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Published online 1 January 2005
Published in Crop Sci 45:251-258 (2005)
© 2005 Crop Science Society of America
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Criteria for Assessing Salinity Tolerance of the Halophytic Turfgrass Seashore Paspalum

Geungjoo Lee, Robert N. Carrow* and Ronny R. Duncan

Department of Crop and Soil Sciences, University of Georgia, Griffin, GA30223-1797

* Corresponding author (rcarrow{at}griffin.uga.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Turfgrass improvement for salinity tolerance requires reliable assessment of salt-tolerance variability among grasses. Using seashore paspalum (Paspalum vaginatum Sw.) as a model halophytic grass, the objectives are to discuss problems of applying traditional direct salinity classification criteria used for glycophytic to halophytic turfgrasses and to provide a set of evaluation criteria for halophytic turfgrasses for which traditional criteria at the soil salinity of ECe (electrical conductivity of soil) <30 dS m–1 are not sufficient. The most salt-tolerant ecotypes exhibit cubic salinity-growth response curves, usually at ECw (electrical conductivity of water) >30 dS m–1, which confound the use of the traditional criteria-threshold ECe, slope factor based on decline in growth with increasing salinity, and ECe for 50% growth reduction. Using absolute growth rather than relative growth (percent of nonsaline control) is important for halophytic grass screening, especially for ecotypes within a species, since plant vigor is necessary in salt-affected sites. Salinity-growth curves for shoot, root, verdure, and total plant growth provide the most information for separation of ecotypes for salt-tolerance, with shoot and root response curves the most critical. Minimum screening criteria that require the least time and cost for large numbers of ecotypes are recommended as absolute growth at nonsaline conditions, growth at the highest salinity level expected for use of the grass, and leaf firing. For the limited number of the most tolerant ecotypes, more detailed salinity-growth curves are necessary, including data at salinities of ECw >30 dS m–1. Classical salt-tolerance ranks together all grasses as "tolerant" with no classes above the levels of a threshold ECw >10 dS m–1 or an ECw for 50% reduction of >22 dS m–1 (Maas, 1994). Halophytic grasses can exhibit considerable variation in salinity tolerance at well above these salinities and ranking classes should be developed that are appropriate to distinguish these classes.

Abbreviations: ECw, electrical conductivity of water • ECe, electrical conductivity of soil/saturated paste • CV, coefficient of variation • HI, Hawaii • K, Kopec • SIPV, Sea Island Paspalum vaginatum


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN RESPONSE TO SALINITY, plants can be classified into two groups. Halophytes are defined as salt-tolerant, salt-loving, or saltwater plants that have genetically, morphologically, and physiologically well-adapting characters in high salinity conditions where most of domesticated crops (salt-sensitive glycophytes) rarely survive (Glenn et al., 1999; Khan and Duke, 2001). Halophytes generally accumulate Na+ and Cl– in vacuoles and synthesize organic osmolytes in the cytoplasm, which facilitates water uptake into the plant and turgor-driven growth enhancement at low to moderate salinity levels (Flowers et al., 1977; Flowers and Yeo, 1986; Bell and O'Leary, 2003). In contrast, glycophytes (glyco or sweet) generally are ion excluders or lack sequestration and compartment of toxic ions into vacuoles or other tissues, which leads to lower water uptake and decrease in growth continuously with increasing salinity.

The initial step in development of salinity-tolerant cultivars is to identify genetic variability for salinity tolerance within ecotypes of a species. Direct selection criteria are based on growth or yield and provide important information for management decisions, such as the salinity level where 50% growth reduction is expected (Ayers and Westcot, 1976). Indirect selection criteria include ion uptake and allocation, osmotic adjustment, or other physiological parameters (Nobel and Rogers, 1992; Shannon, 1994). But, a salinity tolerance selection system based on realistic assessment criteria is a requirement (Subbarao and Johansen, 1994). However, before appropriate physiological criteria that salinity tolerance can be established, very tolerant ecotypes must first be identified on the basis of changes in growth characteristics (Duncan and Carrow, 1999; Flowers, 2004).

Direct selection criteria, such as shoot growth, crop yield, or biomass yield, have been the traditional means of salinity assessment for agronomic and horticultural crops expressed on a relative basis as a percentage of the nonsaline yield using salinities of ECe (electrical conductivity of a saturated soil paste extract) <30 dS m–1 (Maas and Hoffman, 1977; Mass, 1994; Shannon, 1994). For these glycophytic crops, the yield response curve to increasing salinity has been presented as two linear sections or a single linear line if growth immediately declines with increasing salinity. The first linear section is where increasing salinity does not affect growth, followed by a linear section where growth declines with increasing salinity. The two-phase growth response curve has three essential parameters used for classifying plant salinity tolerance (Maas, 1994; Shannon et al., 1994; Marcum, 2002): (i) threshold ECe, the maximum soil salinity that does not reduce yield below that obtained under nonsaline conditions (Maas, 1994); (ii) the slope of the section where increasing salinity reduces growth, which is represented as the yield decline per unit increase in salinity beyond the threshold ECe; and (iii) the ECe related to 50% growth reduction is used. Based on this traditional concept, a high threshold ECe value, a small decline in growth per unit of salinity, and/or a high ECe for 50% growth reduction would be associated with greater salinity tolerance (Ashraf, 1994; Subbarao and Johansen, 1994; Igartua, 1995).

Perennial turfgrasses require consideration of shoot, root, and total plant growth data in contrast to grain or fruit yield. Maintenance of ample shoot tissues, root plasticity, and a functional crown region are essential for maintaining uptake of water and nutrients and for the sufficient storage of carbohydrate reserves necessary for recovery from stress damage (Carrow and Duncan, 1996; Duncan and Carrow, 1997, 1999). Although researches investigating shoot, root, and verdure tissue at salinity levels >30 dS m–1 have been conducted on turfgrasses, no standardized evaluation criteria have been developed, especially for halophytic turfgrass ecotypes within a species (Dudeck and Peacock, 1985; Peacock and Dudeck, 1985; Marcum and Murdoch, 1994; Marcum, 1999). Marcum (1999) noted that many different criteria have been used to measure salinity tolerance of turfgrasses, such as shoot and root weight, shoot weight reduction relative to a nonsaline control, shoot/leaf length, visual scores of salinity injuries such as leaf firing, plant survival, and seed germination. Also, the ECe at 25 or 50% growth reduction of shoot or root tissues have been used for relative tolerance rankings (Maas and Hoffmann, 1977; Carrow and Duncan, 1998).

Assessment of salinity tolerance of halophytes is more complicated than in agronomic and horticultural plants that are primarily glycophytes or weak halophytes. When evaluating detailed growth response versus salinity curves for a number of seashore paspalum ecotypes for salinity tolerance, Lee (2000) observed that there are great variation in salinity tolerance among ecotypes, and many ecotypes exhibited the typical halophytic response of increased total plant growth with increasing salinity within the 5 to 20 dS m–1 range, followed by a decrease in growth (O'Leary, 1995; Marcum, 2002). Also, variations in salt tolerance and diverse screening methods have been investigated in other species including salt-sensitive to tolerant plants (Hester et al., 1996; Ulery et al., 1998; Munns et al., 2002; Bell and O'Leary, 2003; Munns and James, 2003). Due to diverse genetic and environmental variables involved in salinity responses, many researches were conducted to have the best set of evaluation criteria for the particular plants. Unfortunately, no recommendation of evaluation criteria has been made in halophytic turfgrasses which are increasingly demanding resources for the irrigation areas using poor quality water (brackish water, seawater, wastewater).

Specific issues were raised, which can influence salt-tolerance ranking or selection (Lee, 2000). First, the nature of the growth curves may differ from the standard two-linear phase model, especially for the most salt-tolerant types, where cubic curves have been noted at higher salinities (i.e., >30 dS m–1). The cubic growth curve prevents determining a slope factor for the most tolerant types, but not for those types exhibiting more normal growth curve. Second, threshold ECe could be determined in two different manners that would differ from one another. One can be based on when growth declines from the maximum (termed "threshold max ECe" in this paper), with the second based on when growth declines from the nonsaline control (termed "threshold ECe" in this paper). The selection of the threshold ECe complicates where the slope factor should be determined (i.e., starting at threshold max ECe or threshold ECe). Another issue is to use absolute growth that was much more informative than percent relative growth to the nonsaline control, because the most salt tolerance ecotypes were identified to exhibit sufficient growth yield to overcome such high salinity levels. Finally, since the traditional salinity classification criteria are less applicable to strong halophytes, the next question arises concerning what are appropriate direct screen criteria. A problem with classification of plants responding to salinity arises when we use the traditional system where any plant with a threshold ECe >10 dS m–1 or an ECe for 50% reduction of relative yield of >22 dS m–1 would be classified as "tolerant" with no classes above these levels (Maas, 1994).

As more emphasis is placed on selection and breeding of halophytic plants, it will be necessary to develop better classification and assessment criteria that address the inherent differences between glycophytic and halophytic growth responses (Subbarao and Johansen, 1994). In this paper we discuss how the issues previously noted may influence salinity assessment and suggest potential criteria that could improve salinity tolerance assessment or ranking for halophytic turfgrasses, as well as provide information to assist management decisions. The purpose of the current paper, therefore, is to use selected data to illustrate issues that arise in development of salinity assessment and classification criteria for halophytic grasses.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Growth Conditions and Treatments
This greenhouse study was conducted using a solution/sand culture system at the Georgia Agricultural Experiment Station (Griffin). Turfgrass materials included 93 seashore paspalum ecotypes. Plugs of each ecotype (2.5-cm diam) collected with a soil profile sampler in the nursery plots were established in a sand and peat mixture and allowed to acclimate under greenhouse conditions. After about 2 mo, the uniform plugs with washed roots were retransplanted to a slightly cone-shaped plastic pot, 5-cm diam by 20-cm depth filled with sand. Pots in a plywood frame were submerged up to sand surface in a modified, half-strength Hoaglands No. 2 solution contained in 28-L plastic containers. Fe-EDTA (Sprint 138, 6% Fe, Becker-Underwood, Ames, IA) at 2 mg L–1 was supplied to the solution (Hoagland and Arnon, 1950). Algae growth was controlled by covering all exposed surfaces with styrofoam pieces. The nutrient solution was aerated constantly, changed weekly, and maintained at a constant volume by adding tap water.

The sea-salt mixture (Dudeck and Peacock, 1985) was added at 0, 5.13, 12.42, 19.72, 27.02, and 34.32 g L–1 to each nutrient solution to achieve six salinity levels, where average ECw (electrical conductivity of water where ECw would approximately equal ECe under the test conditions) levels measured were 1.1, 8.5, 17.2, 24.8, 33.1, and 41.4 dS m–1, which were measured in solution (25°C) with an Orion 160 conductivity meter (Boston, MA). ECw levels were gradually increased by the addition of 7.3 g L–1 of sea salt mixture every day to avoid salt shock until the final ECw was achieved (Peacock and Dudeck, 1985).

Evaluation of Salinity Tolerance
Shoots were clipped at 2.5-cm height and discarded until final salinity levels were attained. Thereafter, shoot clippings above 2.5-cm height were collected biweekly and dried for the later measurement of total shoot growth. Grass materials including verdure (crown plus stem up to the mowing height of 2.5 cm) and roots were carefully removed from the pot, thoroughly washed, and immediately dried in the paper bag at the final shoot harvest time for determination of root and verdure dry weight. All harvested tissues were dried at 70°C for 48 h. Data from root and verdure dry weight were then combined with the shoot data to determine total growth.

For the evaluation of salinity tolerance of shoot, root and total growth, measurements included dry weight at ECw0, ECw24, ECw32, and ECw40 [biomass yield at ECw0 (control), 24.8, 33.1, and 41.4 dS m–1, respectively]; threshold ECw (salinity level from which growth declined below the nonsaline control, which is the traditional threshold ECw); threshold max ECw (salinity level from which growth started to decline from the point of maximum growth); ECw50% or ECw25% (salinity level indicating 50 or 25% growth reduction from the ECw0 growth baseline) for shoot growth and ECw25% for roots; and verdure growth at ECw0 and ECw40. Leaf firing (LF) based on percentage of leaves exhibiting visual symptoms of chlorosis or tissue desiccation, were rated on control and the 41.4 dS m–1 plots during the midpoint and at the end of the experiment.

Experimental Design and Statistical Analysis
The experimental design was a split-plot design with randomized arrangement of containers with 93 grasses, which were representing salinity levels in six replications. All data were statistically analyzed (SAS institute, 1988), by least significant difference (LSD) to separate the means among ecotypes within a salinity level, with emphasis on identifying the top (best) statistical grouping for the measured parameters. Therefore, all grasses that were found to be statistically similar to the best entry for the criterion were considered in the top group (Trenholm et al., 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nature of Growth Curves
Adalayd is a seashore paspalum that demonstrates a typically glycophytic response for shoot and total growth with decreasing growth as salinity increases (Fig. 1 and 2) . In contrast, Lee (2000) found that the most salt-tolerant seashore paspalums, such as SI 92 and SI 93-1, exhibited cubic shoot and total plant growth curves. For plant breeders desiring to identify the most salt-tolerant ecotypes of a species, knowledge of such response curves occurring for SI 92 at 18 dS m–1 and SI 93-1 at 24 dS m–1 (Fig. 2) is important because the nature of the cubic curves influence interpretation of the threshold ECw and slope factor used in ranking of ecotypes and management decisions (Ayers and Westcot, 1976). Changes in slope for SI 92 and SI 93-1 suggest the presence of salinity tolerance mechanism(s) that allows continued growth or even enhanced growth. Such mechanisms are present in only the most tolerant types and these mechanisms are the most important ones that may serve as indirect screening criteria (Nobel and Rogers, 1992; Shannon, 1994) or as important characteristics for biotechnological enhancement (Duncan and Carrow, 1999).


Figure 1
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  		Fig. 1. Growth responses of shoot dry weight of more tolerant (SI 93-1 and SI 92) and less tolerant (Adalayd) seashore paspalums at different salinity levels. Bars represent standard errors of the mean (n = 6).

 

Figure 2
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  		Fig. 2. Growth responses of total plant (shoot + root + verdure) of more tolerant (SI 93-1 and SI 92) and less tolerant (Adalayd) seashore paspalums at different salinity levels. Bars represent standard errors of the mean (n = 6).

 
The unique growth responses in highly salt-tolerant halophytes have several implications. Detailed response curves based on direct growth measurements provide considerable information relative to classification, management, and refinement of future screening protocols. Also, salinities can be identified related to when specific mechanisms may be triggered that are of physiological and biotechnological significance. While less detailed information may be required to screen a large number of selections, once the most salt-tolerant types are identified, development of detailed growth response curves are very important for these ecotypes to provide full information.

Threshold ECw, Threshold Max ECw, and the Slope Factor
Threshold ECe (or threshold ECw in the sand/water media) indicates "the maximum soil salinity that does not reduce yield below that achieved under nonsaline conditions" (Maas, 1994), where nonsaline growth would be the maximum growth for glycophytes. Threshold ECe is an important criterion used to determine salinity tolerance classification and it also has important management implications, since it identifies the highest salt level that the plant can maintain maximum growth before growth declines. Adalayd has a glycophytic growth curve with a threshold ECw of 1.1 dS m–1 for shoot and total plant growth (Fig. 1 and 2). The slope factor is determined starting at the point where growth first starts to decline, which is the threshold ECw for glycophytes, and ECw for 50% growth reduction would also be determined based on threshold ECw growth.

Halophytes have traditionally been considered to exhibit a bell-shaped response curve such as SI 93-1 (shoot growth, Fig. 1) and SI 91 and SIPV 43-2 (shoot growth, Fig. 3A) (Bell and O'Leary, 2003). For halophytes with bell-shaped salinity response curves, the salinity level corresponding to threshold ECw differs from that in glycophytes where maximum growth occurs. Instead, maximum shoot growth occurs within the 5 to 20 dS m–1 ranges and, thereafter, shoot growth declines with increasing salinity. At salinity levels higher than the maximum growth point, growth again equals nonsaline conditions and then declines to below nonsaline conditions, with this point being the threshold ECw. Threshold ECw for halophytes demonstrating a bell-shaped growth curve can be easily determined. For example, the shoot threshold ECw was 20, 17, and 37 dS m–1 for SI 93-1, SI 91, and SIPV 43-2, respectively (Fig. 1 and 3A), while the threshold max ECw measured at the point of maximum growth was 11, 9, and 11 dS m–1, respectively. However, calculation of the slope factor is less straight forward since it may differ depending on whether the initial starting point is at maximum growth or when it again equals nonsaline conditions. Also, calculation of ECw50% growth reduction is dependent on whether threshold ECw or threshold max ECw is used. It would seem logical for halophyte grasses demonstrating a bell-shaped curve to use the threshold ECe in a similar manner as glycophytes, with the slope factor determined from this point as well as the ECe50% growth reduction.


Figure 3
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  		Fig. 3. Comparison of absolute shoot dry weight (g) (A: top) and relative shoot dry weight (%) (B: bottom) of more tolerant (SI 91) and less tolerant (SIPV 43-2) seashore paspalums. Bars represent standard errors of the mean (n = 6).

 
Determination of threshold ECw, slope factor, and ECw for 50% growth reduction are even more confusing for highly salt-tolerant types that demonstrate a cubic curve, and includes ecotypes that show two equal growth maxima inflection points, such as total plant growth with maxima at ECw 1.1 and 18.0 (SI 93-1) and 1.1 and 33.0 dS m–1 (SI 92) (Fig. 2). Use of the first threshold ECw for total plant growth would indicate that neither of these grasses had good salt-tolerance, while use of the second threshold ECw would classify both as very salt-tolerant. For SI 92, the threshold ECw for shoot growth was only 8 dS m–1 (Fig. 1), yet this grass was the most salt-tolerant paspalum when all direct selection criteria were used (Table 1).


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  		Table 1. Salinity tolerance ranking of the top 15 seashore paspalum ecotypes determined on the basis of overall ranking across all parameters and rankings, which were based on only shoot, root, verdure, or total plant growth parameters and the data presented by Lee (2000).

 
The slope factor may differ depending on the choice of the threshold ECw used. In the case of these most salt-tolerant halophytes, observation of the growth curves would logically suggest that the threshold ECw at the highest salinity be chosen for the correct threshold ECw with the slope and ECw for 25 or 50% growth reduction determined from this point. But, growth curves must be developed over a sufficiently wide range of salinities to encompass all potential threshold ECw values, where some may occur at ECw >30 dS m–1 such as for SI 92 total plant growth (Fig. 2). For the most salt-tolerant ecotypes, the slope after the threshold ECw may not be consistent as demonstrated for SI 92 shoot growth (Fig. 1) and SI 93-1 total plant growth (Fig. 2), which would preclude the use of this parameter as classification criteria for these grasses. Thus, for the most salt-tolerant ecotypes, which have cubic growth curves for total plant growth or shoot growth, the traditional salinity classification criteria become more complicated in interpretation and use.

From the evaluation trial of salinity tolerance of 93 seashore paspalum ecotypes, a wide diversity in threshold ECw and threshold max ECw values were found among ecotypes for different tissues (Lee, 2000). Threshold ECw based on when growth declined below the nonsaline control resulted in a significant F test (P < 0.001 for shoot and 0.001 for total plant) and the CV was 82 and 104, respectively. However, when these criteria were used to classify salinity tolerance, 41 (shoot basis) and 57 (total plant basis) paspalums were ranked in the top statistical group out of 93 total entries, so threshold ECw was not very discriminating in terms of separating grasses for salinity tolerance. Threshold max ECw data did not demonstrate a significant F-test (P of 0.39 for shoots and 0.41 for total plant) and, therefore, could not be used to separate ecotypes for salinity tolerance (Lee et al., 2004a, 2004b).

Absolute Growth or Relative Growth by Salinity Level
Traditionally, relative salinity tolerance is based on crop tolerance expressed as yield (grain or shoot growth) decline at specific salinity levels compared with yield of the nonsaline control with results on a percentage or relative basis (Maas and Hoffmann, 1977). In terms of interactions among plant, soil, and surrounding environmental factors during field evaluation, relative yield response is beneficial where comparing salinity tolerance across crop species and environments. Under controlled environmental conditions (climatic and edaphic), sand–nutrient culture can provide initial preliminary information on genetic-based ecotype differences at specific salinity levels. In such controlled saline environments, absolute growth rate rather than relative growth is suggested as a better tool to separate salinity-tolerant ecotypes from less salinity-tolerant ones.

The difficulty in assessing salinity tolerance occurred when evaluation was based on relative growth for halophytic seashore paspalum ecotypes. Examples in Fig. 3 illustrate how evaluation on a relative versus absolute basis differ and influence salt-tolerance classification. On an absolute basis (Fig. 3A), the salinity-tolerant ecotypes, SI 91, exhibited significantly higher yields across all salinity levels and six out of seven evaluation categories were in the top statistical group, whereas less salinity-tolerant SIPV 43-2 exhibited a relatively low absolute growth rate and was found only one time in the top group (Lee, 2000). In contrast, on the basis of the traditional relative growth basis (Fig. 3B), shoot growth of the less tolerant SIPV 43-2 was significantly enhanced by higher salinity levels relative to ECw0, while salinity-tolerant SI 91 exhibited less shoot yield at salinity level >15 dS m–1 compared with that of nonsaline control. Since these two ecotypes had similar threshold ECw value (about 8 dS m–1), SIPV 43-2 might be selected as a higher salt-tolerant ecotype if the relative growth curve was used.

Inherently higher growth ability is important for perennial turfgrasses exposed to salinity stress, which often reduces growth and may enhance wear stress or decrease recovery from other injuries. Within the same species, selection of inherently higher yielding ecotypes is a meaningful strategy for enhancement of salinity tolerance, but this aspect is masked when the traditional relative growth approach is used (Shannon, 1982; Richards, 1983; Shannon, 1994). Improvement of salinity tolerance of agronomic crops has been achieved by simply selecting cultivars that had greater biomass yields (absolute growth) at the nonsaline control. Two turfgrass species with the greatest salinity tolerance, seashore paspalum (Hawaii selection) and St. Augustinegrass (Hawaii selection) (Stenotaphrum secundatum Walt.), also had the highest inherent shoot growth at the nonsaline control (Marcum and Murdoch, 1994). Glenn et al. (1997) and Pasternak (1987) noted that selections with inherently higher growth often demonstrated greater salt tolerance compared to those with lower growth.

Halophytic seashore paspalum ecotypes (i.e., SI 93-1 and SI 92 in Fig. 1 and 2, SI 91 in Fig. 3) also demonstrated significantly higher absolute shoot or total plant growth rates at ECw 0 compared to less tolerant ecotypes (Lee, 2000; Lee et al., 2004a). Across all grasses, absolute growth at ECw0 for shoots, total plant, and roots exhibited significant association with ECw40 absolute growth with an r = 0.37, 0.67, and 0.61 (P < 0.001), respectively, while nonsignificant with leaf firing (r = –0.11) (Lee et al., 2004a, 2004b). Thus, using absolute growth at nonsaline conditions may be useful for predicting salinity tolerance of halophytes, not because it implies salt tolerance but it does imply plant vigor necessary for salt-affected sites.

Absolute growth at threshold max ECw rather than the actual threshold ECw value would seem to be a more useful standard for comparison of superior salt tolerant cultivars or ecotypes when correlated with growth at higher salinity levels. Absolute growth at threshold max ECw indicates a maximum inherent growth rate that is higher than the nonsaline growth rate for the halophytic ecotypes such as SI 93-1 (Fig. 1). A relatively high correlation of growth rate at threshold max ECw with actual growth rate at ECw40 (r = 0.75, 0.80, and 0.70 for shoot, total plant, and root, respectively, with P < 0.001) was observed by Lee et al. (2004a)(2004b) and Lee (2000).

Unlike agronomic crops, perennial turfgrasses do not require 100% shoot yield at the higher salinity levels but they require reasonable shoot and root growth levels to maintain photosynthetic ability for carbohydrate production and wear tolerance (Carrow and Duncan, 1998). As a "single" criterion associated with salinity assessment among seashore paspalum ecotypes, absolute growth at the highest salinity level (i.e., ECw40) was the most critical parameter that was correlated with other parameters, such as growth at ECw0, ECw24, ECw32, and leaf firing, with r = 0.37, 0.88, 0.91, and –0.68, respectively, for shoot growth at P < 0.001 (Lee et al., 2004a, 2004b). For root growth at ECw 40, r = 0.61, 0.85, and 0.88, respectively, for root growth at ECw0, ECw24, and ECw 32.

Screening of Salinity Tolerance of Seashore Paspalum Ecotypes
Since absolute growth is a more meaningful way than relative growth to compare halophytic grass performance under increasing salinity stress, the question becomes "which screening parameters best differentiate seashore paspalums for salinity tolerance?" Basically we tried to use all parameters from shoot, root, verdure, and total plant growth and to screen ecotypes that have higher frequencies in the top statistical categories. Basing salinity tolerance screening only on shoot growth resulted in seven parameters including absolute growth at ECw0, 24, 32, and 40; ECw25%; ECw50%; and leaf firing exhibiting significant F-test values among entries. Nine ecotypes among 93 total entries were selected to be tolerant in shoot responses since those ranked in top statistical group in seven or six shoot parameters (8.6% of entries) (Table 1). Root growth parameters were also useful in selection of ecotypes for salinity tolerance with 5 parameters with significant F test and the top two categories (ranked in the top group in five or four parameters) included six grasses (6.1% of entries).

In cases where more rapid results are desired and growth data are sacrificed, less comprehensive screening criteria would be useful, such as when screening a large number of ecotypes. For assessment of salinity tolerance for halophytic turfgrasses, the minimum suggested criteria are absolute growth rate at nonsaline conditions and at the highest salinity level (i.e., ECw40 or other higher salinity level chosen) along with leaf firing at the highest salinity level. Leaf firing data can easily be obtained and is nondestructive to plant tissues. Leaf firing alone without growth information does not equate to salinity tolerance because a turfgrass that does not exhibit leaf firing may demonstrate very little growth at the same ECw (i.e., a stunted but green plant) and leaf firing represents only shoot performance. Growth at ECw40 and percentage leaf firing have a reasonable correlation coefficient (r = –0.68, P < 0.001), but using only leaf firing data would result in a substantial number of slow growing ecotypes being selected.

The current salt-tolerance ranking system that is usually used for glycophytic crops categorizes all grasses with a threshold ECe >10 dS m–1 or an ECe for 50% reduction of relative yield of >22 dS m–1 as "tolerant" with no classes above these levels (Maas, 1994). Halophytic grasses can exhibit considerable variation in salinity tolerance at well above these salinities and ranking system for the halophytic grasses should be developed, which are appropriate to distinguish those grasses in the classes where the current classification method is not enough. For example in seashore paspalums ecotypes, the most salt-tolerant ecotypes can be irrigated with seawater (ECw = 54 dS m–1) with 30 to 50% growth reduction, while Adalayd types tolerate about ECw = 20 to 25 dS m–1 for 50% growth reduction.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Appropriate, realistic salinity assessment criterion are essential to define the salinity tolerance and growth responses of halophytes (Subbarao and Johansen, 1994), but no consistent criterion has been used for halophytes as are used for glycophytes (Maas, 1994; Marcum, 2002). In this paper, seashore paspalum is used as a model halophytic grass to discuss issues related to development of better direct selection criteria. The assessment of salinity tolerance for halophytes should address growth at salinities of ECw >30 dS m–1 range, since they will often be used at high salinities. The bell-shaped growth typical of many seashore paspalums and especially the cubic curves typical of the most salt-tolerant ecotypes (top 4–5%) observed by Lee (2000) present challenges in using traditional salinity classification criteria- threshold ECe, slope of the growth curve where growth declines with increasing salinity, and ECw for 50% growth reduction. As summarized in Table 1, salinity tolerance may vary on tissue type used (shoot, root, and total plant) even among ecotypes in the same species. Traditional evaluation approaches using only top growth, therefore, might mislead, especially from the long-term field performance viewpoint. Comprehensive assessment (evaluation at low and high salinity levels as well as consideration of shoot, root, verdure, and total plant growth) would provide better insight into more defined salt-tolerance ranking of given ecotypes or species and expected field responses under a particular salinity level. Comprehensive salinity-growth curves for shoot and root responses also provide the most information for management decisions, including decisions of leaching requirement or turfgrass selection (Ayers and Westcot, 1976), and are suggested as essential for the most salt-tolerant ecotypes, especially since these types are the most likely to exhibit growth curves that make traditional salinity tolerance criteria less reliable. For screening of large numbers of ecotypes in a species, the minimum criteria to provide a high degree of salinity tolerance are suggested as: absolute growth at nonsaline conditions and at the highest salinity level where the grass would be expected to be exposed to, and leaf firing. The conclusions in this paper have been based on seashore paspalum ecotype responses to salinity. Many of the principles should apply to other halophytic grasses.


   ACKNOWLEDGMENTS
 
Funding from the U.S. Golf Association, Georgia Turfgrass Foundation Trust, and The University of Georgia is gratefully acknowledged.

Received for publication December 2, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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Crop Science 2005 45: xi. [Full Text]  




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