Tolerance of a Strandline Plant, Alternanthera maritima (mart.) A.ST.-HIL to Foliar Salt Spray: Suitable for Beach Landscaping  

Otitoloju Kekere
Department of Plant Science and Biotechnology, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria
Author    Correspondence author
International Journal of Horticulture, 2014, Vol. 4, No. 7   doi: 10.5376/ijh.2014.04.0007
Received: 24 Mar., 2014    Accepted: 09 Apr., 2014    Published: 15 Apr., 2014
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This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Kekere et al., 2014, Tolerance of a Strandline Plant, Alternanthera maritima (mart.) A.ST.-HIL to Foliar Salt Spray: Suitable for Beach Landscaping, International Journal of Horticulture, 2014, Vol.4, No.7 32-39 (doi: 10.5376/ijh.2014.04.0007)


Salt spray tolerant plants are important to landscapers due to high mortality and low aesthetic value of ornamental plants in coastal communities. Alternanthera maritima (Mart.) A.St.-Hil. a widely distributed strandline plant in West Africa, was treated with salt spray to investigate its response and suitability for seaside landscaping. Plants were sprayed with seawater at: two sprays/week (2SS), four sprays/week (4SS), six sprays/week (6SS) and control (CSS)-sprayed with deionized water. All plants survived, shoot growth and total biomass increased, rootshoot ratio and leaf total chlorophyll decreased, while root growth was unaffected under seawater treatments. Salt spray disrupted water balance through reduced xylem water potential and increased leaf and stem succulence for ion dilution. It also caused nutrient imbalance by accumulating Na+ and Cl- in the aerial parts and reduced some essential nutrients. N increased in the aerial parts showing the presence of salt-related nitrogen compounds for osmotic adjustment. Plants had reduced leaf area, stomata density and stomata number/leaf to minimize water loss. Except at 6SS, percentage necrotic leaf area and plant visual ratings did not differ significantly from the control. Alternanthera maritima is a salt spray tolerant, with adaptations for survival in the strandline. It is suitable for sea side landscaping.

Growth; Ecophysiology; Adaptations; Necrosis; Landscape

Strandlines are environments where litter, debris and many discarded items are left behind by the previous receding tide above the high water mark along the seashore (Rozema et al., 1982). They are usually colonized by few plant species due to the severity of the abiotic factors affecting growth (Rozema et al., 1982). Unlike the salt marsh, where plant species are exposed to tidal inundation and thus to high salinity (Flowers and Colmer 2008), the strandline is out of reach of mean high tide and only rarely flooded with seawater (Rozema et al., 1985). Thus, salt exposure at the strandline is mainly composed of salt sprays (Rozema et al., 1985; Griffiths et al., 2006; Griffiths, 2006; De Vos et al., 2010).

Much of the research conducted on salinity tolerance have focused on saline soil or saline irrigation (Alshammary et al., 2004; Hunter and Wu, 2005; Marcum et al., 2005). However, very little attention has been given to research on plant exposure to salt spray under non-saline irrigation conditions. It is well documented that plants are often more sensitive to saline spray than to salt applied at the root zone (Grattan et al., 1981; Elhaak et al., 1997). Salt spray can suppress plant growth because it causes water stress, disrupts membranes and enzyme systems, inhibits the uptake of nutrients, causes necrosis or loss of leaves and can lead to mortality (Scheiber et al., 2008). Salt spray has been reported to cause reduced shoot and root growth in Triplasis purpurea (Cheplick and Demetri, 1999), Leymus mollis (Gagne and Houle, 2002), Myrica pensylvanica (Griffiths and Orians, 2003) and Crambe maritima (De Vos et al.,2010). Recently, reduced leaf size, number of leaves and lateral branches and biomass were reported in Diodia maritima (Kekere and Bamidele, 2012), Commelina erecta subsp maritima (Kekere, 2013) and Kylinga peruviana (Kekere, 2014) sprayed with seawater. De Vos et al (2010) stated that the reduced leaf size in Crambe maritimasprayed with seawater minimized water loss through a reduction in surface area available for transpiration. They also reported accumulation of chloride and sodium ions which led to ion toxicity, and nutrient deficiency, which resulted in chlorophyll reduction. Salt spray was said to increase water content in some plants, which is an adaptation for ion dilution (Rozema et al., 1985; De Vos et al., 2010). Also, salt spray disrupts water balance in plants, and only the tolerant species can adjust osmotically through reduced xylem water potential (Griffiths and Orians, 2003; Griffiths, 2006). Thus, plant species growing in the strandline have adapted to salt spray in various ways (Rozema et al., 1985; De Vos et al., 2010).
Alternanthera maritima (Mart.) A.St.-Hil. (Beach Alternanthera) belongs to the family Amaranthaceae. It is an herbaceous, dicotyledonous and perennial plant with fleshy creeping stem, procumbent or prostrate and glaborous, rooting at nodes. Its leaves are narrow to base, sessile, blade oblong, oblanceolate, obovate or oval, succulent, apex obtuse to acute, mucronate and glabrous. Its Inflorescences are axillary, sessile, heads white to stramineous, globose to ovoid. Flower clusters axillary up to about 1.5 cm long, inconspicuous and silvery white on erect shoot. Seeds are subglobose and about 1.5 mm in size (Hutchinson et al., 1968). It is widely distributed in the coast of Africa where it often forms part of the major contributors to the biomass in the strandline. Since it grows naturally in the strandline, I hypothesized that it has some adaptations for survival in the area. A greenhouse experiment was therefore undertaken to determine the effect of different levels of salt spray on the growth of Alternanthera maritima and to have an insight into the ecophysiological adaptations underlying the responses.
Furthermore, landscaping and gardening projects in coastal regions have called for selection of plants that are tolerant to seawater sprays considering the high level of the death of sea side horticultural plants (Scheiber et al., 2008; Conolly et al., 2010). For landscape plantings to be successful, they must not only survive, but meet high aesthetic standards (Marcum et al., 2005). Many coastal plants have shown necrotic damage due to salt sprays as found in Solidago puberula, Solidago rugosa, Gaylussacia baccata and Quercus ilicifolia (Griffiths and Orians, 2003), Pinus rigida (Griffiths and Orians, 2004), Deschampsia caespitosa and Melica californica (Hunter and Wu, 2005), Miscanthus sinensis and Pennisetum Alopecuroides (Scheiber et al.,2008) and Diodia maritima (Kekere and Bamidele, 2012). However, there have been reports of plants that showed high resistance to necrotic damage (Griffiths and Orians, 2003; Kekere, 2013; Kekere, 2014). Landscape value is largely determined by the physical appearance of individual plants, and plants with high necrotic damage are not attractive in gardens and landscapes (Bernstein et al., 1972). In view of this, leaf necrosis, an aesthetically important symptom of damage was also assessed on the leaves of Alternanthera maritima in response to salt spray, in order to ascertain its suitability for landscape projects in coastal beaches.
1 Results
The physicochemical properties of the soil include: 5.48 pH, 20.42 ppm N, 3.56 ppm P, 3.56 (meg/100g) K, 2.32 (meg/100g) Ca, 2.60 (meg/100g) Mg, 8.2 (meg/100g) CEC, 3.67% C, 80.68% sand, 12.06% silt and 8.36% clay, which was characterized as a sandy soil. Survivorship was 100% for both plants sprayed with seawater and deionized water (Table 1). Saltwater sprays significantly decreased leaf area with increasing level of applications. Plants that were exposed to salt spray showeda greater stem girth, number of leaves, number of branches and plant height. Only stem girth and number of leaves were however significant when compared to the control (Table 1). Seawater treatments had effect on shoot but not root growth (Table 1). In fresh and dry mass variables (Table 2), stem and shoot mass were significantly higher under seawater treatment than in the control. Although, salt spray increased leaf mass, it was not significantly different from those sprayed with deionized water. In addition, total biomass and relative growth rate increased while root: shoot ratio and leaf total chlorophyll (LTC) decreased as a result of seawater application (Table 2).

Table 1 Percentage survival and some agronomic parameters of Alternanthera maritima after 12 weeks of exposure to different levels of salt spray

Table 2 Fresh and dry mass, root: shoot ratio, relative growth rate (RGR) and leaf total chlorophyll (LTC) of Alternanthera maritima after 12 weeks of exposure to different levels of salt spray

Except the root, air-borne salt application increased succulence in plant parts (Table 3). Leaf and stem moisture content increased over the control by approximately 19.80% and 5.69% respectively at the highest level of salt spray (6SS). Plant xylem water potential was lower under air-borne salinity treatment than did control plants, and they were significantly different from each other as the application level increased (Table 3). Mid-day xylem water potential values were lower than those of the predawn. Except for Ca2+ and Fe2+ in the stem, salt spray decreased the concentration of the essential elements in the shoot. N content increased in the aerial parts of salt-sprayed plants, but significantly differ from the control only in the leaf. Na+ and Cl- ions accumulated in the aerial parts of salt-treated plants with increasing level of salt applications, resulting in higher total ion uptake (Table 4) and percentage ash content (Figure 1). Na+ accumulation inhibited K+ uptake in the shoot leading to higher Na: K ratio in the leaf and stem (Table 4). In the root however, air-borne salt led to a decrease in Ca2+, Mg2+ and Fe2+, and an increase in Na+ and K+ content, none of the nutrients and their derivatives differ significantly from the control (Table 4).

Table 3 Water status of Alternanthera maritima after 12 weeks of exposure to different levels of salt spray

Table 4 Effect of salt spray on nutrient content (mmol/g dry weight) in the leaf, stem and root of Alternanthera maritima after 12 weeks of exposure to salt spray

Figure1 Percentage ash content in the shoot and root of Alternanthera maritima after 12 weeks of exposure to salt spray

Salt spray treatment reduced the stomata density (Figure 2) and number of stomata per leaf by as much as 69.23% and 80.79% respectively at the highest level of application (Table 5). The necrotic leaf area increased while the visual ratings decreased with increasing level of air-borne salinity, but differed significant from the control at 6SS (Table 5).

Figure 2 Stomata appearance on the abaxial leaf surface of Alternanthera maritima after 12 weeks of exposure to salt spray

Table 5 Some anatomical parameters measured on the leaf of Alternanthera maritima after 12 weeks of exposure to salt spray

The soil used for planting was a sandy soil with low nutrient. Beach plants grow naturally in very sandy soil that tends to be nutrient deficient, and because the soil is porous, leaching rate is high and salt does not accumulate in the root zone (Griffiths and Orians, 2003). The plant maintained high survivorship like the control despite salt sprays. This agrees with earlier findings on Leymus mollis by Gagne and Houle (2002). Salt spray tolerant plants occupy sea-side while sensitive species are eliminated and are found inland far away from the beach (Scheiber et al., 2008). Growth reduction has been reported on many coastal plants sprayed with seawater such as Leymus mollis (Gagne and Houlem, 2002) and Myrica. pensylvanica (Griffiths and Orians, 2003). Reduction in shoot elongation, number of branches and leaves was observed in Miscanthus sinensis and Pennisetum alopecuroides (Scheiber et al., 2008), Crambe maritima (De Vos et al., 2010) and Diodia maritima (Kekere and Bamidele, 2012). However, increase in growth suggested that the plant is tolerant to seawater spray, which affirmed that growth in some plant species is stimulated when sprayed with salt (Rozema et al., 1982). Similarly, Triplasis purpurea closest to sea shore (30~40 m) typically showed greater growth and reproduction relative to those farther from shore (80~90 m) (Cheplick and Demetri, 1999).
Reduction in leaf under salt stress in this study is similar to that of Pinus rigida (Griffiths and Orians, 2004), Crambe maritima(De Vos et al., 2010), Diodia maritima (Kekere and Bamidele, 2012) and Commelina erecta subsp maritima (Kekere, 2013) following exposure to salt sprays. Reduced leaf sizewas due to inhibition of leaf expansion and hence reduction of light interception (De Vos et al., 2010). Interestingly, leaf size was reduced but the general plant growth was enhanced. Reduction in leaf area can therefore be an adaptation for growth. Reduced leaf size provides decreased surface for salt deposition and water loss through transpiration,which are adaptations for water stress (Morant-Manceau et al., 2004). Reduction in chlorophyll content can be attributed to necrotic damage caused by Na+ and Cl- ions toxicity. When there are necrotic spots on the leaf, total photosynthesis and carbohydrate stored in the plant decrease Chlorophyll reduction can also be due to the deficiency of certain nutrients, some of which are important for normal growth and are part of chlorophyll ultrastructure (Touchette, 2009). Application of NaCl to plant foliage induced fragmented cuticles, disrupted stomata, collapsed cell walls, coarsely granulated cytoplasm, disintegrated chloroplasts and nuclei, and disorganized phloem thus reducing biomass (Touchette, 2009). Biomass increase in this study was as a result of the increase in growth parameters. This study showed that salt spray induced leaf and stem succulence. Also, leaf succulence increased in Crambe maritimasubjected to air-borne seawater spray (De Vos et al., 2010).

Increased succulence in the presence of salt is an adaptive mechanism for ion dilution (Rozema et al., 1985). In a previous study,
Griffiths and Orians (2003) reported a significant reduction in xylem water potential in Solidago puberula, Solidago rugosa, Gaylussacia baccata, Myrica pensylvanica, Pinus rigida and Quercus ilicifolia by salt sprays, indicating that salt spray caused water stress and might be inhibiting physiological processes in the plant. The increase of some nutrients and accumulation of Na+ and Cl- ions indicated that high concentrations of seawater can influence ions distribution, so that they can contribute to the osmotic potential, and thereby increase the protection against osmotic stress (Touchette, 2009). Much of N contents under NaCl salinity were probably used in synthesis of specific N compounds such as amino acids (e.g. proline and aspartic acids), amides (glutamine and asparagine) and the stress-related proteins (Ashraf and Harris, 2004). Plants exposed to salt usually absorb a large amount of Na+, which causes a decrease in the contents of K+ (Al–Karaki, 2000). Most salt tolerant plants accumulate Na+ in their shoots whereas sensitive plants do not, and a more efficient K+ uptake represents plant adaptation to salinity. Not only Na+ and K+ contents, but also the Na: K ratio can be used as phyto-physiological parameters for screening less sensitive plants for NaCl stress (Al–Karaki, 2000). A high Na: K ratio indicates metabolic disorders such as a reduction in protein synthesis and enzyme activities and an increase in membrane permeability (Al–Karaki, 2000). Moreover, elevated K+ levels act osmotically, preventing Na+ influx into roots and shoots (Al–Karaki, 2000). Reduced stomata density and stomata number/leaf under salt stress has also been reported on Kandelia candel (Hwang and Chen 1995). The fewer stomata on the leaves were to reduce entry points to salt spray and to minimize water loss through transpiration. Lower visual ratings in plants sprayed with seawater were the results of the presence of chlorotic and necrotic leaves, which conforms to the earlier studies on Miscanthus sinensis and Pennisetum alopecuroides (Scheiber et al., 2008).
3 Conclusion
This study gives an insight into the ecophysiological adaptations underlying the growth responses of Alternanthera maritima to air-borne salinity. Alternanthera maritima is a salt spray tolerant plant with some adaptations for survival in the strandline: (1). Reduction of water loss through decreased leaf size, absence of stomata on the adaxial surface and reduction of stomata density/stomata number per leaf in the abaxial leaf surface. (2). Adjustment to osmotic stress, by salt accumulation, probable production of quaternary amino compounds in the shoot and reduction of water potential. (3). Adaptation to ion toxicity through increased leaf and stem succulence for ion dilution. (4). Increase in K+ content, which acts osmotically to prevent Na+ influx into roots and shoots. Also, since landscape value is largely determined by the physical appearance of individual plants, Alternanthera maritima has high aesthetic value under salt spray, I recommend it to be planted as a landscaping plant on sites where salt spray is known to pose a problem.
4 Materials and Methods
4.1 Preparation of experimental plants
Uniform plants were raised in 20×26 cm perforated plastic pots filled with 2:1 mixture (v/v) of river sand to topsoil (Cheplick and Demetri, 1999; Khan et al., 2000) from the vegetative stem cuttings of Alternanthera maritima collected from Lekki Beach in Lagos, Southern Nigeria.
4.2 Experimental location and plant treatment
This experiment was carried out in the greenhouse of Plant Science and Biotechnology Department, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria (Lat. 70 N 281, Long. 5441 E). Filtered seawater was collected from Lekki Beach in Southern Nigeria on a single day in late July 2013 following the method described by Cheplick and Demetri (1999) and used by Griffiths and Orians (2003), De Vos et al. (2010) and Conolly et al. (2010). The seawater had salinity of 31 ppt and pH of 8.21 with sodium and chloride accounting for approximately 86% of the ions present. The seawater was stored in a 5-L plastic jug and kept in a refrigerator at 4°C and used forthe duration of the experiment. Meanwhile, before treatment commenced, 5 plants were randomly selected and used for the determination of initial growth parameters. Saltwater sprays were initiated on July 30 and lasted for 12 weeks. Plants were sprayed with seawater at: two sprays/week (2SS) -one spray on each of the two days), four sprays/week (4SS) -2 sprays on each of the two days, or six sprays/week (6SS) -three sprays on each of the two days, while plants sprayed with deionized water three times on each of the two days served as control. The control plants were sprayed to account for any physical effect that the spraying might have on plants. Before each salt spray treatment, plastic discs were placed over the soil surface and around the base of each plant to prevent salt deposition on the soil. Plants were sprayed at an interval of 4 hours beginning from 08:00 am in case of two and three sprays. At each spray, plants were taken outside and individual plant was sprayed with seawater to run off with a portable plant mist bottle heldabout20 cm from the side of each shoot. Salt deposited onto shoot was estimated following the method described by Cheplick and Demetri (1999) by using five plants not used in the experiment but grown with the experimental plants. Salt deposition onto shoot for 1 spray, 2 sprays and 3 sprays equaled on average 4, 8 and 12 mg NaCl dm-2 leaf area day-1, which fall within the levels found in the natural habitat of strandline plants (Rozema et al., 1982; Cheplick and Demetri, 1999; Griffiths, 2006). Plants in all treatments were randomly located onto a singlegreenhouse bench and randomly repositioned after each saltwatertreatment (twice weekly). Also, plants were watered from the top of the soil surface at the base of the plants once per week to flush out any salts that might have been deposited onto the soil during misting (Rozema et al., 1982, Cheplick and Demetri, 1999, Griffiths, 2006), so that the relative level of airborne salt deposited onto the shoots would be the primary cause of any observed effect rather than soil salinity or combined effect of both. This basal method of watering did not remove the salts deposited onto the shoots during the application of salt sprays. Salt was allowed to accumulate throughout the experiment, which is realistic in the field because in years with infrequent rain, salt spray is not washed off during the summer growing season (Cheplick and Demetri, 1999).
4.3 Measurement of agronomic parameters
Variables recorded were percentage survival, shoot length and stem girth. Stem length was measured from the soil level to the terminal bud with meter rule. Leaf area meter (LI-COR 300 model) was used to measure the area of the first three fully expanded leaves. Stem girth was measured at about 5 cm from stem base using a digital vernier caliper (model 0~200 mm). Only the leaves on individual plants and number of branches were counted.
4.4 Fresh and dry mass determination
After 12 weeks, plants were destructively harvested, partitioned into leaves, stems and roots. The roots were rinsed with water and the major ones were counted and their length measured. Fresh plant parts were weighed separately before their dry mass was measured after oven-drying to constant weight at 70°C. Derived variables were Root: Shoot ratio (root mass/shoot mass) and the relative growth rate-RGR- (ln mass2-ln mass1)/ time.
4.5 Water status and chlorophyll content
Percentage moisture content of plant parts was calculated with the formula: [(fresh mass– dry mass) / dry mass]×100. Plant xylem water potential was measured with a plant moisture-stress instrument (PMS Instrument Co. Oregon, USA) on six randomly selected stems from each treatment. Pre-dawn xylem water potential was taken between 06.00 and 07.00 am while mid-day xylem water potential was measured between 12:00 noon and 1:00 pm. Leaf total chlorophyll was extracted following the method of Arnon (1945) and calculated with the formula: (20.2×D645+8.02×D663) × (50/1000)×(100/5)×½, where D=absorbance.
4.6 Soil and phytochemical analyses
Dried plant and soil samples were assayed for mineral contents following the standard methods of the Association of Official Analytical Chemists (AOAC, 1985) in the Central Laboratory of The National Institute for Oil Palm Research (NIFOR), Nigeria.
4.7 Stomata number, necrosis and visual ratings determination
Leaf stomata and necrosis were estimated on the leaves used for the determination of moisture content. The adaxial and abaxial surfaces were covered with a layer of nail polish following the method used by Hwang and Chen (1995). This was left to dry and carefully removed from the leaf surface, placed on a microscopic slide and observed under the light microscope with digital camera attachment. The stomata was counted and expressed per unit leaf area. The stomata number per leaf was estimated as the product of stomata density and leaf area. The area of leaf tissue with necrotic damage was measured using a dot grid and expressed as the percentage of total leaf area showing necrosis (Griffiths and Orians, 2003; Griffiths et al., 2006). Visual ratings was conducted by six observers based on foliage appearance, with 1=no green foliage, 2=25% green foliage, 3=50% green foliage, 4=75% green foliage and 5=all green foliage (Scheiber et al., 2008). Although quality standards differ, researchers deem ratings of 1 and 2 as unacceptable, 3 as marginally acceptable, and ratings of 4 and 5 as acceptable in most professionally maintained landscape situations (Scheiber et al., 2008).
4.8 Experimental design and data analysis
The experiment was conducted in a completely randomized design with six single-plant replicates. Data were subjected to single factor ANOVA and means were separated with Turkey Honest Significant Difference (HSD) test with SPSS version 17.0 (SPSS Inc. Chicago, IL, USA) at P≤0.05.
I acknowledge Dr O. F. Olotuah and Dr. O. A. Obembe in the Department of Plant Science and Biotechnology, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria for proofreading the manuscript painstakingly and offer of valuable suggestions. I appreciate Mr Okerenwogba M. B. Mr Joseph Jack Udoh and Mr Omoruyi Efe in the research laboratories of the National Institute for Oil Palm Research (NIFOR) Nigeria for their support during the analyses of soil and plant samples.
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