, the seedling RGR of Sarcobatus was the same as that of the less salt-tolerant Chrysothamnus. This result has two potential implications. The first is on an evolutionary time scale: these data run contrary to the idea that species adaptation to stressful habitats comes at the cost of an inherently lower maximum growth rate under optimal conditions (Grime, 1979
; Ball, 1988
; Chapin, 1991
; Chapin, Autumn, and Pugnaire, 1993
; Ball and Pidsley, 1995
). The second implication is on an ecological time scale: the lack of differences in maximum RGR under nonsaline conditions suggests that these species may not differ in competitive ability in nonsaline soils (Grime and Hunt, 1975
; Grime, 1979
; Ball, 1988
; van Andel and Biere, 1989
; Chapin, 1991
). However, these implications need to be further explored using a wider range of water availability and salinity conditions and using more ontogenetic stages. For example, the RGRs in our study were higher by a factor of ten than those found by Brown (1997)
for older (5 mo) seedlings of these species in pot and field studies. In those studies, which were done under nonsaline conditions, Chrysothamnus did tend to have a higher RGR than Sarcobatus, although the study was not designed for a species comparison.
For the RGR experiment, elemental concentrations were based on small sample sizes for combined plants and thus should be interpreted with caution. However, they suggest that seedlings of both species took up Na and sequestered it in leaves, at the expense of other cations, and that effect was larger for Sarcobatus than for Chrysothamnus. The Na uptake was expected for Sarcobatus seedlings because mature plants of this species have extremely high Na concentration under field and experimental conditions (Glenn and O'Leary, 1984
; Richards, 1994
; Donovan, Richards, and Muller, 1996
; Donovan, Richards, and Schaber, 1997
). Both juvenile and mature plants accumulate leaf Na as part of their ability to actively osmotically adjust (Glenn and O'Leary, 1984
; Romo and Haferkamp, 1989
). Accumulation of leaf Na was not expected for Chrysothamnus seedlings since mature plants of these species do not have high leaf Na concentrations under field conditions (Donovan, Richards, and Muller, 1996
). Seedling Na uptake promotes the water potential gradient between seedling and substrate and thus enhances the seedling ability to maintain turgor for growth.
Overall, osmotic and ionic effects of salinity on seeds and seedlings of Chrysothamnus and Sarcobatus could contribute to observed distribution patterns at Mono Lake, California. At the nonsaline end of the gradient, both species could do well in terms of germination and early seedling growth. At the extreme saline end of the gradient, where a salt crust often forms on the surface, only a halophyte species such Sarcobatus could germinate and grow. For the intermediate areas of the Mono Lake gradient, our results complement those of Schaber (1994)
. Although her results indicated no species-limiting differences in Na toxicity under well-watered conditions (but no Na leaching), our results additionally look at Na s effects and ion enhancement when soil w declines. In the field, surface soils are wettest in the spring, but rarely stay wet for extended periods of time. Seeds and seedlings at the surface would be subjected to drying cycles that would decrease the soil w both due to decreasing m and s. Under moderately saline field conditions, w stress inhibition of Chrysothamnus germination and seedling growth would be exacerbated by the presence of Na. Faced with the same declining w, Sarcobatus seeds and seedlings would take up Na ions and proceed with greater germination and growth than would have been possible in a drying soil without Na.
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FOOTNOTES |
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2
Author for correspondence (donovan{at}dogwood.botany.uga.edu)
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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