Published in J. Environ. Qual. 32:2238-2244 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Plant and Environment Interactions
Contrasting Physiological Responses of Dwarf Sea-Lavender and Marguerite to Simulated Sea Aerosol Deposition
M. J. Sánchez-Blanco*,a,
P. Rodríguezb,
M. A. Moralesa and
A. Torrecillasa
a Centro de Edafología y Biología Aplicada del Segura (CSIC), P.O. Box 4195, E-30080 Murcia, Spain and Unidad Asociada al CSIC de Horticultura Sostenible en Zonas Aridas (UPCT-CEBAS)
b Centro de Edafología y Biología Aplicada del Segura (CSIC), P.O. Box 4195, E-30080 Murcia, Spain and Instituto Nacional de Ciencias Agrícolas (INCA), Gaveta Postal 1, 32700 San José de Las Lajas, La Habana, Cuba
* Corresponding author (quechu{at}cebas.csic.es).
Received for publication November 6, 2002.
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ABSTRACT
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Plants of two wild native species from littoral areas, marguerite [Argyranthemum coronopifolium (Willd.) C.J. Humphries] and dwarf sea-lavender [Limonium pectinatum (Aiton) O. Kuntze], grown in an unheated plastic greenhouse, were sprayed 2 to 3 min per day over a 7-d period with different aqueous solutions containing (i) an anionic surfactant (S1); (ii) a solution simulating the composition of sea aerosol (S2); (iii) a solution simulating sea aerosol with anionic surfactant (S3), and (iv) deionized water alone (control). The plant resistance to sea aerosol and the ability to recover from treatments were studied. By the end of the spraying period, marguerite showed a significant reduction in growth compared with control. However, most of the growth parameters were significantly unaffected in dwarf sea-lavender when plants were treated with sea aerosol containing surfactant. Measurements of water relations variables in marguerite showed a slight decrease in leaf turgor potential after spraying with sea aerosol containing surfactant. The surfactant enhanced the foliar absorption of salt in marguerite plants, inducing reductions in leaf stomatal conductance and causing such damage in the photosynthetic apparatus that the level of net photosynthesis decreased and had not recovered by the end of the experiment. The treatments had no effect on leaf stomatal conductance and photosynthesis rate in dwarf sea-lavender plants. The response of the species studied to sea aerosol was related to the degree of salinity tolerance. Although both species are wild native plants from littoral areas, marguerite is not salt tolerant and was the most sensitive to the sea aerosol treatments, while dwarf sea-lavender, a halophyte species, was more efficient at decreasing the toxic salt content of the tissues as its growth and ornamental characteristics were not affected.
Abbreviations: gl, leaf stomatal conductance • Pn, photosynthetic rate • RWC, relative water content • 
l, leaf water potential • p, leaf turgor potential • s, leaf osmotic potential • 
100%, leaf osmotic potential at full turgor
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INTRODUCTION
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IN COASTAL ECOSYSTEMS, abiotic factors including exposure to sea aerosol and saltwater infiltration of the ground water may well reduce plant growth and affect their reproduction (Hesp, 1991; Cheplick and Demetri, 1999). The contamination of seawater by synthetic surfactants has been the topic of many research studies during the past 30 years (Lapucci et al., 1972), since they may have a phytotoxic effect and produce severe damage in plants. The presence of surfactant enhances the foliar absorption of sea salt through stomatal (Greene and Bukovac, 1974) and cuticular (Schönherr and Bauer, 1992) penetration. Salt and surfactants may interfere with the regulation of stomatal aperture and thus upset the water balance of the leaf or the whole plant (Robinson et al., 1998). The effect of pollutants varies considerably and the same pollutant can, in different circumstances, cause stomata to open or close. Some of these alterations can be explained by disturbance of the mechanical functioning of the stomatal apparatus within the epidermis (Robinson et al., 1998).
It is difficult to generalize about the nature of the physiological changes caused by these surfactants because the variation in the plants' responses is considerable between and within species (Schönherr, 1993). In an experimental investigation of 29 species associated with the coastal habitat, all were described as tolerant to salt spray (Sykes and Wilson, 1988). However, some researchers have noted that the effects of salt spray ranges from increased mortality and leaf damage and/or decreased growth to relatively little or no damage in species that exhibit some degree of salt tolerance (Cartica and Quinn, 1980; Barbour et al., 1985; Sykes and Wilson, 1988). Other authors distinguish tolerance to sea aerosol from tolerance of soil salinity (Wang and Redmann, 1996; Greipsson and Davy, 1996.)
Therefore, the purpose of this study was to investigate the response to sea aerosol of species from saline habitats showing different levels of adaptation to soil salinity (Morales et al., 1997) and to examine the ability of treated plants to recover. Dwarf sea-lavender is a wild halophyte plant that can tolerate a wide range of salinity by controlling the salt content of leaves by means of salt glands (Morales et al., 2001). Marguerite is also a wild native species from littoral areas, but it is not salt tolerant (Morales et al., 1997).
The alterations in growth, water relations, and gas exchange that occur in dwarf sea-lavender and marguerite plants treated with sea aerosol, with and without surfactant, were investigated. The correlations between stomatal conductance and photosynthesis rate, relative water content and photosynthesis rate, and the effect of the sea aerosol and anionic surfactant applied to the leaves on these relationships were studied.
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MATERIALS AND METHODS
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Treatments
The experiment was conducted from April to June 1999 in an unheated plastic greenhouse placed in Santomera (Murcia, Spain). Plants were grown individually in 8- to 10-cm plastic containers filled with a mixture of silt loam, perlite, and clay (equal parts per volume). Plants of approximately equal size were selected at the beginning of the experiment. A randomized complete block design was set up using controls and treatments with three replications. The plants were irrigated daily with an automated drip irrigation system, which provided 500 mL per plant of nutrient solution (0.3 g L-1 NH4NO3, 0.15 g L-1 KNO3, 0.15 g L-1 NH4PO4H, 0.06 g L-1 Ca, 0.005 g L-1 Fe, and 0.005 g L-1 Zn). The maximum and minimum average temperatures were 30 and 10°C and the relative humidity ranged between 30 and 90%. The average maximum photosynthetically active radiation (PAR) was 1250 µmol m-2 s-1.
Plants of different species (dwarf sea-lavender and marguerite) were sprayed with different aqueous solutions: a solution containing an anionic surfactant (sodium dodecylbenzenesulfonate 82.5%, 50 mg L-1) (S1), a solution simulating the composition of sea aerosol (S2), and a solution with sea aerosol and anionic surfactant (S3). Another group of plants was used as control and was treated with deionized water alone. The treatment consisted of spraying the different solutions on the leaves until dripping point. Each treatment lasted 2 to 3 min and was repeated once per day over a 7-d period.
Measurements of Growth and Water Status
At the end of the application period (7 d) (t1) and 15 d later (t2), 48 plants per species (four per treatment and replicate) were harvested and stem, root, and leaf dry weight and leaf area (Delta-T Devices Ltd., Cambridge, UK) were determined.
Leaf water potential (l), leaf osmotic potential (s), leaf osmotic potential at full turgor (100%), and relative water content (RWC) were measured at midday in 15 plants of each species and treatment at the end of the application period (7 d) (t1) and 15 d later (t2) using leaves of the same age. Leaf water potential (l) was determined using a pressure chamber (Soil Moisture Equipment Co., Santa Barbara, CA) according to Scholander et al. (1965). Leaf osmotic potential (s) was estimated using a Wescor (Logan, UT) Model 5500 vapor pressure osmometer in excised leaves harvested and immediately frozen and stored at -30°C. Before the measurements, samples were thawed and leaf sap extracted for immediate osmotic potential determination according to Gucci et al. (1991). The leaves used for determining osmotic potential at full turgor (100%) were placed in plastic bags and allowed to reach full turgor by dipping the petioles in distilled water overnight. After that, the leaves were submitted to the procedure described above until they were measured in the vapor pressure osmometer. Leaf turgor potential (p) was estimated as the difference between l and s. The relative water content of leaves (RWC) was measured according to Barrs (1968). The leaf stomatal conductance (gl) and the photosynthesis rate (Pn) of attached leaves were measured using a LI-COR (Lincoln, NE) LI-1600 steady state porometer and a closed gas-exchange LI-COR LI-6200 portable photosynthesis system, respectively, and all measurements were taken immediately before each harvest.
Mineral Content
Five plants of each species (marguerite and dwarf sea-lavender) per treatment were harvested and washed with distilled water, dried at 70°C, ground, and stored at room temperature for inorganic solute analyses. The Cl- contents were analyzed in the aqueous extracts by potentiometric titration with 0.01 M AgNO3 in an automatic Mettler (Columbus, OH) DL40GP with the Lambert and Du Bois (1971) method. The Na+ contents were determined by atomic absorption spectrometry (Model AA-6701; Shimadzu, Kyoto, Japan) after a digestion of plant parts with HNO3 and HClO4 (2:1, v/v).
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RESULTS
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Sea aerosol affected the growth of each species differently. The total dry mass of marguerite was decreased significantly by treatment with sea aerosol, both with and without surfactant. Dry mass of dwarf sea-lavender was not affected (Table 1).
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Table 1. Root, stem, leaf, and total dry mass and total leaf area of marguerite and dwarf sea-lavender plants under four different spray treatments.
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In dwarf sea-lavender plants none of the variables related to growth (root, stem, and leaf dry mass) was affected significantly by any of treatments at t1. A small but significant reduction was noted in leaf area in dwarf sea-lavender treated with salt plus surfactant (S3) but other variables were not affected (Table 1). However, the treated plants of marguerite showed significant reductions in these parameters compared with control plants (Table 1). Stem and leaf dry mass, leaf area (Table 1), and leaf number (data not shown) decreased in all treatments, but more markedly in S2 and S3. This last treatment also caused a significant reduction in root dry mass (1.74 g compared with 3.06 g in control plants). Severe visual symptoms consisting of leaf chlorosis and necrosis were observed on plants of marguerite treated with sea aerosol (S2 and S3). Fifteen days after the end of treatments (t2) shoot, leaf, and root dry mass values recovered only in surfactant-treated plants (S1) (Table 1). In this treatment, the total leaf area and leaf number were similar to those observed in the control treatment. In the rest of the treatments (S2 and S3) no recovery was observed in any of the growth variables studied (Table 1). For dwarf sea-lavender plants, the differences detected in total dry mass in both sea aerosol treatments (S2 and S3) (Table 1) were due to the differences occurring in the dry weight of flowers (data not shown). The significant reduction in total leaf area observed previously in the combined treatment of surfactant plus salt (S3) was maintained at this time (t2) (Table 1).
Chemical analysis of the treated leaves of marguerite and dwarf sea-lavender showed higher Cl- and Na+ content in both sea aerosol treatments (S2 and S3) at the end of the application period (Table 2). While sodium content in marguerite was 1.4 times greater than observed in control treatment, chloride content was 3.3 times greater than present in control plants. However, the increases in sodium and chloride content observed in treated plants of dwarf sea-lavender were not as high as in marguerite (Table 2). Fifteen days after the plants were treated, a reduction in sodium and chloride content in S2 and S3 with respect to those found in the same treatments at t1 was observed in both species. Nevertheless, at that time (t2) those levels were greater than that of control plants. The highest concentrations of Cl- were detected in the sea aerosol treatments (S2 and S3) in marguerite leaves while the highest Na+ amounts were observed in treated (S2 and S3) dwarf sea-lavender plants (Table 2).
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Table 2. Concentrations of Na+ and Cl- in leaves of marguerite and dwarf sea-lavender plants under four different spray treatments.
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The leaf water potential (l) showed a slight but significant decrease in the marguerite treated plants (S1, S2, and S3) at t1. However, the leaf turgor potential (p) was significantly lower only in the plants treated with sea aerosol plus surfactant (Fig. 1)
. At the end of the experiment (t2) slight differences in l values of saline plants with respect to the control were found. The behavior of l and p in dwarf sea-lavender plants was very similar in all the treatments assayed and no changes in the variables studied were observed throughout the experimental period. Only a slight decrease of l in the treatment S2 at t2 was noted. No osmotic adjustment (decrease of osmotic potential at full turgor, 100%) was observed in either species in our experimental conditions (Fig. 1).
The responses of stomatal conductance (gl) and photosynthetic rate (Pn) in both species are shown in Fig. 2
. By the end of the application period, the gl values in marguerite had an important decrease in sea aerosol with surfactant treatment (S3). For Pn values, a decrease with similar extent in both sea aerosol treatments was observed. The photosynthetic rate in treated plants (S2 and S3) had fallen to 40% of that of control plants at the end of the spraying period (Fig. 2) and it did not recover to reach similar values to the control plants.
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Fig. 2. Leaf stomatal conductance (gl) and photosynthesis rate (Pn) in marguerite (A. coronopifolium) and dwarf sea-lavender (L. pectinatum) plants under control, S1, S2, and S3 treatments, at the end of the application period (t1) and 15 d later (t2). Each histogram represents the mean of five plants. Vertical bars on each histogram represent the standard error.
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However, in dwarf sea-lavender plants, there were no differences in gl or Pn between treatments at any time of the experiment. A similar behavior was observed in the photosynthesis rate in this species (Fig. 2). The gl and Pn levels found in this species were always lower than in marguerite.
The relationship between midday Pn and gl during the end of the application and recovery periods was linear for all treatments of marguerite (Fig. 3)
, indicating, in general, that an increase in gl is associated with an increase in Pn. However, the relationship was different depending on the treatment applied. The covariance test performance showed differences in the slopes of the different treatments. The same increase in gl was associated with a higher increase in Pn in plants of control and S1 treatments than in S2 and S3 treatments. The values of Pn for both sea aerosol treatments for a similar gl value were lower than in the corresponding control and S1 treatments. The combined data for dwarf sea-lavender plants indicate a uniform behavior in all treatments (Fig. 4)
. The covariance test showed no differences in the slopes or intercepts between treatments, thus, a given value of gl was associated with a similar value of Pn, independently of the treatment.
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Fig. 3. Relationship between leaf stomatal conductance (gl) and photosynthesis rate (Pn) levels at midday in marguerite (A. coronopifolium) plants under control (Pn = 0.04gl + 3.45; r2 = 0.84, significant at the 0.001 probability level [***]), S1 (Pn = 0.04gl + 0.70; r2 = 0.96***), S2 (Pn = 0.02gl + 1.54; r2 = 0.87***), and S3 treatments (Pn = 0.01gl + 4.15; r2 = 0.75***) during the experimental period.
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Fig. 4. Relationship between leaf stomatal conductance (gl) and photosynthesis rate (Pn) levels at midday in dwarf sea-lavender (L. pectinatum) plants under control (Pn = 0.04gl + 0.56; r2 = 0.92, significant at the 0.001 probability level [***]), S1 (Pn = 0.03gl + 1.19; r2 = 0.95***), S2 (Pn = 0.04gl + 0.005; r2 = 0.95***), and S3 treatments (Pn = 0.05gl - 0.13; r2 = 0.86***) during the experimental period.
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A similar behavior was observed in the relationship between Pn and relative water content (RWC) during the experimental period (Fig. 5)
. In marguerite an increase in RWC provoked higher levels of Pn in the control and S1 treatments than in S2 and S3 (Fig. 5). For a given RWC, the photosynthetic rate was significantly lower in the presence of sea aerosol treatments. In contrast, this relationship was similar for all treatments in dwarf sea-lavender plants (Fig. 6)
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Fig. 5. Relationship between relative water content (RWC) and photosynthesis rate (Pn) levels at midday in marguerite (A. coronopifolium) plants under control (Pn = 0.98RWC - 75.35; r2 = 0.92, significant at the 0.001 probability level [***]), S1 (Pn = 0.60RWC - 43.74; r2 = 0.79***), S2 (Pn = 0.39RWC - 30.30; r2 = 0.80***), and S3 treatments (Pn = 0.31RWC - 20.54; r2 = 0.61***) during the experimental period.
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Fig. 6. Relationship between relative water content (RWC) and photosynthesis rate (Pn) levels at midday in dwarf sea-lavender (L. pectinatum) plants under control (Pn = 0.07RWC - 1.68; r2 = 0.94, significant at the 0.001 probability level [***]), S1 (Pn = 0.13RWC - 7.6; r2 = 0.92***), S2 (Pn = 0.11RWC - 6.0; r2 = 0.94***), and S3 treatments (Pn = 0.06RWC - 1.52; r2 = 0.95***) during the experimental period.
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DISCUSSION
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Trials conducted in a controlled environment showed the different degrees of susceptibility of the two species tested. The most sensitive to sea aerosols was marguerite, in which saltwater sprays reduced its growth and dry mass, while the effect on dwarf sea-lavender plant growth was not pronounced (Table 1). Researchers have noted that the effect of airborne salt depends on the species, ranging from increased mortality, leaf damage, and/or decreased growth, as it was seen in marguerite, to relatively little damage as in the case of dwarf sea-lavender. According to Sykes and Wilson (1988) and Greipsson et al. (1997), the second type of behavior is associated with species that exhibit some level of salt tolerance. In our case, the effect was clear, since marguerite, although a wild native from coastal areas, is not a salt-tolerant species (Morales et al., 1998). However, salts can be directly excreted from the leaves of Limonium spp. when plants are submitted to saline nutrient solution (Alarcón et al., 1999). In such conditions, dwarf sea-lavender was not affected to any significant degree by salts because the Na+ and Cl- excretion rate through salt glands is higher even than in other Limonium genotypes (Morales et al., 2001).
The effect of the synthetic surfactant alone (S1) was evident in marguerite plants, since the plants submitted to this treatment presented a significant reduction in the stem and leaf growth (Table 1). In this sense, some authors have also discussed the possibility that surfactants may have a direct phytotoxic effect (Bussotti et al., 1995). However, the fact that saline-treated plants with and without anionic surfactant (S2 and S3) presented greater alterations in these growth variables than in plants treated only with surfactant (S1) suggests that there was a toxic effect due to the salt deposition on the leaves. This hypothesis was confirmed by the chemical analysis of the leaves sampled from the marguerite plants (Table 2). In this species, a correlation between leaf tissue Na+ and Cl- accumulation, especially by the chloride content and the severity of the symptoms like the reductions in the growth (Table 1) was observed. Polizzi (1995) also noted that the damage was mainly caused by chloride accumulation in many species exposed to sea aerosol treatments.
De Herralde et al. (1998) pointed out that marguerite undergoes important morphological changes during saline stress, even when the time period of the treatments was relatively short (15 d). Morales et al. (1997) working with marguerite exposed to 70 mmol L-1 NaCl nutrient solution observed reductions in the relative growth rate, net assimilation rate, and leaf area ratio indicating that both the leaf expansion and photosynthetic rate were affected by the salt. Nevertheless, the tolerance to salt exposure depends on the prevention of salt accumulation in the shoot (Crawford, 1989) and the strategies that may minimize the salinity effects (Yeo and Flowers, 1986; Erdei et al., 1990). Cheplick and Demetri (1999) also suggested that the relative level of airborne salt deposited onto the shoot was the primary cause of the growth reduction in greenhouse conditions. In our experiment, salt was not effectively excluded from the mesophyll in marguerite in response to salinity. One important aspect of salt tolerance is the ability of a plant to exclude and compartmentalize toxic ions, such as Na+ and Cl- (Boursier and Läuchli, 1989).
The study of the leaf water relations showed that no osmotic adjustment occurred in either of the species studied during the experimental period (Fig. 1). In spite of this and because dehydration (decrease of l) was slight in S1 and S2 treated marguerite plants, the turgor potential was maintained at the end of the application (Fig. 1). However, in the plants treated with salt and surfactants (S3), the higher foliar dehydration provoked a significant decrease in the values of leaf turgor potential. Nevertheless, the maintenance of turgor in the other treated plants (S1 and S2) did not permit the maintenance of growth (Table 1), confirming that cell enlargement is not always closely correlated with the maintenance of turgor because it also depends on metabolic processes (Kramer, 1988; Van Volkenburgh and Boyer, 1985). In this experiment, the presence of Na+ and Cl- in the sea aerosol produced a toxic effect in marguerite plants, and reduced growth, leaf stomatal conductance, and photosynthesis (Table 1 and Fig. 2). This effect was more marked by the presence of surfactant.
The foliar absorption of sea salt via leaves occurs through both stomatal (Greene and Bukovac, 1974) and cuticular (Schönherr and Bauer, 1992) penetration. Such absorption reduced leaf stomatal conductance and photosynthesis but especially Pn in marguerite plants (Fig. 2). Since for a given increase in the stomatal aperture, the increase in the photosynthesis rate was lower for sea aerosol treatments (Fig. 3), it is possible that there was direct toxicity or damage to the photosynthetic apparatus (e.g., choloroplasts). In this sense, the decrease in Pn in treated plants seems to be due to nonstomatal factors. According to Morales et al. (1998), the cellular structure of these plants is affected even when the salt is taken through the roots, inducing variations in the chloroplasts. In our treatments, the injuries sustained in marguerite did not allow the level of net photosynthesis to recover by the end of the experimental period (Fig. 2). This fact is not observed in dwarf sea-lavender plants (Fig. 2), in which the salt content of treated plants did not increase as much as treated plants of marguerite (Table 2), probably because the toxic ions could be directly excreted from leaves via salt glands or excluded from roots (Gorham, 1996; Morales et al., 2001). The salt-treated plants of marguerite (S2 and S3) presented lower values of photosynthesis for the same tissue water status (Fig. 5).
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CONCLUSIONS
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The response of the studied species to the sea aerosol is related to the degree of salinity tolerance. Although both species are wild natives from coastal areas, marguerite, a non-salt-tolerant species, was the more sensitive to the sea aerosol treatments, while dwarf sea-lavender, a halophyte species, was more efficient at decreasing the toxic salt content of the tissues by excretion through the salt glands, as pointed out by Morales et al. (2001). Its growth and ornamental characteristics were not affected. The accumulation of salts in the tissue of marguerite from the sea aerosol, especially by the chloride content, reduced photosynthesis, an effect that was independent of plant relative water content.
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ACKNOWLEDGMENTS
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The authors are grateful to M.D. Velasco and M. Garcia for their technical assistance. This research was supported by FEDER (1FD97-0420-C02-02) and Séneca (PI- 75/00819/FS/01). P. Rodríguez was a recipient of a MUTIS research fellowship from the Agencia Española de Cooperación Internacional (AECI).
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ABSTRACT
INTRODUCTION
